The origin of both the Ising chain magnetism and ferroelectricity in Ca$_3$CoMnO$_6$ is studied by $ab$ $initio$ electronic structure calculations and x-ray absorption spectroscopy. We find that Ca$_3$CoMnO$_6$ has the alternate trigonal prismatic Co$^{2+}$ and octahedral Mn$^{4+}$ sites in the spin chain. Both the Co$^{2+}$ and Mn$^{4+}$ are in the high spin state. In addition, the Co$^{2+}$ has a huge orbital moment of 1.7 $μ_B$ which is responsible for the significant Ising magnetism. The centrosymmetric crystal structure known so far is calculated to be unstable with respect to exchange striction in the experimentally observed $\uparrow\uparrow\downarrow\downarrow$ antiferromagnetic structure for the Ising chain. The calculated inequivalence of the Co-Mn distances accounts for the ferroelectricity.
Structural, magnetic, and dielectric properties of Sm$_2$CoMnO$_6$ have been studied. X-ray diffraction and Rietveld analysis show that the sample crystallizes in the monoclinic structure with \textit{P2$_1$/n} space group. Magnetic study shows that the sample undergoes a paramagnetic to ferromagnetic phase transition around $T_c$ $\sim$148 K and a low-temperature ordering of Sm$^{3+}$ moments. Temperature-dependent Raman study shows spin-phonon coupling present in this material. Dielectric response of Sm$_2$CoMnO$_6$ shows strong frequency dependence with large dispersion and large dielectric…
We report on the magnetic and structural properties of ferromagnetic-insulating La$_2$CoMnO$_6$ thin films grown on top of (001) STO substrates by means of RF sputtering technique. Careful structural analysis, by using synchrotron X-ray diffraction, allows identifying two different crystallographic orientations that are closely related to oxygen stoichiometry and to the features (coercive fields and remanence) of the hysteresis loops. Both Curie temperature and magnetic hysteresis turn out to be dependent on the oxygen stoichiometry. In situ annealing conditions allow tailoring the oxygen cont…
We report multiferroic behavior in double perovskite Y2CoMnO6 with ferroelectric transition temperature Tc = 80K. The origin of ferroelectricity is associated with magnetic ordering of Co2+ and Mn4+ moments in a up-up-down-down arrangement. The saturation polarization and magnetization are estimated to be 65 uC/m2 and 6.2 Bohr magneton/f.u. respectively. The magnetoelectric coupling parameter, on the other hand, is small as a 5 Tesla field suppresses the electric polarization by only ~8%. This is corroborated with observed hysteretic behaviour at 5K that remains unsaturated even upto 7 Tesla. …
Multiferroics - materials which are simultaneously (ferro)magnetic and ferroelectric, and often also ferroelastic, attract now considerable attention, both because of the interesting physics involved and as they promise important practical applications. In this paper I give a survey of microscopic factors determining the coexistence of these properties, and discuss different possible routes to combine them in one material. In particular the role of the occupation of d-states in transition metal perovskites is discussed, possible role of spiral magnetic structures is stressed and the novel mech…
The spin-induced ferroelectricity in quasi-1D spin chain system is little known, which could be fundamentally different from those in three-dimensional (3D) system. Here, we report the ferroelectricity driven by a tilted screw spin order and its exotic dynamic in the spin-chain compound Ba9Fe3Se15. It is found that the spin-induced polarization has already occurred and exhibits magnetoelectric coupling behavior far above the long-range spin order (LRSO) at TN = 14 K. The polarized entities grow and their dynamic responses slow down gradually with decreasing temperature and permeate the whole l…
We employ electron paramagnetic resonance (EPR) of the spin probe Mn2+ to study the paraelectric ferroelectric transition in DMAMnF and Mn2 doped DMZnF, which are considered to be model metal organic frameworks (MOF) with a Pb free perovskite architecture. In DMAMnF, we study the variation of the Mn2+ EPR line shape and intensity at the X-band (9.4 GHz) and over 80 to 300 K, and we show the absence of magnetoelectric coupling at the ferroelectric transition. At the antiferromagnetic transition in DMMnF, we detect a magnetoelectric coupling caused by weak ferromagnetism in the AFM phase. In DMZ…
Ca3CoMnO6 is composed of CoMnO6 chains made up of face-sharing CoO6 trigonal prisms and MnO6 octahedra. The structural, magnetic, and ferroelectric properties of this compound were investigated on the basis of density functional theory calculations. Ca3CoMnO6 is found to undergo a Jahn-Teller distortion associated with the CoO6 trigonal prisms containing high-spin Co2+ (d7) ions, which removes the C3 rotational symmetry and hence uniaxial magnetism. However, the Jahn-Teller distortion is not strong enough to fully quench the orbital moment of the high-spin Co2+ ions thereby leading to an elect…
Magnetics, ferroelectrics and multiferroics have attracted great attentions because they are not only extremely important for investigating fundamental physics, but also have important applications in information technology. Here, recent computational studies on magnetism and ferroelectricity are reviewed. We first give a brief introduction to magnets, ferroelectrics, and multiferroics. Then, theoretical models and corresponding computational methods for investigating these materials are presented. In particular, a new method for computing the linear magnetoelectric coupling tensor without app…
Tungstates $A$WO$_4$ with the wolframite structure characterized by the $A$O$_6$ octahedral zigzag chains along the $c$-axis, can be magnetic if $A$=Mn, Fe, Co, Cu, Ni. Among them, MnWO$_4$ is a unique member with a cycloid Mn$^{2+}$ spin order developed at low temperature, leading to an interesting type-II multiferroic behavior. However, so far no other multiferroic material in the tungstate family has been found. In this work, we present the synthesis and the systematic study of the double tungstate LiFe(WO$_4$)$_2$. Experimental characterizations including structural, thermodynamic, magneti…
DyFeO3 exhibits a weak ferromagnetism (TNFe ~ 645 K) that disappears below a spin-reorientation (Morin) transition at TSRFe ~ 50 K. It is also known that applied magnetic field induces ferroelectricity at the magnetic ordering temperature of Dy-ions (TNDy ~ 4.5 K). Here, we show that the ferroelectricity exists in the weak ferromagnetic state (TSRFe < T < TN,C) without applying magnetic field, indicating the crucial role of weak ferromagnetism in inducing ferroelectricity. 57Fe Mössbauer studies show that hyperfine field (Bhf) deviates from mean field-like behaviour that is observed in the wea…
Ferroelectricity has been found to occur in several insulating systems, such as TbMnO$_3$ (TMO) and Ni$_3$V$_2$O$_8$ (NVO) which have more than one phase with incommensurately modulated long-range magnetic order. Here we give a phenomenological model which relates the symmetries of the magnetic structure as obtained from neutron diffraction to the development and orientation of a spontaneous ferroelectric moment induced by the magnetic ordering. This model leads directly to the formulation of a microscopic spin-phonon interaction which gives explains the observed phenomena. The results are giv…
Multiferroics with coupling between ferroelectricity and magnetism have been pursued for decades. However, their magnetoelectric performances remain limited due to the common trade-off between ferroelectricity and magnetism. Here, a family of nitride perovskites is proposed as multiferroics with prominent physical properties and nontrivial mechanisms. Taking GdWN$_3$ as a prototype, our first-principles calculations found that its perovskite phases own large polarizations (e.g. $111.3$ $μ$C/cm$^2$ for the $R3c$ phase) and a magnetic moment $7$ $μ_{\rm B}$/Gd$^{3+}$. More interestingly, its fer…
Nominally pure nanocrystalline KTaO3 was thoroughly investigated by micro-Raman and magnetic resonance spectroscopic techniques. In all samples the defect driven ferroelectricity and magnetism are registered. Both ordering states are suggested to appear due to the iron atoms and oxygen vacancies. The concentration of defects was estimated to be 0.04 and 0.06-0.1 mole %, respectively. Note that undoped single crystals of KTaO3 are nonmagnetic and have never exhibited ferromagnetic properties. The results enable us to refer a nanosized KTa(Fe)O3 to the class of multiferroics and assume that it c…
We report ferroelectricity with fairly large spontaneous polarization (P ~ 0.2 - 0.8 \muC/cm2) below the Néel temperature of chromium (TNCr) in weakly ferromagnetic rare-earth orthochromites, RCrO3 (R=rare-earth) only when the rare-earth ion is magnetic. Intriguingly, the ferroelectricity in ErCrO3 (TC = 133 K) disappears at a spin-reorientation (Morin) transition (TSR ~ 22 K) below which the weak ferromagnetism associated with the Cr-sublattice also disappears, demonstrating the crucial role of weak ferromagnetism in inducing ferroelectricity. Further, the ferroelectric polarization (P) could…
The realization of multiferroicity in 2D nanomaterials is crucially important for designing advanced nanoelectronic devices such as non-volatile multistate data storage. In this work, the coexistence of ferromagnetism and ferroelectricity is reported in monolayer SnTe system by transition metal (TM) doping. Based on first-principles calculations, the spontaneous spin polarization could be realized by TM doping in ferroelectric SnTe monolayer. In addition to in-plane ferroelectric polarization, the out-of-plane ferroelectric polarization emerges in Mn (Fe)-doped SnTe monolayer due to the intern…
First principles calculations based on the density functional theory within the local spin density approximation plus U(LSDA+U)scheme, show rhombohedral Bi$_2$FeTiO$_6$ is a potential multiferroic in which the magnetism and ferroelectricity coexist . A ferromagnetic configuration with magnetic moment of 4 $μ_B$ per formula unit have been reported with respect to the minimum total energy. Spontaneous polarization of 27.3 $μ$ C/cm$^2$, caused mainly by the ferroelectric distortions of Ti, was evaluated using the berry phase approach in the modern theory of polarization. The Bi-6s stereochemical …
The complete irreducible co-representations of the paramagnetic space group provide a simple and direct path to explore the symmetry restrictions of magnetically driven ferroelectricity. The method consists of a straightforward generalization of the method commonly used in the case of displacive modulated systems and allows us to determine, in a simple manner, the full magnetic symmetry of a given phase originated from a given magnetic order parameter. The potential ferroic and magneto-electric properties of that phase can then be established and the exact Landau free energy expansions can be …
We use first-principles calculations based on density functional theory to investigate the interplay between oxygen vacancies, A-site cation size / tolerance factor, epitaxial strain, ferroelectricity and magnetism in the perovskite manganite series, AMnO3 (A=Ca2+, Sr2+, Ba2+). We find that, as expected, increasing the volume through either chemical pressure or tensile strain generally lowers the formation energy of neutral oxygen vacancies consistent with their established tendency to expand the lattice. Increased volume also favors polar distortions, both because competing rotations of the o…
Multiferroics, where two or more ferroic order parameters coexist, is one of the hottest fields in condensed matter physics and materials science[1-9]. However, the coexistence of magnetism and conventional ferroelectricity is physically unfavoured[10]. Recently several remedies have been proposed, e.g., improper ferroelectricity induced by specific magnetic[6] or charge orders[2]. Guiding by these theories, currently most research is focused on frustrated magnets, which usually have complicated magnetic structure and low magnetic ordering temperature, consequently far from the practical appli…
Titanates with the perovskite structure, including ferroelectrics (e.g., BaTiO$_3$) and ferromagnetic ones (e.g., YTiO$_3$), are important functional materials. Recent theoretical studies predicted multiferroic states in strained EuTiO$_3$ and titanate superlattices, the former of which has already been experimental confirmed. Here, a first-principles calculation is performed to investigate the structural, magnetic, and electronic properties of Y half-substituted LaTiO3. Our results reveal that the magnetism of Y$_{0.5}$La$_{0.5}$TiO$_3$ sensitively depends on its structural details because of…
BiFeO3 is the most famous multiferroic material, but its G-type antiferromagnetism is highly desirable to be replaced by strong macroscopic magnetism beyond room temperature. Here we obtain double perovskite Bi2FeMoO6 with R3 (#146) space group by substituting Mo for 50% Fe in BiFeO3. Our first-principles calculated results show that it is a semiconductor with gap reaching to 0.725 eV, its net magnetic moment is 2μ_B per formula unit, and its ferroelectric polarization is 85μC/cm^2. This ferroelctricity is comparable with that of BiFeO3, but here the magnetism is a strong ferrimagnetism with C…
Two-dimensional electron gas due to semiconductor interfaces can have high mobility and exhibits superconductivity, magnetism, and other exotic properties that are unexpected in constituent bulk materials. We study crystal structures, electronic states, and magnetism of short-period (BTO)$_m$/(GTO)$_2$ ($m$=2 and 4) superlattices consisting of ferroelectric BaTiO$_3$ (BTO) and ferrimagnetic insulating polar GdTiO$_3$ (GTO) by first principles calculations. Our investigation shows that the middle Ti-O monolayer in the GTO layer becomes metallic because the ferroelectricity in the insulating BTO…
Charge dipole moment and spin moment rarely coexist in single-phase bulk materials except in some multiferroics. Despite the progress in the past decade, for most multiferroics their magnetoelectric performance remains poor due to the intrinsic exclusion between charge dipole and spin moment. As an alternative approach, the oxide heterostructures may evade the intrinsic limits in bulk materials and provide more attractive potential to realize the magnetoelectric functions. Here we perform a first-principles study on LaAlO$_3$/PbTiO$_3$ superlattices. Although neither of the components is magne…
Multiferroics with intrinsic ferromagnetism and ferroelectricity are highly desired but rather rare, while most ferroelectric magnets are antiferromagnetic. A recent theoretical work [Phys. Rev. B {\bf 99}, 195434 (2019)] predicted that oxyhalides VO$X_2$ ($X$: halogen) monolayers are two-dimensional multiferroics by violating the empirical $d^0$ rule. Most interestingly, the member VOI$_2$ are predicted to exhibit spontaneous ferromagnetism and ferroelectricity. In this work, we extend the previous study on the structure and magnetism of VOI$_2$ monolayer by using density functional theory an…
The discoveries of magnetism and ferroelectricity in 2D van der Waals (vdW) materials have brought important functionalities to the 2D materials family, and may trigger a revolution in next generation nanoelectronics and spintronics. In this perspective article, we briefly review the recent progress in the field of 2D ferroelectrics, focusing on the mechanisms that drive spontaneous polarizations in 2D systems, unique properties brought about by the reduced lattice dimensionality, and promising applications of 2D ferroelectrics. At the end, we provide an outlook for challenges that need to be …
Multiferroic materials, with their combined and coupled magnetism and ferroelectricity, providea playground for studying new physics and chemistry as well as a platform for development ofnovel devices and technologies. Based on my July 2017 Royal Society Inaugural Lecture, I review recent progress and propose future directions in the fundamentals and applications of multiferroics, with a focus on unanticipated developments outside of the core activity of electric-field control of magnetism.
Achieving multiferroic two-dimensional (2D) materials should enable numerous functionalities in nanoscale devices. Until now, however, predicted 2D multiferroics are very few and with coexisting yet only loosely coupled (type-I) ferroelectricity and magnetism. Here, a type-II multiferroic MXene Hf$_{2}$VC$_{2}$F$_{2}$ monolayer is identified, where ferroelectricity originates directly from its magnetism. The noncollinear Y-type spin order generates a polarization perpendicular to the spin helical plane. Remarkably, the multiferroic transition is estimated to occur above room temperature. Our i…
Highly ordered Bi2FeMnO6 epitaxial thin films have been successfully grown on SrTiO3 substrate. Both synchrotron X-ray reciprocal space mapping and high resolution transmission electron microscopy confirmed the alternative alignment of Fe and Mn along [111] direction of Bi2FeMnO6 films. Magnetic and ferroelectric properties of Bi2FeMnO6 films are characterized and analyzed. The room-temperature ferroelectricity is well kept in Bi2FeMnO6 film as expected. However, it is very interesting that Bi2FeMnO6 film exhibits a typical spin-glass behavior and very weak magnetism rather than a ferri/ferrom…
This work shows an unconventional route for spin-driven ferroelectricity originating from a metastable magnetic field-induced canting of chromium sublattice in the presence of gadolinium moments in GdCrTiO5 at low temperatures. Compared to the isostructural neodymium compound, significant differences of magnetism and magnetoelectric effects are seen. We present the results of thorough investigations of temperature and magnetic field dependent magnetization as well as ac and dc magnetic susceptibility. These bulk measurements are complemented by local-probe spectroscopy utilizing electron-spin …
Ferroelectricity occurs in crystals with broken spatial inversion symmetry. In conventional perovskite oxides, concerted ionic displacements within a three-dimensional network of transition metal-oxygen polyhedra (MOx) manifest spontaneous polarization. Meanwhile, some two-dimensional networks of MOx can foster geometric ferroelectricity with magnetism, owing to the distortion of the polyhedra. Because of the fundamentally different mechanism of ferroelectricity in a two-dimensional network, one can further challenge an uncharted mechanism of ferroelectricity in a one-dimensional channel of MO…
Controlling functionalities, such as magnetism or ferroelectricity, by means of oxygen vacancies (VO) is a key issue for the future development of transition metal oxides. Progress in this field is currently addressed through VO variations and their impact on mainly one order parameter. Here we reveal a new mechanism for tuning both magnetism and ferroelectricity simultaneously by using VO. Combined experimental and density-functional theory studies of Eu0.5Ba0.5TiO3-δ, we demonstrate that oxygen vacancies create Ti3+ 3d1 defect states, mediating the ferromagnetic coupling between the localize…
Realizing effective manipulation and explicit identification of topological spin textures are two crucial ingredients to make them as information carrier in spintronic devices with high storage density, high data handling speed and low energy consumption. Electric-field manipulation of magnetism has been achieved as a dissipationless method compared with traditional regulations. However, the magnetization is normally insensitive to the electric field since it does not break time-reversal symmetry directly, and distribution of topological magnetic quasiparticles is difficult to maintain due to …
Antisymmetric Dzyaloshinskii-Moriya (DM) interactions generating from the spin-orbit coupling induce various fascinating properties, like magnetoelectric (ME) effect, weak ferromagnetism and non-trivial topological spin textures like skyrmions, in real materials. Compared to their symmetric isotropic exchange counterpart, these interactions are generally of a weaker order of strength, creating modest twisting in the spin structure which results in weak ferromagntism or weak linear ME effect. Our proposed two-sublattice model, in contrast, predicts a hitherto unobserved, charge ordered non-coll…
We report a systematic study of the structure, electric and magnetic properties of Ca$_3$Co$_{2-x}$Mn$_x$O$_6$ single crystals with $x =$ 0.72 and 0.26. The DC and AC magnetic susceptibilities display anomalies with characteristic of the spin freezing. The crystals show ferroelectric transition at 40 K and 35 K ($T_{FE}$) for $x =$ 0.72 and 0.26, respectively, with a large value of 1400 $μ$C/m$^2$ at 8 K for electric polarization ($P_c$) along the spin-chain ($c$-axis) direction. Interestingly, the electric polarization perpendicular to the chain direction ($P_{ab}$) can also be detected and h…
Within the broad class of multiferroics (compounds showing a coexistence of magnetism and ferroelectricity), we focus on the subclass of "improper electronic ferroelectrics", i.e. correlated materials where electronic degrees of freedom (such as spin, charge or orbital) drive ferroelectricity. In particular, in spin-induced ferroelectrics, there is not only a {\em coexistence} of the two intriguing magnetic and dipolar orders; rather, there is such an intimate link that one drives the other, suggesting a giant magnetoelectric coupling. Via first-principles approaches based on density functiona…
Chirality, i.e., the right- and left-handedness of structure, is one of the key concepts in many fields of science including biology, chemistry and physics, and its manipulation is an issue of vital importance. The electron spins in solids can form chiral configurations. In perovskite manganites RMnO3 (R=Tb, Dy,...etc), the Mn-spins form a cycloidal structure, which induces ferroelectric polarization (P) through the relativistic spin-orbit interaction. This magnetism-induced ferroelectricity (multiferroics) and associated infrared-active spin waves (electromagnons) open a promising route to co…
Magnetoelectric coupling has been a trending research topic in both organic and inorganic materials and hybrids. The concept of controlling magnetism using an electric field is particularly appealing in energy efficient applications. In this spirit, ferroelectricity has been introduced to organic spin valves to manipulate the magneto transport, where the spin transport through the ferromagnet/organic spacer interfaces (spinterface) are under intensive study. The ferroelectric materials in the organic spin valves provide a knob to vary the interfacial energy alignment and the interfacial crysta…
Controlling the ferroelectricity and magnetism in multiferroic materials has been an important research topic. We report the formation of a highly polarized state in multiferroic DyMnO3 single crystals which develops well above the magnetic transition temperatures, and we attribute it the thermally stimulated depolarization current effect of excess holes forming Mn4+ ions in the system. We also show that this high temperature polarized state intimately correlates with the lower temperature ferroelectric state that is induced by the incommensurate spiral magnetic order of Mn spins. This study d…
The research landscape of magnetism has been recently enriched by the discovery of altermagnetism. It is an unconventional phase of matter characterized by a d-wave (or higher even-parity-wave) collinear compensated spin ordering, which enables strongly spin-polarized currents in the absence of magnetization, and features fast spin dynamics. Simultaneously, on the applied magnetism front, spintronic memories based on conventional ferromagnets are currently turning from a niche to a mass produced integrated-circuit technology as they start to complement semiconductors on advanced-node microproc…
We report the results of various measurements, namely magnetization, complex dielectric permittivity and electric polarization (P) on Dy2BaNiO5 as a function of temperature (T) and magnetic-field (H), apart from heat-capacity (C), with the primary motivation of exploring the existence of magnetoelectric (ME) coupling among Haldane spin-chain systems. The M(T) and C(T) data establish long range magnetic ordering at 58K. The most noteworthy observations are: (i) Distinct anomalies are observed in dielectric constant (ε') vs T and loss (tanδ) vs T at different temperatures, i.e. 12.5, 30, 50 and …
