Moscow State University, Leninskie gory, 119899 Moscow, Russia
Lately the magnetoelectric effect in the magnetoferroelectrics BiFeO3 attracted interest in the context of the spatially modulated spin structure (SMSS) inherent in this material [1]. The SMSS in BiFeO3 has the cycloid character and caused by the relativistic Dzyaloshinskii-Moria like interaction, which is linear in the spatial derivatives of the antiferromagnetic vector.
The occurrence of the SMSS handers the detection of the linear magnetolectric effect and weak ferromagnetic moment which are allowed in the BiFeO3 by magnetic symmetry. To reveal these effects we attempted to destroy the SMSS, i.e. induce the magnetic phase transition the SMSS-UAFS, where UAFS is the uniform antiferromagnetic state. Two ways to reach it have been sampled. First way is connected with the action of the high magnetic field, which according to the theoretical estimates may destroy the SMSS in the fields H>Hc, where critical field Hc is of order 20T. On the other hand destroying the SMSS is possible due to doping of the BiFeO3 rare earth ions, which increase the magnetic anisotropy. To check these ideas the single crystals of the compounds RxBi1-xFeO3 (R=La, Nd, Sm, Dy) were obtained and studied. The electric polarization of these samples have been measured depending on the magnetic field up to 30T in the temperature region 5-200K. The new field-induced phase transition SMCC-UAFS was discovered at the critical field Hc=20T. This transition is accompanied by the great jump of the electric polarization due to initiation of the linear magnetoelectric effect [2].
We studied also the H-T phase diagrams of the mixed magnetic ferroelectrics (RBi)FeO3. The value of critical field Hc decreased considerably in these compounds in comparison to BiFeO3. Critical field is also decreased strongly with decreasing of the temperature in the (NdBi)FeO3 and (DyBi)FeO3. It seems this fact indicates on the existing of the spontaneous phase transition SMSS-UAFS.
1. I. Sosnovska. Ferroelectrics, 79, 127 (1988).
2. Yu.F. Popov, A.K. Zvezdin, G.P. Vorob'ev, A.M. Kadomtseva,
V.A. Murashov, D.N. Rakov. JETP Lett., v. 57, N 1, p. 69-73 (1993).
This work had financial support from the Russian Basic Research Foundation (93-02-3214).