arXiv subjects

Florencio Sánchez

Publications and source records attributed to Florencio Sánchez.

17 recordsLinked to original sources

arXiv2026

Computing at the Edge Enabled by Indoor Photovoltaics and Ferroelectrics

The exponential, pervasive rise of artificial intelligence (AI), hosted in large data centres, will test the limits of the underpinning infrastructure, including electricity and water supplies, which are becoming increasingly precious resources. Rather than performing all AI computations in centralized servers, for many functionalities, these can be performed locally across a network of billions of small, autonomous nodes, saving substantial energy costs associated with communication. This alternative paradigm is known as edge computing, or distributed intelligence (DI), but has been held back by the lack of availability of reliable local energy supplies matching the energy requirements of the computational infrastructure. Opportunities to address this challenge are emerging with rapid advances in high-performance indoor photovoltaics (IPVs) for local energy harvesting, as well as reductions in computational cost through neuromorphic or in-memory computing devices. In this perspective, we discuss the requirements of IPVs for DI, the extent to which emerging materials fulfil these requirements, and how current gaps could be addressed. We make the case that (anti)ferroelectric materials can surpass bottlenecks for both electrostatic energy storage and low-power in-memory neuromorphic computing. The core theme of this perspective is that co-creation between energy harvesting, storage and computing is essential for making DI a reliable and more sustainable alternative to centralized AI.

cond-mat.mtrl-sci↗

arXiv2025

Carbon magneto-ionics: Control of magnetism through voltage-driven carbon transport

Control of magnetism through voltage-driven ionic processes (i.e., magneto-ionics) holds potential for next-generation memories and computing. This stems from its non-volatility, flexibility in adjusting the magnitude and speed of magnetic modulation, and energy efficiency. Since magneto-ionics depends on factors like ionic radius and electronegativity, identifying alternative mobile ions is crucial to embrace new phenomena and applications. Here, the feasibility of C as a prospective magneto-ionic ion is investigated in a Fe-C system by electrolyte gating. In contrast to most magneto-ionic systems, Fe-C presents a dual-ion mechanism: Fe and C act as cation and anion, respectively, moving uniformly in opposite directions under an applied electric field. This leads to a 7-fold increase in saturation magnetization with magneto-ionic rates larger than 1 emu cm-3 s-1, and a 25-fold increase in coercivity. Since carbides exhibit minimal cytotoxicity, this introduces a biocompatible dimension to magneto-ionics, paving the way for the convergence of spintronics and biotechnology.

cond-mat.mtrl-sci↗

arXiv2023

Disentangling stress and strain effects in ferroelectric HfO2

Ferroelectric HfO2 films are usually polycrystalline and contain a mixture of polar and nonpolar phases. This challenges the understanding and control of polar phase stabilization and ferroelectric properties. Several factors such as dopants, oxygen vacancies, or stress, among others, have been investigated and shown to have a crucial role on optimizing the ferroelectric response. Stress generated during deposition or annealing of thin films is a main factor determining the formed crystal phases and influences the lattice strain of the polar orthorhombic phase. It is difficult to discriminate between stress and strain effects on polycrystalline ferroelectric HfO2 films, and the direct impact of orthorhombic lattice strain on ferroelectric polarization has yet to be determined experimentally. Here, we analyze the crystalline phases and lattice strain of several series of doped HfO2 epitaxial films. We conclude that stress has a critical influence on metastable orthorhombic phase stabilization and ferroelectric polarization. On the contrary, the lattice deformation effects are much smaller than those caused by variations in the orthorhombic phase content. The experimental results are confirmed by density functional theory calculations on HfO2 and Hf0.5Zr0.5O2 ferroelectric phases.

cond-mat.mtrl-sci↗

arXiv2021

Flexible antiferromagnetic FeRh tapes as memory elements

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 oC. 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.

cond-mat.mtrl-sci↗

arXiv2020

Blocking of conducting channels widens window for ferroelectric resistive switching in interface-engineered Hf0.5Zr0.5O2 tunnel devices

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 layers 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 functioning 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.

cond-mat.mtrl-sci↗

arXiv2020

Unraveling ferroelectric polarization and ionic contributions to electroresistance in epitaxial Hf0.5Zr0.5O2 tunnel junctions

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 by 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. Here is shown the fundamental role of the microstructure of HZO films setting the balance between those contributions. The oxide film presents coherent or incoherent grain boundaries, associated to the existence of monoclinic and orthorhombic phases in HZO films, 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 (1000-100000 %) electroresistance when both phases coexist.

cond-mat.mtrl-sci↗

arXiv2020

Domain Matching Epitaxy of Ferroelectric Hf0.5Zr0.5O2(111) on La2/3Sr1/3MnO3(001)

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 have been 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 the 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.

cond-mat.mtrl-sci↗

arXiv2020

High polarization, endurance and retention in sub-5 nm Hf$_{0.5}$Zr$_{0.5}$O$_2$ films

Ferroelectric HfO$_2$ is a promising material for new memory devices, but significant improvement of important properties is necessary to reach applications. However, precedent literature shows that a dilemma between polarization, endurance and retention exists. 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 uC/cm2 in the pristine state), endurance (2Pr > 6 uC/cm2 after E11 cycles) and retention (2Pr > 12 uC/cm2 extrapolated at 10 years) using 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.

physics.app-ph↗

arXiv2020

Reversible and magnetically unassisted voltage-driven switching of magnetization in FeRh/PMN-PT

Reversible control of magnetization by electric fields without assistance from a subsidiary magnetic field or electric current could help reduce the power consumption in spintronic devices. When increasing temperature above room temperature, FeRh displays an uncommon antiferromagnetic to ferromagnetic phase transition linked to a unit cell volume expansion. Thus, using the strain exerted by an adjacent piezoelectric layer, the relative amount of antiferromagnetic and ferromagnetic regions can be tuned by an electric field applied to the piezoelectric material. Indeed, large variations in the saturation magnetization have been observed when straining FeRh films grown on suitable piezoelectric substrates. In view of its applications, the variations in the remanent magnetization rather than those of the saturation magnetization are the most relevant. Here, we show that in the absence of any bias external magnetic field, permanent and reversible magnetization changes as high as 34% can be induced by an electric field, which remain after this has been zeroed. Bulk and local magnetoelectric characterization reveals that the fundamental reason for the large magnetoelectric response observed at remanence is the expansion (rather than the nucleation) of ferromagnetic nanoregions.

cond-mat.mtrl-sci↗

arXiv2019

Unconventional Ferroelectric Switching via Local Domain Wall Motion in Multiferroic $ε$-Fe2O3 Films

Deterministic polarization reversal in ferroelectric and multiferroic films is critical for their exploitation in nanoelectronic devices. While ferroelectricity has been studied for nearly a century, major discrepancies in the reported values of coercive fields and saturation polarization persist in literature for many materials. This raises questions about the atomic-scale mechanisms behind polarization reversal. Unconventional ferroelectric switching in $ε$-Fe2O3 films, a material that combines ferrimagnetism and ferroelectricity at room temperature, is reported here. High-resolution in-situ scanning transmission electron microscopy (STEM) experiments and first-principles calculations demonstrate that polarization reversal in $ε$-Fe2O3 occurs around pre-existing domain walls only, triggering local domain wall motion in moderate electric fields of 250 - 500 kV/cm. Calculations indicate that the activation barrier for switching at domain walls is nearly a quarter of that corresponding to the most likely transition paths inside $ε$-Fe2O3 domains. Moreover, domain walls provide symmetry lowering, which is shown to be necessary for ferroelectric switching. Local polarization reversal in $ε$-Fe2O3 limits the macroscopic ferroelectric response and offers important hints on how to tailor ferroelectric properties by domain structure design in other relevant ferroelectric materials.

cond-mat.mtrl-sci↗

arXiv2019

Growth Window of Ferroelectric Epitaxial Hf0.5Zr0.5O2 Thin Films

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 around 24 uC/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 coercive electric field (Ec) - thickness (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.

cond-mat.mtrl-sci↗

arXiv2019

Epitaxial Integration on Si(001) of Ferroelectric Hf0.5Zr0.5O2 Capacitors with High Retention and Endurance

Epitaxial ferroelectric Hf0.5Zr0.5O2 films have been successfully integrated in a capacitor heterostructure on Si(001). The orthorhombic Hf0.5Zr0.5O2 phase, [111] 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 writing field of around 5 MV/cm, and the capacitors show endurance up to 1E9 cycles for 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 towards 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.

physics.app-ph↗

arXiv2016

Strain-controlled responsiveness of slave half-doped manganite La0.5Sr0.5MnO3 layers inserted in BaTiO3 ferroelectric tunnel junctions

Insertion of layers displaying field-induced metal-to-insulator (M/I) transition in ferroelectric tunnel junctions (FTJs) has received attention as a potentially useful way to enlarge junction tunnel electroresistance (TER). Half-doped manganites being at the verge of metal-insulator character are thus good candidates to be slave layers in FTJs. However, the phase diagram of these oxides is extremely sensitive to strain and thus can be radically different when integrated in epitaxial FTJs. Here we report a systematic study of large-area (A = 4 to 100 um2) Pt/La0.5Sr0.5MnO3/BaTiO3/La0.7Sr0.3MnO3 (Pt/HD/BTO/LSMO) FTJs, having different thicknesses of the ferroelectric (2-3nm) and HD layers (1-2nm), grown on substrates imposing either tensile (SrTiO3) or compressive (LaAlO3) strains. Room-temperature electric characterization of the FTJs shows polarization-controlled ON/ OFF states. Clear evidences of field-induced M/I transition (difference between junction resistance in OFF and ON state is increased of more than one order of magnitude) are observed in junctions prepared on SrTiO3 but the HD layer is generally metallic on LaAlO3. Moreover, the M/I transition is only confined in an interfacial layer of the slave film thus entailing an overall reduction of TER. The orderly results reported here give some hints towards selection of HD materials and substrates for optimal FTJ responsiveness.

cond-mat.mtrl-sci↗

arXiv2016

Spin Hall magnetoresistance as a probe for surface magnetization in Pt/CoFe$_2$O$_4$ bilayers

We study the spin Hall magnetoresistance (SMR) in Pt grown $\textit{in situ}$ on CoFe$_2$O$_4$ (CFO) ferrimagnetic insulating (FMI) films. A careful analysis of the angle-dependent and field-dependent longitudinal magnetoresistance indicates that the SMR contains a contribution that does not follow the bulk magnetization of CFO but it is a fingerprint of the complex magnetism at the surface of the CFO layer, thus signaling SMR as a tool for mapping surface magnetization. A systematic study of the SMR for different temperatures and CFO thicknesses gives us information impossible to obtain with any standard magnetometry technique. On one hand, surface magnetization behaves independently of the CFO thickness and does not saturate up to high fields, evidencing that the surface has its own anisotropy. On the other hand, characteristic zero-field magnetization steps are not present at the surface while they are relevant in the bulk, strongly suggesting that antiphase boundaries are the responsible of such intriguing features. In addition, a contribution from ordinary magnetoresistance of Pt is identified, which is only distinguishable due to the low resistivity of the $\textit{in-situ}$ grown Pt.

cond-mat.mes-hall↗

arXiv2016

Giant Optical Polarization Rotation Induced by Spin-Orbit Coupling in Polarons

We have uncovered a giant gyrotropic magneto-optical response for doped ferromagnetic manganite La2/3Ca1/3MnO3 around the near room-temperature paramagnetic-to-ferromagnetic transition. At odds with current wisdom, where this response is usually assumed to be fundamentally fixed by the electronic band structure, we point to the presence of small polarons as the driving force for this unexpected phenomenon. We explain the observed properties by the intricate interplay of mobility, Jahn-Teller effect and spin-orbit coupling of small polarons. As magnetic polarons are ubiquitously inherent to many strongly correlated systems, our results provide an original, general pathway towards the generation of gigantic gyrotropic responses that can be harnessed for nonreciprocal devices that exploit the polarization of light.

cond-mat.str-el↗

arXiv2014

Spin Hall magnetoresistance at Pt/CoFe2O4 interfaces and texture effects

We report magnetoresistance measurements on thin Pt bars grown on epitaxial (001) and (111) CoFe2O4 (CFO) ferrimagnetic insulating films. The results can be described in terms of the recently discovered spin Hall magnetoresistance (SMR). The magnitude of the SMR depends on the interface preparation conditions, being optimal when Pt/CFO samples are prepared in situ, in a single process. The spin-mixing interface conductance, the key parameter governing SMR and other relevant spin-dependent phenomena such as spin pumping or spin Seebeck effect, is found to be different depending on the crystallographic orientation of CFO, highlighting the role of the composition and density of magnetic ions at the interface on spin mixing.

cond-mat.mes-hall↗

arXiv2012

High mobility conduction at (110) and (111) LaAlO3/SrTiO3 interfaces

In recent years, striking discoveries have revealed that two-dimensional electron liquids (2DEL) confined at the interface between oxide band-insulators can be engineered to display a high mobility transport. The recognition that only few interfaces appear to suit hosting 2DEL is intriguing and challenges the understanding of these emerging properties not existing in bulk. Indeed, only the neutral TiO2 surface of (001)SrTiO3 has been shown to sustain 2DEL. We show that this restriction can be surpassed: (110) and (111) surfaces of SrTiO3 interfaced with epitaxial LaAlO3 layers, above a critical thickness, display 2DEL transport with mobilities similar to those of (001)SrTiO3. Moreover we show that epitaxial interfaces are not a prerequisite: conducting (110) interfaces with amorphous LaAlO3 and other oxides can also be prepared. These findings open a new perspective both for materials research and for elucidating the ultimate microscopic mechanism of carrier doping.

cond-mat.mtrl-sci↗