Polarized-neutron-scattering studies on the chiral magnetism in multiferroic MnWO₄

Abstract

Neutron diffraction with spherical polarization analysis is a powerful tool for studying the multiferroic materials where the ferroelectric polarization arises from a complex magnetic structure. Analyzing the off-diagonal terms in the polarization matrix one may directly detect the chiral contributions even in a multidomain arrangement. In MnWO4 one can control the chiral magnetism by varying an electric field at constant temperature. The analysis of multiferroic hysteresis cycles at four equivalent magnetic Bragg peaks fully agrees with a nearly monodomain chiral arrangement controlled by the electric field. A pronounced asymmetry of the hysteresis cycles and memory effects point to strong pinning of the chiral magnetism in MnWO 4. We find a second-order harmonic modulation which exhibits both magnetic and structural character and which may be related with the domain pinning. The observed interference between the nuclear and the magnetic modulation is another manifestation of the coupling between the crystal structure and the magnetism in the multiferroic oxides.

Kinetics of the multiferroic switching inMnWO4

“…A higher field is needed to force the sample in the notpreferred state and a lower field can reverse it into its preferred state. This is in accordance with previous results on the control of multiferroic domains by varying an electric field at constant temperature 17,27 .…”

“…Since this transition is of clear first order, one would not expect any slowing down of excitations, which could melt the pinning. Instead the lower transition is related to the anharmonic distortions of the magnetic modulation, which increase upon cooling in the multiferroic phase 27 . This increase is relatively stronger than that of the first-order magnetic modulation.…”

“…In reference 27 it was shown, that this half-period structural modulation is sizeable in MnWO 4 and that it is accompanied by a second-order modulation of the magnetic structure. The two modulations exhibit coherent interference underlining their tight coupling 27 . This structural distortion can possess a higher pinning potential than the weak ferroelectric distortion itself.…”

Robust charge and magnetic orders under electric field and current in multiferroicLuFe2O4

“…Nevertheless, there have been only a few observations of such effects [5][6][7][8][9][10] in known multiferroic systems, and all of them can be attributed to electric field realigning ferroelectric domains and therefore causing a macroscopic magnetic response. In LuFe 2 O 4 , the ferroelectric polarization is chargevalence driven, and the charge-valence order also couples strongly to the magnetic order.…”

Control of Chiral Magnetism Through Electric Fields in Multiferroic Compounds above the Long-Range Multiferroic Transition

“…External magnetic fields imply a flop of electric polarization, and electric fields can control chiral magnetic domains [1][2][3][4][5]. Various neutron experiments [6][7][8][9][10][11][12] as well as resonant and nonresonant x-ray studies [13,14] show that cooling in electric fields enforces a monodomain chiral state, and varying external electric fields at constant temperature drives the chiral magnetic order [9][10][11][12], which corresponds to the most promising application as a data storage medium. In addition, time resolved soft x-ray diffraction showed that chiral magnetism can be manipulated by THz-radiation pulses at an electromagnon energy [15].…”