Yunwei Sheng, Ignasi Fina, Marin Gospodinov, and Josep Fontcuberta
Appl. Phys. Lett. 118, 232902 (2021); DOI: doi.org/10.1063/5.0053379
Hexagonal manganites, such as h-LuMnO3, are ferroelectric with its polar axis along the hexagonal axis and have a narrow electronic bandgap (≈1.5 eV). Using Pt electrodes, h-LuMnO3 single crystals display a strong rectification, characteristic of a Schottky diode, and a large photoresponse. It is found that the short circuit photocurrent density Jsc along the polar axis is modulated (up to 25%) by the direction of the ferroelectric polarization P, leading to a short circuit photocurrent loop that mimics the ferroelectric polarization. However, a non-switchable Jsc persists. Diffusion photocurrent is shown to dominate current-in-plane measurements and contributes to the non-switchable Jsc. This observation illustrates the dramatic role of the large optical absorption in hexagonal manganites. The accompanying optical dichroism might challenge disentangling a genuine bulk photovoltaic response in h-LuMnO3 single crystals, contributing to the non-switchable Jsc. Epitaxial thin films may offer a suitable alternative.
Mathieu Mirjolet , Hari Babu Vasili, Adrian Valadkhani, José Santiso , Vladislav Borisov, Pierluigi Gargiani, Manuel Valvidares, Roser Valentí , and Josep Fontcuberta
Phys Rev. Materials 5, 095002 (2021), DOI:10.1103/PhysRevMaterials.5.09500212K
Strain is a knob that allows tuning the electronic bandwidth or the electronic distribution within the atomic orbitals is solids, to promote dramatic changes of their properties or even to induce emerging ones, such as metal-insulator transitions, ferroelectricity, ferromagnetism or superconductivity, to mention a few.
SrVO3 is an excellent metal of the highest interest in technology. Epitaxial strain affects the electronic conductivity supposedly by changing the electronic distribution within the 3d(t2g) orbitals and their corresponding bandwidth.
Here, we show that the role of strain is more involved. It turns out that the V-O hybridization is substantially modified by unbalancing the 3d(t2g)-2p molecular orbital mixtures. It follows that not only the orbital hierarchy of the (xy, xz and yz) orbitals of the t2g manifold is modified but also the amount of electrons at 3d(t2g) orbitals of the metal ion also change with strain. In other words, a rigid band picture does not hold even in this seemingly simple metallic oxide
Tingfeng Song, Romain Bachelet, Guillaume Saint-Girons, Nico Dix, Ignasi Fina and Florencio Sanchez
J. Mater. Chem. C, 2021, DOI: doi.org/10.1039/D1TC02512K
Stabilization of the orthorhombic phase of HfO2 with La allows very high polarization and endurance. However, these properties have not been confirmed yet in films having thickness of less than 10 nm. We have grown (111)-oriented La (2 at%) doped epitaxial HfO2 films on SrTiO3(001) and Si(001) substrates, and we report on the thickness dependence of their ferroelectric properties.
Films of less than 7 nm have a high remanent polarization of about 30 µC cm-2, show slight wake-up, endurance of at least 1010 cycles and retention of more than 10 years, both latest properties measured at the same poling voltage. La-doped HfO2 films even as thin as 4.5 nm also show robust ferroelectric properties.
M. Mirjolet, F. Rivadulla, P. Marsik, V. Borisov, R. Valentí, J. Fontcuberta,
J. Adv. Sci. 2021, 2004207. https://doi.org/; DOI:doi.org/10.1002/advs.202004207
- The nature of electron-electron and electron-lattice interactions in metallic oxides is revised. The common wisdom is that the strong correlations among electrons determine their properties.
- Here we argue that the unavoidable coupling between free electrons and the lattice in ionic materials leads to the formation of polarons. These are carriers dressed by a lattice distortion that travel with them and largely determine the transport and some optical properties.
- Moreover, we argue that in early transition metal oxides, the Fermi surface has a cylindrical shape that limits the phonons available for scattering.
- Taking SrVO3as illustrative example of a Fermi liquids, we demonstrate that both mechanisms can contribute to the ubiquitously observed quasi-T2temperature dependence of the electrical resistivity in many metallic oxides.
- A new twist on the physics of Fermi liquids in ionic lattices.
Mathieu Mirjolet, Mikko Kataja, Tommi K. Hakala, Philipp Komissinskiy, Lambert Alff, Gervasi Herranz, Josep Fontcuberta,
Adv. Opt. Materials First published: 14 June 2021; DOI:doi.org/10.1002/adom.202100520
From catalysis and flat panel displays to photovoltaics, transparent and conducting transition metal oxides are gaining momentum toward more sustainable and cost-efficient applications.
Here it is shown that, without using phase-matching arrangements, bulk plasmons can be excited in continuous epitaxial films of metallic SrVO3 and SrNbO3, with plasma absorption edges at visible range, and tuned mainly by electron correlations and phonon dressing.
Films can be made reflective or transparent at whish.
Tingfeng Song, Huan Tan, Nico Dix, Rahma Moalla, Jike Lyu, Guillaume Saint-Girons, Romain Bachelet, Florencio Sánchez, and Ignasi Fina
ACS Appl. Electron. Mater. 2021, 3, 5, 2106–2113 (2021); DOI:doi.org/10.1021/acsaelm.1c00122
Systematic studies on polycrystalline Hf1–xZrxO2 films with varying Zr contents show that HfO2 films are paraelectric (monoclinic). If the Zr content is increased, films become ferroelectric (orthorhombic) and then antiferroelectric (tetragonal). HfO2 shows very good insulating properties and it is used in metal-oxide-semiconductor field-effect devices, while ZrO2 shows good piezoelectric properties, but it is antiferroelectric. In between, Hf0.5Zr0.5O2 shows good ferroelectric properties at the expense of poorer insulating and piezoelectric properties than HfO2 and ZrO2, respectively.
Here, we explore the ferroelectric, insulating, and piezoelectric properties of a series of epitaxial films of Hf1–xZrxO2 with different compositions. We show that epitaxial growth permits the stabilization of the ferroelectric phase in a whole range of Zr content (from x = 0 to x = 1). In epitaxial ZrO2 films, ferroelectricity coexists with better piezoelectric and insulating properties than Hf0.5Zr0.5O2, and in HfO2 epitaxial films, ferroelectricity coexists with better insulating properties than Hf0.5Zr0.5O2. For the case of ZrO2 films, large electroresistance is also observed. In both cases, the ferroelectric endurance is poorer than that for Hf0.5Zr0.5O2.
Saúl Estandía, Jaume Gazquez, Maria Varela, N. Dix, Mengdi Qian, Raul Solanas, Ignasi Fina and Florencio Sanchez
J. Mater. Chem. C, 9, 3486 – 3492 (2021); DOI:doi.org/10.1039/D0TC05853J
Epitaxial orthorhombic Hf0.5Zr0.5O2 (HZO) films on La0.67Sr0.33MnO3 (LSMO) electrodes show robust ferroelectricity, with high polarization, endurance and retention. However, no similar results have been achieved using other perovskite electrodes so far. Here, LSMO and other perovskite electrodes are compared.
A Small amount of orthorhombic phase and low polarization is found in HZO films grown on La-doped BaSnO3 and Nb-doped SrTiO3, while null amounts of orthorhombic phase and polarization are detected in films on LaNiO3 and SrRuO3.The critical effect of the electrode on the stabilized phases is not consequence of differences in the electrode lattice parameter. The interface is critical, and engineering the HZO bottom interface on just a few monolayers of LSMO permits the stabilization of the orthorhombic phase. Furthermore, while the specific divalent ion (Sr or Ca) in the manganite is not relevant, reducing the La content causes a severe reduction of the amount of orthorhombic phase and the ferroelectric polarization in the HZO film.
Ignasi Fina* and Florencio Sánchez*
ACS Appl. Electron. Mater. 2021, 3, 4, 1530-1549; DOI:doi.org/10.1021/acsaelm.1c00110
About ten years after ferroelectricity was first reported in doped HfO2 polycrystalline films, there is tremendous interest in this material and ferroelectric oxides are once again in the spotlight of the memories industry. Great efforts are being made to understand and control ferroelectric properties. Epitaxial films, which have fewer defects and a more controlled microstructure than polycrystalline films, can be very useful for this purpose. Epitaxial films of ferroelectric HfO2 have been much less investigated, but after the first report in 2015 significant progress has been achieved.
This review summarizes and discusses the main advances on epitaxial HfO2, considering growth, study of structural and ferroelectric properties, identification of the ferroelectric phase, and fabrication of devices. We hope this review will help researchers investigating epitaxial HfO2. It can also help extend the interest of the ferroelectric HfO2 community, now basically focused on polycrystalline samples, to epitaxial films.
Xiao Long, Huan Tan, Florencio Sánchez, Ignasi Fina, Josep Fontcuberta
Nature Communications volume 12, Article number: 382 (2021); DOI:10.1038/s41467-020-20660-9
In the quest for energy efficient and fast memory elements, optically controlled ferroelectric memories are promising candidates. Here, we show that, by taking advantage of the imprint electric field existing in the nanometric BaTiO3 films and their photovoltaic response at visible light, the polarization of suitably written domains can be reversed under illumination. We exploit this effect to trigger and measure the associate change of resistance in tunnel devices. We show that engineering the device structure by inserting an auxiliary dielectric layer, the electroresistance increases by a factor near 2 × 103%, and a robust electric and optic cycling of the device can be obtained mimicking the operation of a memory device under dual control of light and electric fields.
J. W. Adkins, I. Fina, F. Sanchez, S. R. Bakaul, and J. T. Abiade
Appl. Phys. Lett. 117, 142902 (2020);
Herein, we report a cryogenic-temperature study on the evolution of the ferroelectric properties of epitaxial Hf0.5Zr0.5O2 thin films on silicon. Wake-up, endurance, and fatigue of these films are found to be intricately correlated, strongly hysteretic, and dependent on available thermal energy. Field-dependent measurements reveal a decrease in polarization with temperature, which has been determined not to be an intrinsic change of the material property, rather a demonstration of the increase in the coercive bias of the material. Our findings suggest that a deficiency in thermal energy suppresses the mobility of defects presumed to be oxygen vacancies during wake-up and trapped injected charge during fatigue, which is responsible for polarization evolution during cycling. This permits accelerated wake-up and fatigue effects at high temperatures where thermal energy is abundant but delays these effects at cryogenic temperatures.
Judith Oró‐Solé, Ignasi Fina, Carlos Frontera, Jaume Gàzquez, Clemens Ritter, Marina Cunquero, Pablo Loza‐Alvarez, Sergio Conejeros, Pere Alemany, , Enric Canadell, Josep Fontcuberta, Amparo Fuertes;
Angew. Chem. Int. Ed. 2020, 59, 18395–18399; DOI:10.1002/anie.202006519
In the quest for materials of narrow band gap, oxynitrides are excellent candidates. Aiming at obtaining a polar materials of narrow gap, oxynitides are developed here having an hexagonal structure and anion ordering that leads to a non centrosymmetric structures. As such, here BaWON2 is synthesized and we demonstrate its polar character and large photo absorptivity.
F. Macià and A. D. Kent
Journal of Applied Physics 128, 100901 (2020)
Magnetic droplet solitons are dynamical magnetic textures that form due to an attractive interaction between spin waves in thin films with perpendicular magnetic anisotropy. Spin currents and the spin torques associated with these currents enable their formation as they provide a means to excite non-equilibrium spin-wave populations and compensate their decay. Recent years have seen rapid advances in experiments that realize and study magnetic droplets. Important advances include the first direct x-ray images of droplets, determination of their threshold and sustaining currents, measurement of their generation and annihilation time, and evidence for drift instabilities, which can limit their lifetime. This perspective discusses these studies and contrasts these solitons to other types of spin-current excitations, such as spin-wave bullets, and static magnetic textures, including magnetic vortices and skyrmions. Magnetic droplet solitons can also serve as current controlled microwave frequency oscillators with potential applications in neuromorphic chips as nonlinear oscillators with memory.
B. von Boehn, M. Foerster, J. Prat , F. Macià, B. Casals, M. Waqas Khaliq, A. Hernández-Mínguez, L. Aballe, R. Imbih
Surface acoustic waves (SAW) allow to manipulate surfaces with potential applications in catalysis, sensor and nanotechnology. SAWs were shown to cause a strong increase in catalytic activity and selectivity in many oxidation and decomposition reactions on metallic and oxidic catalysts. However, the promotion mechanism has not been unambiguously identified. Using stroboscopic X‐ray photoelectron spectro‐microscopy we were able to evidence a sub‐nanosecond work function change during propagation of 500 MHz SAWs on a 9 nm thick platinum film. We quantify the work function change to 455 µeV. Such a small variation rules out that electronic effects due to elastic deformation (strain) play a major role in the SAW‐induced promotion of catalysis. In a second set of experiments SAW‐induced intermixing of a five monolayers thick Rh film on top of polycrystalline platinum was demonstrated to be due to enhanced thermal diffusion caused by an increase of the surface temperature by about 75 K when SAWs were excited. Reversible surface structural changes are suggested to be a major cause for catalytic promotion.
J. Lyu, I. Fina, and F. Sánchez
Appl. Phys. Lett. 117, 072901 (2020);
The growth window of epitaxial Hf0.5Zr0.5O2 is established taking into account the main ferroelectric properties that films have to present simultaneously: high remanent polarization, low fatigue, and long retention. Defects in the film and imprint field depend on deposition temperature and oxygen pressure, with an impact on fatigue and retention, respectively. Fatigue increases with substrate temperature and pressure, and retention is short if low temperature is used. The growth window of epitaxial stabilization of ferroelectric Hf0.5Zr0.5O2 is narrower when all major ferroelectric properties (remanence, endurance, and retention) are considered, but deposition temperature and pressure ranges are still sufficiently wide.
Y. Chen, Y. Lechaux, B. Casals, B. Guillet, A. Minj, J. Gázquez, L. Méchin, and G. Herranz
Phys. Rev. Lett. 124, 246804 (2020)
Persistent photoconductance is a phenomenon found in many semiconductors, by which light induces long-lived excitations in electronic states. Commonly, persistent photoexcitation leads to an increase of carriers (accumulation), though occasionally it can be negative (depletion). Here, we present the quantum well at the LaAlO3/SrTiO3 interface, where in addition to photoinduced accumulation, a secondary photoexcitation enables carrier depletion. The balance between both processes is wavelength dependent, and allows tunable accumulation or depletion in an asymmetric manner, depending on the relative arrival time of photons of different frequencies. We use Green’s function formalism to describe this unconventional photoexcitation, which paves the way to an optical implementation of neurobiologically inspired spike-timing-dependent plasticity.
M. Cervo Sulzbach, S. Estandía, J. Gàzquez, F. Sánchez, I. Fina, and J. Fontcuberta
Adv. Funct. Mater. 2002638 (2020)
Films of Hf0.5Z0.5O2 (HZO) contain a network of grain boundaries. In (111) HZO epitaxial films on (001) SrTiO3, for instance, twinned orthorhombic (o‐HZO) ferroelectric crystallites coexist with grain boundaries between o‐HZO and a residual paraelectric monoclinic (m‐HZO) phase. These grain boundaries contribute to the resistive switching response in addition to the genuine ferroelectric polarization switching and have detrimental effects on device performance. Here, it is shown that, by using suitable nanometric capping layer deposited on HZO film, a radical improvement of the operation window of the tunnel device can be achieved. Crystalline SrTiO3 and amorphous AlOx are explored as capping layers. It is observed that these layers conformally coat the HZO surface and allow to increase the yield and homogeneity of ferroelectric junctions while strengthening endurance. Data show that the capping layers block ionic‐like transport channels across grain boundaries. It is suggested that they act as oxygen suppliers to the oxygen‐getters grain boundaries in HZO. In this scenario it could be envisaged that these and other oxides could also be explored and tested for fully compatible CMOS technologies.
M. Foerster, E. Menéndez, E. Coy, A. Quintana, C. Gómez-Olivella, D. Esqué de los Ojos, O. Vallcorba, C. Frontera, L. Aballe, J. Nogués, J. Sort, and I. Fina
Materials Horizons, 2020,
Among metamagnetic materials, FeRh alloys are technologically appealing due to their uncommon antiferromagnetic-to-ferromagnetic metamagnetic transition which occurs at a temperature T* just above room temperature. Here, a controlled increase of T* (ΔT* ∼ 20 °C) is induced in pre-selected regions of FeRh films via mechanical strain nanopatterning. Compressive stresses generated at the vicinity of pre-defined nanoindentation imprints cause a local reduction of the FeRh crystallographic unit cell parameter, which leads to an increase of T* in these confined micro-/nanometric areas. This enhances the stability of the antiferromagnetic phase in these localized regions. Remarkably, generation of periodic arrays of nanopatterned features also allows modifying the overall magnetic and electric transport properties across large areas of the FeRh films. This approach is highly appealing for the design of new memory architectures or other AFM-spintronic devices.
S. Estandía, N. Dix, M.F. Chisholm, I. Fina, and F. Sánchez
Crystal Growth & Design 20, 3801 (2020)
Epitaxial ferroelectric HfO2 films are the most suitable to investigate intrinsic properties of the material and for prototyping emerging devices. Ferroelectric Hf0.5Zr0.5O2(111) films were epitaxially stabilized on La2/3Sr1/3MnO3(001) electrodes. This epitaxy, considering the symmetry dissimilarity and the huge lattice mismatch, is not compatible with conventional mechanisms of epitaxy. To gain insight into the epitaxy mechanism, scanning transmission electron microscopy characterization of the interface was performed, revealing arrays of dislocations with short periodicities. These observed periodicities agree with those expected for domain matching epitaxy, indicating that this unconventional mechanism could be the prevailing factor in the stabilization of ferroelectric Hf0.5Zr0.5O2 with (111) orientation in the epitaxial Hf0.5Zr0.5O2(111)/La2/3Sr1/3MnO3(001) heterostructure.
High polarization, endurance and retention in sub-5 nm Hf0.5Zr0.5O2 films
J. Lyu, T. Song, I. Fina, and F. Sánchez
Nanoscale 12, 11280 (2020)
Ferroelectric HfO2 is a promising material for new memory devices, but significant improvement of its important properties is necessary for practical application. However, previous literature shows that a dilemma exists between polarization, endurance and retention. Since all these properties should be simultaneously high, overcoming this issue is of the highest relevance. Here, we demonstrate that high crystalline quality sub-5 nm Hf0.5Zr0.5O2 capacitors, integrated epitaxially with Si(001), present combined high polarization (2Pr of 27 μC cm−2 in the pristine state), endurance (2Pr > 6 μC cm−2 after 1011 cycles) and retention (2Pr > 12 μC cm−2 extrapolated at 10 years) using the same poling conditions (2.5 V). This achievement is demonstrated in films thinner than 5 nm, thus opening bright possibilities in ferroelectric tunnel junctions and other devices.
A. Rubano, M. Scigaj, F. Sánchez, G. Herranz and D. Paparo.
Journal of Physics: Condensed Matter, Vol 32, Num 32, 135001 (2020)
Oxide growth with semiconductor-like accuracy allows the fabrication of atomically precise thin films and interfaces displaying a wide range of phases and functionalities that are absent in the corresponding oxide bulk materials. Among the other properties it was found that a two-dimensional electronic gas is formed under some circumstances at the LaAlO3/SrTiO3(0 0 1) interface separating two typical insulating perovskite crystals. The origin of this conducting state has been discussed at length, since different doping mechanisms can act in these material systems. Many experimental results point to the so-called polar catastrophe scenario as the principal mechanism driving the formation of the two-dimensional electronic gas. According to this mechanism, the existence of an interfacial polar discontinuity is the key ingredient to drive an electronic reconstruction at the LaAlO3/SrTiO3(0 0 1) interface and the consequent formation of a two-dimensional electron gas. This simple picture has been often questioned by the existence of material systems whose interface are predicted being non-polar according to the simplistic ‘ionic’ limit but that display an electrical behavior analogous to that of LaAlO3/SrTiO3(0 0 1) interfaces. This is the case of the LaAlO3/SrTiO3(1 1 0), i.e., a LaAlO3/SrTiO3 interface with a different in-plane orientation. It is evident that to solve such kind of controversies a detailed investigation of the polar or non-polar state of these interfaces is needed, although this is not simple for the lack of experimental tools that are specifically sensitive to interfacial polarity. Here we apply Optical Second Harmonic Generation to investigate LaAlO3/SrTiO3 interfaces with different in-plane orientations to bridge this gap. By comparing our results with recent theoretical findings, we will arrive to the conclusion that the real LaAlO3/SrTiO3(1 1 0) interface is strongly polar.
X. Torrelles, G. Cantele, G. M. De Luca, R. Di Capua, J. Drnec, R. Felici, D. Ninno, G. Herranz, and M. Salluzzo.
Phys. Rev. B. 99, 205421 (2019).
Polar surfaces are known to be unstable due to the divergence of the surface electrostatic energy. Here we report on the experimental determination, by grazing incidence x-ray diffraction, of the surface structure of polar Ti-terminated (111) SrTiO3 single crystals. We find that the polar instability of the 1×1 surface is solved by a pure electronic reconstruction mechanism, which induces out-of-plane ionic displacements typical of the polar response of SrTiO3 layers to an electron confining potential. On the other hand, the surface instability can be also eliminated by a structural reconstruction driven by a change in the surface stoichiometry, which induces a variety of 3×3 (111) SrTiO3 surfaces consisting in an incomplete Ti (surface)–O2 (subsurface) layer covering the 1×1 Ti-terminated (111) SrTiO3 truncated crystal. In both cases, the TiO6 octahedra are characterized by trigonal distortions affecting the structural and the electronic symmetry of several unit cells from the surface. These findings show that the stabilization of the polar (111) SrTiO3 surface can lead to the formation of quasi-two-dimensional electron systems characterized by radically different ground states which depend on the surface reconstructions.
R. Cichelero; M. Kataja; G. Herranz
Proceedings of SPIE, Vol 11090, Spintronics XII; 1109039 (2019)
Magnetoplasmonic gratings offer a suitable platform to analyze fundamental aspects of the interaction of plasmons with magnetization. In particular, broken time-reversal symmetry induces frequency shifts in plasmon resonances that result in large magneto-optic responses at the plasmon frequency. Here we study the excitation of plasmons with light at oblique incidence, focusing on the effect of plasmon excitation on the diffracted modes of the gratings. This way, we were able to select one of the two counterpropagating plasmons allowed for each diffraction order. Interestingly, in these conditions we found large magneto-optic amplitudes driven by plasmons near grating diffraction modes. Finite-difference timedomain simulations confirm these experimental results and corroborate that the sign of the magneto-optic enhancement is exclusively dependent on the surface plasmon wavevector and does not depends on the angle of incidence.
Kataja, M., Cichelero, R., Herranz, G.
J. Vis. Exp. (153), e60094 (2019)
Photonic crystals are periodic nanostructures that can support a variety of confined electromagnetic modes. Such confined modes are usually accompanied by local enhancement of electric field intensity that strengthens light-matter interactions, enabling applications such as surface-enhanced Raman scattering (SERS) and surface plasmon enhanced sensing. In the presence of magneto-optically active materials, the local field enhancement gives rise to anomalous magneto-optical activity. Typically, the confined modes of a given photonic crystal depend strongly on the wavelength and incidence angle of the incident electromagnetic radiation. Thus, spectral and angular-resolved measurements are needed to fully identify them as well as to establish their relationship with the magneto-optical activity of the crystal. In this article, we describe how to use a Fourier-plane (back focal plane) microscope to characterize magneto-optically active samples. As a model system, here we use a plasmonic grating built out of magneto-optically active Au/Co/Au multilayer. In the experiments, we apply a magnetic field on the grating in situ and measure its reciprocal space response, obtaining the magneto-optical response of the grating over a range of wavelengths and incident angles. This information enables us to build a complete map of the plasmonic band structure of the grating and the angle and wavelength dependent magneto-optical activity. These two images allow us to pinpoint the effect that the plasmon resonances have on the magneto-optical response of the grating. The relatively small magnitude of magneto-optical effects requires a careful treatment of the acquired optical signals. To this end, an image processing protocol for obtaining magneto-optical response from the acquired raw data is laid out.
H. B. Vasili, D. Pesquera, M. Valvidares, P. Gargiani, E. Pellegrin, F. Bondino, E. Magnano, A. Barla, and J. Fontcuberta
Phys. Rev. Materials 4, 044404
The different occupancy of electronic orbitals, the so-called orbital polarization, is a key parameter determining electric and magnetic properties of materials. Here we report on the demonstration of in operando voltage-controlled tuning of the orbital occupation in LaNiO3 epitaxial thin films grown on piezoelectric substrates. The different static contributions to the orbital occupation are disentangled, namely the epitaxial strain and the surface symmetry breaking, and the superimposed electric-field controlled orbital polarization are determined by x-ray linear dichroism at the Ni L2,3 edges. The voltage-controlled orbital polarization allows changing the orbital polarization by about an additional 50%.
B. Casals, N. Statuto, M. Foerster, A. Hernández-Mínguez, R. Cichelero, P. Manshausen, A. Mandziak, L. Aballe, J. Manel Hernàndez, and F. Macià
Phys. Rev. Lett. 124, 137202 (2020)
Using hybrid piezoelectric-magnetic systems we have generated large amplitude magnetization waves mediated by magnetoelasticity with up to 25 degrees variation in the magnetization orientation. We present direct imaging and quantification of both standing and propagating acoustomagnetic waves with different wavelengths, over large distances up to several millimeters in a nickel thin film.
I. Fina, N. Dix, E. Menéndez, A. Crespi, M. Foerster, L. Aballe, F. Sánchez, J. Fontcuberta
ACS Appl. Mater. Interfaces 2020, 12, 13, 15389-15395
The antiferromagnetic to ferromagnetic transition occurring above room temperature in FeRh is attracting interest for applications in spintronics, with perspectives for robust and untraceable data storage. Here, we show that FeRh films can be grown on a flexible metallic substrate (tape shaped), coated with a textured rock-salt MgO layer, suitable for large-scale applications. The FeRh tape displays a sharp antiferromagnetic to ferromagnetic transition at about 90 °C. Its magnetic properties are preserved by bending (radii of 300 mm), and their anisotropic magnetoresistance (up to 0.05%) is used to illustrate data writing/reading capability.
J. Oro-Sole, C. Frontera, A. P. Black, A. Castets, K. L. Velasquez-Mendez, J. Fontcuberta, A. Fuertes
Journal of Solid State Chemistry 286 (2020) 121274
EuTi0.5W0.5O3-xNx oxynitrides with nitrogen contents between 0.87 and 1.63 have been synthesized by ammonolysis of EuTi0.5W0.5O4 scheelite at high temperature. They are B-site disordered perovskites that show two different crystal symmetries as a function of the nitrogen content, changing at x near 1.3 from cubic Pm-3m to orthorhombic Pbnm. The nitriding degree is tuned by the ammonia flow rate and synthesis temperature, and determines the oxidation states of the cations and the equilibrium Eu2+ + W6+ ↔ Eu3++W5+ promoting the decrease of the tolerance factor and the structural transition for large x values and electronic reconstructions. Accompanying the symmetry lowering, the nature of the magnetic order of Eu2+ moments changes from ferromagnetic to antiferromagnetic as a result of a complex interplay of antiferromagnetic/ferromagnetic superexchange interactions balanced by structural effects and the carrier-mediated magnetic exchange between Eu2+ ions.
Phys. Rev. Research 1, 032025(R) – Published 26 November 2019
Domain motion during ferroelectric switching has been recently suggested to follow scale-invariant avalanche dynamics. An interesting question concerns the dynamics of ferroelastic materials where the bulk material is nonpolar, while the polarity arises at domain walls only. We tackle this issue by investigating the dynamics of ferroelastic twins in SrTiO3 where the movement of domains is driven mainly by the anisotropic dielectric response at low temperatures. We find that the dynamics of the twin reconfiguration under electric field proceeds by jerks, where the energy distribution is power-law distributed, indicating avalanche dynamics. Avalanche exponents are sensitive to the complexity of the twin pattern structure, reflecting glassiness when twins are interwoven and forming junctions at the intersections between domain walls. This “glassy” behavior is attributed to the pinning originated by these self-generated defects during jamming between twins.
M. Cervo Sulzbach, S. Estandía, X. Long, J. Lyu, N. Dix, J. Gàzquez, M. F. Chisholm, F. Sánchez, I. Fina,* and J. Fontcuberta*
Adv. Electron. Mater. 2019, 1900852
Tunnel devices based on ferroelectric Hf0.5Zr0.5O2 (HZO) barriers hold great promises for emerging data storage and computing technologies. The resistance state of the device can be changed through use of a suitable writing voltage. However, the microscopic mechanisms leading to the resistance change are an intricate interplay between ferroelectric polarization controlled barrier properties and defect‐related transport mechanisms. The fundamental role of the microstructure of HZO films determining the balance between those contributions is demonstrated. The HZO film presents coherent or incoherent grain boundaries, associated to the coexistance of monoclinic and orthorhombic phases, which are dictated by the mismatch with the substrates for epitaxial growth. These grain boundaries are the toggle that allows to obtain either large (up to ≈450%) and fully reversible genuine polarization controlled electroresistance when only the orthorhombic phase is present or an irreversible and extremely large (≈103–105%) electroresistance when both phases coexist.
Ignasi Fina and Josep Fontcuberta
Journal of Physics D: Applied Physics, Volume 53, Number 2
FeRh based alloys may display an uncommon transition from a ferromagnetic to an antiferromagnetic state upon cooling. The transition takes place roughly above room temperature and it can be sensitively modulated by composition and external parameters, including pressure and strain. Consequently, thin films of FeRh have received a great deal of attention for its potential applications in spintronics, antiferromagnetic spintronics and sensing. Interestingly, the extreme sensitivity of its properties to strain has created expectations for energy friendly voltage-control of the magnetic state of FeRh, with a number of potential applications at the horizon. Here, after summarizing the current understanding of strain effects on the magnetic properties of FeRh thin films, we review the achievements of exploiting piezoelectric substrates for in operando tuning of their magneto-electric properties. We conclude with a brief summary and an outlook for future initiatives.
S. González-Casal, I. Fina, F. Sánchez, J. Fontcuberta
ACS Appl. Electron. Mater. 2019, 191937-1944
Achieving large magnetoelectric coupling is of interest for memory and communication applications. In multiferroic hybrid structures (combining ferroelectric and magnetic materials) in the presence of a magnetoelectric coupling, the ferroelectric properties can be modulated by a magnetic field. This is called the direct magnetoelectric effect. Measuring the ferroelectric properties in multiferroic materials most commonly requires using metallic electrodes that sandwich the ferroelectric material. In the present work, we use the series resistance introduced by the metallic electrode (La2/3Sr1/3MnO3) to manipulate one relevant ferroelectric parameter, i.e., the coercive voltage, of an adjacent ferroelectric layer (BaTiO3) by a magnetic field. We will show that the variations are fully reversible and more apparent at high frequencies; thus, of particular interest for applications, where high commutation rates are required.
M. Qian, I. Fina, M. Cervo Sulzbach, F. Sánchez, J. Fontcuberta
Advanced Electronic Materials, Volume 5, Issue 3, 1800646 (2019)
Advanced use of ferroelectric capacitors in data storage and computing relies on the control of their electrical resistance (electroresistance, ER) by the change of the electrostatic potential profile across the capacitor occurring upon electric field–driven polarization switching. Here it is reported the observation that BaTiO3‐based capacitors, sandwiched between Pt and La2/3Sr1/3MnO3 electrodes, display a large ER, whose magnitude (near 104% at room temperature) and sign (ER > 0, ER < 0) are determined by the writing pulse duration and temperature. Temperature‐dependent measurements have been instrumental to obtain evidence of the presence of a thermally activated process coexisting with the electronic changes produced by ferroelectric polarization switching, both contributing to ER. Detailed analysis allows concluding that the thermally activated process can be attributed to field‐assisted ionic motion. It is argued that the relative balance between purely electronic and ionic diffusion processes modulate the height of the interfacial Schottky barriers and, consequently, are responsible for the observed variations of magnitude and sign of ER.
M. Mirjolet, H. B. Vasili, L. López-Conesa, S. Estradé, F. Peiró, J. Santiso, F. Sánchez, P. Machado, P. Gargiani, M. Valvidares, and J. Fontcuberta*
Adv. Funct. Mater. 2019, 1904238
Transparent metallic oxides are pivotal materials in information technology, photovoltaics, or even in architecture. They display the rare combination of metallicity and transparency in the visible range because of weak interband photon absorption and weak screening of free carriers to impinging light. However, the workhorse of current technology, indium tin oxide (ITO), is facing severe limitations and alternative approaches are needed. AMO3 perovskites, M being a nd1 transition metal, and A an alkaline earth, have a genuine metallic character and, in contrast to conventional metals, the electron–electron correlations within the nd1 band enhance the carriers effective mass (m*) and bring the transparency window limit (marked by the plasma frequency, ωp*) down to the infrared. Here, it is shown that epitaxial strain and carrier concentration allow fine tuning of optical properties (ωp*) of SrVO3 films by modulating m* due to strain‐induced selective symmetry breaking of 3d‐t2g(xy, yz, xz) orbitals. Interestingly, the DC electrical properties can be varied by a large extent depending on growth conditions whereas the optical transparency window in the visible is basically preserved. These observations suggest that the harsh conditions required to grow optimal SrVO3 films may not be a bottleneck for their future application.
Y. Chen, B. Casals, F. Sánchez, G. Herranz
ACS Appl. Electron. Mater. 1, 7, 1189-1197 (2019)
Recently, inspired by neurobiological information processing, correlation-based learning has been expressed physically in nonbiological systems by exploiting the time causality of electric signals. Yet, the capability to learn from visual events requires extending these concepts to optical stimuli. Here we show a solid-state system that is sensitive to 100 ms-scale timing of pairs of light stimuli with complementary short/long visible wavelengths, causing asymmetric changes of photoconductance. This property endows optical signals with time causality, leading to wavelength-sensitive time correlations with time scales comparable with those of perceptual recognition. On the basis of these observations, we propose that complex information can be extracted from visual patterns imprinted as spatiotemporal modulations of persistent photoconductance. We suggest that this capability may stimulate neuromorphic hybrid electronic/photonic systems to construct biomimetic spatial memory and navigation maps inspired from neurobiology.
J. Fontcuberta, H. B. Vasili, J. Gàzquez and Dr. F. Casanova.
Adv. Mater. Interfaces, 6, 190047 (2019)
Spin currents have emerged as a new tool in spintronics, with promises of more efficient devices. A pure spin current can be generated in a nonmagnetic metallic (NM) layer by a charge current (spin Hall effect). When the NM layer is placed in contact with a magnetic material, a magnetoresistance (spin Hall magnetoresistance) develops in the former via the inverse spin Hall effect (ISHE). In other novel spin‐dependent phenomena, such as spin pumping or spin Seebeck effect, spin currents are generated by magnetic resonance or thermal gradients and detected via ISHE in a neighboring normal metal layer. All cases involve spin transport across interfaces between nonmagnetic metallic layers and magnetic materials; quite commonly, magnetic insulators. The structural, compositional, and electronic differences between these materials and their integration to form an interface, challenge the control and understanding of the spin transport across it, which is known to be sensitive to sub‐nanometric interface features. Here, the authors review the tremendous progress in material’s science achieved during the last few years and illustrate how the spin Hall magnetoresistance can be used as a probe for surface magnetism. The authors end with some views on concerted actions that may allow further progress.
S. Estandía, N. Dix, J. Gazquez, I. Fina, J. Lyu, M. F. Chisholm, J. Fontcuberta, and F. Sánchez
ACS Applied Electronic Materials, 1, 8, 1449-1457 (2019)
The critical impact of epitaxial stress on the stabilization of the ferroelectric orthorhombic phase of hafnia is proved. Epitaxial bilayers of Hf0.5Zr0.5O2(HZO) and La0.67Sr0.33MnO3 (LSMO) electrodes were grown on a set of single crystalline oxide (001)-oriented (cubic or pseudocubic setting) substrates with a lattice parameter in the 3.71–4.21 Å range. The lattice strain of the LSMO electrode, determined by the lattice mismatch with the substrate, is critical in the stabilization of the orthorhombic phase of HZO. On tensilely strained LSMO electrodes, most of the HZO film is orthorhombic, whereas the monoclinic phase is favored when LSMO is relaxed or compressively strained. Therefore, the HZO films on TbScO3 and GdScO3 substrates present substantially enhanced ferroelectric polarization in comparison to films on other substrates, including the commonly used SrTiO3. The capability of having epitaxial doped HfO2 films with controlled phase and polarization is of major interest for a better understanding of the ferroelectric properties and paves the way for fabrication of ferroelectric devices based on nanometric HfO2 films.
Y. Chen, B. Casals, G. Herranz
ACS Appl. Electron. Mater. 1, 810-816 (2019)
We report on transport measurements under optical stimulation of persistent photoconductance (PPC) at the interface between amorphous LaAlO3 films and SrTiO3 crystals. The spectral response in the visible region was analyzed under varying illumination conditions and exposure times down to milliseconds. The PPC is plastically modulated by optical stimuli of varying strength and duration, demonstrating fine-tuned photoconductive responsivity over a diversity of cumulated timespans. Interestingly, under optimal conditions, the photoconductance is sensitive to intensity contrasts under conditions comparable to bright-sunlight environments. The prospects of exploiting photoconductance—including potential strategies to reach higher sensitivity—are discussed in the context of neuromorphic applications.
J. Lyu, I. Fina, R. Bachelet, G. Saint-Girons, S. Estandıa, J. Gazquez, J. Fontcuberta, and F. Sanchez
Appl. Phys. Lett. 114, 222901 (2019)
SrTiO3 templates have been used to integrate epitaxial bilayers of ferroelectric Hf0.5Zr0.5O2and La2/3Sr1/3MnO3 bottom electrodes on Si(001). The Hf0.5Zr0.5O2 films show enhanced properties in comparison to equivalent films on SrTiO3(001) single crystalline substrates. The films, thinner than 10 nm, have a very high remnant polarization of 34 μC/cm2. Hf0.5Zr0.5O2capacitors at an operating voltage of 4 V present a long retention time well beyond 10 years and high endurance against fatigue up to 109 cycles. The robust ferroelectric properties displayed by the epitaxial Hf0.5Zr0.5O2 films on Si(001) using SrTiO3 templates pave the way for the monolithic integration on silicon of emerging memory devices based on epitaxial HfO2.
Financial support from the Spanish Ministry of Economy, Competitiveness and Universities, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (No. SEV-2015-0496) and the MAT2017-85232-R (AEI/FEDER, EU) and MAT2015-73839-JIN projects and from Generalitat de Catalunya (2017 SGR 1377) is acknowledged. I.F. acknowledges Ramón y Cajal Contract No. RYC-2017-22531. J.L. is financially supported by the China Scholarship Council (CSC) with No. 201506080019. S.E. acknowledges the Spanish Ministry of Economy, Competitiveness and Universities for his Ph.D. Contract (No. SEV-2015-0496-16-3) and its cofunding by the ESF. The work of J.L. and S.E. has been done as a part of their Ph.D. program in Materials Science at Universitat Autònoma de Barcelona. INL authors acknowledge the financial support from the European Commission through the Project TIPS (No. H2020-ICT-02-2014-1-644453) and from the French national research agency (ANR) through the projects DIAMWAFEL (No. ANR-15-CE08-0034), LILIT (No. ANR-16-CE24-0022), and MITO (No. ANR-17-CE05-0018). They are also grateful to the joint laboratory INL-RIBER and P. Regreny, C. Botella, and J. B. Goure for the MBE technical support on the Nanolyon technological platform.
Guest editors: M. Coll, J. Fontcuberta, N. Pryds and F. Miletto Granozio.
Applied Surface Science, 482, 1-93 (2019)
At the end of a rush lasting over half a century, in which CMOS technology has been experiencing a constant and breathtaking increase of device speed and density, Moore’s law is approaching the insurmountable barrier given by the ultimate atomic nature of matter. A major challenge for 21st century scientists is finding novel strategies, concepts and materials for replacing silicon-based CMOS semiconductor technologies and guaranteeing a continued and steady technological progress in next decades. Among the materials classes candidate to contribute to this momentous challenge, oxide films and heterostructures are a particularly appealing hunting ground. The vastity, intended in pure chemical terms, of this class of compounds, the complexity of their correlated behaviour, and the wealth of functional properties they display, has already made these systems the subject of choice, worldwide, of a strongly networked, dynamic and interdisciplinary research community.
Oxide science and technology has been the target of a wide four-year project, named Towards Oxide-Based Electronics (TO-BE), that has been recently running in Europe and has involved as participants several hundred scientists from 29 EU countries. In this review and perspective paper, published as a final deliverable of the TO-BE Action, the opportunities of oxides as future electronic materials for Information and Communication Technologies ICT and Energy are discussed. The paper is organized as a set of contributions, all selected and ordered as individual building blocks of a wider general scheme. After a brief preface by the editors and an introductory contribution, two sections follow. The first is mainly devoted to providing a perspective on the latest theoretical and experimental methods that are employed to investigate oxides and to produce oxide-based films, heterostructuresand devices. In the second, all contributions are dedicated to different specific fields of applications of oxide thin films and heterostructures, in sectors as data storage and computing, optics and plasmonics, magnonics, energy conversion and harvesting, and power electronics.
Nahuel Statuto, Christian Hahn, Joan Manel Hernàndez, Andrew D. Kent, and Ferran Macià
PHYSICAL REVIEW B 99, 174436 (2019)
Droplet solitons are large amplitude localized spin-wave excitations that can be created in magnetic thin films with uniaxial anisotropy by a spin-polarized current flowing through an electrical nanocontact. Here, we report a low-temperature (4 K) experimental study that shows there are multiple and, under certain conditions, combinations of droplet modes, each mode with a distinct high-frequency spin precession (tens of gigahertz). Low-frequency (≲1GHz) voltage noise is used to assess the stability of droplet modes. It is found that droplets are stable only in a limited range of applied field and current, typically near the current and field at which they nucleate, in agreement with recent predictions. Applied fields in the film plane favor multiple droplet modes, whereas fields perpendicular to the film plane tend to stabilize a single droplet mode. Micromagnetic simulations are used to show that spatial variation in the energy landscape in the nanocontact region (e.g., spatial variation of magnetic anisotropy or magnetic field) can lead to quantized droplet modes and low-frequency mode modulation, characteristics observed in our experiments.
G. Singh, A. Jouan, G. Herranz, M. Scigaj, F. Sánchez, L. Benfatto, S. Caprara, M. Grilli, G. Saiz, F. Couëdo, C. Feuillet-Palma, J. Lesueur & N. Bergeal
Nature Materials, 18, 948 – 954 (2019)
In multi-orbital materials, superconductivity can exhibit several coupled condensates. In this context, quantum confinement in two-dimensional superconducting oxide interfaces offers new degrees of freedom to engineer the band structure and selectively control the occupancy of 3d orbitals by electrostatic doping. Here, we use resonant microwave transport to extract the superfluid stiffness of the (110)-oriented LaAlO3/SrTiO3 interface in the entire phase diagram. We provide evidence of a transition from single-condensate to two-condensate superconductivity driven by continuous and reversible electrostatic doping, which we relate to the Lifshitz transition between 3d bands based on numerical simulations of the quantum well. We find that the superconducting gap is suppressed while the second band is populated, challenging Bardeen–Cooper–Schrieffer theory. We ascribe this behaviour to the existence of superconducting order parameters with opposite signs in the two condensates due to repulsive coupling. Our findings offer an innovative perspective on the possibility to tune and control multiple-orbital physics in superconducting interfaces.
Roberta Ceravola, Carlos Frontera, Judith Oro ́-Solé, Ashley P. Black, Clemens Ritter, Ignasi Mata, Elies Molins, Josep Fontcuberta* and Amparo Fuertes
Chem.Commun. 55, 3105 (2019)
The topotactic nitridation of cation ordered, tetragonal Sr2FeMoO6 in NH3 at moderate temperatures leads to cubic, Fmm double perovskite oxynitride Sr2FeMoO4.9N1.1 where double-exchange interactions determine ferromagnetic order with TC ≈ 100 K. Substitution of oxide by nitride induces bond asymmetries and local electronically driven structural distortions, which combined with Fermi level lowering restricts charge itinerancy to confined regions and preclude spontaneous long-range magnetic order. Under a magnetic field, ferromagnetic correlations expand, favoring charge delocalization and a negative magnetoresistance is observed.
Michael Foerster, Ignasi Fina, Simone Finizio, BlaiCasals, AniaMandziak, FrancoisFauthand Lucia Aballe
J. Phys.: Condens. Matter 31 084003 (2019)
An odd-symmetry magnetic response of multiferroic composites comprising ultrathin Co layers on Pt electrodes on [Pb(Mg0.33Nb0.67)O3](1−x)[PbTiO3] x (PMN-PT) (0 1 1) piezoelectric substrates is observed upon electrical poling of the PMN-PT substrates: the magnetic easy axis of the Co rotates by 90° in-plane between oppositely poled ferroelectric states, mimicking the signature of a surface polarization charge driven effect, which however can be excluded from the presence of the thick Pt interlayer. The origin for this unexpected behavior is as an odd symmetry piezoelectric response of the PMN-PT substrate, as indicated by x-ray diffraction with applied poling, in combination with conventional magnetoelastic coupling. Ferroelectric characterization reveals corresponding features, possibly related to an unswitchable polarization component.
Mirjolet, Florencio Sánchez, and Josep Fontcuberta
Adv. Funct. Materials, 29, 1808432 (2019)
The urgent need for more performant transparent conducting electrodes is stimulating intensive research on oxide thin films based on early transition metals (e.g., V, Nb, Mo, etc.), where it is expected that the partially occupied (i.e., nd1, nd2…) conduction band will give rise to metallic conductivity. Growing thin films of these oxides typically requires an extremely low oxygen pressure. However, in growth methods involving hyperthermal kinetics (such as pulsed laser deposition), this may have severe detrimental effects on the electrical and optical properties of the film. Here, it is shown that the use of a nonreactive gas during a pulsed laser deposition process allows epitaxial SrVO3 films to be obtained with low room temperature resistivity (ρ ≈ 31 μΩ cm), large carrier mobility (μ ≈ 8.3 cm2V−1 s−1), and large residual resistivity ratio (RRR ≈ 11.5), while improving optical transparency in the visible range. It is argued that the success of this growth strategy relies on the modulation of energetics of plasma species and a concomitant reduction of defects in the films. These findings may find applications in other oxide‐based thin film technologies (i.e., ferroelectric tunnel memories, etc.) where growth‐induced point effects may compromise functionality.
Mengdi Qian, Ignasi Fina, Florencio Sánchez, Josep Fontcuberta
Small 15, 1805042 (2019)
Complementary resistive switching (CRS) devices are receiving attention because they can potentially solve the current‐sneak and current‐leakage problems of memory arrays based on resistive switching (RS) elements. It is shown here that a simple anti‐serial connection of two ferroelectric tunnel junctions, based on BaTiO3, with symmetric top metallic electrodes and a common, floating bottom nanometric film electrode, constitute a CRS memory element. It allows nonvolatile storage of binary states (“1” = “HRS+LRS” and “0” = “LRS+HRS”), where HRS (LRS) indicate the high (low) resistance state of each ferroelectric tunnel junction. Remarkably, these states have an identical and large resistance in the remanent state, characteristic of CRS. Here, protocols for writing information are reported and it is shown that non‐destructive or destructive reading schemes can be chosen by selecting the appropriate reading voltage amplitude. Moreover, this dual‐tunnel device has a significantly lower power consumption than a single ferroelectric tunnel junction to perform writing/reading functions, as is experimentally demonstrated. These findings illustrate that the recent impressive development of ferroelectric tunnel junctions can be further exploited to contribute to solving critical bottlenecks in data storage and logic functions implemented using RS elements.
Jike Lyu, Ignasi Fina, Josep Fontcuberta, and Florencio Sanchez
ACS Appl. Mater. Interfaces, 11 (6), 6224 – 6229 (2019)
Epitaxial ferroelectric Hf0.5Zr0.5O2 films have been successfully integrated in a capacitor heterostructure on Si(001). The orthorhombic Hf0.5Zr0.5O2phase,  out-of-plane oriented, is stabilized in the films. The films present high remnant polarization Pr close to 20 μC/cm2, rivaling with equivalent epitaxial films on single crystalline oxide substrates. Retention time is longer than 10 years for a writing field of around 5 MV/cm, and the capacitors show endurance up to 109 cycles for a writing voltage of around 4 MV/cm. It is found that the formation of the orthorhombic ferroelectric phase depends critically on the bottom electrode, being achieved on La0.67Sr0.33MnO3 but not on LaNiO3. The demonstration of excellent ferroelectric properties in epitaxial films of Hf0.5Zr0.5O2 on Si(001) is relevant toward fabrication of devices that require homogeneity in the nanometer scale, as well as for better understanding of the intrinsic properties of this promising ferroelectric oxide.
Jike Lyu, Ignasi Fina, Raul Solanas, Josep Fontcuberta, and Florencio Sanchez.
ACS Appl. Electron. Mater. 1 (2), 220 – 228 (2019)
The metastable orthorhombic phase of hafnia is generally obtained in polycrystalline films, whereas in epitaxial films, its formation has been much less investigated. We have grown Hf0.5Zr0.5O2 films by pulsed laser deposition, and the growth window (temperature and oxygen pressure during deposition and film thickness) for epitaxial stabilization of the ferroelectric phase is mapped. The remnant ferroelectric polarization, up to ∼24 μC/cm2, depends on the amount of orthorhombic phase and interplanar spacing and increases with temperature and pressure for a fixed film thickness. The leakage current decreases with an increase in thickness or temperature, or when decreasing oxygen pressure. The coercive electric field (EC) depends on thickness (t) according to the EC – t–2/3 scaling, which is observed for the first time in ferroelectric hafnia, and the scaling extends to thicknesses down to around 5 nm. The proven ability to tailor the functional properties of high-quality epitaxial ferroelectric Hf0.5Zr0.5O2 films paves the way toward understanding their ferroelectric properties and prototyping devices.
R. Cichelero, M. Kataja, M. Campoy-Quiles, and G. Herranz.
Optics Express, 26, Issue 26, 34842 – 34852 (2018)
Phase-matching conditions—used to bridge the wave vector mismatch between light and surface plasmon polaritons (SPPs)—have been exploited recently to enable nonreciprocal optical propagation and enhanced magneto-optic responses in magnetoplasmonic systems. Here we show that using diffraction in conjunction with plasmon excitations leads to a photonic system with a more versatile and flexible response. As a testbed, we analyzed diffracted magneto-optical effects in magnetoplasmonic gratings, where broken time-reversal symmetry induces frequency shifts in the energy and angular spectra of plasmon resonance. These result in exceptionally large responses in the diffracted magneto-optical effect. The concepts presented here can be used to develop non-reciprocal optical devices that exploit diffraction, in order to achieve tailored electromagnetic responses.
R. Cichelero, M.A. Oskuei, M. Kataja, S.M. Hamidi, G. Herranz.
Journal of Magnetism and Magnetic Materials 476, 54 – 58 (2019)
We investigate the transverse magneto-optic Kerr effect (TMOKE) of magnetoplasmonic crystals grown on top of commercial optical disks. From full angle-resolved scans we can identify Wood’s anomalies related to the excitation of plasmons of different orders. From these maps we also detect a wide range of wavelengths and angles of incidence for which the TMOKE signal is increased due to the interaction of light with surface propagating plasmons. Remarkably, conditions are established for unexpectedly large responses at quasi-normal incidence, where, by fundamental symmetry reasons, the intrinsic TMOKE should be vanishingly small. The key towards this unexpected outcome is to engineer the geometry of magnetoplasmonic crystals, so that first-order plasmon dispersion lines run up towards quasi-normal angles of incidence. These results provide general rules for magneto-optic enhancement and, in particular, show the potential of standard commercial disks as platforms for enhanced magneto-optic devices.
Michael Foerster, Nahuel Statuto, Blai Casals, Alberto Hernández-Mínguez, Simone Finizio, Ania Mandziak, Lucia Aballe, Joan Manel Hernàndez Ferràs and Ferran Macià
Journal of Synchrotron Radiation, 26, 184 – 193 (2019)
The quantification of surface acoustic waves (SAWs) in LiNbO3 piezoelectric crystals by stroboscopic X-ray photoemission electron microscopy (XPEEM), with a temporal smearing below 80 ps and a spatial resolution below 100 nm, is reported. The contrast mechanism is the varying piezoelectric surface potential associated with the SAW phase. Thus, kinetic energy spectra of photoemitted secondary electrons measure directly the SAW electrical amplitude and allow for the quantification of the associated strain. The stroboscopic imaging combined with a deliberate detuning allows resolving and quantifying the respective standing and propagating components of SAWs from a superposition of waves. Furthermore, standing-wave components can also be imaged by low-energy electron microscopy (LEEM). Our method opens the door to studies that quantitatively correlate SAWs excitation with a variety of sample electronic, magnetic and chemical properties.
M. Qian, I. Fina, F. Sánchez, and J. Fontcuberta
Adv. Electron. Mater. 5, 1800407, 2019
The resistive switching associated with polarization reversal is studied in detail in ferroelectric BaTiO3 tunnel junctions, with focus on the dynamics of the ferroelectric domain switching. It is observed that the transition between the high‐resistance state (HRS) and the low‐resistance state (LRS) is largely asymmetric being smooth from LRS to HRS, but proceeds via avalanches in the HRS‐to‐LRS transitions. It is shown that this distinct behavior is related to the presence of an imprint field in the junction and has important consequences on the junction’s performance.
L Vistoli, W Wang, A Sander, Q Zhu, B Casals, R Cichelero, A Barthélémy, S Fusil, G Herranz, S Valencia, R Abrudan, E Weschke, K Nakazawa, H Kohno, J Santamaria, W Wu, V Garcia & M Bibes
Nature Physics 15, 67 – 72 (2019)
Strong electronic correlations can produce remarkable phenomena such as metal–insulator transitions and greatly enhance superconductivity, thermoelectricity or optical nonlinearity. In correlated systems, spatially varying charge textures also amplify magnetoelectric effects or electroresistance in mesostructures. However, how spatially varying spin textures may influence electron transport in the presence of correlations remains unclear. Here we demonstrate a very large topological Hall effect (THE) in thin films of a lightly electron-doped charge-transfer insulator, (Ca,Ce)MnO3. Magnetic force microscopy reveals the presence of magnetic bubbles, whose density as a function of magnetic field peaks near the THE maximum. The THE critically depends on carrier concentration and diverges at low doping, near the metal–insulator transition. We discuss the strong amplification of the THE by correlation effects and give perspectives for its non-volatile control by electric fields.
M. Foerster , L. Aballe , J. M. Hernàndez , and F. Macià
MRS BULLETIN , volume 43, (2018)
I. Fina, A. Quintana, X. Martí, F. Sánchez, M. Foerster, L. Aballe, J. Sort, and
Applied Physics Letters 113, 152901 (2018)
J. Lyu, I. Fina, R. Solanas, J. Fontcuberta and F. Sanchez
J. Lyu, I. Fina, R. Solanas, J. Fontcuberta, and F. Sánchez
Appl. Phys. Lett. 113, 082902 (2018)
J. Lyu, S. Estandía, J. Gazquez, M. F. Chisholm, I. Fina, N. Dix, J. Fontcuberta, and F. Sánchez
ACS Applied Materials & Interfaces
Fanmao Liu, Ignasi Fina, Guillaume Sauthier, Florencio Sánchez, Andrew M. Rappe, and Josep Fontcuberta
ACS Appl. Mater. Interfaces, Article ASAP, June 18, 2018
Nahuel Statuto, Joan Manel Hernàndez , Andrew D Kent and Ferran Macià
Nanotechnology, 29 325302 (2018)
Blai Casals, Andrea Schiaffino, Arianna Casiraghi, Sampo J. Hämäläinen, Diego López González, Sebastiaan van Dijken, Massimiliano Stengel, and Gervasi Herranz
Phys. Rev. Lett. 120, 217601 – Published 25 May 2018
J. Hang, C. Hahn, N. Statuto, F. Macià, A. D. Kent
Scientific Reports 8, 6847 (2018)
H. B. Vasili, M. Gamino, J. Gàzquez, F. Sánchez, M. Valvidares, P. Gargiani, E. Pellegrin, and J. Fontcuberta.
ACS Appl. Mater. Interfaces 2018, 10, 12031−12041
J. Lyu, I. Fina, R. Solanas, J. Fontcuberta, and F. Sánchez
Scientific Reports 8, 495 (2018)
J. Gazquez, M. Stengel, R. Mishra, M. Scigaj, M. Varela, M. A. Roldan, J. Fontcuberta, F. Sánchez, and G. Herranz
Phys. Rev. Lett. 119, 106102 (2017)
Michael Foerster, Ferran Macià, Nahuel Statuto, Simone Finizio, Alberto Hernández-Mínguez, Sergi Lendínez, Paulo V. Santos, Josep Fontcuberta,
Joan Manel Hernàndez, Mathias Kläui & Lucia Aballe
Nature Communications 8, Article number: 407(2017)
Erik Wahlström, Ferran Macià, Jos E Boschker, Åsmund Monsen, Per Nordblad, Roland Mathieu, Andrew D Kent and Thomas Tybell
New Journal of Physics New J. Phys. 19 (2017) 063002, 1-8
Sergi Lendínez, Jinting Hang, Saül Vélez, Joan Manel Hernández, Dirk Backes, Andrew D. Kent, and Ferran Macià
Phys. Rev. Applied 7, 054027 – Published 31 May 2017
Ignasi Fina, Alberto Quintana, Alberto, Jessica Padilla-Pantoja, Xavier Marti, Ferran Macià, Florencio Sanchez, Michael Foerster, Lucia Aballe, Josep Fontcuberta, Jordi Sort
ACS Appl. Mater. Interfaces, Article ASAP
C. Coll, L. López-Conesa, J. M. Rebled, C. Magén, F. Sánchez, J. Fontcuberta, S. Estradé, and F. Peiró
J. Phys. Chem. C, April 6, 2017
L. López-Conesa, J. M. Rebled, D. Pesquera, N. Dix, F. Sánchez, G. Herranz, J. Fontcuberta, C. Magén, M. J. Casanove, S. Estradé and F. Peiró
Phys. Chem. Chem. Phys. DOI: 10.1039/C7CP00902J
Greta Radaelli, Diego Gutiérrez, Mengdi Qian, Ignasi Fina, Florencio Sánchez, Lorenzo Baldrati, Jakoba Heidler, Cinthia Piamonteze, Riccardo Bertacco, and Josep Fontcuberta.
Adv. Electron. Mater. 2016, 2, 1600368.
M Lorenz, M S Ramachandra Rao, T Venkatesan, E Fortunato, P Barquinha, R Branquinho, D Salgueiro, R Martins, E Carlos, A Liu, F K Shan, M Grundmann, H Boschker, J Mukherjee, M Priyadarshini,N DasGupta, D J Rogers, F H Teherani, E V Sandana, P Bove, K Rietwyk, A Zaban, A Veziridis,A Weidenkaff, M Muralidhar, M Murakami, S Abel, J Fompeyrine, J Zuniga-Perez, R Ramesh,N A Spaldin, S Ostanin, V Borisov, I Mertig, V Lazenka, G Srinivasan, W Prellier, M Uchida,M Kawasaki, R Pentcheva, P Gegenwart, F Miletto Granozio, J Fontcuberta and N Pryds.
J. Phys. D: Appl. Phys. 49 (2016) 433001 (53pp). DOI: 10.1088/0022-3727/49/43/433001
Miren Isasa, Saül Vélez, Edurne Sagasta, Amilcar Bedoya-Pinto, Nico Dix, Florencio Sánchez, Luis E. Hueso, Josep Fontcuberta, and Fèlix Casanova.
PHYS. REV. APPLIED 6, 034007 (2016). DOI: 10.1103/PhysRevApplied.6.034007
D. Pesquera, A. Barla, M. Wojcik, E. Jedryka, F. Bondino, E. Magnano, S. Nappini, D. Gutiérrez, G. Radaelli, G. Herranz, F. Sánchez, and J. Fontcuberta.
PHYSICAL REVIEW APPLIED 6, 034004 (2016). DOI: 10.1103/PhysRevApplied.6.034004
M. Scigaj, C. H. Chao, J. Gázquez, I. Fina, R. Moalla, G. Saint-Girons, M. F. Chisholm, G. Herranz, J. Fontcuberta, R. Bachelet, and F. Sánchez.
Appl. Phys. Lett. 109, 122903 (2016). DOI: 10.1063/1.4962836
Vegard Flovik, Ferran Macià & Erik Wahlström.
Scientific Reports, 6:32528 (2016). DOI: 10.1038/srep32528
Mateusz Scigaj, Nico Dix, Jaume Gázquez, María Varela, Ignasi Fina, Neus Domingo, Gervasi Herranz, Vassil Skumryev, Josep Fontcuberta & Florencio Sánchez.
Scientific Reports, 6:31870 (2016). DOI: 10.1038/srep31870
M. C. Weber, M. Guennou, N. Dix, D. Pesquera, F. Sánchez, G. Herranz, J. Fontcuberta, L. López-Conesa, S. Estradé, F. Peiró, Jorge Íniguez, and J. Kreisel.
PHYSICAL REVIEW B 94, 014118 (2016). DOI: 10.1103/PhysRevB.94.014118
Ekaterina Khestanova, Nico Dix, Ignasi Fina, Mateusz Scigaj, José Manuel Rebled, César Magén, Sonia Estradé, Francesca Peiró, Gervasi Herranz, Josep Fontcuberta, and Florencio Sánchez*
Adv. Funct. Mater. 2016, 26, 6446–6453. DOI: 10.1002/adfm.201602084
In this work, published in Physical Review Letters, we demonstrate that the polarization of light is changed dramatically when its energy resonates with the polaron-hopping energy of self-trapped electrons. In the presence of spin-orbit coupling, the self-trapped hopping electron may reverse its spin producing a large gyrotropic effect, which is observed close to room temperature. The effect is observed in La2∕3Ca1∕3MnO3 thin films and, plausibly, should be present in other manganites.
Since straining this kind of materials can change the polaron concentration, our finding suggests a route to mechanical or electro-mechanical control of the magneto-optical effect, paving the way to unconventional magnetoelectric effects.
This work has been highlighted with a Synopsis on the Physics website:
Vegard Flovik, Ferran Macià, Sergi Lendínez, Joan Manel Hernàndez, Ingrid Hallsteinsen, Thomas Tybell
M. Valvidares, N. Dix, M. Isasa, K. Ollefs, F. Wilhelm, A. Rogalev, F. Sánchez, E. Pellegrin, A. Bedoya-Pinto, P. Gargiani, L. E. Hueso, F. Casanova, and J. Fontcuberta.
PHYSICAL REVIEW B 93, 214415 (2016). DOI: 10.1103/PhysRevB.93.214415
Laura Cabana, Maxime Bourgognon, Julie T.-W. Wang, Andrea Protti, Rebecca Klippstein, Rafael T. M. de Rosales, Ajay M. Shah, Josep Fontcuberta, Ester Tobías-Rossell, Jane K. Sosabowski, Khuloud T. Al-Jamal,* and Gerard Tobias*
Small 2016, 12, No. 21, 2893–2905. DOI: 10.1002/smll.201502721
Blai Casals,Marina Espínola, Rafael Cichelero, Stephan Geprägs, Matthias Opel, Rudolf Gross, GervasiHerranz and JosepFontcuberta
Applied Physics Letters 108, 102407 (2016) http://dx.doi.org/10.1063/1.4943515
OndřejVlašín, RomainJarrier, Rémi Arras, Lionel Calmels, BénédicteWarot-Fonrose, Cécile Marcelot, MatthieuJamet, Philippe Ohresser, Fabrice Scheurer, Riccardo Hertel, GervasiHerranz, and SaliaCherifi-Hertel
ACS Appl. Mater. Interfaces 8, 553 (2016) DOI: 10.1021/acsami.5b12777
Fanmao Liu, I. Fina, R. Bertacco, and J. Fontcuberta
Scientific Reports 6, 25028 (2016) DOI: 10.1038/srep25028
Fanmao Liu, Ignasi Fina, Riccardo Bertacco & Josep Fontcuberta.
Scientific Reports, 6:25028 (2016). DOI: 10.1038/srep25028
OndrejVlasin, Blai Casals, Nico Dix, Diego Gutiérrez, Florencio Sánchez & GervasiHerranz
Scientific Reports 5, 15800 (2015)| DOI: 10.1038/srep15800
Lucía Aballe,Sonia Matencio,Michael Foerster,Esther Barrena, Florencio Sánchez,Josep Fontcubertaand Carmen Ocal.
ChemistryMaterials, 27, 6198-6204 (2015); DOI: 10.1021/acs.chemmater.5b00661
FanmaoLiu, IgnasiFina, Diego Gutiérrez, Greta Radaelli, RiccardoBertacco, Josep Fontcuberta
Adv. Electron. Mater. 2015, 1500171 ; DOI:10.1002/aelm.201500171
Josep Fontcuberta, Vassil Skumryev, Vladimir Laukhin, Xavier Granados & Ekhard K. H. Salje
Scientific Reports 5, Article number: 13784 (2015); doi:10.1038/srep13784
Greta Radaelli , Diego Gutiérrez , Florencio Sánchez , Riccardo Bertacco , Massimiliano Stengel , and Josep Fontcuberta
Adv. Mater. 2015, DOI: 10.1002/adma.201405117
Pt/BaTiO3/La0.7Sr0.3MnO3 tunnel junctions.
It is demonstrated that reversing the polarization direction of a ferroelectric barrier in a tunnel junction leads to a change of junction conductance and capacitance, with concomitant variations on the barrier height and effective thickness, both contributing to produce larger electroresistance
Romain Bachelet, Patricia de Coux, Bénédicte Warot-Fonrose, Vassil Skumryev, Gang Niu, Bertrand Vilquin, Guillaume Saint-Girons and FlorencioSánchez
CrystEngComm,16, 10741-10745 (2014), DOI: 10.1039/C4CE01817F
Novel spinel heteroepitaxial structures composed of ferrimagnetic spinel CoFe2O4 and ultrathin γ-Al2O3 buffer layers are integrated on Si(111).
The development of today’s electronics is reaching fundamental limits and novel concepts and materials are researched to achieve ever faster and more efficient electronic devices. In this regard, oxides are well positioned, as they display a fascinating complexity of electronic phases –not present in conventional semiconductors–that may open new pathways.
The interface between LaAlO3 and SrTiO3 is the oxide quantum well (QW) par excellence, where superconductivity and magnetism emerge. In LaAlO3/SrTiO3 QWs electrons move in two dimensions forming a two-dimensional electron gas (2DEG, Box a). However, not all the electrons reside in the same QW subband (Box b) and, indeed, they behave differently depending on which orbital are they occupying. We demonstrate that the symmetry of the conduction band inside the QWs can be selected, so that the properties of electrons vary largely. In particular, we show that in the superconductive state Cooper pairs do not bind in the same way either when we change the number of electrons in the 2DEG by electric fields (Box a) or when the direction of confinement is changed (Box b). We also show that choosing the confinement plane allows manipulating the spin-dependent transport in these QWs.
Our work opens a groundbreaking route to manipulate selectively electrons in two dimensions. See more @ G. Herranz at al., Engineering two-dimensional superconductivity and Rashba spin–orbit coupling in LaAlO3/SrTiO3 quantum wells by selective orbital occupancy, Nature Communications 2014, doi:10.1038/ncomms7028.
OndřejVlašín, OanaPascu, Anna Roig, and GervasiHerranz
Phys. Rev. Applied 2, 054003 (2014) DOI: http://dx.doi.org/10.1103/PhysRevApplied.2.054003
We show that very diluted dispersions of ferromagnetic metal nanoparticles in diamagnetic hosts show a large, linear response under an applied magnetic field, which is of interest for emerging applications in sensing, integrated optical communications, and magneto-optical current transformers and transducers. We present also a theory that describes accurately these results and that can be further extended to describe the optical properties of e.g., metal inclusions in polymers or glasses.
D. Pesquera, M. Scigaj, P. Gargiani, A. Barla, J. Herrero-Martín, E. Pellegrin, S. M. Valvidares, J. Gázquez, M. Varela, N. Dix, J. Fontcuberta, F. Sánchez, and G. Herranz
Phys. Rev. Lett. 113, 156802 (2014), DOI: http://dx.doi.org/10.1103/PhysRevLett.113.156802
Quantum wells at the LaAlO3/SrTiO3 interface with tunable electron states
Quantum wells with d-electrons at the LaAlO3(LAO)/SrTiO3(STO) interface exhibit physical properties, such as superconductivity or magnetism, unseen in conventional semiconductors (Si, Ge, GaAs, …). We have demonstrated that the symmetry of the conduction band inside the QWs can be selected at whish. This opens up novel perspectives to manipulate the electronic properties of QWs at the LAO/STO interface.
MirenIsasa, Amilcar Bedoya-Pinto, Saül Vélez, Federico Golmar, Florencio Sánchez, Luis E. Hueso, Josep Fontcuberta, and Fèlix Casanova
Applied Physics Letters 105, 142402 (2014)
SPINTRONICS WITH NON-MAGNETIC MATERIALS. The neighborhood makes all difference
It has been discovered that electric resistance of a nonmagnetic metal (Pt in our case) is depending on magnetization state of neighboring insulating ferromagnetic layer, namely on the atomic planes forming the interface and their crystallographic orientation.
I. Fina, X. Marti, D. Yi, J. Liu, J. H. Chu, C. Rayan-Serrao, S. Suresha, A. B. Shick, J. Zelezny, T. Jungwirth, J. Fontcuberta, R. Ramesh
Nature Communications 5, Article number: 4671; doi:10.1038/ncomms5671
M. Scigaj, N. Dix, M. Cabero, A. Rivera-Calzada, J. Santamaria, J. Fontcuberta, G. Herranz, and F. Sánchez
Applied Physics Letters 104, 251602 (2014)
P. de Coux, R. Bachelet, B. Warot-Fonrose, V. Skumryev, L. Lupina, G. Niu, T. Schroeder, J. Fontcuberta, and F. Sánchez
Applied Physics Letters 105, 012401 (2014)