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

Comparative study of the field-induced and spontaneous AF2′ multiferroic phases in MnWO4and Mn0.90Co0.10WO4within the magnetic symmetry framework

(Mn,Co)WO4compounds are regarded as reference spin-induced multiferroics. A comparative study is presented here, within the magnetic symmetry framework, of the incommensurate magnetic orders responsible for the ferroelectric phases of (i) MnWO4under a magnetic field (H||b) and (ii) Mn0.90Co0.10WO4in the absence of an external field. On the one hand, although these two ferroelectric phases are stabilized under different external physical conditions, both present the sameXc1′(α0γ)ssmagnetic symmetry and practically the same modulation vector. The magnetic ordering in both phases is an elliptical helix with the magnetic moments (as the polarization vector,P) perpendicular to thebaxis, although in most of the ferroelectric compositions of the (Mn,Co)WO4family the spins rotate in planes containingb(and haveP||b). On the other hand, the anisotropy of the resulting magnetic modulations is extraordinarily different in the two phases. This is described and explained in the light of the different anisotropies of Co and Mn ions.

Electric polarization of magnetic domain walls in magnetoelectrics

Two prominent magnetoelectrics MnWO4 and CuO possess low-temperature commensurate paraelectric magnetically ordered phase. Here using Monte Carlo simulations we show that the walls between the domains of this phase are ferroelectric with the same electric polarization direction and value as those in the magnetoelectric phases of these compounds. We also suggest that experimental observation of electric polarization of domain walls in MnWO4 should help to determine the macroscopic interactions responsible for its magnetoelectric properties.

Chiral domains in cycloidal multiferroic thin films: switching and memory effects

Cycloidal magnetic order occurring in some AMnO(3) perovskites is known to induce ferroelectricity. The polarization is perpendicular to the propagation vector direction of the cycloid and its chirality, and therefore it is directly related to the chiral domain structure. We show that the switching process of chiral domains is sensitively dependent on the magnetoelectric history of the sample. Moreover, by appropriate field cycling, magnetic order can display partial chiral memory. We argue that memory results from electric field coupling of cycloidal domain and nucleation and pinning of chiral domain walls, much like the domain structure in other ferroic systems.

Magnetic phase diagram of multiferroic MnWO4 probed by ultrasound

The magnetic phase diagram of multiferroic MnWO(4) is studied in magnetic fields up to 60 T using sound velocity and sound attenuation measurements. Anomalies are observed at temperatures T(N1) = 7.6 K, T(N2) = 12.6 K and T(N3) = 13.6 K that separate commensurate antiferromagnetic (AF1) to helical AF2 and commensurate AF3 to paramagnetic phases, respectively. The anomalies are significantly different and shed light on the spin-phonon coupling and evolution of the various order parameters in this multiferroic material. For temperatures below T(N2) pronounced field hysteresis effects are also observed in the sound velocity, indicating field-induced transformations. In the temperature dependence of the attenuation we observe anomalies distinctively different from the usual maxima related to relaxation effects. They are attributed to the combination of dispersion effects due to domain walls and the discontinuously changing sound velocity. In total, six different magnetic phases, at various temperatures and fields including a novel high-field phase, are revealed and analyzed.

Directional magnetoelectric effects in MnWO4: magnetic sources of the electric polarization

The ferroelectric order and magnetic field induced effects observed in the spiral phase of MnWO4 are described theoretically. It is demonstrated explicitly that the Dzyaloshinskii-Moriya antisymmetric interactions contribute to the correlation between spins and electric dipoles in the incommensurate and commensurate ferroelectric phases of magnetic multiferroics. However, other single-site symmetric interactions are shown to be involved in the magnetoelectric process, suggesting the possible existence of an electric polarization originating from purely symmetric effects.

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.

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.