Session W9: Focus Session: Complex Bulk Oxides: Multiferroics

Evolution of spin wave excitations in Sr$_{2}$FeSi$_{2}$O$_{7}$ under an external magnetic field

Evolution of static and dynamic spin correlations in a new multiferroics material Sr$_{2}$FeSi$_{2}$O$_{7}$ under an external magnetic field was investigated by elastic and inelastic neutron scattering techniques. An external magnetic field up to $B$ = 14 Tesla induces four different magnetic and ferroelectric phases in Sr$_{2}$FeSi$_{2}$O$_{7}$. The static magneto-electric coupling can be understood as the spin-dependent metal-ligand hybridization proposed for a related material Ba$_{2}$CoGe$_{2}$O$_{7}$. By analyzing the inelastic neutron scattering data obtained from a single crystal of Sr$_{2}$FeSi$_{2}$O$_{7}$ without field, we have determined the effective spin Hamiltonian in this material that includes isotropic nearest neighbor exchange interaction in the two-dimensional Fe square plane and easy plane single ionic anisotropies. The spin wave excitations show interesting changes as upon ramping up the system enters the field-induced phases for $B \quad >$ 6.5 Tesla, which will also be discussed.   

THz spectroscopy of spin waves in multiferroic Ba$_2$CoGe$_2$O$_7$ in high magnetic fields

By applying external magnetic field the square-lattice antiferromagnet Ba$_2$CoGe$_2$O$_7$ can be transformed to a chiral form, evidenced by large optical activity when the light is in resonance with spin excitations at sub-terahertz frequencies. We found that the magnetochiral effect, the absorption difference for the light beams propagating parallel and anti-parallel to the applied magnetic field, has an exceptionally large amplitude close to 100\% and persists to fields up to 30\,T. All these features are ascribed to the magnetoelectric nature of spin excitations as they interact both with the electric and magnetic components of light. We observe a spin flop at 15\,T, that is consistent with our theoretical calculations.   

Ground State Properties and Magnetodielectric Coupling in Sm$_{0.5}$Nd$_{0.5}$Fe$_{3}$(BO$_{3})_{4}$

We report x-ray scattering and polarized infrared reflectivity measurements of the substituted ferroborate Sm$_{0.5}$Nd$_{0.5}$Fe$_{3}$(BO$_{3})_{4}$. Below T$_{N}$ = 33 K, a new set of commensurate peaks with resonant enhancements at the rare earth L edges indicates a doubling of the magnetic structure along the c-axis and simultaneous ordering of the rare earth and iron ions. Rare earth spin polarizations decrease rapidly with increasing temperature, in contrast to that of the iron ions. Shell-specific measurements of the rare earth spin polarizations indicate similar behaviors of the Sm and Nd 5d states, while the Sm 4f and 5d states have different temperature dependences. Along the c-axis we observe negative thermal expansion below $\sim $75 K and a strong phonon softening from room temperature down to T$_{N}$, at which it freezes in frequency. Also at T$_{N}$ we observe the appearance of an electromagnon in the ab-plane that gets its spectral weight from the lowest frequency phonon. These results indicate a lattice instability linked to magnetism with a strong coupling between magnetic and elastic properties.   

Spin-driven ferroelectricity in ferroaxial crystals

Spin-driven ferroelectricity in most non-collinear magnets, such as TbMnO3, is induced by the so-called inverse Dzyalonshinskii-Moriya mechanism and requires a cycloidal magnetic structure, an ordered magnetic state that is not truly chiral (or lacks helicity). Conversely, in a truly chiral magnetic state (proper helix), the pseudo-scalar helicity can not couple directly to the electric polarization, and therefore can't induce ferroelectric order. However, in systems of specific crystal symmetry, named here ``ferroaxials,'' the presence of collective structural rotations mediates an indirect coupling between magnetic helicity and ferroelectricity. I will review our recent experimental results for new compounds of this class, obtained by magnetic X-ray and neutron diffraction techniques, including a clear demonstration that the magnetic helicity can be controlled by an electric field.   

Multiferroic behavior in trimerized Mott insulators

We demonstrate multiferroic behavior in trimerized Mott insulators through the interplay between spins and electric dipole moments resulting from electronic charge fluctuations in frustrated units [L. N. Bulaevskii, C. D. Batista, M. V. Mostovoy, and D. I. Khomskii, Phys. Rev. B 78, 024402 (2008)]. The model consists of stacked triangular layers of trimers with small inter-trimer exchange interactions $J'$ and $J''$. We construct a phase diagram using a semi-classical approach. Ferroelectric states coexist with ferro- or antiferromagnetic orderings depending on the value of the magnetic field $H$ and the sign of the inter-layer exchange $J''$. The electric polarization undergoes abrupt changes as a function of $H$.   

Electric field control of magnetic chiralities in ferroaxial multiferroic RbFe(MoO$_{4})_{2}$

The onset of ferroelectric polarisation in high symmetry, proper-screw type multiferroic materials cannot be explained in terms of conventional microscopic mechanisms or symmetry analysis since the direction of the magnetic propagation vector is orthogonal to the plane of the spins. RbFe(MoO$_{4})_{2 }$undergoes a structural distortion at T$_{c}$ = 190 K in which the oxygen tetrahedra rotate, imposing an ``axiality'' to the crystal structure. We show that a simple symmetric-exchange driven coupling of this axiality with the magnetic chiralities below T$_{N}$ = 4 K explains the appearance of a ferroelectric polarisation parallel to both the axial vector and direction of magnetic propagation. We present spherical neutron polarimetry data that are sensitive to both helical and triangular chiral domains of the magnetic structure. We are able to distinguish the magnetic order in the two axial domains that are present in equal proportions, and to demonstrate direct control of the magnetic structure by an applied electric field. The above formalism may be readily generalised to other `ferroaxial' systems in which the magnetic ordering breaks the inversion symmetry in a point group supporting an axial vector.   

Electric field control of nonvolatile four-state magnetization at room temperature

We find the realization of large converse magnetoelectric (ME) effects at room temperature in a multiferroic hexaferrite Ba$_{0.52}$Sr$_{2.48}$Co$_{2}$Fe$_{24}$O$_{41}$ single crystal, in which rapid change of electric polarization in low magnetic fields (about 5 mT) is coined to a large ME susceptibility of 3200 ps/m. The modulation of magnetization then reaches up to 0.62 $\mu _{B}$/f.u. in an electric field of 1.14 MV/m. We find further that four ME states induced by different ME poling exhibit unique, nonvolatile magnetization versus electric field curves, which can be described by an effective free energy with a distinct set of ME coefficients. *These authors contributed equally to this work.   

Magnetoelectric Effects and Related Phenomena in Spin-spiral Hexaferrites

Among various multiferroics, extensive studies of ferroelectrics originating from magnetic orders, i.e., \textit{magnetically-induced ferroelectrics }in which the inversion simmetry breaking and resultant ferroelectricity are induced by complex magnetic orders, have been triggered almost a decade ago by the discovery of multiferroic nature in a perovskite-type rare-earh manganites TbMnO$_{3}$. The magnetically-induced ferroelectrics often show giant magnetoelectric effects, remarkable changes in electric polarization in response to a magnetic field, since the origin of their ferroelectricity is driven by magnetism which sensitively responds to an applied magnetic field. Though a large number of new magnetically-induced ferroelectrics have been reported in the past decade, so far there has been no practical application employing the magnetoelectric effect of the magnetically-induced ferroelectrics. This is partly because none of the existing magnetically-induced ferroelectrics have combined large and robust electric and magnetic polarizations at room temperature until quite recently. The situation is changed by the discoveries of magnetoelectricity in hexagonal ferrites (\textit{hexaferrites}) with spin-spiral structures.\footnote{T. Kimura, G. Lawes, and A. P. Ramirez, Phys. Rev. Lett. 94, 137201 (2005).}$^,$\footnote{Y. Kitagawa \textit{et al.}, Nature Mater. 9, 797 (2010).}$^,$\footnote{K. Okumura \textit{et al.}, Appl. Phys. Lett. 98, 212504 (2011).} In this presentation, I show our recent studies on magnetoelectric effects and related phenomena in the new series of magnetically-induced ferroelectrics which are promising candidates for multiferroics operating at room temperature and low fields. This work has been done in collaboration with Y. Hiraoka, T. Ishikura, K. Okumura, Y. Kitagawa, H. Nakamura, Y. Wakabayashi, M. Soda, T. Asaka, and Y. Tanaka.   

Pressure-induced Polarization Reversal in Z-type Hexaferrite Single Crystal

Multiferroic materials with a gigantic magnetoelectric (ME) coupling at room temperature have been searched for applications to novel devices. Recently, large direct and converse ME effects were realized at room temperature in the so-called Z-type hexaferrite (Ba,Sr)$_{3}$Co$_{2}$Fe$_{24}$O$_{41}$ single crystals [1,2]. To obtain a new control parameter for realizing a sensitive ME tuning, we studied ME properties of the crystals under uniaxial pressure. Upon applying a tiny uniaxial pressure of about 0.6 GPa, magnetic field-driven electric polarization reversal and anomaly in a $M-H$ loop start to appear at 10 K and gradually disappear at higher temperature above 130 K. By comparing those results with longitudinal magnetostriction at ambient pressure, we propose the pressure-dependent variations of transverse conical spin configuration as well as its domain structure under small magnetic field bias, and point out the possibility of having two different physical origins of the ME coupling in this system. [1] Y. Kitagawa \textit{et al}., Nat. Mater. \textbf{9}, 797 (2010) [2] S. H. Chun \textit{et al}., submitted.   

The complex multiferroic phase diagram of Mn$_{1-x}$Co$_x$WO$_4$

MnWO$_4$ is a classical multiferroic where ferroelectricity is induced by an inversion symmetry breaking helical spin order. The origin of the helical order is found in competing magnetic exchange interactions with strong uniaxial anisotropy, resulting in magnetic frustration. The microscopic parameters can be tuned by chemical substitution of Fe, Zn, or Co for Mn. The effects of Co substitution up to 30\% on the magnetic structure and the ferroelectric (FE) phase are investigated. The multiferroic phase diagram of Mn$_{1-x}$Co$_x$WO$_4$ is completely resolved. At low Co content, the FE polarization is oriented along the b-axis and decreases with increasing x. At doping levels between 7.5\% and 12\% Co, the polarization points along the a-axis and it reaches a maximum value of 90 $\mu$C/m$^2$ at x=10\%. With further increasing x, the FE polarization rotates back to the b-axis. Its magnitude decreases continuously and vanishes above 30\% Co content. This complex behavior comes along with a delicate sequence of different magnetic phases which are explored by magnetization, heat capacity, and neutron scattering experiments.   

Spin flop transition in the multiferroic Mn$_{1-x}$Co$_{x}$WO$_{4}$ studied by neutron diffraction

Elastic neutron diffraction is employed to investigate the ground state magnetic structure of the multiferroic Mn$_{1-x}$Co$_{x}$WO$_{4}$. Unlike the undoped MnWO$_{4 }$that has low-$T$ collinear spin structure, the doped MnWO$_{4}$ exhibits complex evolution of magnetic structure. Samples at lower concentration ($x<$0.10) have similar elliptical spiral structure as MnWO$_{4}$ in the multiferroic phase. With increasing $x$, the magnetic structure undergoes a sudden spin-flop transition which switches electric polarization from crystalline $b$-axis to $a$-axis. Polarized neutron scattering is also used to study the correlation between the bulk electricity and magnetic helicity.