No abstract

No abstract

LiCuVO4 is a model system of a 1D spin-1/2 chain that enters a planar spin-spiral ground state below its Néel temperature of 2.4 K due to competing nearest and next nearest neighbor interactions. The spin-spiral state is multiferroic with an electric polarization along the a axis which has been proposed to be caused purely by the spin supercurrent mechanism. With external magnetic fields in c direction TN can be suppressed down to 0 K at 7.4 T. Here we report dynamical measurements of the polarization from P(E)-hysteresis loops, magnetic field dependent pyro-current and non-linear dielectric spectroscopy as well as thermal expansion and magnetostriction measurements at very low temperatures. The multiferroic transition is accompanied by strong anomalies in the thermal expansion and magnetostriction coefficients and we find slow switching times of electric domain reversal. Both observations suggest a sizable magnetoelastic coupling in LiCuVO4. By analyzing the non-linear polarization dynamics we derive domain sizes in the nm range that are probably caused by Li defects.

Polarized neutron scattering experiments reveal that type-II multiferroics allow for controlling the spin chirality by external electric fields even in the absence of long-range multiferroic order. In the two prototype compounds TbMnO 3 and MnWO 4 , chiral magnetism associated with soft overdamped electromagnons can be observed above the long-range multiferroic transition temperature T MF , and it is possible to control it through an electric field. While MnWO 4 exhibits chiral correlations only in a tiny temperature interval above T MF , in TbMnO 3 chiral magnetism can be observed over several kelvin up to the lock-in transition, which is well separated from T MF . DOI: 10.1103/PhysRevLett.119.177201 Multiferroic materials with coupled magnetic and ferroelectric order bear considerable application potential [1,2]. In type-II multiferroics, magnetic order directly induces ferroelectric polarization and giant magnetoelectric coupling. 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].So far, studies of the multiferroic coupling and hysteresis curves were restricted to the phases with long-range magnetic order on bulk or film materials [16], while only small multiferroic blocks would be vital for applications. Also, from the fundamental point of view, one may ask whether multiferroic hysteresis and control can be achieved in short-range systems above the long-range static multiferroic transition, and how far spin chirality persists above the static and long-range multiferroic order. The mixed system Ni 0.42 Mn 0.58 TiO 3 already indicates that magnetoelectric coupling can persist in cluster systems with competing magnetic structures [17], but until now there has been no information about the control and multiferroic coupling of chiral ordering that is limited in space and time. Here, we study two prototype type-II multiferroics, TbMnO 3 [1, 3,4] and MnWO 4 [18-20], above the long-range ferroelectric transition at zero electric field T MF , where it is still possible to pole and control chiral magnetic correlations. Although the two materials exhibit a similar sequence of magnetic transitions, it turns out that only in TbMnO 3 can chiral scattering be controlled over a large temperature interval of several kelvin.TbMnO 3 (MnWO 4 ) both exhibit a first magnetic transition at T N ¼ 42 K (13.5 K), followed by a second transition at lower temperature, at which cycloid order develops at T MF ¼ 27.6 K (12.6 K). For ...

No abstract