Correlation between dielectric and magnetic properties was investigated on the triangular lattice antiferromagnets ACrO2 (A= Cu, Ag, Li, or Na) showing 120-degree spiral spin structure with easy-axis anisotropy. For the A= Cu and Ag compounds with delafossite structure, ferroelectric polarization emerges upon the spiral spin order, implying the strong coupling between the ferroelectricity and spiral spin structure. On the other hand, for the A = Li and Na compounds with ordered rock salt structure, no polarization but only clear anomalies in dielectric constant can be observed upon the spiral spin order. The absence of polarization can be interpreted as the antiferroelectric state induced by the alternate stacking of Cr3+ layer with opposite spin vector chirality. These results imply that a vast range of trigonally stacked triangular-lattice systems with 120-degree spin structure can be multiferroic, irrespective of their magnetic anisotropy.
Dielectric responses have been investigated on the triangular-lattice antiferromagnet CuFeO2 and its site-diluted analogs CuFe1-xAlxO2 (x=0.01 and 0.02) with and without application of magnetic field. We have found a ferroelectric behavior at zero magnetic field for x=0.02. At any doping level, the onset field of the ferroelectricity always coincides with that of the noncollinear magnetic structure while the transition field dramatically decreases to zero field with Al doping. The results imply the further possibility of producing the ferroelectricity by modifying the frustrated spin structure…
Ferroelectric properties of cubic chiral magnet Cu2OSeO3 can emerge due to the spin noncollinearity induced by antiferromagnetic cantings. These cantings are the result of the Dzyaloshinskii-Moriya interaction and in many ways similar to the ferromagnetic cantings in weak ferromagnets. An expression for the local electric polarization is derived, including terms with gradients of magnetization M(r). When averaged over the crystal the electric polarization has a non-vanishing part associated with the anisotropy of the crystal point group 23. In the framework of the microscopic theory, it is sho…
We have investigated the dielectric response in an antiferromagnetic dimer-Mott insulator beta'-(BEDT-TTF)2ICl2 with square lattice, compared to a spin liquid candidate kappa-(BEDT-TTF)2Cu2(CN)3. Temperature dependence of the dielectric constant shows a peak structure obeying Curie-Weiss law with strong frequency dependence. We found an anisotropic ferroelectricity by pyrocurrent measurements, which suggests the charge disproportionation in a dimer. The ferroelectric actual charge freezing temperature is related to the antiferromagnetic interaction, which is expected to the charge-spin coupled…
Cu3Nb2O8 is an unusual multiferroic compound that undergoes a series of magnetic ordering at low temperatures. Concurrent development of electric polarization has been reported at TN1 ~25 K corresponding to a non-collinear helicoidal ordering (Phys. Rev. Lett. 107, 137205 (2011)). But questions remain on the microscopic origin of such phenomena. In this communication, we report a detailed study of induced polar ordering in Cu3Nb2O8 by performing polarization, magnetization, dielectric constant and heat capacity measurements over a broad range of temperature and magnetic field. The dielectric c…
Multiferroics, showing simultaneous ordering of electrical and magnetic degrees of freedom, are remarkable materials as seen from both the academic and technological points of view. A prominent mechanism of multiferroicity is the spin-driven ferroelectricity, often found in frustrated antiferromagnets with helical spin order. There, similar to conventional ferroelectrics, the electrical dipoles arise from an off-centre displacement of ions. However, recently a different mechanism, namely purely electronic ferroelectricity, where charge order breaks inversion symmetry, has attracted considerabl…
The spiral magnetic structure and underlying spin lattice of BiMnFe2O6 are investigated by low-temperature neutron powder diffraction and density functional theory band structure calculations. In spite of the random distribution of the Mn3+ and Fe3+ cations, this compound undergoes a transition into an incommensurate antiferromagnetically ordered state below TN ~ 220 K. The magnetic structure is characterized by the propagation vector k=[0,beta,0] with beta ~ 0.14 and the P22_12_11'(0 β0)0s0s magnetic superspace symmetry. It comprises antiferromagnetic helixes propagating along the b-axis. The…
Aiming at understanding the origin of the electronic contribution to ferroelectric polarization in undoped manganites, we evaluate the Berry phase of orbital-polarizable Bloch electrons as an orbital ordering (OO) establishes in the background of an antiferromagnetic E-type configuration. The onset of OO is tuned by the Jahn-Teller (JT) interaction in a tight-binding model for interacting electrons moving along zigzag chains. A finite polarization is found as soon as the JT coupling is strong enough to induce OO, supporting the large electronic contribution predicted from first principles.
We have investigated the variation of induced ferroelectric polarization under magnetic field with various directions and magnitudes in a staggered antiferromagnet Ba$_2$CoGe$_2$O$_7$. While the ferroelectric polarization cannot be explained by the well-accepted spin current model nor exchange striction mechanism, we have shown that it is induced by the spin-dependent $p$-$d$ hybridization between the transition-metal (Co) and ligand (O) via the spin-orbit interaction. On the basis of the correspondence between the direction of electric polarization and the magnetic state, we have also demonst…
The hexagonal RMnO$_3$ (h-RMnO$_3$) are multiferroic materials, which exhibit the coexistence of a magnetic order and ferroelectricity. Their distinction is in their geometry that both results in an unusual mechanism to break inversion symmetry, and also produces a 2D triangular lattice of Mn spins, which is subject to geometrical magnetic frustration due to the antiferromagnetic interactions between nearest neighbour Mn ions. This unique combination makes the h-RMnO$_3$ a model system to test ideas of spin-lattice coupling, particularly when the both the improper ferroelectricity and the Mn t…
We report experimental and theoretical evidence that Rb$_2$Cu$_2$Mo$_3$O$_{12}$ has a nonmagnetic tetramer ground state of a two-leg ladder comprising antiferromagnetically coupled frustrated spin-$1/2$ chains and exhibits a Haldane spin gap of emergent spin-1 pairs. Three spin excitations split from the spin-1 triplet by a Dzyaloshinskii-Moriya interaction are identified in inelastic neutron-scattering and electron spin resonance spectra. A tiny magnetic field generates ferroelectricity without closing the spin gap, indicating a novel class of ferroelectricity induced by a vector spin chirali…
The ferroelectric polarization of triangular-lattice antiferromagnets induced by helical spin-spiral order is not explained by any existing model of magnetic-order-driven ferroelectricity. We resolve this problem by developing a general theory for the ferroelectric polarization induced by spin-spiral order and then by evaluating the coefficients needed to specify the general theory on the basis of density functional calculations. Our theory correctly describes the ferroelectricity of triangular-lattice antiferromagnets driven by helical spin-spiral order, and incorporates known models of magne…
Chirality or the handedness of objects is of prime importance in life science, biology, chemistry and physics. It is also a major symmetry ingredient in frustrated magnets revealing spin-spiral ground states. Vector chiral phases, with the twist (either clock- or counter clock-wise) between neighbouring spins being ordered, but with disorder with respect to the angles between adjacent spins, have been predicted almost five decades ago. Experimental proofs, however, are rare and controversial. Here, we provide experimental evidence for such a phase in LiCuVO$_4$, a one-dimensional quantum magne…
Using {\it ab initio} density functional theory and single-orbital Hubbard model calculations via the density matrix renormalization group method, we systematically studied the monolayer VOI$_2$ with a $3d^1$ electronic configuration. Our phonon calculations indicate that the orthorhombic $Pmm2$ FE-II phase is the most likely ground state, involving a ferroelectric distortion along the $a$-axis and V-V dimerization along the $b$-axis. Specifically, the "pseudo Jahn-Teller" effect caused by the coupling between empty V ($d_{xz/yz}$ and $d_{3z^2-r^2}$) and O $2p$ states is proposed as the mechan…
We have investigated orthorhombic $R$MnO$_{3}$ ($R$=(Gd$_{1-y}$Tb$_{y}$)) crystals near the phase boundary between the paraelectric $A$-type-antiferromagnetic (AF) phase (PA) and the ferroelectric transverse-spiral-AF one (FS). The spiral AF structure breaks inversion symmetry and induces the ferroelectric polarization through the inverse Dzyaloshinskii-Moriya (DM) interaction. We have found that the PA-FS phase boundary is located at around 0.15$<$$y$$<$0.2. In $y$=0.15 compound, PA and FS phases appear in cooling scan, while FS is not observed in warming scan. This result suggests that FS ob…
The tetragonal copper oxide Bi$_2$CuO$_4$ has an unusual crystal structure with a three-dimensional network of well separated CuO$_4$ plaquettes. This material was recently predicted to host electronic excitations with an unconventional spectrum and the spin structure of its magnetically ordered state appearing at T$_N$ $\sim$43 K remains controversial. Here we present the results of detailed studies of specific heat, magnetic and dielectric properties of Bi$_2$CuO$_4$ single crystals grown by the floating zone technique, combined with the polarized neutron scattering and high-resolution X-ray…
SmFeO3 has attracted considerable attention very recently due to the reported multiferroic properties above room-temperature. We have performed powder and single crystal neutron diffraction as well as complementary polarization dependent soft X-ray absorption spectroscopy measurements on floating-zone grown SmFeO3 single crystals in order to determine its magnetic structure. We found a k=0 G-type collinear antiferromagnetic structure that is not compatible with inverse Dzyaloshinskii-Moriya interaction driven ferroelectricity. While the structural data reveals a clear sign for magneto-elastic …
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 st…
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…
Competing interactions and geometric frustration provide favourable conditions for exotic states of matter. Such competition often causes multiple phase transitions as a function of temperature and can lead to magnetic structures that break inversion symmetry, thereby inducing ferroelectricity [1-4]. Although this phenomenon is understood phenomenologically [3-4], it is of great interest to have a conceptually simpler system in which ferroelectricity appears coincident with a single magnetic phase transition. Here we report the first such direct transition from a paramagnetic and paraelectric …
By studying the dielectric properties of the geometrically frustrated spinel CdV2O4, we observe ferroelectricity developing at the transition into the collinear antiferromagnetic ground state. In this multiferroic spinel, ferroelectricity is driven by local magnetostriction and not by the more common scenario of spiral magnetism. The experimental findings are corroborated by ab-initio calculations of the electric polarization and the underlying spin and orbital order. The results point towards a charge rearrangement due to dimerization, where electronic correlations and the proximity to the in…
In order to clarify the mechanism associated with pressure/magnetic-field-induced giant ferroelectric polarization in TbMnO3, this work investigated changes in magnetic ordering brought about by variations in temperature, magnetic field, and pressure. This was accomplished by means of neutron diffraction analyses under high pressures and high magnetic fields, employing a single crystal. The incommensurate magnetic ordering of a cycloid structure was found to be stable below the reported critical pressure of 4.5 GPa. In contrast, a commensurate E-type spin ordering of Mn spins and a noncollinea…
We have investigated the magnetic and dielectric properties of orthorhombic Eu$_{1-y}$Y$_y$MnO$_3$ ($0\leq y\leq 0.6$) single crystals without the presence of the 4$f$ magnetic moments of the rare-earth ions. In $y\geq 0.2$, the magnetic-structure driven ferroelectricity is observed. The ferroelectric transition temperature is steeply reducing with increasing $y$. In $y\geq 0.52$, two ferroelectric phases ($P\parallel a$ and $P\parallel c$) are coexistent at low temperatures. In these phases, ferroelectricity has different origin, which is evidenced by the distinctive poling-electric-field dep…
Vector-chiral (VC) antiferromagnetism is a spiral-like ordering of spins which may allow ferroelectricity to occur due to loss of space inversion symmetry. In this paper we report direct experimental observation of ferroelectricity in the VC phase of $β$-TeVO$_4$, a frustrated spin chain system with pronounced magnetic anisotropy and a rich phase diagram. Saturation polarization is proportional to neutron scattering intensities that correspond to the VC magnetic reflection. This implies that inverse Dzyaloshinskii-Moriya mechanism is responsible for driving electric polarization. Linear magnet…
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…
Magnetic and dielectric properties of the hexagonal triangular lattice antiferromagnet 2H-AgFeO2 have been studied by neutron diffraction, magnetic susceptibility, specific heat, pyroelectric current, and dielectric constant measurements. The ferroelectric polarization, P ~ 5 μC/m2, has been found to appear below 11 K due to a polar nature of the magnetic ground state of the system. In the temperature range of 11 K < T < 18 K, an incommensurate spin density wave (ICM1) with the nonpolar magnetic point group mmm1' and the k1 = (0,q1_b,0; q1_b = 0.390-0.405)propagation vector takes place. Below …
We discuss the particle size driven tunability of the coexistence of ferromagnetism and ferroelectricity in Pr0.67Ca0.33MnO3(PCMO) with the help of x ray diffraction, magnetization, impedance spectroscopy, and remanent polarization measurements. The remanent polarization measurements using the positive up negative down method clearly prove the existence of ferroelectricity in PCMO with phase separation between Zener polaron(ZP) ordered P21nm and disordered Pbnm structures. We also find that the ferroelectric response is enhanced in nanocrystalline samples so long as ZP ordering is not destroye…
Strongly correlated materials with multiple order parameters provide unique insights into the fundamental interactions in condensed matter systems and present opportunities for innovative technological applications. A class of antiferromagnetic honeycomb lattices compounds, A4B2O9 (A = Co, Fe, Mn; B = Nb, Ta), have been explored owing to the occurrence of linear magnetoelectricity. We observe a highly nonlinear magnetoelectric effect on single crystals of Co4Ta2O9 (CTO), distinctive from the linear behavior in the isostructural Co4Nb2O9. Ferroelectricity emerges primarily along the [110] direc…
We consider a class of one-dimensional compass models with antisymmetric Dzyaloshinskii-Moriya exchange interaction in an external magnetic field. Based on the exact solution derived by means of Jordan-Wigner transformation, we study the excitation gap, spin correlations, ground-state degeneracy, and critical properties at phase transitions. The phase diagram at finite electric and magnetic field consists of three phases: ferromagnetic, canted antiferromagnetic, and chiral. Dzyaloshinskii-Moriya interaction induces an electrical polarization in the ground state of the chiral phase, where the n…
We report the magnetic and electric properties of Ba$_3$NiNb$_2$O$_9$, which is a quasi-two-dimensional spin-1 triangular lattice antiferromagnet (TLAF) with trigonal structure. At low $T$ and with increasing magnetic field, the system evolves from a 120 degree magnetic ordering phase (A phase) to an up-up-down ($uud$) phase (B phase) with a change of slope at 1/3 of the saturation magnetization, and then to an "oblique" phase (C phase). Accordingly, the ferroelectricity switches on at each phase boundary with appearance of spontaneous polarization. Therefore, Ba$_3$NiNb$_2$O$_9$ is a unique T…
The microscopic origin of the magnetically-driven ferroelectricity in collinear AFM-E orthorhombic manganites is explained by means of first-principles Wannier functions. We show that the polarization is mainly determined by the "asymmetric electron hopping" of orbitally-polarized Mn eg states, implicit in the peculiar in-plane zig-zag spin arrangement in the AFM-E configuration. In ortho-HoMnO3, Wannier function centers are largely displaced with respect to corresponding ionic positions, implying that the final polarization is strongly affected by a purely electronic contribution, at variance…
The orthovanadate DyVO3 crystal, known to exhibit multiple structural, spin and orbital ordering transitions, is presently investigated on the basis of magnetization, heat capacity, resistivity, dielectric and polarization measurements. Our main result is experimental evidence for the existence of multiferroicity below a high TC of 108 K over a wide temperature range including different spin-orbital ordered states. The onset of ferroelectricity is found to coincide with the antiferromagnetic C-type spin ordering transition taking place at 108 K, which indicates that DyVO3 belongs to type II mu…
Multiferroics, defined for those multifunctional materials in which two or more kinds of fundamental ferroicities coexist, have become one of the hottest topics of condensed matter physics and materials science in recent years. The coexistence of several order parameters in multiferroics brings out novel physical phenomena and offers possibilities for new device functions. The revival of research activities on multiferroics is evidenced by some novel discoveries and concepts, both experimentally and theoretically. In this review article, we outline some of the progressive milestones in this st…
Trirutile-type LiFe$_2$F$_6$ is a charge-ordered material with Fe$^{2+}$/Fe$^{3+}$ configuration. Here its physical properties, including magnetism, electronic structure, phase transition, and charge ordering, are studied theoretically. On one hand, the charge ordering leads to improper ferroelectricity with a large polarization. On the other hand, its magnetic ground state can be tuned from the antiferromagnetic to ferrimagnetic by moderate compressive strain. Thus, LiFe$_2$F$_6$ can be a rare multiferroic with both large magnetization and polarization. Most importantly, since the charge orde…
Here we investigate the temperature evolution of the structural parameters of a potential magnetoelectric material, MnTiO$_{3}$. The experimental results reveal interesting temperature dependence of the $c/a$ ratio and the Mn-O bonds which can be divided into three regions. In region I (300 K to 200 K), the above parameters are seen to decrease with decrease in temperature due to thermal effect. In the region II (200 K to 95 K), the decrement in the structural parameters are reduced due the competing intra layer antiferromagnetic interaction setting in $\sim$ 200 K. The $c/a$ ratio are seen to…
We derive an effective Hamiltonian for the ionic Hubbard model at half filling, extended to include nearest-neighbor repulsion. Using a spin-particle transformation, the effective model is mapped onto simple spin-1 models in two particular cases. Using another spin-particle transformation, a slightly modified model is mapped into an SU(3) antiferromagnetic Heisenberg model whose exact ground state is known to be spontaneously dimerized. From the effective models several properties of the dimerized phase are discussed, like ferroelectricity and fractional charge excitations. Using bosonization …
CuCrO2 offers insights into the different types of spiral magnetic orderings that can form spontaneously due to frustration in triangular-lattice antiferromagnets. We explore the magnetic phase diagram up to 65 T along all the principle axes, and also use electric polarization to probe changes in the spiral order at high magnetic fields. It is known that at zero magnetic field a proper-screw spiral of the Cr S = 3/2 spins forms that in turn induces electric polarization with six possible orientations ab-plane. Applied magnetic fields in the (hard) ab-plane have been shown to induce a transitio…
Magnetic susceptibility, specific heat, dielectric, and electric polarization of LiCuFe2(VO4)3 have been investigated. Two sequential antiferromagnetic transitions at TN1 ~ 9.95 K and TN2 ~ 8.17 K are observed under zero magnetic field. While a dielectric peak at TN1 is clearly identified, the measured pyroelectric current also exhibits a sharp peak at TN1, implying the magnetically relevant ferroelectricity. Interestingly, another pyroelectric peak around TN2 with opposite signal is observed, resulting in the disappearance of electric polarization below TN2. Besides, the electric polarization…
Collective mode dynamics of the helical magnets coupled to electric polarization via spin-orbit interaction is studied theoretically. The soft modes associated with the ferroelectricity are not the transverse optical phonons, as expected from the Lyddane-Sachs-Teller relation, but are the spin waves hybridized with the electric polarization. This leads to the Drude-like dielectric function $ε(ω)$ in the limit of zero magnetic anisotropy. There are two more low-lying modes; phason of the spiral and rotation of helical plane along the polarization axis. The roles of these soft modes in the neutr…
In order to investigate nature of the antiferromagnetic structures in perovskite RMnO3, we study a Heisenberg J1-J2 model with bond alternation using analytical and numerical approaches. The magnetic phase diagram which includes incommensurate spiral states and commensurate collinear states is reproduced. We discuss that the magnetic structure with up-up-down-down spin configuration (E-type structure) and the ferroelectricity emerge cooperatively to stabilize this phase. Magnetoelastic couplings are crucial to understand the magnetic and electric phase diagram of RMnO3.
Magnetoelectric properties were investigated for an S=1/2 chain antiferromagnet CuCl2, which turns out to be the first example of non-chalcogen based spiral-spin induced multiferroics. Upon the onset of helimagnetic order propagating along the b-axis under zero magnetic field (H), we found emergence of ferroelectric polarization along the c-axis. Application of H along the b-axis leads to spin-flop transition coupled with drastic suppression of ferroelectricity, and rotation of H around the b-axis induces the rotation of spin-spiral plane and associated polarization direction. These behaviors …
We have recently reported that the Haldane spin-chain system, Er2BaNiO5, undergoing antiferromagnetic order below 32 K, is characterized by the onset of ferroelectricity near 60K due to magnetoelectric coupling induced by short-range magnetic-order within spin-chains. We have carried out additional magnetic and dielectric studies to understand the properties well below antiferromagnetic ordering temperature. We emphasize here on the following: (i) A strong frequency dependent behaviors of ac magnetic susceptibility and complex dielectric properties have been observed at much lower temperatures…
Lattice modulation and magnetic structures in magnetoelectric compounds Tb1-xDyxMnO3 have been studied around the ferroelectric (FE) Curie temperature T_C by x-ray and neutron diffraction. Temperature-independent modulation vectors through T_C are observed for the compounds with 0.50< x < 0.68. This indicates that ferroelectricity with a polarization (P) along the c axis in the RMnO3 series cannot be ascribed to such an incommensurate-commensurate transition of an antiferromagnetic order as was previously anticipated. Neutron diffraction study of a single crystal with x=0.59 shows that the FE …
There has been tremendous research activity in the field of magneto-electric (ME) multiferroics after Kimura et al. [1] showed that antiferromagnetic and ferroelectric order coexist in orthorhom- bically distorted perovskite TbMnO3 and are strongly coupled. It is now generally accepted that ferroelectricity in TbMnO3 is induced by magnetic long range order that breaks the symmetry of the crystal and creates a polar axis [2]. One remaining key question is whether magnetic order can induce ferroelectric polarization that is as large as that of technologically useful materials. We show that ferro…
We present the muon spin relaxation/rotation spectra in the multiferroic compound (Cu,Zn)$_{3}$Mo$_{2}$O$_{9}$. The parent material Cu$_{3}$Mo$_{2}$O$_{9}$ has a multiferroic phase below $T_{\rm N}$ = 8.0 K, where the canted antiferromagnetism and the ferroelectricity coexist. The asymmetry time spectra taken at RIKEN-RAL pulsed muon facility show a drastic change at $T_{\rm N}$. At low temperatures the weakly beating oscillation caused by the static internal magnetic fields in the antiferromagnetic phase was observed in Cu$_{3}$Mo$_{2}$O$_{9}$ and the lightly ($0.5\%$) Zn-doped sample. From t…
We investigate magnetoelectric coupling and low-energy magnetic excitations in multiferroic $α$-Cu$_2$V$_2$O$_7$ by detailed thermal expansion, magnetostriction, specific heat and magnetization measurements in magnetic fields up to 15~T and by high-field/high-frequency electron spin resonance studies. Our data show negative thermal expansion in the temperature range $\leq 200$~K under study. Well-developed anomalies associated with the onset of multiferroic order (canted antiferromagnetism with a significant magnetic moment and ferroelectricity) imply pronounced coupling to the structure. We d…
Spin supersolids and spin superfluids reveal complex canted spin structures with independent order of longitudinal and transverse spin components. This work addresses the question whether these exotic phases can exhibit spin-driven ferroelectricity. Here we report the results of dielectric and pyrocurrent measurements of MnCr2S4 as function of temperature and magnetic field up to 60 T. This sulfide chromium spinel exhibits a Yafet-Kittel type canted spin structure at low temperatures. As function of external magnetic field, the manganese spins undergo a sequence of ordering patterns of the tra…
The spin-$1/2$ Heisenberg chain with a ferromagnetic first-neighbor exchange coupling $J_1$ and an antiferromagnetic second-neighbor $J_2$ has a Haldane dimer ground state with an extremely small spin gap. Thus, the ground state is readily altered by perturbations. Here, we investigate the effects of XXZ exchange magnetic anisotropy of both the easy-axis and easy-plane types and an alternation in $J_1$ on the ground state, the spin gap, and magnetic properties of the frustrated ferromagnetic spin-$1/2$ chain. It is found that there are two distinct dimerized spin-gap phases, in one of which th…
ABO3 perovskite multiferroic La0.5Bi0.5Mn0.5Fe0.5O3.09 where the B-site cations is responsible for the magnetic properties and the A-site cation with lone pair electron is responsible for the ferroelectric properties was synthesized at normal conditions. This oxide exhibits a ferromagnetic transition around 240 K with a well defined hysteresis loop, and a significant reversible remnant polarization below 67K similar to ferroelectric behavior. The magnetic interaction is interpreted by the ferromagnetic Fe3+-O-Mn3+ and antiferromagnetic Fe3+(Mn3+)-O-Fe3+(Mn3+) interactions competed each other, …
The magnetic properties of CuO encompass several contemporary themes in condensed matter physics, including quantum magnetism, magnetic frustration, magnetically-induced ferroelectricity and orbital currents. Here we report polarized and unpolarized neutron inelastic scattering measurements which provide a comprehensive map of the cooperative spin dynamics in the low temperature antiferromagnetic (AFM) phase of CuO throughout much of the Brillouin zone. At high energies ($E \gtrsim 100$\,meV) the spectrum displays continuum features consistent with the des Cloizeax--Pearson dispersion for an i…
We present the first-principles investigation of the structural, electronic, and magnetic properties of SrCoO$_{3-δ}$ ($δ=0, 0.25, 0.5$) to understand the multivalent nature of Co ions in SrCoO$_{3-δ}$ along the line of topotactic transition between perovskite SrCoO$_{3}$ and brownmillerite SrCoO$_{2.5}$. From the on-site Coulomb interaction $U$-dependent ground state of stoichiometric SrCoO$_{3}$, we show the proximity of its metallic ferromagnetic ground state to other antiferromagnetic states. The structural and magnetic properties of SrCoO$_{3-δ}$ depending on their oxygen-content provide …
We report on structural, magnetic, dielectric, and thermodynamic properties of (Eu:Y)MnO3 for Y doping levels 0 <= x < 1. This system resembles the multiferroic perovskite manganites RMnO3 (with R= Gd, Dy, Tb) but without the interference of magnetic contributions of the 4f-ions. In addition, it offers the possibility to continuously tune the influence of the A-site ionic radii. For small concentrations x <= 0.1 we find a canted antiferromagnetic and paraelectric groundstate. For higher concentrations x <= 0.3 ferroelectric polarization coexists with the features of a long wavelength incommens…
Ab initio calculations of the electric polarization of correlation-driven insulating materials, namely Mott insulators, have not been possible so far. Using a combination of density functional theory and dynamical-mean-field theory we study the electric polarization of the Mott insulator Sr$_{0.5}$Ba$_{0.5}$MnO$_{3}$. We predict a ferroelectric polarization of $\simeq 16.5 μC/cm^2$ in the high temperature paramagnetic phase and recover the measured value of $\simeq 13.3 μC/cm^2$ in the low temperature antiferromagnetic phase. Our calculations reveal that the driving force for the ferroelectric…
The magnetic, dielectric and structural properties of type-I multiferroic KNiPO$_4$ have been investigated by neutron powder diffraction, magnetization, dielectric and high temperature synchrotron-XRD measurements. Below the Néel transition of $T_\mathrm{N}$ = 25 K, KNiPO$_4$ displays a weakly non-collinear antiferromagnetic (AFM) structure with the orientation of the Ni$^{2+}$ magnetic moments mainly along $a$ axis. The compound crystallizes in the polar orthorhombic $Pna2_1$ space group at room temperature. A second-order structural phase transition corresponding to the onset ferroelectricit…
To understand the collinear-magnetism-driven ferroelectricity in multiferroic Ca3CoMnO6 compound, we have established an elastic diatomic Ising spin-chain model with axial-next-nearest-neighbor interaction to describe its magnetoelectric properties. By employing magneto-phonon decoupling and transfer-matrix method, the possible ground-state configurations and thermodynamic behaviors of the system have been exactly determined. From the perspective of the ground-state configuration, we analyze the computational results and make a detail comparison with experimental data. The parameter relation f…
The lacunar spinel GeV4S8 undergoes orbital and ferroelectric ordering at the Jahn-Teller transition around 30 K and exhibits antiferromagnetic order below about 14 K. In addition to this orbitally driven ferroelectricity, lacunar spinels are an interesting material class, as the vanadium ions form V4 clusters representing stable molecular entities with a common electron distribution and a well-defined level scheme of molecular states resulting in a unique spin state per V4 molecule. Here we report detailed x-ray, magnetic susceptibility, electrical resistivity, heat capacity, thermal expansio…
