Magnetic reversal of the ferroelectric polarization in a multiferroic spinel oxide

Current Control of Spin Helicity and Nonreciprocal Charge Transport in a Multiferroic Conductor

A multiferroic state with both electronic polarity (P) and magnetization (M) shows the inherently strong P-M coupling when P is induced by cycloidal (Néel-wall like) spin modulation. The sign of P is determined by the clockwise or counterclockwise rotation of spin, termed the spin helicity. Such a multiferroic state is not limited to magnetic insulators but can be broadly observed in conductors. Here, the current control of the multiferroics is reported in a helimagnetic metal YMn6Sn6 and its detection through nonreciprocal resistivity (NRR). The underlying concept is the coupling of the current with the toroidal momentT ∼ P × M ∼ ( q ̂ × χ v ) × M $\bm{T}\sim \bm{P}\ensuremath{\times{}}\bm{M}\sim (\widehat{\bm{q}}\ensuremath{\times{}}{\bm{\chi}}_{v})\ensuremath{\times{}}\bm{M}$ as well as with the magneto-chirality χv · M, whereq ̂ $\hspace*{0.28em}\widehat{\bm{q}}$ and χv are the unit modulation wave vector and the vector spin chirality, respectively. An enhancement of NRR is furthermore observed by the spin-cluster scattering via χv and its fluctuation. These findings may pave the way to an exploration of multiferroic conductors and the application of the spin-helicity degree of freedom as a state variable.

Electrifying energy storage by investigating the electrochemical behavior of CoCr2O4/graphene-oxide nanocomposite as supercapacitor high performance electrode material

This study reports novel three-step electrochemical fabrication of CoCr2O4/graphene-oxide nanocomposite on glassy carbon electrode including sequential synthesis of graphene-oxide using modified Hummer's method, CoCr2O4 nanoparticles using sol-gel method and the cost-effective co-precipitation technique for nanocomposite formation. The resulting nanocomposite was subjected to comprehensive analytical and morphological analysis. X-ray diffractometry (XRD) confirms nanocomposite formation with reduced average crystallite size value of 26.9 nm whereas SEM indicates spherical grains existence within nanocomposite. Energy-dispersive X-ray spectrometry (EDS) confirms no impurity peak existence whereas Raman spectroscopy clearly indicated D and G band existence at 1345 and 1587 cm-1 respectively. Photoluminescent spectra reveals decreasing trend in band gap value about 3.49 eV. The electrochemical properties of CoCr2O4/graphene-oxide nanocomposite electrode explored, showcasing remarkable capacitance with a surface area of merely 0.068 cm2, 574.8 F/g specific capacitance in alkaline 1M KOH and 488.6 F/g specific capacitance in acidic 0.1M H2SO4 electrolyte. Moreover, synthesized nanocomposite demonstrates remarkable electrochemical stability resulting capacitance retention about 95 % after 100 cycles in 1M KOH electrolyte. GCD analysis reveals impressive power and energy density values of 2489 W/kg and 14.88 Wh/kg respectively. These outstanding properties make the nanocomposite an attractive material for next-generation supercapacitors and energy storage solutions.

Magnetic Field Effect on the Electric and Dielectric Properties of the Linear Magnetoelectric Compound Co4Nb2O9

Using Green's function theory and a microscopic model, the multiferroic properties of Co4Nb2O9 are investigated theoretically. There are some discrepancies in the discussion of the electric and dielectric behavior of CNO with and without external magnetic fields. We try to clarify them. It is observed that the polarization and the dielectric constant do not show a peak at the antiferromagnetic phase transition temperature TN without an external magnetic field h. But applying h, there appears a peak around the Neel temperature TN, which increases with increasing h and then shifts to lower temperatures. The magneto-dielectric coefficient MD(T,h) is also calculated. Moreover, the magnetization rises with an increasing external electric field below the Neel temperature. This shows strong magnetoelectric coupling in Co4Nb2O9. The obtained results are compared with the existing experimental data. There is a good qualitative agreement.

Effect of Ho3+ Substitution on Magnetic Properties of ZnCr2Se4

A series of ZnCr2-xHoxSe4 microcrystalline spinels (where x = 0.05, 0.075, and 0.10) containing holmium ions in octahedral coordination were obtained by sintering of adequate reactants at high temperatures. The obtained doped materials were characterized by X-ray diffraction, Scanning Electron Microscopy, UV-Vis-NIR, molecular field approximation, and XPS spectroscopies. Their thermal properties were also investigated. The doping of the ZnCr2S4 matrix with paramagnetic Ho3+ ions with a content of not more than 0.1 and a screened 4f shell revealed a significant effect of orbital and Landau diamagnetism, a strong reduction in short-range ferromagnetic interactions, and a broadening and shift of the peak of the first critical field by simultaneous stabilization of the sharp peak in the second critical field. These results correlate well with FPLO calculations, which show that Cr sites have magnetic moments of 3.19 µB and Ho sites have significantly larger ones with a value of 3.95 µB. Zn has a negligible magnetic polarization of 0.02 µB, and Se induces a polarization of approximately -0.12 µB.

Spin-Chirality-Driven Multiferroicity in van der Waals Monolayers

Driven by the expected contribution of two-dimensional multiferroic systems with strong magnetoelectric coupling to the development of multifunctional nanodevices, here we propose, by means of first-principles calculations, vanadium-halide monolayers as a new class of spin-chirality-driven van der Waals multiferroics. The frustrated 120-deg magnetic structure in the triangular lattice induces a ferroelectric polarization perpendicular to the spin-spiral plane, whose sign is switched by a spin-chirality change. It follows that, in the presence of an applied electric field perpendicular to the monolayers, one magnetic chirality can be stabilized over the other, thereby allowing the long-sought electrical control of spin textures. Moreover, we demonstrate the remarkable role of spin-lattice coupling on magnetoelectricity, which adds to the expected contribution of spin-orbit interaction determined by an anion. Indeed, such compounds exhibit sizeable spin-driven structural distortions, thereby promoting the investigation of multifunctional spin-electric-lattice couplings.

Temperature dependent structural properties of Mn1.90M0.10O3(M = Cr and Fe)

The polycrystalline samples of Mn1.90Cr0.10O3(MCO) and Mn1.90Fe0.10O3(MFO) have been investigated for their temperature dependent magnetic and structural properties. The Cr and Fe substitutions have significant effect on the magnetic and structural properties of Mn2O3. Like pristine Mn2O3, the Cr and Fe substituted samples MCO and MFO also exhibit two antiferromagnetic transitions; one at ∼77 K, ∼80 K, respectively and another at ∼40 K. Our room temperature synchrotron x-ray powder diffraction (SXRD) results confirm that both the MCO and MFO samples crystallize in cubic symmetry. The temperature dependent SXRD results demonstrate the cubic to orthorhombic structural transition for the studied samples. The pristine Mn2O3shows cubic to orthorhombic transition around 310 K, whereas this structural transition shifted towards lower temperature side with these substitutions i.e. around 240 K for MCO and 260 K for MFO. Interestingly, the centrosymmetricPcabto non-centrosymmetricPca21change in symmetry is also resolved at the ferroelectric ordering temperature for MCO.

Broadband magnetic resonance spectroscopy in MnSc[Formula: see text]S[Formula: see text]

Recent neutron scattering experiments suggested that frustrated magnetic interactions give rise to antiferromagnetic spiral and fractional skyrmion lattice phases in MnSc[Formula: see text]S[Formula: see text] . Here, to trace the signatures of these modulated phases, we studied the spin excitations of MnSc[Formula: see text]S[Formula: see text] by THz spectroscopy at 300 mK and in magnetic fields up to 12 T and by broadband microwave spectroscopy at various temperatures up to 50 GHz. We found a single magnetic resonance with frequency linearly increasing in field. The small deviation of the Mn[Formula: see text] ion g-factor from 2, g = 1.96, and the absence of other resonances imply very weak anisotropies and negligible contribution of higher harmonics to the spiral state. The significant difference between the dc magnetic susceptibility and the lowest-frequency ac susceptibility in our experiment implies the existence of mode(s) outside of the measured frequency windows. The combination of THz and microwave experiments suggests a spin gap opening below the ordering temperature between 50 GHz and 100 GHz.

The Zn1−xPbxCr2Se4—Single Crystals Obtained by Chemical Vapour Transport—Structure and Magnetic, Electrical, and Thermal Properties

Monocrystalline chalcogenide spinels ZnCr₂Se₄ are antiferromagnetic and semiconductor materials. They can be used to dope or alloy with related semiconducting spinels. Therefore, their Pb-doped display is expected to have unique properties and new potential applications. This paper presents the results of dc and ac magnetic measurements, including the critical fields visible on the magnetisation isotherms, electrical conductivity, and specific heat of the ZnCr₂S₄:Pb single crystals. These studies showed that substituting the diamagnetic Pb ion with a large ion radius for the Zn one leads to strong short-range ferromagnetic interactions in the entire temperature range and spin fluctuations in the paramagnetic region at H_dc = 50 kOe.

Tuning Exchange Coupling in a New Family of Nanocrystal-Based Granular Multiferroics Using an Applied Electric Field

In this work, we demonstrate an experimental realization of a granular multiferroic composite, where the magnetic state of a nanocrystal array is modified by tuning the interparticle exchange coupling using an applied electric field. Previous theoretical models of a granular multiferroic composite predicted a unique magnetoelectric coupling mechanism, in which the magnetic spins of the ensemble are governed by interparticle exchange. The extent of these exchange interactions can be controlled by varying the local dielectric environment between grains. We specifically utilize the strong dielectric dependence of ferroelectric materials to modify the interparticle coupling of closely spaced magnetic nanoparticles using either a change in temperature or an electric field. This coupling modifies the ensemble magnetic coercivity and thus the superparamagnetic-to-ferromagnetic phase transition temperature. Through the use of two different ferroelectrics, our results suggest that this magnetoelectric coupling mechanism could be generalized as a new class of multiferroic material, applicable to a broad range of ferroelectric/magnetic nanocrystal composites.

Magnetic-Field-Induced Dielectric Anomalies in Cobalt-Containing Garnets

Here we present a comparative study of the magnetic and crystal chemical properties of two Co²⁺ containing garnets. CaY₂Co₂Ge₃O₁₂ (which has been reported previously) and NaCa₂Co₂V₃O₁₂ both exhibit the onset of antiferromagnetic order around 6 K as well as field-induced transitions around 7 and 10 T, respectively, that manifest as anomalies in the dielectric properties of the material. We perform detailed crystal-chemistry analyses and complementary density functional theory calculations to show that very minor changes in the local environment of the Co ions explain the differences in the two magnetic structures and their respective properties.

Domain Wall Patterning and Giant Response Functions in Ferrimagnetic Spinels

The manipulation of mesoscale domain wall phenomena has emerged as a powerful strategy for designing ferroelectric responses in functional devices, but its full potential is not yet realized in the field of magnetism. This work shows a direct connection between magnetic response functions in mechanically strained samples of Mn3 O4 and MnV2 O4 and stripe-like patternings of the bulk magnetization which appear below known magnetostructural transitions. Building off previous magnetic force microscopy data, a small-angle neutron scattering is used to show that these patterns represent distinctive magnetic phenomena which extend throughout the bulk of two separate materials, and further are controllable via applied magnetic field and mechanical stress. These results are unambiguously connected to the anomalously large magnetoelastic and magnetodielectric response functions reported for these materials, by performing susceptibility measurements on the same crystals and directly correlating local and macroscopic data.

Influence of Polymorphism on the Magnetic Properties of Co5TeO8 Spinel

This study presents the influence of polymorphism on the magnetic properties of Co₅TeO₈. This compound with a spinel-like structure [Co₂]^A[Co₃Te]^BO₈ was synthesized into two polymorphs: one disordered within a cubic Fd3̅m structure, where Co²⁺ and Te⁶⁺ ions are randomly distributed on the octahedral B sites [the disordered polymorph can also be presented as an inverse spinel of the formula Co(Co_1.5Te_0.5)O₄] and the other ordered with a cubic P4₃32 structure where Co²⁺ and Te⁶⁺ ions are ordered on the B sites. The macroscopic magnetic measurements showed that both polymorphs present a ferrimagnetic ordering, below ∼40 K, and a second transition is also observed at 27 K for the ordered polymorph. Neutron powder diffraction data between room temperature and 1.7 K showed as well the presence of short-range magnetic ordered clusters, which appears for both polymorphs below 200 K. At lower temperature, these short-range orders are transformed into long-range ferrimagnetic orders. Below T_C = 40 K, the colinear ferrimagnetic structure of the disordered polymorph is described with the I4₁/am'd' space group. The ordered polymorph undergoes an incommensurate ferrimagnetic spiral spin ordering below T_C1 = 45 K, followed by a second magnetic phase transition at T_C2 = 27 K. This last transition is associated with the emergence of an additional ferrimagnetic component and an abrupt change in the magnitude of the magnetic propagation vector k = [0, 0, γ] from γ = 0.086 at T = 30 K to γ ≈ 0.14 in the range between 27 and 1.7 K. The magnetic symmetry of the ordered polymorph is described with the P4₃(00γ)0 magnetic superspace group. We evidenced that the ordering of Co²⁺/Te⁶⁺ on the B sites changes all of the Co-Co and Co-O distances and thus all J_AB, J_AA, and J_BB exchange interactions, between the A and B sites, which are able to stabilize the incommensurate spin modulation in the ordered polymorph.

Synthesis and Fabrication of Co1−xNixCr2O4 Chromate Nanoparticles and the Effect of Ni Concentration on Their Bandgap, Structure, and Optical Properties

In the present work, cobalt-chromite-based pigment Co1-xNixCr2O4 chromate powder and nanoparticles with various transition metal concentrations (x = 0.2, 0.4, 0.6, and 0.8) were manufactured by applying aqueous synthesis approaches and sol–gel synthesis routes. XRD analysis of the powder shows that all samples formulated by the sol–gel method were crystalline with a spinel structure. Chromites show green color with a higher nickel concentration, while Co-substituent shows blackish pigments. Samples were annealed at distinct temperatures ranging from 600 °C to 750 °C. The nanoparticles obtained were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy (RS), photoluminescence (PL), and energy-dispersive X-ray spectroscopy (EDS). The particle size of the parent compound (CoCr2O4) ranges from 100 nm to 500 nm, as measured by SEM. The tendency of particles to form aggregates with increasing annealing temperature was observed. These compounds may be successfully used as an effective doped nickel-cobalt ceramic pigment.

Magnetic Texture in Insulating Single Crystal High Entropy Oxide Spinel Films

Magnetic insulators are important materials for a range of next-generation memory and spintronic applications. Structural constraints in this class of devices generally require a clean heterointerface that allows effective magnetic coupling between the insulating layer and the conducting layer. However, there are relatively few examples of magnetic insulators that can be synthesized with surface qualities that would allow these smooth interfaces and precisely tuned interfacial magnetic exchange coupling, which might be applicable at room temperature. In this work, we demonstrate an example of how the configurational complexity in the magnetic insulator layer can be used to realize these properties. The entropy-assisted synthesis is used to create single-crystal (Mg_0.2Ni_0.2Fe_0.2Co_0.2Cu_0.2)Fe₂O₄ films on substrates spanning a range of strain states. These films show smooth surfaces, high resistivity, and strong magnetic responses at room temperature. Local and global magnetic measurements further demonstrate how strain can be used to manipulate the magnetic texture and anisotropy. These findings provide insight into how precise magnetic responses can be designed using compositionally complex materials that may find application in next-generation magnetic devices.

Proximate Quantum Spin Liquid on Designer Lattice

Complementary to bulk synthesis, here we propose a designer lattice with extremely high magnetic frustration and demonstrate the possible realization of a quantum spin liquid state from both experiments and theoretical calculations. In an ultrathin (111) CoCr₂O₄ slice composed of three triangular and one kagome cation planes, the absence of a spin ordering or freezing transition is demonstrated down to 0.03 K, in the presence of strong antiferromagnetic correlations in the energy scale of 30 K between Co and Cr sublattices, leading to the frustration factor of ∼1000. Persisting spin fluctuations are observed at low temperatures via low-energy muon spin relaxation. Our calculations further demonstrate the emergence of highly degenerate magnetic ground states at the 0 K limit, due to the competition among multiply altered exchange interactions. These results collectively indicate the realization of a proximate quantum spin liquid state on the synthetic lattice.

Modeling and prediction of lattice parameters of binary spinel compounds (AM2X4) using support vector regression with Bayesian optimization

The lattice constants of spinel compounds AM2X4 are correlated with the constituent elemental properties using support vector regression (SVR) optimized with Bayesian optimization.

Modulating the optical and magnetic properties of geometrically frustrated ZnV2O4 by the introduction of indium (nonmagnetic ions), iron, and chromium (magnetic ions)

The current study is aimed at understanding the effects of diluting the magnetic properties of a geometrically frustrated normal spinel, ZnV2O4, with the incorporation of nonmagnetic In3+ in place of V3+. Samples with the formula, ZnV2-xInxO4 (x = 0.00, 0.25, 0.50, 1.00 and 1.50), were synthesized following an epoxide mediated gel method and characterized extensively. The monophasic cubic spinel structure was retained up to 50 mol% of vanadium with indium, beyond which phase separation took place. The occupancy of indium at the octahedral site and the near-linear increment of the cubic unit cell constant were confirmed from the successful structural refinements. The optical bandgap increased from 2.80 (ZnV2O4) to 3.06, 3.19, and 3.35 eV for x values of 0.25, 0.50, and 1.00 in ZnV2-xInxO4. ZnVInO4 exhibited paramagnetic behavior down to 2 K in both the field-dependent and temperature-dependent magnetic measurements. However, the magnetization values were lower than those of ZnV2O4. A frustration index of 42 was estimated for ZnVInO4. Samples containing magnetic Cr3+ and Fe3+ ions in place of V3+ were synthesized and characterized to compare and contrast the magnetic ions' influence. For both chromium and iron substituted samples, the optical bandgap was higher than that of ZnV2O4. ZnVCrO4 showed an antiferromagnetic ordering of spins with a TN of 12.3 K. In contrast, the randomization of Zn, V, and Fe among the available crystallographic sites increased the ferrimagnetic transition temperature (TF) to 31.9 K. ZnVInO4 catalyzed the photodegradation of rhodamine-6G under UV-vis radiation to a greater extent than ZnVCrO4 and ZnVFeO4.

Disclosing the behavior under hydrostatic pressure of rhombohedral MgIn2Se4 by means of first-principles calculations

AM2X4 crystalline materials display important technological electronic, optical and magnetic properties that are sensitive to general stress effects. In this paper, the behavior under hydrostatic pressure of the ambient condition rhombohedral phase of MgIn2Se4 is investigated in detail for the first time. We carried out first-principles calculations within the density functional theory framework aimed at determining the pressure-induced polymorphic sequence of this selenide. To accurately evaluate transition pressures at room temperature, thermal corrections have been included after the computation of phonon dispersion curves in potential candidate phases, namely the initial rhombohedral R3[combining macron]m one, inverse and direct spinels, LiTiO2-type and defective I4[combining macron] structures. Only the transition from the R3[combining macron]m to the inverse spinel phase was found to fulfill the thermodynamic and mechanical stability criteria. The direct spinel could appear as metastable if kinetic effects hinder the above transition. Additionally, electronic band structures and chemical bonding properties were analyzed from the outcome of our quantum-mechanical solutions reporting band gap values and ionicity and noncovalent interaction indexes. It is shown that the investigated compound keeps behaving as a semiconductor, loses its van der Waals interactions, and becomes more covalent as hydrostatic pressure is applied.

Evidence of anomalous conventional and spontaneous exchange bias, high coercivity in Fe doped NiCr2O4 spinel

NiCr2-xFexO4 (x = 0 and 0.2) polycrystalline ceramics have been synthesized successfully through a simple co-precipitation technique to study the evolution of structural and magnetic properties by doping Fe. X-ray diffraction (XRD) reveals that the high-temperature cubic phase (space group Fd3[combining macron]m) observed at 320 K in bulk NiCr2O4 is stabilized at room temperature by decreasing the particle size to nanometer in x = 0 as well as after incorporating 20 at% Fe in the NiCr2O4 lattice. The cation distribution obtained from X-ray absorption fine structure (XAFS) analysis illustrates that while in x = 0, Ni2+ and Cr3+ ions occupy the tetrahedral (A) and octahedral (B) sites, respectively, x = 0.2, Fe3+ and Cr3+ ions occupy the A and B sites, respectively, and Ni2+ ions are distributed among the A and B sites. This transformation from the normal to mixed spinel structure strongly affects the magnetic properties. While the paramagnetic to long-range ferrimagnetic ordering temperature TC is enhanced from 71 to 192 K, significantly large coercive field (HC) of ∼29 kOe is observed for x = 0.2 as compared to the HC ∼13 kOe for x = 0. Moreover, unusually large conventional and spontaneous exchange bias fields of ∼26 and ∼2.6 kOe are observed for x = 0.2, which is absent for x = 0. The presence of anomalous exchange bias field is ascribed to the unidirectional exchange anisotropy between the two magnetic sublattices at A and B sites. The training effect of the exchange bias field is discussed using a phenomenological model, which considers the contribution from irreversible uncompensated spins that modify the exchange anisotropy at the interface between A and B magnetic sublattices. In addition, diffuse neutron scattering (DNS) with XYZ analysis is employed for both compositions to clearly illustrate the low-temperature peculiar magnetic phase transitions such as spin spiral transition, TS and spin lock-in transition, Tl. The DNS demonstrates that while Tl decreases from 10 K to 7 K with the incorporation of Fe in the NiCr2O4 lattice, TS significantly increases from 28 K to 50 K.

Role of Ni substitution on structural, magnetic and electronic properties of epitaxial CoCr2O4 spinel thin films

Cubic spinel CoCr2O4 has recently attained attention due to its multiferroic properties. However, the Co site substitution effect on the structural and magnetic properties has rarely been studied in thin film form. In this work, the structural and magnetic properties of Co1-x Ni x Cr2O4 (x= 0, 0.5) epitaxial thin films deposited on MgAl2O4 (100) and MgO (100) substrates to manipulate the nature of strain in the films using pulsed laser deposition (PLD) technique are presented. The epitaxial nature of the films was manifested through x-ray diffraction (XRD), reciprocal space mapping (RSM) and Rutherford backscattering spectrometry (RBS) measurements. Raman measurements revealed a disappearance of characteristic A 1 g and F 2 g modes of the CoCr2O4 with increase in the Ni content. Atomic force microscopy (AFM) and field emission scanning electron microscopy (FE-SEM) studies show a modification of the surface morphology upon Ni substitution. Magnetic measurements disclose that the ferrimagnetic Curie temperature (T C) of the CoCr2O4 in thin film grown on MgAl2O4 (100) and MgO (100) substrates were found to be 100.6 ± 0.5 K and 93.8 ± 0.2 K, respectively. With Ni substitution the T C values were found to be enhanced to 104.5 ± 0.4 K for MgAl2O4 (100) and 108.5 ± 0.6 K for MgO (100) substrates. X-ray photoelectron spectroscopy (XPS) suggests Cr3+ oxidation states in the films, while Co ions are present in a mixed Co2+/Co3+ oxidation state. The substitution of Ni at Co site significantly modifies the line shape of the core level as well as the valence band. Ni ions are also found to be in a mixed 2+/3+ oxidation state. O 1s core level display asymmetry related to possible defects like oxygen vacancies in the films.

Effect of Fe substitution on structure and exchange interactions within and between the sublattices of frustrated CoCr2O4

Obtaining tunable magnetic states in geometrically frustrated multiferroic compound CoCr₂O₄ by tuning the sublattice magnetic coupling is indeed of high interest from the fundamental and applied points of view.

Emergent Magnetic State in (111)-Oriented Quasi-Two-Dimensional Spinel Oxides

We report on the emergent magnetic state of (111)-oriented CoCr₂O₄ ultrathin films sandwiched between Al₂O₃ spacer layers in a quantum confined geometry. At the two-dimensional crossover, polarized neutron reflectometry reveals an anomalous enhancement of the total magnetization compared to the bulk value. Synchrotron X-ray magnetic circular dichroism measurements demonstrate the appearance of a long-range ferromagnetic ordering of spins on both Co and Cr sublattices. Brillouin function analyses and ab-initio density functional theory calculations further corroborate that the observed phenomena are due to the strongly altered magnetic frustration invoked by quantum confinement effects, manifested by the onset of a Yafet-Kittel-type ordering as the magnetic ground state in the ultrathin limit, which is unattainable in the bulk.

Destruction of multiferroicity in Tb2BaNiO5 by Sr-doping and its implication to magnetodielectric coupling

The Haldane spin-chain compound, Tb₂BaNiO₅, with two antiferromagnetic transitions, one at T ₁  =  63 K and the other at T ₂  =  25 K, has been recently shown to be an exotic multiferroic below T ₂. Here, we report the results of our investigation of Sr doping at the Ba site by magnetization, heat-capacity, magnetodielectric (MDE) and pyrocurrent measurements. An intriguing finding, which we stress, is that the ferroelectricity is lost even for a doping level of ten atomic percent, though magnetic ordering prevails. The doped specimens however retain significant MDE behaviour, but with reduced magnitudes and qualitative changes with respect to the behaviour of the parent compound. This implies that ferroelectric order is also crucial for the anomalously large MDE in the parent compound, in addition to the role of 4f single-ion anisotropy.

Spiral spin structures and skyrmions in multiferroics

In this article, we focus on (1) type-II multiferroics driven by spiral spin orderings and (2) magnetoelectric couplings in multiferroic skyrmion-hosting materials. We present both phenomenological understanding and microscopic mechanisms for spiral spin state, which is one of the essential starting points for type-II multiferroics and magnetic skyrmions. Two distinct mechanisms of spiral spin states (frustration and Dzyaloshinskii–Moriya [DM] interaction) are discussed in the context of the lattice symmetry. We also discuss the spin-induced ferroelectricity on the basis of the symmetry and microscopic atomic configurations. We compare two well-known microscopic models: the generalized inverse DM mechanism and the metal-ligand d-p hybridization mechanism. As a test for these models, we summarize the multiferroic properties of a family of triangular-lattice antiferromagnets. We also give a brief review of the magnetic skyrmions. Three types of known skyrmion-hosting materials with multiferroicity are discussed from the view point of crystal structure, magnetism, and origins of the magnetoelectric couplings. For exploration of new skyrmion-hosting materials, we also discuss the theoretical models for stabilizing skyrmions by magnetic frustration in centrosymmetric system. Several basic ideas for material design are given, which are successfully demonstrated by the recent experimental evidences for the skyrmion formation in centrosymmetric frustrated magnets.

Spin–orbit coupling in magnetoelectric Ba3(Zn1−xCox)2Fe24O41 hexaferrites

Z-type hexaferrites Ba₃(Zn_1−xCo_x)₂Fe₂₄O₄₁ (x = 0.2, 0.4, 0.6, 0.8, defined as Z1–Z4) were synthesized by a sol–gel method.

Lock-in spin structures and ferrimagnetism in polar Ni2-xCoxScSbO6 oxides

The new phase Co₂ScSbO₆ and Ni_2-xCo_xScSbO₆ solid solutions adopt the polar Ni₃TeO₆-type structure and order magnetically below 60 K. A series of long-period lock-in [0 0 1/3n] spin structures with n = 5, 6, 8 and 10 is discovered, coexisting with a ferrimagnetic [0 0 0] phase at high Co-contents. The presence of electrical polarisation and spontaneous magnetisations offers possibilities for multiferroic properties.

Synthesis, Structure, and Physical Properties of the Polar Magnet DyCrWO6

It has recently been reported that the ordered aeschynite-type polar ( Pna2₁) magnets RFeWO₆ (R = Eu, Tb, Dy, Y) exhibit type II multiferroic properties below T_N ∼ 15-18 K. Herein, we report a comprehensive investigation of the isostructural oxide DyCrWO₆ and compare the results with those of DyFeWO₆. The cation-ordered oxide DyCrWO₆ crystallizes in the same polar orthorhombic structure and undergoes antiferromagnetic ordering at T_N = 25 K. Contrary to DyFeWO₆, only a very weak dielectric anomaly and magnetodielectric effects are observed at the Néel temperature and, more importantly, there is no induced polarization at T_N. Furthermore, analysis of the low-temperature neutron diffraction data reveals a collinear arrangement of Cr spins but a noncollinear Dy-spin configuration due to single-ion anisotropy. We suggest that the collinear arrangement of Cr spins may be responsible for the absence of electric polarization in DyCrWO₆. A temperature-induced magnetization reversal and magnetocaloric effects are observed at low temperatures.

Magnetoelectric inversion of domain patterns

The inversion of inhomogeneous physical states has great technological importance; for example, active noise reduction relies on the emission of an inverted sound wave that interferes destructively with the noise of the emitter¹, and inverting the evolution of a spin system by using a magnetic-field pulse enables magnetic resonance tomography². In contrast to these examples, inversion of a distribution of ferromagnetic or ferroelectric domains within a material is surprisingly difficult: field poling creates a single-domain state, and piece-by-piece inversion using a scanning tip is impractical. Here we report inversion of entire ferromagnetic and ferroelectric domain patterns in the magnetoelectric material Co₃TeO₆ and the multiferroic material Mn₂GeO₄, respectively. In these materials, an applied magnetic field reverses the magnetization or polarization, respectively, of each domain, but leaves the domain pattern intact. Landau theory indicates that this type of magnetoelectric inversion is universal across materials that exhibit complex ordering, with one order parameter holding the memory of the domain structure and another setting its overall sign. Domain-pattern inversion is only one example of a previously unnoticed effect in systems such as multiferroics, in which several order parameters are available for combination. Exploring these effects could therefore advance multiferroics towards new levels of functionality.

A theoretical insight into an isentropic strategy for enhancing magnetoelectric coupling of organic multiferroics

The cross-coupling between magnetic and ferroelectric orders in spin-driven organic multiferroics provides great potential for realizing multi-state logic memory. Creating strong magnetoelectric coupling around room-temperature is the key to eliminate the main roadblock for practical application. Herein, quantum correlation controlled means are employed to tune the transition temperature TC = 300 K, as the optimal operating temperature. After that, based on the magnetocaloric or electrocaloric effect, a temperature mediated mechanism is proposed to enhance magnetoelectric coupling within an isentropic rather than an isothermal process. Furthermore, a moderate magnetic field combined with a relatively weak electric field can jointly control and dramatically enhance the isentropic magnetoelectric coupling around room-temperature.

Theoretical spin-wave dispersions in the antiferromagnetic phase AF1 of MnWO4 based on the polar atomistic model in P2

The spin wave dispersions of the low temperature antiferromagnetic phase (AF1) MnWO₄ have been numerically calculated based on the recently reported non-collinear spin configuration with two different canting angles. A Heisenberg model with competing magnetic exchange couplings and single-ion anisotropy terms could properly describe the spin wave excitations, including the newly observed low-lying energy excitation mode [Formula: see text] meV appearing at the magnetic zone centre. The spin wave dispersion and intensities are highly sensitive to two differently aligned spin-canting sublattices in the AF1 model. Thus this study reinsures the otherwise hardly provable hidden polar character in MnWO₄.

Switching of the Chiral Magnetic Domains in the Hybrid Molecular/Inorganic Multiferroic (ND4)2[FeCl5(D2O)]

(ND₄)₂[FeCl₅(D₂O)] represents a promising example of the hybrid molecular/inorganic approach to create materials with strong magneto-electric coupling. Neutron spherical polarimetry, which is directly sensitive to the absolute magnetic configuration and domain population, has been used in this work to unambiguously prove the multiferroicity of this material. We demonstrate that the application of an electric field upon cooling results in the stabilization of a single-cycloidal magnetic domain below 6.9 K, while poling in the opposite electric field direction produces the full population of the domain with opposite magnetic chirality. We prove the complete switchability of the magnetic domains at low temperature by the applied electric field, which constitutes a direct proof of the strong magnetoelectric coupling. Additionally, we refine the magnetic structure of the ordered ground state, deducing the underlying magnetic space group consistent with the direction of the ferroelectric polarization, and we provide evidence of a collinear amplitude-modulated state with magnetic moments along the a-axis in the temperature region between 6.9 and 7.2 K.

Coexistence of long-range cycloidal order and spin-cluster glass state in the multiferroic BaYFeO4

We report the presence of spin glass state below the cycloidal spin ordering in the multiferroic BaYFeO₄. This compound is known to crystallize in an orthorhombic structure with a centrosymmetric space group Pnma and exhibits two successive antiferromagnetic phase transitions. Upon cooling, it undergoes a spin density wave (SDW)-like antiferromagnetic ordering at T _N1 ~ 48 K and a cycloidal ordering at T _N2 ~ 35 K. Using dc magnetic memory effect and magnetization relaxation studies, we have shown that this oxide undergoes a reentrant spin glass transition below T ^* ~ 17 K. Our analysis suggests the presence of spin clusters in the glassy state. The coexistence of spin-cluster glass and long-range cycloidal ordered states results in an exchange bias effect at 2 K. The origin of the glassy state has been attributed to freezing of some Fe³⁺ moments, which do not participate in the long-range ordering.

Experimental investigation and micromagnetic simulations of hybrid CoCr2O4/Ni coaxial nanostructures

Multiphase CoCr₂O₄/Ni core-shell nanowires (NWs) have been synthesized within anodic aluminum oxide membranes by the combination of the sol-gel method with electrodeposition techniques. X-ray diffraction and x-ray photoemission spectroscopy results confirmed the formation of a cubic spinel structure of CoCr₂O₄ shell with space group Fd-3m (227). The morphology and composition of the as-grown NWs were studied by field emission scanning electron microscopy, as well as transmission electron microscopy. The magnetic properties of the CoCr₂O₄ NT shell and hybrid CoCr₂O₄/Ni NWs were measured at low temperature using a physical property measurement system. The temperature dependence of the magnetization curves showed that CoCr₂O₄ NTs undergo a transition from a paramagnetic state to a ferrimagnetic state at about 90 K and a spiral ordering transition temperature near 22 K. An enhanced coercivity and saturation field were observed for the CoCr₂O₄/Ni core-shell NWs compared to the single-phase Ni NWs. Micromagnetic simulation results indicated that there is a strong coupling between the shell and core layers during the magnetization reversal process. The combination of hard CoCr₂O₄ and soft Ni in a single NW structure may have potential applications in future multifunctional devices.

Coexisting exchange bias effect and ferroelectricity in geometrically frustrated ZnCr2O4

Concomitant occurrence of exchange bias effect and ferroelectric order is revealed in antiferromagnetic spinel ZnCr₂O₄. The exchange bias effect is observed below antiferromagnetic Neél temperature (T _N) with a reasonable value of exchange bias field ([Formula: see text] Oe at 2 K). Intriguingly, the [Formula: see text] ratio is found unusually high as  ∼2.2, where H _C is the coercivity. This indicates that large H _C is not always primary for obtaining large exchange bias effect. Ferroelectric order is observed at T _N, where non-centrosymmetric magnetic structure with [Formula: see text] space group associated with the magnetoelectric coupling correlates the ferroelectric order, proposing that, ZnCr₂O₄ is an improper multiferroic material. Rare occurrence of exchange bias effect and ferroelectric order in ZnCr₂O₄ attracts the community for fundamental interest and draws special attention in designing new materials for possible electric field control of exchange bias effect.

LiNbO3 surfaces from a microscopic perspective

A large number of oxides has been investigated in the last twenty years as possible new materials for various applications ranging from opto-electronics to heterogeneous catalysis. In this context, ferroelectric oxides are particularly promising. The electric polarization plays a crucial role at many oxide surfaces, and it largely determines their physical and chemical properties. Ferroelectrics offer in addition the possibility to control/switch the electric polarization and hence the surface chemistry, allowing for the realization of domain-engineered nanoscale devices such as molecular detectors or highly efficient catalysts. Lithium niobate (LiNbO₃) is a ferroelectric with a high spontaneous polarization, whose surfaces have a huge and largely unexplored potential. Owing to recent advances in experimental techniques and sample preparation, peculiar and exclusive properties of LiNbO₃ surfaces could be demonstrated. For example, water films freeze at different temperatures on differently polarized surfaces, and the chemical etching properties of surfaces with opposite polarization are strongly different. More important, the ferroelectric domain orientation affects temperature dependent surface stabilization mechanisms and molecular adsorption phenomena. Various ab initio theoretical investigations have been performed in order to understand the outcome of these experiments and the origin of the exotic behavior of the lithium niobate surfaces. Thanks to these studies, many aspects of their surface physics and chemistry could be clarified. Yet other puzzling features are still not understood. This review gives a résumé on the present knowledge of lithium niobate surfaces, with a particular view on their microscopic properties, explored in recent years by means of ab initio calculations. Relevant aspects and properties of the surfaces that need further investigation are briefly discussed. The review is concluded with an outlook of challenges and potential payoff for LiNbO₃ based applications.

Position Assignment and Oxidation State Recognition of Fe and Co Centers in Heterometallic Mixed-Valent Molecular Precursors for the Low-Temperature Preparation of Target Spinel Oxide Materials

A series of mixed-valent, heterometallic (mixed-transition metal) diketonates that can be utilized as prospective volatile single-source precursors for the low-temperature preparation of M_xM'_3-xO₄ spinel oxide materials is reported. Three iron-cobalt complexes with Fe/Co ratios of 1:1, 1:2, and 2:1 were synthesized by several methods using both solid-state and solution reactions. On the basis of nearly quantitative reaction yields, elemental analyses, and comparison of metal-oxygen bonds with those in homometallic analogues, heterometallic compounds were formulated as [Fe^III(acac)₃][Co^II(hfac)₂] (1), [Co^II(hfac)₂][Fe^III(acac)₃][Co^II(hfac)₂] (2), and [Fe^II(hfac)₂][Fe^III(acac)₃][Co^II(hfac)₂] (3). In the above heteroleptic complexes, the Lewis acidic, coordinatively unsaturated Co^II/Fe^II centers chelated by two hexafluoroacetylacetonate (hfac) ligands maintain bridging interactions with oxygen atoms of acetylacetonate (acac) groups that chelate the neighboring Fe^III metal ion. Preliminary assignment of Fe and Co positions/oxidation states in 1-3 drawn from X-ray structural investigation was corroborated by a number of complementary techniques. Single-crystal resonant synchrotron diffraction and neutron diffraction experiments unambiguously confirmed the location of Fe and Co sites in the molecules of dinuclear (1) and trinuclear (2) complexes, respectively. Direct analysis in real time mass spectrometry revealed the presence of Fe^III- and Co^II-based fragments in the gas phase upon evaporation of precursors 1 and 2 as well as of Fe^III, Fe^II, and Co^II species for complex 3. Theoretical investigation of two possible "valent isomers", [Fe^III(acac)₃][Co^II(hfac)₂] (1) and [Co^III(acac)₃][Fe^II(hfac)₂] (1'), provided an additional support for the metal site/oxidation state assignment giving a preference of 6.48 kcal/mol for the experimentally observed molecule 1. Magnetic susceptibility measurements data are in agreement with the presence of high-spin Fe^III and Co^II magnetic centers with weak anti-ferromagnetic coupling between those in molecules of 1 and 2. Highly volatile heterometallic complexes 1-3 were found to act as effective single-source precursors for the low-temperature preparation of iron-cobalt spinel oxides Fe_xCo_3-xO₄ known as important materials for diverse energy-related applications.

Coupled multiferroic domain switching in the canted conical spin spiral system Mn2GeO4

Despite remarkable progress in developing multifunctional materials, spin-driven ferroelectrics featuring both spontaneous magnetization and electric polarization are still rare. Among such ferromagnetic ferroelectrics are conical spin spiral magnets with a simultaneous reversal of magnetization and electric polarization that is still little understood. Such materials can feature various multiferroic domains that complicates their study. Here we study the multiferroic domains in ferromagnetic ferroelectric Mn₂GeO₄ using neutron diffraction, and show that it features a double-Q conical magnetic structure that, apart from trivial 180^o commensurate magnetic domains, can be described by ferromagnetic and ferroelectric domains only. We show unconventional magnetoelectric couplings such as the magnetic-field-driven reversal of ferroelectric polarization with no change of spin-helicity, and present a phenomenological theory that successfully explains the magnetoelectric coupling. Our measurements establish Mn₂GeO₄ as a conceptually simple multiferroic in which the magnetic-field-driven flop of conical spin spirals leads to the simultaneous reversal of magnetization and electric polarization.

First-principles investigations into the thermodynamics of cation disorder and its impact on electronic structure and magnetic properties of spinel Co(Cr1-x Mn x )2O4

Cation disorder over different crystallographic sites in spinel oxides is known to affect their properties. Recent experiments on Mn doped multiferroic [Formula: see text] indicate that a possible distribution of Mn atoms among tetrahedrally and octahedrally coordinated sites in the spinel lattice give rise to different variations in the structural parameters and saturation magnetisations in different concentration regimes of Mn atoms substituting the Cr. A composition dependent magnetic compensation behaviour points to the role conversions of the magnetic constituents. In this work, we have investigated the thermodynamics of cation disorder in [Formula: see text] system and its consequences on the structural, electronic and magnetic properties, using results from first-principles electronic structure calculations. We have computed the variations in the cation-disorder as a function of Mn concentration and the temperature and found that at the annealing temperature of the experiment many of the systems exhibit cation disorder. Our results support the interpretations of the experimental results regarding the qualitative variations in the sub-lattice occupancies and the associated magnetisation behaviour, with composition. We have analysed the variations in structural, magnetic and electronic properties of this system with variations in the compositions and the degree of cation disorder from the variations in their electronic structures and by using the ideas from crystal field theory. Our study provides a complete microscopic picture of the effects that are responsible for composition dependent behavioural differences of the properties of this system. This work lays down a general framework, based upon results from first-principles calculations, to understand and analyse the substitutional magnetic spinel oxides [Formula: see text] in presence of cation disorder.

Study of the sign change of exchange bias across the spin reorientation transition in Co(Cr1-x Fe x )2O4 (x = 0.00-0.125)

We present the evolution of novel phenomena of magnetic compensation effect, exchange bias (EB) effect and the field induced anomalies in '[Formula: see text]' substituted multiferroic compound [Formula: see text]. A few percent of '[Formula: see text]' substitution for '[Formula: see text]' in [Formula: see text] results in the reversal of field cooled magnetization under low applied fields below compensation temperature T _comp. Further, increase in the field leads to the spin reorientation transition (T _SR). Signature of EB in a narrow temperature window in the vicinity of T _SR and its sign change across T _SR is observed. Magnitude of EB depends on the amount of compensation and rigidity of the spin reorientation. We also notice the appearance of positive EB below the lock-in transition (T _L). Presence of unidirectional anisotropy developed in the commensurate spin-spiral below T _L could be responsible for the appearance of EB below T _L.

Multiferroic Nanopatterned Hybrid Material with Room-Temperature Magnetic Switching of the Electric Polarization

A nanopatterned hybrid layer is designed, wherein the electric polarization can be flipped at room temperature by a magnetic field aided by an electrical field. This is achieved by embedding ferromagnetic nanopillars in a continuous organic ferroelectric layer, and amplifying the magnetostriction-generated stress gradients by scaling down the supracrystalline cell of the material.

Cation distribution dependent magnetic properties in CoCr2−xFexO4 (x = 0.1 to 0.5): EXAFS, Mössbauer and magnetic measurements

Evolution of structure and rich magnetic transitions such as paramagnetic to ferrimagnetic phase transition at Curie temperature (T_C), spiral ordering temperature (T_S) and lock-in temperature (T_L) have been discussed in CoCr₂O₄ spinel multiferroic after substituting Fe.

Magnetoelectric Response in Multiferroic SrFe12O19 Ceramics

We report here realization of ferroelectricity, ferromagnetism and magnetocapacitance effect in singleSrFe12O19ceramic at room temperature. The ceramics demonstrate a saturated polarization hysteresis loop, two nonlinear I-V peaks and large anomaly of dielectric constant near Curie temperature, which confirm the intrinsic ferroelectricity of SrFe12O19 ceramicswith subsequent heat-treatment in O2atmosphere. The remnant polarization of the SrFe12O19 ceramic is estimated to be 103μC/cm2. The ceramic also exhibits strong ferromagnetic characterization, the coercive field and remnant magnetic moment are 6192Oe and 35.8emu/g, respectively. Subsequent annealing SrFe12O19 ceramics in O2 plays a key role on revealing its intrinsic ferroelectricity and improving the ferromagnetism through transforming Fe2+ into Fe3+. By applying a magnetic field, the capacitance demonstrates remarkable change along with B field, the maximum rate of change in ε (Δε(B)/ε(0)) is 1174%, which reflects a giant magnetocapacitance effect in SrFe12O19. XPS and molecular magnetic moment measurements confirmed the transformation of Fe2+ into Fe3+ and removal of oxygen vacancies upon O2 heat treatment. These combined functional responses in SrFe12O19 ceramics opens substantial possibilities for applications in novel electric devices.

Systematic analysis of structural and magnetic properties of spinel CoB2O4 (B = Cr, Mn and Fe) compounds from their electronic structures

The structural and magnetic properties of spinel compounds CoB2O4 (B  =  Cr, Mn and Fe) are studied using the DFT+U method and generalized gradient approximation. We concentrate on understanding the trends in the properties of these materials as the B cation changes, in terms of relative strengths of crystal fields and exchange fields through an analysis of their electronic densities of states. We find that the electron-electron correlation plays a significant role in obtaining the correct structural and electronic ground states. Significant structural distortion in CoMn2O4 and 'inverted' sublattice occupancy in CoFe2O4 affects the magnetic exchange interactions substantially. The trends in the magnetic exchange interactions are analysed in terms of the structural parameters and the features in their electronic structures. We find that the Fe states in CoFe2O4 are extremely localised, irrespective of the symmetry of the site, which makes it very different from the features of the states of the B cations in two other compounds. These results provide useful insights into the trends in the properties of CoB2O4 compounds with variation of B cation, which would help in understanding the results of recent experiments on doping of Mn and Fe in multiferroic CoCr2O4.

Superadiabatic quantum heat engine with a multiferroic working medium

A quantum thermodynamic cycle with a chiral multiferroic working substance such as LiCu_{2}O_{2} is presented. Shortcuts to adiabaticity are employed to achieve an efficient, finite-time quantum thermodynamic cycle, which is found to depend on the spin ordering. The emergent electric polarization associated with the chiral spin order, i.e., the magnetoelectric coupling, renders possible steering of the spin order by an external electric field and hence renders possible an electric-field control of the cycle. Due to the intrinsic coupling between the spin and the electric polarization, the cycle performs an electromagnetic work. We determine this work's mean-square fluctuations, the irreversible work, and the output power of the cycle. We observe that the work mean-square fluctuations are increased with the duration of the adiabatic strokes, while the irreversible work and the output power of the cycle show a nonmonotonic behavior. In particular, the irreversible work vanishes at the end of the quantum adiabatic strokes. This fact confirms that the cycle is reversible. Our theoretical findings evidence the existence of a system inherent maximal output power. By implementing a Lindblad master equation we quantify the role of thermal relaxations on the cycle efficiency. We also discuss the role of entanglement encoded in the noncollinear spin order as a resource to affect the quantum thermodynamic cycle.