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We have shown that in the chromium oxide the components of the tensor of magnetoelectric susceptibility f 33 ( T ) in the original phase and f 31 ( T ) in the spin-flop phase exhibit similar temperature dependencies. It's proved in the present paper that noted above behavior of these components can be explained in the framework of the phenomenological theory if one takes into account invariants of the third and fifth order of magnitude in the components of the antiferromagnetic vector in the phenomenological thermodynamic potential with explicitly separated exchange and relativistic terms. Also, the form of thermodynamic potentials elaborated in this paper suggests that the transition to the spin-flop phase in the chromium oxide is incomplete and f 33 cannot be equal to zero in the spin-flop phase.

The electric polarization of single crystal YMn 2 O 5 was investigated in strong pulsed magnetic fields up to H = 250 kOe at low temperatures. It was shown that at T ∼ 40 K magnetic and structural phase transitions occur, with the structural changes taking place improperly (magnetically induced). We have also shown the existence of the structural phase transition at T ∼ 20 K with the onset of spontaneous polarization P b . The quadratic magnetoelectric effect at temperatures T < 40 K was observed (with b-component domination), the magnetoelectric susceptibility below T = 20 K greatly increased for all field orientations. When H A a the magnetoelectric effect changed its sign which in analogue with EuMn 2 O 5 was attributed to non-commensurate to commensurate structural change. The polarization anomalies in strong magnetic fields were observed with the former being due to complex magnetic structure changes and the process of spontaneous polarization induction by magnetic field.

The magnetoelectric effect in (Fe x Cr1-x )2O3 (x=0, 0.02, 0.05, 0.1) single crystals has been studied under a magnetic field of up to 200 kOe. We have found that various magnetic phase transitions due to the restructuring of the spatial spin structure and leading to radical changes in the magnetoelectric effect take place in substitutional alloys. We have also detected a spontaneous phase transition from a conical helix to a cycloidal spin structure taking place at x = 0.1 and a temperature of 45 K. The experimental data are in agreement with the theoretical concept based on the thermodynamic model and can be accounted for in terms of competition among exchange energies of different modes.

The field-dependencies of the ME-effect and longitudinal magnetostriction of single crystals GaxFe2-xO3 in magnetic fields up to 200 kOe have been studied. For analysis of the experimental data the representation about toroidal spin ordering has been used, which is in qualitative conformity with the experimental data.

The field induced transition from the space modulated spin structures to the homogeneous antiferro-magnetic state in magnetic ferroelectrics R x Bi1–x FeO3 (R = La3+, Nd3+, Dy3+) was investigated. It has been established that space modulated spin structures (SMSS) exists in compounds: with La up to concentration x = 0.6; with Nd up to x = 0,1 and Dy up to x = 0.1. For all investigated compounds the polarization curves have linear dependence upon the field above Hcr . The critical field decreases with increase of concentration x and magnetic moment of the R ions. Coexistence of weak ferromagnetic moment and SMSS was observed on the compound La x Bi1–x FeO3 at x ⩾ 0,1.

The discovery of a new magnetic field induced phase transition between the space-modulated spin structure (SMSS) and the homogeneous antiferromagnetic state (HAFS) is reported. This phase transition is accompanied by large magnetoelectric (ME) anomalies. It is well known that BiFeO, shows the ferroelectric ordering (T c = 1083 K) and antiferromagnetic ordering (T N = 673 K). ME properties of BiFeO3 so far remain a challenging experimental and theoretical problem. While the crystal symmetry of BiFeO3 allows a linear ME effect, it cannot be observed because the antiferromagnetic order of BiFeO3 is SMSS (cycloid). We show that there is a Lifshits invariant, which decreases the ground state energy and induces the SMSS. In a high magnetic field the energy of the HAFS decreases more rapidly than the energy of the SMSS. Hence the field induced phase transition between SMSS and HAFS is possible. We stress that a large variation of the ME effect is expected at the transition due to a rising linear ME cont...