M. Balli, D. Matte, S. Jandl, P. Fournier, M. M. Gospodinov
In this paper, we report on the magnetocaloric properties of some selected multiferroic oxides, namely HoMn2O5 and La2(Ni,Co)MnO6 compounds which exhibit transition points from 10K up to almost room temperature. In order to avoid grain boundary effects and structural inhomogeneity observed frequently in polycrystalline samples, only single crystals were considered for this study.
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Relation between rare-earth magnetism and the magnetocaloric effect in multiferroic hexagonal manganites
本文通过超过20 T的脉冲磁场直接测量，研究了多铁性六方锰氧化物的磁热效应，发现稀土4f-磁矩与绝热温度变化直接相关，HoMnO3中温度变化达20.1 K，揭示了稀土磁性在低温制冷中的关键作用。
Solid state-refrigerants have generated worldwide interest owing to their growing potential for use in efficient and green cooling devices. Caloric effects could be obtained by manipulating their degrees of freedom such as magnetization, electric polarization and volume using a variable external field. In conventional magnetocaloric refrigeration systems, the magnetocaloric effect is exploited by moving the active material in and out of the magnetic field source. Here we demonstrate that a giant and reversible magnetocaloric effect can be generated simply by rotating the multiferroic TbMn2O5 s…
Combining multiple degrees of freedom in strongly-correlated materials such as transition-metal oxides would lead to fascinating magnetic and magnetocaloric features. Herein, the strain effects are used to markedly tailor the magnetic and magnetocaloric properties of PrVO3 thin films. The selection of appropriate thickness and substrate enables us to dramatically decrease the coercive magnetic field from 2.4 T previously observed in sintered PVO3 bulk to 0.05 T for compressive thin films making from the PrVO3 compound a nearly soft magnet. This is associated with a marked enhancement of the ma…
AlFeO3, GaFeO3 and Al0.5Ga0.5FeO3, all crystallizing in a non-centrosymmetric space group, are multiferroic and also exhibit magnetoelectric properties, with Al0.5Ga0.5FeO3 showing unusually large magnetocapacitance at 300 K.
The list of materials systems displaying both electric and magnetic long range order is short. Oxides, however, concentrate numerous examples of multiferroicity with, in some cases, a large magnetoelectric coupling. As a result, a fruitful research field has emerged contemporaneously with the consolidation of spintronic. The synergy between multiferroics and spintronics was meant to be inevitable and hence the characterization of spintronic functionalities in multiferroic materials is rather abundant. The aim of the present chapter is to review the oxide heterostructures where magnetoelectric …
Recent development of magnetocaloric effect in Pyrochlore oxides
本文综述了烧绿石氧化物中磁热效应的研究进展，重点分析了磁阻挫与自旋-轨道耦合等机制对磁热性能的影响，并指出ErMnO和YIrCrO等体系分别展现出最大常规磁热效应及常规与逆磁热效应共存现象。
GaFeO3, AlFeO3 and related oxides are ferrimagnetic exhibiting magnetodielectric effect. There has been no evidence to date for ferroelectricity and hence multiferroicity in these oxides. We have investigated these oxides as well as oxides of the composition Al1-x-yGaxFe1+yO3 (x = 0.2, y = 0.2) for possible ferroelectricity by carrying out pyroelectric measurements. These measurements establish the occurrence of ferroelectricity at low temperatures below the Nèel temperature in these oxides. They also exhibit significant magnetoelectric effect. We have tried to understand the origin of ferroel…
Multiferroic materials, in which ferroelectric and magnetic ordering coexist, are of fundamental interest for the development of multi-state memory devices that allow for electrical writing and non-destructive magnetic read-out operation. The great challenge is to create multiferroic materials that operate at room-temperature and have a large ferroelectric polarization P. Cupric oxide, CuO, is promising because of its large P ~ 10^{2} μC.m^{-2}, but is unfortunately only multiferroic in a temperature range of 20 K, from 210 to 230 K. Here, using a combination of density functional theory and M…
Controlling magnetism by using electric fields is a goal of research towards novel spintronic devices and future nano-electronics. For this reason, multiferroic heterostructures attract much interest. Here we provide experimental evidence, and supporting DFT analysis, of a transition in La0.65Sr0.35MnO3 (LSMO) thin film to a stable ferromagnetic phase, that is induced by the structural and strain properties of the ferroelectric BaTiO3 (BTO) substrate, which can be modified by applying external electric fields. X-ray Magnetic Circular Dichroism (XMCD) measurements on Mn L edges with a synchrotr…
In spite of being highly relevant for the development of a new generation of information storage devices, not many single-phase materials displaying magnetic and ferroelectric orders above room temperature are known. Moreover, these uncommon materials typically display insignificant values of the remanent moment in one of the ferroic orders or are complex multicomponent oxides which will be very challenging to integrate in devices. Here we report on the strategy to stabilize the metastable epsilon-Fe2O3 in thin film form, and we show that besides its already known ferrimagnetic nature, the fil…
We construct ferroelectric (LuFeO3)m/(LuFe2O4) superlattices with varying index m to study the effect of confinement on topological defects. We observe a thickness-dependent transition from neutral to charged domain walls and the emergence of fractional vortices. In thin LuFeO3 layers, the volume fraction of domain walls grows, lowering the symmetry from P63cm to P3c1 before reaching the non-polar P63/mmc state, analogous to the high-temperature ferroelectric to paraelectric transition. Our study shows how dimensional confinement stabilizes textures beyond those in bulk ferroelectric systems.
We investigate the electronic, ferroelectric and magnetic properties of KTaO3/PbTiO3 interfaces by using conventional density functional theory (DFT) and advanced DFT such as hybrid functional HSE06. We show that doped holes in valence bands or electrons in conduction bands give rise to ferromagnetism at the interfaces. The ferromagnetic states are ground states for both hole-doped (p-type) and electron-doped (n-type) interfaces by comparison with their corresponding nonmagnetic and antiferromagnetic states. Carriers (holes or electrons) concentrate near the interface to screen the polarizatio…
We report a detailed study of the magnetocaloric effect in heterostructures of La2NiMnO6 and La2/3Sr1/3MnO3. The shape, width and magnitude of the temperature dependence of the magnetic entropy change ($ΔS_{m}$) for these multilayer samples can be tuned and controlled by changing their layout. A large $ΔS_{m}$ over a wide temperature range which goes beyond room temperature is observed in all samples. We observe a temperature-independent table-top-like $ΔS_{m}$ over a temperature range as large as 100 K in the trilayer samples. La2NiMnO6 double perovskite with multiple magnetic phase transitio…
Complex oxides with tunable structures have many fascinating properties, though high-quality complex oxide epitaxy with precisely controlled composition is still out of reach. Here we have successfully developed solution-based single crystalline epitaxy for multiferroic (1-x)BiTi(1-y)/2FeyMg(1-y)/2O3-(x)CaTiO3 (BTFM-CTO) solid solution in large area, confirming its ferroelectricity at atomic-scale with a spontaneous polarization of 79~89uC/cm2. Careful compositional tuning leads to a bulk magnetization of ~0.07uB/Fe at room temperature, enabling magnetically induced polarization switching exhi…
We report visible light detection with in-plane BiFeO3 (BFO) thin films grown on pre-patterned inter-digital electrodes. In-plane configured BFO film displayed photocurrents with a 40:1 photo-to-dark-current ratio and improved photo-sensing ability for >15000 s (4 hrs) under small bias voltage (42V). Nearly sixty percent of the photo-induced charge carriers decay in 1.0 s and follow a double-exponential decay model. At 373 K the effect of light does not significantly increase the dark current, probably due to reduced mobility. Sub-bandgap weak monochromatic light (1 mw/cm2) shows one fold incr…
Ferroics, characterized by a broken symmetry state with nonzero elastic, polar, or magnetic order parameters $\vec{u}$, are recognized platforms for staging and manipulating topologically-protected structures as well as for detecting unconventional topological phenomena. The unrealized possibility of producing ordered topological phases in magnetoelectric multiferroics, exhibiting coupled magnetic and polar order parameters, is anticipated to engender novel functionality and open avenues for manipulating topological features. Here, we report the discovery of an ordered $π_1$-$S_\infty$ vortex …
Detailed magnetic studies including magneto-caloric measurements on magnetic multiferroic quadruple perovskite CaMn7O12 are presented. Based on the collective response of Arrott plots and ΔSM (T), a magnetic phase diagram of CaMn7O12 is suggested. A new magnetic transition at TN3 ~20 K where the system changes from noncollinear AFM to collinear AFM is reported. An anomaly observed in both ΔSM (T) and Arrott plots around 64 K has been attributed to high external field induced spin canting leading to change in magnetic order inducing phase transition. Magneto-caloric effect in this material is p…
Multiferroic tunnel junctions (MFTJs) have already been proved to be promising candidates for application in spintronics devices. The coupling between tunnel magnetoresistance (TMR) and tunnel electroresistance (TER) in MFTJs can provide four distinct resistive states in a single memory cell. Here we show that in an all-oxide asymmetric MFTJ of La0.7Sr0.3MnO3 /PbZr0.2Ti0.8O3 /La0.7Te0.3MnO3 (LSMO/PZT/LTMO) with p-type and n-type electrodes, the intrinsic rectification is observed and can be modified by the ferroelectric polarization of PZT. Owing to the combined TMR, TER and diode effects, two…
We propose that concurrently magnetic and ferroelectric, i.e. multiferroic, compounds endowed with electrically-active magnetic excitations (electromagnons) provide a key to produce large directional dichroism for long wavelengths of light. By exploiting the control of ferroelectric polarization and magnetization in a multiferroic oxide Ba$_2$CoGe$_2$O$_7$, we demonstrate the realization of such a directional light-switch function at terahertz frequecies in resonance with the electromagnon absorption. Our results imply that this hidden potential is present in a broad variety of multiferroics.
It is known that orthorhombic RMnO3 multiferroics (R = magnetic rare earth) with low symmetry exhibit a large rotating magnetocaloric effect because of their strong magnetocrystalline anisotropy. In this paper, we demonstrate that the hexagonal ErMnO3 single crystals also unveils a giant rotating magnetocaloric effect that can be obtained by spinning them in constant magnetic fields around their a or b axes. When the ErMnO3 crystal is rotated with the magnetic field initially parallel to the c-axis, the resulting entropy change reaches maximum values of 7, 17 and 20 J/kg K under a constant mag…
In this work multiferroic thin films of Bi containing perovskite oxides are discussed, where the driving force for ferroelectricity are the Bi3+ lone-pair electrons. First, a brief introduction of Bi containing multiferroic perovskite oxides will be presented to describe the mechanisms for establishing magnetic and ferroelectric order in these compounds and some recent developments in this field of research are reported. The second section addresses experimental aspects of epitaxial thin film growth of BiMnO3, (Bi0.9La0.1)2NiMnO6 (BLNMO) and BiFeO3 thin films by pulsed laser deposition. The th…
The magnetic phase transitions reported below 230 K in cupric oxide are analyzed theoretically at the macroscopic and microscopic levels. The incommensurate multiferroic and lock-in commensurate phases are shown to realize an inverted sequence of symmetry-breaking mechanisms with respect to the usual sequence occurring in low temperature multiferroic compounds. The higher temperature spin-spiral phase results from coupled order-parameters which decouple at the lock-in transition to the commensurate ground state phase. Expressing the order-parameters in function of the magnetic spins allows det…
We propose a modified spin-field-effect transistor fabricated in a two dimensional electron gas (2DEG) formed at the surface of multiferroic oxides with a transverse helical magnetic order. The topology of the oxide local magnetic moments induces a resonant momentum-dependent effective spin-orbit interaction acting on 2DEG. We show that spin polarization dephasing is strongly suppressed which is crucial for functionality. The carrier spin precession phase depend linearly on the magnetic spiral helicity. The latter is electrically controllable by virtue of the magento-electric effect. We also s…
Reversibly Strain Engineering and Electric-Field Control of Crystal Symmetry in Multiferroic Oxides
本研究通过薄膜厚度调控残余应变，在 La 掺杂 BiFeO3 薄膜中稳定反铁电正交相，并实现电场下其与铁电菱方相之间的可逆相变。
A systematic microscopic theory of magnetically induced ferroelectricity and lattice modulation is presented for all electron configurations of Mott-insulating transition-metal oxides. Various mechanisms of polarization are identified in terms of a strong-coupling perturbation theory. Especially, the spin-orbit interaction acting on the ligand p orbitals is shown to give the ferroelectric polarization of the spin-current form, which plays a crucial role particularly in eg systems. Semiquantitative agreements with the multiferroic TbMnO3 are obtained. Predictions for X-ray and neutron scatterin…
Novel phenomena appear when two different oxide materials are combined together to form an interface. For example, at the interface of LaAlO3/SrTiO3, two dimensional conductive states form to avoid the polar discontinuity and magnetic properties are found at such interface. In this work, we propose a new type of interface between two nonmagnetic and nonpolar oxides that could host a conductive state with magnetic properties, where it is the ferroelectric polarization discontinuity instead of the polar discontinuity that leads to the charge transfer, forming the interfacial conductive or magnet…
The strain dependent functional properties of epitaxial transition metal oxide films can be significantly modified via substrate selection. However, large lattice mismatches preclude dislocation-free epitaxial growth on ferroelectric substrates, whose strain states are modified by applied electric fields. Here we overcome this mismatch problem by depositing an epitaxial film of ferromagnetic La0.7Sr0.3MnO3 on a single crystal substrate of well lattice matched SrTiO3 via a film of SrRuO3 that we subsequently dissolved, permitting the transfer of unstrained La0.7Sr0.3MnO3 to a ferroelectric subs…
Layer and spontaneous polarizations in perovskite oxides and their interplay in multiferroic bismuth ferrite
本文从现代极化理论统一视角回顾了铁电极化与离子层电荷引起的表面电荷及极化灾难问题，并以 BiFeO3 为例，结合密度泛函计算和电镜分析揭示了其表面电荷自补偿的特殊机制。
Optimal materials to induce bulk photovoltaic effects should lack inversion symmetry and have an optical gap matching the energies of visible radiation. Ferroelectric perovskite oxides such as BaTiO$_3$ and PbTiO$_3$ exhibit substantial polarization and stability, but have the disadvantage of excessively large band gaps. We use both density functional theory and dynamical mean field theory calculations to design a new class of Mott multiferroics--double perovskite oxides $A_2$VFeO$_6$ ($A$=Ba, Pb, etc). While neither perovskite $A$VO$_3$ nor $A$FeO$_3$ is ferroelectric, in the double perovskit…
We have performed a systematic study of the electronic structures of BiMeO3 (Me = Sc, Cr, Mn, Fe, Co, Ni) series by soft X-ray emission (XES) and absorption (XAS) spectroscopy. The band gap values were estimated for all compounds in the series. For BiFeO3 a band gap of ~0.9 eV was obtained from the alignment of the O Ka XES and O 1s XAS. The O 1s XAS spectrum of BiNiO3 indicates that the formation of holes is due to a Ni2+ valency rather than a Ni3+ valency. We have found that the O Ka XES and O 1s XAS of BiMeO3 probing partially occupied and vacant O 2p states, respectively, are in agreement …
We report the magnetic entropy change (Delta Sm) in magnetoelectric Eu1-xBaxTiO3 for x = 0.1- 0.9. We find - delta Sm = 11 (40) J/kg.K in x = 0.1 for a field change of 1 (5) Tesla respectively, which is the largest value among all Eu-based oxides. Delta Sm arises from the field-induced suppression of the spin entropy of Eu2+:4f7 localized moments. While -delta Sm decreases with increasing x, -DeltaSm = 6.58 J/kg.K observed in the high spin diluted composition x = 0.9 is larger than that in many manganites. Our results indicate that these magnetoelectrics are potential candidates for cryogenic …
Double perovskites provide a unique opportunity to induce and control multiferroic behaviors in oxide systems. The appealing possibility to design materials with a strong coupling between the magnetization and the polarization fields may be achieved in this family since these magnetic insulators can present structural self-ordering in the appropriate growth conditions. We have studied the functional properties of La2CoMnO6 and Bi2CoMnO6 epitaxial thin films grown by pulsed laser deposition. Cation-ordered La2CoMnO6 films display a magnetic Curie temperature of 250 K while cation-disordered Bi2…
Our first-principles results show that geometric frustration can be induced in thin films of multiferroic BiFeO3. We find that competing magnetic interactions occur in the so-called super-tetragonal phase of this material, which can be grown on strongly-compressive substrates. We show that the frustration level can be tuned by appropriately choosing the substrate; in fact, the three-dimensional spin order gets totally annihilated in a narrow range of epitaxial strains. We argue that the effects revealed here are not exclusive to BiFeO3, and predict that they also occur in multiferroic BiCoO3.
We present a study based on several advanced First-Principles methods, of the recently synthesized PbNiO3 [J. Am. Chem. Soc 133, 16920 (2011)], a rhombohedral antiferromagnetic insulator which crystallizes in the highly distorted R3c crystal structure. We find this compound electrically polarized, with a very large electric polarization of about 100 (\muC/cm)^2, thus even exceeding the polarization of well-known BiFeO3. PbNiO3 is a proper ferroelectric, with polarization driven by large Pb-O polar displacements along the [111] direction. Contrarily to naive expectations, a definite ionic charg…
Over the last two decades, the research activities on magnetocalorics have been exponentially increased leading to the discovery of a wide category of materials including intermetallics and oxides. Even though the reported materials were found to show excellent magnetocaloric properties on laboratory scale, only a restricted family among them could be upscaled toward industrial levels and implemented as refrigerants in magnetic cooling devices. On the other hand, in the most of reported reviews, the magnetocaloric materials are usually discussed in terms of their adiabatic temperature and entr…
Hexagonal ferrites (h-RFeO$_3$, R=Y, Dy-Lu) have recently been identified as a new family of multiferroic complex oxides. The coexisting spontaneous electric and magnetic polarizations make h-RFeO$_3$ rare-case ferroelectric ferromagnets at low temperature. Plus the room-temperature multiferroicity and predicted magnetoelectric effect, h-RFeO$_3$ are promising materials for multiferroic applications. Here we review the structural, ferroelectric, magnetic, and magnetoelectric properties of h-RFeO$_3$. The thin film growth is also discussed because it is critical in making high quality single cr…
Frustrated lanthanide oxides with dense magnetic lattices are of fundamental interest for their potential in cryogenic refrigeration due to a large ground state entropy and suppressed ordering temperatures, but can often be limited by short-range correlations. Here, we present examples of frustrated fcc oxides, Ba$_2$GdSbO$_6$ and Sr$_2$GdSbO$_6$ and the new site-disordered analog Ca$_2$GdSbO$_6$ ([CaGd]$_A$[CaSb]$_B$O$_6$), in which the magnetocaloric effect is influenced by minimal superexchange ($J_1 \sim 10$ mK). We report on the crystal structures using powder x-ray diffraction and the bu…
Recently, the perovskite BiCoO$_3$ has been shown experimentally to be isostructural with PbTiO$_3$, while simultaneously the $d^6$ Co$^{3+}$ ion has a high spin ground state with $C$-type antiferromagnetic ordering. Using hybrid density functional calculations, we investigate the atomic, electronic and magnetic structure of BiCoO$_3$ to elucidate the origin of the multiferroic state. To begin with, we perform a qualitative trend sudy of the role of $d$ electrons in affecting the tendency for perovskite materials to exhibit a ferroelectric distortion; this work initially explores a qualitative…
The recent study of oxides led to the discovery of several new fascinating physical phenomena. High-temperature superconductivity, colossal magnetoresistance, dilute magnetic doping, or multiferroicity were discovered and investigated in transition-metal oxides, representing a prototype class of strongly correlated electronic systems. This development was accompanied by an enormous progress regarding thin film fabrication. Within the past two decades, epitaxial thin films with crystalline quality approaching semiconductor standards became available using laser molecular beam epitaxy. This evol…
Magnetic Tunnel Junctions whose basic element consists of two ferromagnetic electrodes separated by an insulating non-magnetic barrier have become intensely studied and used in non-volatile spintronic devices. Since ballistic tunnel of spin-polarized electrons sensitively depends on the chemical composition and the atomic geometry of the lead/barrier interfaces their proper design is a key issue for achieving the required functionality of the devices such as e.g. a high tunnel magneto resistance. An important leap in the development of novel spintronic devices is to replace the insulating barr…
A series of superlattices composed of ferromagnetic La$_{0.7}$Ca$_{0.3}$MnO$_3$ (LCMO) and ferroelectric/paraelectric Ba$_{1-x}$Sr$_x$TiO$_3$ (0$\leq $x$\leq $1) were deposited on SrTiO$_3$ substrates using the pulsed laser deposition. Films of epitaxial nature comprised of spherical mounds having uniform size are obtained. Magnetotransport properties of the films reveal a ferromagnetic Curie temperature in the range of 145-158 K and negative magnetoresistance as high as 30%, depending on the type of ferroelectric layers employed for their growth (\QTR{it}{i.e.} '\QTR{it}{x'} value). Ferroelec…
3D oxygen vacancy order and defect-property relations in multiferroic (LuFeO$_3$)$_9$/(LuFe$_2$O$_4$)$_1$ superlattices
研究多铁性 (LuFeO$_3$)$_9$/(LuFe$_2$O$_4$)$_1$ 超晶格，利用原子探针断层扫描揭示三维氧空位有序结构，证明其对室温多铁性的关键作用并建立原子级缺陷量化方法。
The electric-field control of $d$-electron magnetism in multiferroic transition metal oxides is attracting widespread interest for the underlying fundamental physics and for next generation spintronic devices. Here, we report an extensive study of the $3d$ magnetism in magnetoelectric Ga$_{0.6}$Fe$_{1.4}$O$_3$ (GFO) epitaxial films by polarization dependent x-ray absorption spectroscopy. We found a non-zero integral of the x-ray magnetic circular dichroism, with a sign depending upon the relative orientation between the external magnetic field and the crystallographic axes. %By reliably enlarg…
Thermally Configurable Multi-Order Polar Skyrmions in Multiferroic Oxide Superlattices
通过热调制策略，在多铁氧化物超晶格中稳定多阶极性Skyrmion，实现拓扑阶的可逆调控，为存储设备提供可变拓扑配置方法。
The magneto-electrocaloric effect which can be defined as the coupling between magnetocaloric and electrocaloric effects attracts currently considerable attention due to the advantages provided by the caloric effect in designing solid-state refrigeration technologies. The magneto-electrocaloric effect of a multiferroic antiferromagnetic spin system with the Dzyaloshinskii Moriya (DM) interaction is investigated in this paper. The DM interaction is assimilated to a coupling between an external site-dependent electric field and a local electric polarization. The external magnetic is also conside…
Prediction of Room Temperature Electric Field Reversal of Magnetization in the Family of $A_4B_3\rm{O}_9$ Layered Oxides
基于群论引导的第一性原理计算，发现$A_4B_3\ m{O}_9$层状氧化物的新型极性相可通过罕见的$Γ$点磁电-多铁耦合机制实现室温电场反转磁化，为下一代存储器件提供材料基础。
We used first-principles methods to perform a systematic search for potentially-stable phases of multiferroic BiFeO3. We considered a simulation cell compatible with the atomic distortions that are most common among perovskite oxides, and found a large number of local minima of the energy within 100 meV/f.u. of the ferroelectric ground state. We discuss the variety of low-symmetry structures discovered, as well as the implications of these findings as regards current experimental (e.g., on thin films displaying {\em super-tetragonal} phases) and theoretical (on models for BiFeO3's structural p…
Oxide thin films exhibit versatile physical properties such as magnetism, ferroelectricity, piezoelectricity, metal-insulator transition (MIT), multiferroicity, colossal magnetoresistivity, switchable resistivity, etc. More importantly, the exhibited multifunctionality could be tuned by various external fields, which has enabled demonstration of novel electronic devices. In this article, recent studies of the multi-fields modulation of physical properties in oxide thin films have been reviewed. Some of the key issues and prospects about this field are also addressed.
Tunneling magnetoresistance (TMR) via oxides or molecules includes fruitful physics, such as spin filtering and hybridized interface states, in addition to various practical applications using large TMR ratio at room temperature. Then, a larger TMR effect with a new fundamental physics is awaited because further progress on spintronics can be realized. Here we report a discovery of a gigantic TMR ratio of 1,400,000% in a C60-Co nanocomposite spin device. The observed effect is induced by a combination of a Coulomb blockade effect and a novel magnetic switching effect. Theoretical investigation…
The electronic valence state of Mn in Pb(Zr0.2Ti0.8)O3/La0.8Sr0.2MnO3 multiferroic heterostructures is probed by near edge x-ray absorption spectroscopy as a function of the ferroelectric polarization. We observe a temperature independent shift in the absorption edge of Mn associated with a change in valency induced by charge carrier modulation in the La0.8Sr0.2MnO3, demonstrating the electronic origin of the magnetoelectric effect. Spectroscopic, magnetic, and electric characterization shows that the large magnetoelectric response originates from a modified interfacial spin configuration, ope…
We show that the presence of free carriers in a substance can generate the multiferroic behavior. Namely, if the substance has mixed-valence ions, which can supply free carriers and have electric dipole and spin moments, all three types of long-range order (ferromagnetic, ferroelectric and magnetoelectric (ME)) can occur at low temperature. The physical origin of the effect is that charge carriers can mediate the multiferroic behavior via spin - spin (RKKY), dipole-dipole and dipole - spin interactions. Our estimate of the interaction magnitude shows that there exist an optimal carrier concent…
Topological textures and emergent altermagnetic signatures in ultrathin BiFeO3
研究通过超薄BiFeO3实现多铁性，维持两种序参数并展现反磁特征，为氧化物电子学的标度极限提供新路径。
Structural, electronic and dielectric properties of high-quality ultrathin BaTiO3 films are investigated. The films, which are grown by ozone-assisted molecular beam epitaxy on Nb-doped SrTiO3 (001) substrates and having thicknesses as thin 8 unit cells (3.2 nm), are unreconstructed and atomically smooth with large crystalline terraces. A strain-driven transition to 3D island formation is observed for films of of 13 unit cells thickness (5.2 nm). The high structural quality of the surfaces, together with the dielectric properties similar to bulk BaTiO3 and dominantly TiO2 surface termination, …
Giant photovoltaic effect due to bulk photovoltaic effect observed in multiferroic BiFeO3 thin films has triggered a renewed interest on photoferroelectric materials for photovoltaic applications. Tremendous advance has been done to improve power conversion efficiency (up to up to 8.1%) in photoferroelectrics via absorption increase using narrow bandgap ferroelectrics. Other strategies, as it is the more efficient use of ferroelectric internal electric field, are ongoing. Moreover, as a by-product, several progress have been also achieved on photostriction that is the photo-induced deformation…
We address a systematic microscopic theory of spin-dependent electric polarization in 3d oxides starting with a generic three-site two-hole cluster. A perturbation scheme realistic for 3d oxides is applied which implies the quenching of orbital moments by low-symmetry crystal field, strong intra-atomic correlations, the dp-transfer effects, and rather small spin-orbital coupling. An effective spin operator of the electric dipole moment is deduced incorporating both nonrelativistic and relativistic terms. The nonrelativistic electronic polarization mechanism related with the effects of the redi…
We report that CaMn$_7$O$_{12}$ is a new magnetic multiferroic material. The appearance of a ferroelectric polarization coinciding with the magnetic phase transition ($\sim90$ K) suggests the presence of ferroelectricity induced by magnetism, further confirmed by its strong magnetoelectric response. With respect to other known magnetic multiferroics, CaMn$_7$O$_{12}$ displays attractive multiferroic properties, such as a high ferroelectric critical temperature and large polarization. More importantly, these results open a new avenue to search for magnetic multiferroics in the catalogue of dope…
There have been a large number of papers on bismuth ferrite (BiFeO3) over the past few years, trying to exploit its room-temperature magnetoelectric multiferroic properties. Although these are attractive, BiFeO3 is not the ideal multiferroic, due to weak magnetization and the difficulty in limiting leakage currents. Thus there is an ongoing search for alternatives, including such materials as gallium ferrite (GaFeO3). In the present work we report a comprehensive study of the perovskite PbTi1-xPdxO3 with 0 < x < 0.3. Our study includes dielectric, impedance and magnetization measurements, cond…
Magnetic properties of a series of (La0.7Sr0.3MnO3/SrRuO3) superlattices, where the SrRuO3 layer thickness is varying, are examined. A room-temperature magnetocaloric effect is obtained owing to the finite size effect which reduces the TC of La0.7Sr0.3MnO3 layers. While the working temperature ranges are enlarged,, -DeltaSmax values remains similar to the values in polycrystalline La0.7Sr0.3MnO3. Consequently, the relative cooling powers are significantly improved, the microscopic mechanism of which is related to the effect of the interfaces at La0.7Sr0.3MnO3/SrRuO3 and higher nanostructural d…
Cupric oxide is multiferroic at unusual high temperatures. From density functional calculations we find that the low-T magnetic phase is paraelectric and the higher-T one ferroelectric, with a size and direction of polarization in good agreement with experiment. By mapping of the ab initio results on an effective spin model we find with Monte Carlo that in the high-T magnetic state non-collinearity and inversion symmetry breaking stabilize each other via the Dzyaloshinskii Moriya interaction. This leads to a novel mechanism for multiferroicity, with the particular property that non-magnetic im…
The alkaline earth binary oxides adopt a simple rocksalt structure and form an important family of compounds because of their large presence in the earth's mantle and their potential use in microelectronic devices. In comparison to the class of multifunctional ferroelectric perovskite oxides, however, their practical applications remain limited and the emergence of ferroelectricity and related functional properties in simple binary oxides seems so unlikely that it was never previously considered. Here, we show using first-principles density functional calculations that ferroelectricity can be …
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