Multiferroic State Research Articles

Mn3TeO6- a new multiferroic material with two magnetic substructures

From magnetic susceptibility, dielectric permittivity, electric polarization and specific heat measurements, we discover spin-induced ferroelectricity and magnetoelectric coupling in Mn3TeO6 and observe two successive magnetic transitions at low temperatures. A non-ferroelectric intermediate magnetic state occurs below 23 K and a multiferroic ground state emerges below 21 K. Moreover, Mn3TeO6 is a candidate for a multiferroic material where two types of incommensurate spin structures, cycloidal and helical, coexist. Theoretically, both spin substructures may contribute to the macro electric polarization via different mechanisms.

Design of a Mott Multiferroic from a Nonmagnetic Polar Metal.

We examine the electronic properties of the newly discovered "ferroelectric metal" LiOsO3 combining density-functional and dynamical mean-field theories. We show that the material is close to a Mott transition and that electronic correlations can be tuned to engineer a Mott multiferroic state in the 1/1 superlattice of LiOsO3 and LiNbO3. We use electronic structure calculations to predict that the (LiOsO3)1/(LiNbO3)1 superlattice exhibits strong coupling between magnetic and ferroelectric degrees of freedom with a ferroelectric polarization of 41.2 μC cm(-2), Curie temperature of 927 K, and Néel temperature of 379 K. Our results support a route towards high-temperature multiferroics, i.e., driving nonmagnetic polar metals into correlated insulating magnetic states.

Raman Spectra of Multiferroics TbMnO<sub>3</sub>

The dream to have multifunctional materials has triggered an intense study of multiferroicity, i.e. of materials in which magnetic and electric polarizations are simultaneously present and strongly coupled. Here we study TbMnO3, a perovskite manganite exhibiting multiferroicity. It has an incommensurate antiferromagnetic ordering transition at 41 K and a transition into a multiferroic spin spiral state at 26 K. In this study, Raman spectroscopy has been used to elucidate the nature of the excitations related to phase transitions. We show that the optical phonons in TbMnO3 resemble the phonon characteristics typical for perovskite manganites, corresponding to MnO6 octahedral vibrations. The appearance of a new low energy modes at 32 cm-1, 40 cm-1, and 63 cm-1 observed below 41 K signifies the antiferromagentic ordering in TbMnO3. The 32 cm- 1 and 63 cm-1 are interpreted as spin ordering activated phonon while the 40 cm-1 is interpreted as spin ordering induced splitting of Tb quasi doublet cystal field level.

Spin-driven multiferroics in BaYFeO4

We report on the spin-driven multiferroic property and magnetoelectric effect in the lately synthesized compound BaYFeO4. Due to its peculiar crystal structure, the system exhibits complex magnetic phases with multiple transitions. The dielectric and pyroelectric measurements evidence a spin-driven multiferroic state raised by the cycloidal spin structure below T1 = 36 K. Strong magnetoelectric effect has also been observed in the multiferroic state. The origin of noncollinear cycloidal spin structure in BaYFeO4 is believed to arise from the interactions between low-dimensional magnetic columns.

Exploiting interfacial and size effects to construct oxide superlattices with robust and tunable magnetoelectric properties at room temperature

We propose a strategy to create materials displaying robust and tunable magnetoelectric multiferroic properties at room temperature. The key idea is to construct heterostructures that combine two different constituents: (1) compound ${\mathrm{BiFeO}}_{3}$, which presents strong ferroelectric and antiferromagnetic orders well above room temperature, but displays a small magnetic moment, and (2) a ferromagnetic insulator (e.g., ${\mathrm{BiMnO}}_{3}$) that is only required to couple magnetically with ${\mathrm{BiFeO}}_{3}$. Our simulations show that it is possible to combine such materials to create superlattices that present (i) a room-temperature multiferroic state with relatively large magnetization (up to $0.3{\ensuremath{\mu}}_{\mathrm{B}}$ per transition metal atom, with the possibility to improve by finding a suitable replacement for ${\mathrm{BiMnO}}_{3}$), (ii) an amply customizable magnetic behavior, and (iii) a strong magnetoelectric coupling. Thus, the design strategy successfully addresses a great challenge in the area of magnetoelectric multiferroics, exploiting interfacial couplings and size (layer-thickness) effects to produce materials apt for applications.

Phase transition and phase separation in multiferroic orthorhombic Dy(1-x)Ho(x)MnO3 (0 ≤ x ≤ 1).

We report on structural, magnetic, ferroelectric, and thermodynamic properties of polycrystalline orthorhombic manganites Dy1−xHoxMnO3 for Ho substitution levels 0 ≤ x ≤ 1. This system offers a possibility to systemically modulate the multiferroicity of RMnO3 via tuning the A-site ionic radii as well as the A-site magnetism. The successive transition of the multiferroic ground state is traced from the bc-cycloidal (DyMnO3) to the E-type antiferromagnetic phase (HoMnO3). In the middle substitution range 0.4 < x < 0.5, the phase separation is prominent, which's residual may survive in an even wider range. Accompanied with the phase transition and phase separation, obvious enhancement of both the polarization and magnetoelectric response is observed. Our experimental study also confirmed that the rare earth (Dy/Ho)-Mn exchange striction is a crucial role in deciding the multiferroicity of manganites.

FeCr₂S₄ in magnetic fields: possible evidence for a multiferroic ground state.

We report on neutron diffraction, thermal expansion, magnetostriction, dielectric, and specific heat measurements on polycrystalline FeCr2S4 in external magnetic fields. The ferrimagnetic ordering temperatures TC ≈ 170 K and the transition at TOO ≈ 10 K, which has been associated with orbital ordering, are only weakly shifted in magnetic fields up to 9 T. The cubic lattice parameter is found to decrease when entering the state below TOO. The magnetic moments of the Cr- and Fe-ions are reduced from the spin-only values throughout the magnetically ordered regime, but approach the spin-only values for fields >5.5 T. Thermal expansion in magnetic fields and magnetostriction experiments indicate a contraction of the sample below about 60 K. Below TOO this contraction is followed by a moderate expansion of the sample for fields larger than ~4.5 T. The transition at TOO is accompanied by an anomaly in the dielectric constant. The dielectric constant depends on both the strength and orientation of the external magnetic field with respect to the applied electric field for T < TOO. A linear correlation of the magnetic-field-induced change of the dielectric constant and the magnetic-field dependent magnetization is observed. This behaviour is consistent with the existence of a ferroelectric polarization and a multiferroic ground state below 10 K.

Cross coupling between electric and magnetic orders in a multiferroic metal-organic framework.

The coexistence of both electric and magnetic orders in some metal-organic frameworks (MOFs) has yielded a new class of multiferroics beyond inorganic materials. However, the coupling between two orders in multiferroic MOFs has not been convincingly verified yet. Here we present clear experimental evidences of cross coupling between electric and magnetic orders in a multiferroic MOF [(CH3)2NH2]Fe(HCOO)3 with a perovskite structure. The dielelectric constant exhibit a hump just at the magnetic ordering temperature TN. Moreover, both the direct (magnetic field control of dielectric properties) and converse (electric field control of magnetization) magnetoelectric effects have been observed in the multiferroic state. This work opens up new insights on the origin of ferroelectricity in MOFs and highlights their promise as magnetoelectric multiferroics.

Influence of nonmagnetic Zn substitution on the lattice and magnetoelectric dynamical properties of the multiferroic material CuO

Dynamic magnetoelectric coupling in the improper ferroelectric ${\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}\mathrm{O} (x=0,x=0.05)$ was investigated using terahertz time-domain spectroscopy to probe electromagnon and magnon modes. Zinc substitution was found to reduce the antiferromagnetic ordering temperature and widen the multiferroic phase, under the dual influences of spin dilution and a reduction in unit-cell volume. The impact of Zn substitution on lattice dynamics was elucidated by Raman and Fourier-transform spectroscopy, and shell-model calculations. Pronounced softenings of the ${A}_{u}$ phonons, active along the direction of ferroelectric polarization, occur in the multiferroic state of ${\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}\mathrm{O}$, and indicate strong spin-phonon coupling. The commensurate antiferromagnetic phase also exhibits spin-phonon coupling, as evidenced by a Raman-active zone-folded acoustic phonon, and spin dilution reduces the spin-phonon coupling coefficient. While the phonon and magnon modes broaden and shift as a result of alloy-induced disorder, the electromagnon is relatively insensitive to Zn substitution, increasing in energy without widening. The results demonstrate that electromagnons and dynamic magnetoelectric coupling can be maintained even in disordered spin systems.

Multiferroicity in the Mott Insulating Charge-Transfer Salt &lt;inline-formula&gt; &lt;tex-math notation="TeX"&gt;$\kappa-({\rm BEDT-TTF})_{2}{\rm Cu}[{\rm N}({\rm CN})_{2}]{\rm Cl}$ &lt;/tex-math&gt;&lt;/inline-formula&gt;

The recently proposed multiferroic state of the charge-transfer salt {\kappa}-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl [P. Lunkenheimer et al., Nature Mater., vol. 11, pp. 755-758, Sept. 2012] has been studied by dc-conductivity, magnetic susceptibility and measurements of the dielectric constant on various, differently prepared single crystals. In the majority of crystals we confirm the existence of an order-disorder-type ferroelectric state which coincides with antiferromagnetic order. This phenomenology rules out scenarios which consider an inhomogeneous, short-range-ordered ferroelectric state. Measurements of the dielectric constant and the magnetic susceptibility on the same crystals reveal that both transitions lie very close to each other or even collapse, indicating that both types of order are intimately coupled to each other. We address issues of the frequency dependence of the dielectric constant {\epsilon}' and the dielectric loss {\epsilon}'' and discuss sample-to-sample variations.

Effects of Al substitution and thermal annealing on magnetoelectric Ba0.5Sr1.5Zn2Fe12O22 investigated by the enhancement factor of 57Fe nuclear magnetic resonance

The magnetoelectric properties of hexaferrite Ba0.5Sr1.5Zn2Fe12O22 are significantly improved by Al substitution and thermal annealing. Measuring the enhancement factor of 57Fe NMR, we found direct microscopic evidence that the magnetic moments of the L and S blocks are rotated by a magnetic field in such a way as to increase the net magnetic moment of a magnetic unit, even after the field is removed. Al substitution makes magnetoelectric property arise easily by suppressing the easy-plane anisotropy. The effect of thermal annealing is to stabilize the multiferroic state by reducing the number of pinning sites and the electron spin fluctuation. The transverse conic structure gradually changes to the alternating longitudinal conic structure where spins fluctuate more severely.

Prediction of a multiferroic state with large electric polarization in tensile-strained TbMnO3

By performing first-principles calculations, we systematically explore the effect of epitaxial strain on the structure and properties of multiferroic TbMnO3. We show that, although the unstrained bulk TbMnO3 displays a non-collinear antiferromagnetic spin order, TbMnO3 can be ferromagnetic under compressive strain, in agreement with the experimental results on TbMnO3 grown on SrTiO3. By increasing the tensile strain up to 5%, we predict that TbMnO3 transforms into a new multiferroic state with a large ferroelectric polarization,two orders of magnitude larger than that in the unstrained bulk, and with a relatively high Neel temperature E-type antiferromagnetic order. We also find that the ferroelectric domain and antiferromagnetic domain are interlocked with each other, thus an external electric field can switch the ferroelectric domain and the antiferromagnetic domain simultaneously. Our work demonstrates that strain engineering can be used to improve the multiferroic properties of TbMnO3.

Evolution of magnetic and crystal structures in the multiferroic FeTe2O5Br

Neutron diffraction and nuclear quadrupole resonance (NQR) measurements were employed to investigate magnetic order in the non-ferroelectric phase preceding the low-temperature multiferroic state in FeTe2O5Br. Refnement of the neutron diffraction data and simulations of 79,81Br NQR spectra reveal that the incommensurate magnetic ordering in the non-ferroelectric state comprises amplitude-modulated magnetic moments, similarly as in the multiferroic state. The two ordered states differ in the orientation of the magnetic moments and phase shifts between modulation waves. Surprisingly, all symmetry restrictions for the electric polarization are absent in both states. The different ferroelectric responses of the two states are thus argued to arise from the differences in the phase shifts between certain modulation waves, which cancel out in the non-ferrolectric state.

Magnetic and Electric Orderings and Intrinsic Magnetoelectric Coupling in Pb(Fe,W)O3 Based Ceramics

In this work structural, electric, dielectric, and anelastic properties of single phase high density (> 97%) (1-x)PFW-xPT ceramics, with 0 ≤ x ≤ 0.30, are investigated. The experimental results show that the temperatures, in which magnetic (TN ) or electric (TFE ) ordering occur, can be interchanged by PbTiO3 addition. Correlations between the experimental results allow to estimate intrinsic ferromagnetoelectroelastic coefficients at the temperatures, where the multiferroic state exist. The intrinsic magnetoelectric coefficient is maximum for compositions, in which the temperatures for magnetic and ferroelectric ordering coincide, no matter in what sequence the orderings (FE or AFM) occur.

Ferroelectricity enhanced by disorder in collinear-magnetism-induced multiferroic state

To understand the fantastic static-disorder-enhanced ferroelectricity observed in Ca3Co2−xMnxO6 compound, the multiferroic state originated from the up-up-down-down spin order in different conditions of disorder is simulated by using Monte Carlo algorithm on a three-dimensional elastic Ising model with dilution. The spin structure factor and the correlation of ionic displacements are calculated to show the detailed variations of microscopic magnetic and ferroelectric structures. It is indicated that in the pure system without disorder the frustration from the long-ranged interchain interaction disrupts the up-up-down-down spin order and the corresponding ferroelectric domain, and hence suppresses the ferroelectric polarization (P). When disorder is introduced by doping a few spin defects, the frustration can be relaxed around these defects. Then the disturbance upon ferroelectric domain is lessened and hereby P is recovered effectively. The result obtained gives a clear microscopic scenario of the static-disorder-enhanced ferroelectricity observed in Ca3Co2−xMnxO6 compound.

Magnetic structure and ferroelectric activity in orthorhombic YMnO3: Relative roles of magnetic symmetry breaking and atomic displacements

We discuss relative roles played by the magnetic inversion symmetry breaking and the ferroelectric (FE) atomic displacements in the multiferroic state of YMnO3. For these purposes we derive a realistic low-energy model, using results of first-principles calculations and experimental parameters of the crystal structure. Then, we solve this model in the Hartree-Fock approximation. We argue that the multiferroic state in YMnO3 has a magnetic origin, and the centrosymmetric Pbnm structure is formally sufficient for explaining details of the noncentrosymmetric magnetic ground state. The relativistic spin-orbit interaction lifts the degeneracy, caused by the frustration of isotropic interactions, and stabilizes a twofold periodic magnetic state, which is similar to the E-state apart from the spin canting. The noncentrosymmetric atomic displacements in the P2_1nm phase reduce the spin canting, but do not change the symmetry of the magnetic state. The effect of the P2_1nm distortion on the FE polarization P_a is twofold: (i) it gives rise to ionic contributions, associated with the Y and O sites; (ii) it affects the electronic polarization, through the change of the spin canting. The relatively small value of P_a, observed in the experiment, is caused by a partial cancelation of the electronic and ionic contributions in the experimental P2_1nm structure. Finally, we theoretically optimize the crystal structure, by using the LSDA+U approach and assuming the collinear E-type alignment. We have found that the agreement with the experimental data in this case is less satisfactory and P_a is largely overestimated. Although the magnetic structure can be formally tuned by varying the Coulomb repulsion U as a parameter, apparently LSDA+U fails to reproduce some fine details of the experimental structure, and the cancelation of different contributions in P_a does not occur.

Structurally tuned multiferroic state in BiFeO3-based compounds

Structurally tuned multiferroic state is demonstrated for BiFeO3-based compounds. The electric and magnetic orders are strongly affected by the coexistence of R3c and Cm phases, i.e., by structural softness through monoclinicity, which leads the multiferroism to be driven by the same cation. The Cm phase enhances the ferroelectric and magnetic responses through Bi/Ba–O and Fe/Ti–O bonds by influencing structural distortions and ion valence. We also show the strong correlations between ferroic orders, structural arrangements, and tuning of the ion valence in the perovskite B site.

Determination of the magnetic order and the crystal symmetry in the multiferroic ground state of Ba2CoGe2O7

Detailed structural investigation of Ba${}_{2}$CoGe${}_{2}$O${}_{7}$ was performed in its low-temperature multiferroic state combining neutron diffraction with magnetization measurements and the optical study of lattice vibrations on single crystals. The crystal structure above (10.4 K) and the crystal and magnetic structures below (2.2 K) the antiferromagnetic transition temperature of ${T}_{N}$ $=$ 6.7 K were determined using neutron diffraction. The tetragonal space group (SG) $P$-42${}_{1}m$, corresponding to the average structure at room temperature, was found to also describe the structure at low temperatures well. Neutron diffraction data and infrared phonon mode analysis imply no structural phase transition down to 2.2 K. Orthorhombic polar SG Cmm2 is proposed as a true crystal structure. Below ${T}_{N}$, the spins of the Co${}^{2+}$ ions form a square-lattice N\'eel order within the ($a$,$b$) plane, while their alignment is ferromagnetic along the $c$ axis. The magnitude of the ordered moment, fully lying within the ($a$,$b$) plane, is found to be 2.9(1) ${\ensuremath{\mu}}_{B}$/Co${}^{2+}$ and the easy axis of the sublattice magnetizations corresponds to the [110] direction. A noncollinear spin structure due to small canting is allowed.

The complex multiferroic phase diagram of Mn1−xCoxWO4

The complete magnetic and multiferroic phase diagram of Mn1−xCoxWO4 single crystals is investigated by means of magnetic, heat capacity and polarization experiments. We show that the ferroelectric (FE) polarization in the multiferroic state abruptly changes its direction twice upon increasing Co content, x. At xc1 = 0.075, rotates from the b-axis into the a–c-plane and at xc2 = 0.15 it flips back to the b-axis. The origin of the multiple polarization flops is identified as an effect of the Co anisotropy on the orientation and shape of the spin helix leading to thermodynamic instabilities caused by the decrease of the magnitude of the polarization in the corresponding phases. A qualitative description of the FE polarization is derived by taking into account the intrachain (c-axis) as well as the interchain (a-axis) exchange pathways connecting the magnetic ions. In a narrow Co concentration range (0.1 ⩽ x ⩽ 0.15), an intermediate phase, sandwiched between the collinear high-temperature and the helical low-temperature phases, is discovered. The new phase exhibits a collinear and commensurate spin modulation similar to the low-temperature magnetic structure of MnWO4.

First-order multi-kphase transitions and magnetoelectric effects in multiferroic Co3TeO6

A theoretical description of the sequence of magnetic phases in Co3TeO6 is presented. The strongly first-order character of the transition to the commensurate multiferroic ground state, induced by coupled order parameters corresponding to different wavevectors, is related to a large magnetoelastic effect with an exchange energy critically sensitive to the interatomic spacing. The monoclinic magnetic symmetry C2' of the multiferroic phase permits spontaneous polarization and magnetization as well as the linear magnetoelectric effect. The existence of weakly ferromagnetic domains is verified experimentally by second harmonic generation measurements.