Magnetic reversal of the ferroelectric polarization in a multiferroic spinel oxide

Magnetic ordering and magnetodielectric phenomena in CoSeO4

CoSeO(4) has a structure consisting of edge-sharing chains of Co(2+) octahedra which are held together by SeO(4)(2-) tetrahedra via shared oxygen atoms at the edges of the octahedra. DC magnetization measurements indicate a transition to an ordered state below 30 K. Powder neutron diffraction refinements suggest an ordered state with two unique antiferromagnetic chains within the unit cell. Isothermal magnetization measurements indicate a temperature-dependent field-induced magnetic transition below the ordering temperature. From neutron diffraction, we find that this corresponds to a realignment of spins from the canted configuration towards the c-axis. The dielectric constant shows a change in slope at the magnetic ordering temperature indicating an interplay between the spin and charge degrees of freedom.

Multiferroic M-type hexaferrites with a room-temperature conical state and magnetically controllable spin helicity

Magnetic and magnetoelectric (ME) properties have been studied for single crystals of Sc-doped M-type barium hexaferrites. Magnetization (M) and neutron diffraction measurements revealed that by tuning Sc concentration a longitudinal conical state is stabilized up to above room temperatures. ME measurements have shown that a transverse magnetic field (H) can induce electric polarization (P) at lower temperatures and that the spin helicity is nonvolatile and endurable up to near the conical magnetic transition temperature. It was also revealed that the response (reversal or retention) of the P vector upon the reversal of M varies with temperature. In turn, this feature allows us to control the relation between the spin helicity and the M vectors with H and temperature.

Simple point-ion electrostatic model explains the cation distribution in spinel oxides

The A2BO4 spinel oxides are distinguished by having either a normal (N) or an inverse (I) distribution of the A, B cations on their sublattices. A point-ion electrostatic model parametrized by the oxygen displacement parameter u and by the relative cation valencies Z{A} vs Z{B} provides a simple rule for the structural preference for N or I: if Z{A}>Z{B} the structure is normal for u>0.2592 and inverse for u<0.2578, while if Z{A}<Z{B} the structure is normal for u<0.2550 and inverse for u>0.2578. This rule is illustrated for the known spinel oxides, proving to be ∼98% successful.

A phenomenological Landau theory for electromagnons in cubic spinel multiferroic CoCr₂O₄

Non-anisotropic free energy is considered which under minimization yields two magnetic phases: a conical spin density wave and a low temperature conical cycloid. Using equations of motion, the excitation spectrum is studied. Knowing the nature of these excitations, the dielectric function as well as the fluctuation specific heat is computed and compared with the experimental spectrum. Due to the electromagnon going soft, the dielectric function (imaginary part) as well as the specific heat capacity show peaks at the temperature where ferroelectricity appears in the system.

Multiferroics with spiral spin orders

Cross correlation between magnetism and electricity in a solid can host magnetoelectric effects, such as magnetic (electric) induction of polarization (magnetization). A key to attain the gigantic magnetoelectric response is to find the efficient magnetism-electricity coupling mechanisms. Among those, recently the emergence of spontaneous (ferroelectric) polarization in the insulating helimagnet or spiral-spin structure was unraveled, as mediated by the spin-exchange and spin-orbit interactions. The sign of the polarization depends on the helicity (spin rotation sense), while the polarization direction itself depends on further details of the mechanism and the underlying lattice symmetry. Here, we describe some prototypical examples of the spiral-spin multiferroics, which enable some unconventional magnetoelectric control such as the magnetic-field-induced change of the polarization direction and magnitude as well as the electric-field-induced change of the spin helicity and magnetic domain.

Polar properties and phase sequence in Eu(0.8)Y(0.2)MnO(3)

In this work, we have studied in detail the temperature dependence of the electric polarization of Eu(0.8)Y(0.2)MnO(3) aimed at clarifying the controversial issues concerning the ferroelectric nature of the lower temperature magnetic phases and hence its multiferroic character. The existence of a spontaneous polarization in 30 K < T < 22 K, provides clear evidence for the ferroelectric character of the re-entrant non-collinear spiral-antiferromagnetic phase, stable in that temperature range. Contrary to results published in previous works, our experimental data clearly show that the weak-ferromagnetic, canted antiferromagnetic phase stable below 20 K is not intrinsically ferroelectric. The misinterpretation, regarding the polar character of the lower temperature magnetic phases, stems from the existence of an induced polarization occurring below 30 K. The mechanisms associated with polar and magnetic properties, and their correlation with both spin and lattice structures are also discussed.

Magnetodielectric coupling in frustrated spin systems: the spinels MCr₂O₄ (M = Mn, Co and Ni)

We have studied the magnetodieletric coupling of polycrystalline samples of the spinels MCr(2)O(4) (M = Mn, Co and Ni). Dielectric anomalies are clearly observed at the onset of the magnetic spiral structure (T(s)) and at the 'lock-in' transition (T(f)) in MnCr(2)O(4) and CoCr(2)O(4), and also at the onset of the canted structure (T(s)) in NiCr(2)O(4). The strength of the magnetodielectric coupling in this system can be explained by spin-orbit coupling. Moreover, the dielectric response in an applied magnetic field scales with the square of the magnetization for all three samples. Thus, the magnetodielectric coupling in this state appears to originate from the P(2)M(2) term in the free energy.

Low temperature incommensurately modulated and noncollinear spin structure in FeCr2S4

FeCr(2)S(4) orders magnetically at T(N)≈ 170 K. According to neutron diffraction, the ordered state down to 4.2 K is a simple collinear ferrimagnet maintaining the cubic spinel structure. Later studies, however, claimed trigonal distortions below ∼ 60 K coupled to the formation of a spin glass type ground state. To obtain further insight, muon spin rotation/relaxation (μSR) spectroscopy was carried out between 5 and 200 K together with new (57)Fe Mössbauer measurements. Below ∼ 50 K, our data point to the formation of an incommensurately modulated noncollinear spin arrangement like a helical spin structure. Above 50 K, the spectra are compatible with collinear ferrimagnetism, albeit with a substantial spin disorder on the scale of a few lattice constants. These spin lattice distortions become very large at 150 K and the magnetic state is now better characterized as consisting of rapidly fluctuating short-range ordered spins. The Néel transition is of second order, but ill defined, extending over a range of ∼ 10 K. The Mössbauer data around 10 K confirm the onset of orbital freezing and are also compatible with the noncollinear order of iron. The absence of a major change in the quadrupole interaction around 50 K renders the distortion of crystal symmetry to be small.

Magnetic digital flop of ferroelectric domain with fixed spin chirality in a triangular lattice helimagnet

The ferroelectric properties in a magnetic field (H) of varying magnitude and direction have been investigated for the triangular-lattice helimagnet CuFe(1-x)Ga(x)O(2) (x = 0.035). The in-plane H was found to induce the rearrangement of six possible multiferroic domains. Upon every 60 degrees rotation of in-plane H around the c axis, a unique 120 degrees flop of electric polarization occurs as a result of the switch of the helical magnetic q vector. The chirality of the spin helix is always conserved upon the q flop. The possible origin is discussed in light of the stable structure of the multiferroic domain wall.

Crossover from incommensurate to commensurate magnetic orderings in CoCr(2)O(4)

The conical spin order of multiferroic CoCr(2)O(4) has been studied by a neutron diffraction to investigate its magnetic phase transitions at temperatures below 40 K. Magnetic order of a spiral spin component with an incommensurate propagation vector of (0.63, 0.63, 0) was observed at 26 K, while at 14.5 K, the incommensurate conical spin order showed a transition into the fixed commensurate propagation vector of (2/3,2/3,0). In addition, two satellite peaks with propagation vectors of (0.035, 0, 0) and (0, 0.035, 0) from the commensurate vector were observed. The widths of these peaks indicate a long-range magnetic order. This new magnetic configuration below 14.5 K may lead to a new model of multiferroic behavior differing from the well-known spin-current model for magnetic ferroelectricity.

Novel multiferroic state of Eu1-xYxMnO3 in high magnetic fields

Magnetic and dielectric properties of Eu1-xYxMnO3 (x=0 and 0.4) are studied in pulsed magnetic fields up to 55 T. For x=0, application of magnetic fields higher than 20 T along the b axis causes magnetic transitions accompanied by generation of electric polarization (P) along the a axis. Similar first-order transitions are also observed in crystals of x=0.4, in which the ground state at zero magnetic field is already a ferroelectric P parallel a phase of different origin. Realistic model calculation indicates the presence of a novel multiferroic state induced by the spin exchange striction mechanism in high magnetic fields as an essential nature of the frustrated Mn spin system in this class of manganites.

Bi-substitution-induced magnetic moment distribution in spinel Bi(x)Co(2-x)MnO(4) multiferroic

We report the near-edge x-ray absorption spectroscopy (NEXAFS) at the Co/Mn L(3,2) edge and oxygen K edge of the well-characterized Bi-substituted Co(2)MnO(4) multiferroic samples. The evolution of peak features in NEXAFS spectra of the Co/Mn L(3,2) edge and O K edge show the Bi-induced redistribution of magnetic cations (Co/Mn). The variation in valence states of Co and Mn in all the substituted compositions is consistent with the observed ferrimagnetic behaviour of the samples. Magnetization data show the decrease in molecular field complementing the ferrimagnetism. The role of Bi in the enhancement of magnetic interactions as well as the appearance of ferroelectricity in Bi(x)Co(2-x)MnO(4) (0≤x≤0.3) is discussed.

Tuning ferroelectric polarization reversal by electric and magnetic fields in CuCrO(2)

The effects of electric and magnetic fields on magnetic and electric properties have been investigated for a triangular lattice antiferromagnet CuCrO(2) showing magnetically induced ferroelectric order. We demonstrate that ferroelectric polarization reversal can be finely tuned by using both magnetic and electric fields in the triangular lattice antiferromagnet. The observed magnetoelectric tunability can be attributed to small in-plane spin anisotropy and a resultant high degree of freedom for the direction of ferroelectric polarization, which is characteristic of a multiferroic triangular lattice antiferromagnet with out-of-plane 120 degrees spin structure.

Magnetic-order-induced crystal symmetry lowering in ACr2O4 ferrimagnetic spinels

We demonstrate that the onset of complex spin orders in ACr2O4 spinels with magnetic and Jahn-Teller active A=Fe and Cu ions lowers the lattice symmetry. This is clearly indicated by the emergence of anisotropic lattice dynamics-i.e., by the pronounced phonon splittings-even when experiments probing static distortions fail. The crystal symmetry in the magnetic phase is reduced from tetragonal to orthorhombic for both compounds. The conical spin ordering in FeCr2O4 is also manifested in the hardening of the phonon frequencies. In contrast, the multiferroic CoCr2O4 with no orbital degrees of freedom shows tiny deviations from cubic structure even in its ground state.

Magnetic phase evolution in the spinel compounds Zn(1-x)Co(x)Cr(2)O(4)

We present the magnetic properties of complete solid solutions of ZnCr(2)O(4) and CoCr(2)O(4): two well studied oxide spinels with very different magnetic ground states. ZnCr(2)O(4), with non-magnetic d(10) cations occupying the A site and magnetic d(3) cations on the B site, is a highly frustrated antiferromagnet. CoCr(2)O(4), with magnetic d(7) cations (three unpaired electrons) on the A site as well, exhibits Néel ferrimagnetism as well as commensurate and incommensurate non-collinear magnetic order. More recently, CoCr(2)O(4) has been studied extensively because of its polar behavior which arises from conical magnetic ordering. Gradually introducing magnetism on the A site of ZnCr(2)O(4) results in a transition from frustrated antiferromagnetism to glassy magnetism at low concentrations of Co, and eventually to ferrimagnetic and conical ground states at higher concentrations. Real-space Monte Carlo simulations of the magnetic susceptibility suggest that the first magnetic ordering transition and features of the susceptibility across x are captured by near-neighbor self-couplings and cross-couplings between the magnetic A and B atoms. We present, as a part of this study, a method for displaying the temperature dependence of magnetic susceptibility in a manner which helps distinguish between compounds possessing purely antiferromagnetic interactions from compounds where other kinds of ordering are present.

Flop of electric polarization driven by the flop of the Mn spin cycloid in multiferroic TbMnO3

Using in-field single-crystal neutron diffraction, we have determined the magnetic structure of TbMnO(3) in the high field P parallel a phase. We unambiguously establish that the ferroelectric polarization arises from a cycloidal Mn spin ordering, with spins rotating in the ab plane. Our results demonstrate directly that the flop of the ferroelectric polarization in TbMnO(3) with applied magnetic field is caused from the flop of the Mn cycloidal plane.

Magnetoelectric memory effect of the nonpolar phase with collinear spin structure in multiferroic MnWO4

The novel memory effect of a nonpolar paraelectric phase with a collinear spin structure has been observed in a magnetoelectric multiferroic material MnWO4. Since the ferroelectric polarization arises from a noncollinear spin structure, in a new class of magnetoelectric multiferroic materials with a spiral-spin structure, the information of ferroelectric domains should be lost in the collinear spin phase. However, in MnWO4, it has been found that the domain states in the ferroelectric phase are memorized even in the nonpolar phase with a collinear spin structure, when the phase transition is of the first-order type. Here we demonstrate a magnetoelectric memory effect that the ferroelectric single-domain state can be reproduced from the paraelectric phase by a magnetic field. We propose the nuclei growth model, in which the small ferroelectric embryos keep the polarization state in the nonpolar collinear spin phase.

Thermally or magnetically induced polarization reversal in the Multiferroic CoCr2O4

We report the unexpected evolution, with thermal and magnetic-field (H) variations, of the interrelation between the polarization P, magnetization M, and spiral wave vector Q in CoCr2O4, which has a ferrimagnetic conical-spiral magnetic order. For example, P suddenly jumps and changes its sign at the magnetic lock-in transition (T_{L}) with thermal variation, or with isothermal variation of H (without changing its direction) at T_{L}, which surprisingly occurs without change in spiral handedness (i.e., the sign of Q). The presence of multiple spiral sublattices may be behind this unusual behavior.

Control of the magnetoelectric domain-wall stability by a magnetic field in a multiferroic MnWO4

The relation between the orientation of the magnetic field and the flopped ferroelectric polarization has been investigated for multiferroic MnWO4. The ferroelectric single-domain state is retained across the polarization flop process when the direction of the applied magnetic field slightly deviates from the b axis within the ab plane. Furthermore, the electric polarization in the high-field P parallela phase is reversed when the P parallelb-to-P parallela transition takes place while decreasing and increasing the magnetic fields oppositely canted from the b axis. These results indicate that the symmetry breaking induced by a canted magnetic field determines the direction of the polarization flop, which corresponds to the direction of the vector spin chirality. The stability of the magnetoelectric domain walls in a canted magnetic field play a key role in the directional control of the electric polarization flop phenomenon.

Rotation of an electric polarization vector by rotating magnetic field in cycloidal magnet Eu0.55Y0.45MnO3

To clarify the microscopic origin of the gigantic magnetoelectric effect in multiferroics, we have investigated the variation of an electric polarization (P) vector under a rotating magnetic field (H) for a cycloidal helimagnet Eu0.55Y0.45MnO3 as a canonical example. P rotates smoothly by rotating H around the magnetic propagation wave vector k, which can be well understood by the rotation of the conical spin structure around k. We also show that the rotation process of the conical spin structure under H is crucial for the retention or reversal of the spin helicity or equivalently of the direction of P.

Cycloidal spin order in the a-axis polarized ferroelectric phase of orthorhombic perovskite manganite

The ferroelectric state in an orthorhombic perovskite RMnO3 (R=Gd0.7Tb0.3) was proved by neutron scattering studies to show the cycloidal spin state with the ab-spiral plane and the spin-helicity dependent polarization vector along the a axis, sharing the microscopic origin (inverse Dzyaloshinskii-Moriya interaction) with the more widely observed P||c state (e.g., for R=Tb and Dy) with the bc-spiral plane. The magnetic-field induced polarization flop from P||c to P||a as well known for RMnO3 is thus assigned to the orthogonal flop of the spin spiral plane from bc to ab.

Magnetic-field-induced ferroelectric state in DyFeO3

Versatile and gigantic magnetoelectric (ME) phenomena have been found for a single crystal of DyFeO3. Below the antiferromagnetic ordering temperature of Dy moments, a linear-ME tensor component as large as alphazz approximately 2.4 x 10(-2) esu is observed. It is also revealed that application of magnetic field along the c axis induces a multiferroic (weakly ferromagnetic and ferroelectric) phase with magnetization [> or =0.5 microB/formula unit (f.u.)] and electric polarization (> or =0.2 microC/cm2) both along the c axis. Exchange striction working between adjacent Fe3+ and Dy3+ layers with the respective layered antiferromagnetic components is proposed as the origin of the ferroelectric polarization in the multiferroic phase.

Spin-driven ferroelectricity in triangular lattice antiferromagnets ACrO2 (A=Cu, Ag, Li, or Na)

The correlation between the dielectric and magnetic properties is investigated on the triangular-lattice antiferromagnets ACrO2 (A=Cu, Ag, Li, or Na) with a 120-degree spiral structure. For the A=Cu and Ag compounds with a delafossite structure, the ferroelectric polarization emerges with a spiral-spin order, implying strong coupling between ferroelectricity and the spiral-spin structure. For the A=Li and Na compounds with an ordered rock salt structure, on the other hand, no spontaneous polarization is discerned, while the clear anomaly in the dielectric constant can be observed upon the transition to the spiral-spin ordered state. This feature can be ascribed to the possible antiferroelectric state induced by the alternate stacking of the Cr-spin sheet with opposite vector spin chirality.

Correlation between spin helicity and an electric polarization vector in quantum-spin chain magnet LiCu2O2

Measurements of polarized neutron scattering were performed on a S=1/2 chain multiferroic LiCu2O2. In the ferroelectric ground state with the spontaneous polarization along the c axis, the existence of transverse spiral spin component in the bc plane was confirmed. When the direction of electric polarization is reversed, the vector spin chirality as defined by C_(ij)=S_(i)xS_(j) (i and j being the neighboring spin sites) is observed to be reversed, indicating that the spin-current model or the inverse Dzyaloshinskii-Moriya mechanism is applicable even to this e_(g)-electron quantum-spin system. Differential scattering intensity of polarized neutrons shows a large discrepancy from that expected for the classical-spin bc-cycloidal structure, implying the effect of large quantum fluctuation.

Ferroelectricity in an ising chain magnet

We report discovery of collinear-magnetism-driven ferroelectricity in the Ising chain magnet Ca3Co2-xMn(x)O6 (x approximately 0.96). Neutron diffraction shows that Co2+ and Mn4+ ions alternating along the chains exhibit an up-up-down-down ( upward arrow upward arrow downward arrow downward arrow) magnetic order. The ferroelectricity results from the inversion symmetry breaking in the upward arrow upward arrow downward arrow downward arrow spin chain with an alternating charge order. Unlike in spiral magnetoelectrics where antisymmetric exchange coupling is active, the symmetry breaking in Ca3(Co,Mn)2O6 occurs through exchange striction associated with symmetric superexchange.

Mechanisms of exchange bias with multiferroic BiFeO3 epitaxial thin films

We have combined neutron scattering and piezoresponse force microscopy to show that the exchange field in CoFeB/BiFeO_{3} heterostructures scales with the inverse of the ferroelectric and antiferromagnetic domain size of the BiFeO3 films, as expected from Malozemoff's model of exchange bias extended to multiferroics. Accordingly, polarized neutron reflectometry reveals the presence of uncompensated spins in the BiFeO3 film at the interface with CoFeB. In view of these results, we discuss possible strategies to switch the magnetization of a ferromagnet by an electric field using BiFeO3.

Polarization reversal in multiferroic TbMnO3 with a rotating magnetic field direction

For the memory application of magnetoelectric multiferroics, not only bistability (i.e., ferroelectricity) but also the switching of the polarization direction with some noneverlasting stimulus is necessary. Here, we report a novel method for the electric polarization reversal in TbMnO3 without the application of an electric field or heat. The direction of the magnetic-field-induced polarization along the a axis (Pa) is memorized even in the zero field where Pa is absent. The polarization direction can be reversed by rotating the magnetic-field direction in the ab plane.

Large magnetostriction and negative thermal expansion in the frustrated antiferromagnet ZnCr2Se4

A detailed investigation of ZnCr2Se4 is presented which is dominated by strong ferromagnetic exchange but orders antiferromagnetically at TN=21 K. Specific heat and thermal expansion exhibit sharp first-order anomalies at the antiferromagnetic transition. TN is shifted to lower temperatures by external magnetic fields and finally is fully suppressed by a field of 65 kOe. The relative length change DeltaL/L(T) is unusually large and exhibits negative thermal expansion alpha below 75 K down to TN indicating strong frustration of the lattice. Magnetostriction DeltaL/L(H) reveals large values comparable to giant magnetostrictive materials. These results point to a spin-driven origin of the structural instability at TN explained in terms of competing ferromagnetic and antiferromagnetic exchange interactions.

Electric control of spin helicity in a magnetic ferroelectric

Magnetic ferroelectrics or multiferroics, which are currently extensively explored, may provide a good arena to realize a novel magnetoelectric function. Here we demonstrate the genuine electric control of the spiral magnetic structure in one such magnetic ferroelectric, TbMnO3. A spin-polarized neutron scattering experiment clearly shows that the spin helicity, clockwise or counterclockwise, is controlled by the direction of spontaneous polarization and hence by the polarity of the small electric field applied on cooling.

Ferroelectricity in an s=1/2 chain cuprate

We report our discovery of ferroelectricity in the spiral-magnetic state in the quantum quasi-one-dimensional (1D) S=1/2 magnet of LiCu2O2. Electric polarization (P) emerges along the c direction below the spiral-magnetic order temperature, but changes from the c to a axis when magnetic fields (H) are applied along the b direction. We also found that P(c) increases with H(c), and P(a) appears with H(a). LiCu2O2 in zero field appears to be the first ferroelectric cuprate and also a prototypical example of the "1D spiral-magnetic ferroelectrics." However, the unexpected behavior in H may demonstrate the complexity of the ordered spin configuration, inherent in the 1D S=1/2 magnet of LiCu2O2.

Multiferroics: a magnetic twist for ferroelectricity

Magnetism and ferroelectricity are essential to many forms of current technology, and the quest for multiferroic materials, where these two phenomena are intimately coupled, is of great technological and fundamental importance. Ferroelectricity and magnetism tend to be mutually exclusive and interact weakly with each other when they coexist. The exciting new development is the discovery that even a weak magnetoelectric interaction can lead to spectacular cross-coupling effects when it induces electric polarization in a magnetically ordered state. Such magnetic ferroelectricity, showing an unprecedented sensitivity to ap plied magnetic fields, occurs in 'frustrated magnets' with competing interactions between spins and complex magnetic orders. We summarize key experimental findings and the current theoretical understanding of these phenomena, which have great potential for tuneable multifunctional devices.

Ferroelectricity in the magnetic E-phase of orthorhombic perovskites

We show that the symmetry of the spin zigzag chain E phase of the orthorhombic perovskite manganites and nickelates allows for the existence of a finite ferroelectric polarization. The proposed microscopic mechanism is independent of spin-orbit coupling. We predict that the polarization induced by the E-type magnetic order can potentially be enhanced by up to 2 orders of magnitude with respect to that in the spiral magnetic phases of TbMnO3 and similar multiferroic compounds.

Ferroelectric Porous Molecular Crystal, [Mn3(HCOO)6](C2H5OH), Exhibiting Ferrimagnetic Transition

A porous molecular crystal with guest ethanol molecules, [Mn3(HCOO)6](C2H5OH), was found to be a new type of multifunctional molecular system, which exhibits a ferroelectric transition at 165 K and a ferrimagnetic transition at 8.5 K. [Mn3(HCOO)6](C2H5OH) will give a hint to design "multiferroic" molecular materials where ferroelectric and ferromagnetic orders coexist.

Ferroelectric polarization flop in a frustrated magnet MnWO4 induced by a magnetic field

The relationship between magnetic order and ferroelectric properties has been investigated for MnWO4 with a long-wavelength magnetic structure. Spontaneous electric polarization is observed in an elliptical spiral spin phase. The magnetic-field dependence of electric polarization indicates that the noncollinear spin configuration plays a key role for the appearance of the ferroelectric phase. An electric polarization flop from the b direction to the a direction has been observed when a magnetic field above 10 T is applied along the b axis. This result demonstrates that an electric polarization flop can be induced by a magnetic field in a simple system without rare-earth 4f moments.