G. P. Vorob'ev

Discovery of the linear magnetoelectric effect in magnetic ferroelectric BiFeo3 in a strong magnetic field

Abstract 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...

Spin density wave and field induced phase transitions in magnetoelectric antiferromagnets

Abstract An investigation was done of the field induced phase transition from the spatially modulated spin structures to the homogeneous antiferromagnetic state in magnetic ferroelectrics R x Bi 1− x FeO 3 (R = La, Nd) and magnetoelectrics Fe x Cr 2− x O 3 in pulsed magnetic field up to 30 T. It has been found that this transition is accompanied by a large change in the magnitude of the magnetoelectric effect. In contrast to the R x Bi 1− x FeO 3 , where the linear magnetoelectric effect appears in strong magnetic fields ( H > H c ) in Fe x Cr 2− x O 3 such effect exists in the low-field phase ( H H c ). This is associated with the different character of the spatially modulated spin structure in these materials (cycloid structure in R x Bi 1− x FeO 3 and cone spiral in Fe x Cr 2− x O 3 ).

Magnetoelectric Effect in YMn 2 O 5 in Strong Pulsed Magnetic Fields

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 Á 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.

Linear magnetostriction and magneto-electric effect in piezoelectric Ga2-xFexO3

Abstract 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.

High magnetic field investigations of the magnetoelectric effect in magnetic ferroelectrics (RBi)FeO3

Abstract 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.

Induced by magnetic field spin reorientation in YMn2O5

Abstract In ferroelectric YMn2O5 weak ferromagnetism was found along a-axis of rhombic crystal. It was shown that the nature of ferromagnetic sublattices cant is determined by magnetoelectric ordering. Reorientation of magnetic moment was observed under strong magnetic field applied along b-axis of crystal, that was accompanied by sharp increase of magnetostriction and jumps of electric polarization. In EuMn2O5, as in YMn2O5, spontaneous and magnetic field induced phase transitions from incommensurate to commensurate phase were observed, accompanied by cardinal changing of magnetoelectrical properties.

Anomalies of magnetic and magnetoelastic properties in Nd1−xCaxMnO3

Abstract Magnetic, anisotropic, and magnetoelastic properties have been investigated for Nd1−xCaxMnO3 monocrystals (x=0; 0.25; 0.05; 0.3) in pulsed magnetic fields up to 250 kOe in temperature range 10– 100 K . Jumps of magnetization and magnetostriction induced by the strong magnetic field H||b have been observed at H>100 kOe in the x=0, 0.25, 0.05 compounds. We attribute this phenomena to a reorientation of the weak ferromagnetic moment from c- to b-axis. Corresponding H–T phase diagrams were obtained.

Magnetoelastic anomalies at spontaneous and field-induced transitions in La1−xSrxMnO3

Abstract Thermal expansion Δl/l(T) and longitudinal and transverse magnetostriction λ||,⊥(H,T) in pulsed magnetic fields H up to 250 kOe were studied in single La1−xSrxMnO3 crystals ( x=0.1, 0.125 and 0.15). A strong increase of the field-induced magnetostriction λ||,⊥(H) (from 10−4 up to 7×10−4) was observed in the Jahn–Teller distorted O′ phase as compared to the charge-ordered and pseudocubic ones, which was accompanied by additional features in the vicinity of the spontaneous transitions. The results may be related to a suppression of the O′ phase and field-induced transitions to a new orbital-ordered ferromagnetic state.

Low-temperature phase transition in EuMn2O5 induced by a strong magnetic field

Abstract The magnetostriction, magnetoelectric effect and phase transitions induced by a strong magnetic field in EuMn 2 O 5 single crystal have been investigated. It is known that EuMn 2 O 5 compounds show structural and magnetic phase transitions and incommensurate magnetic structure around 40 K. New phase transitions from a modulated spin structure to a homogeneous structure in EuMn 2 O 5 at 26 K accompanied by a significant change of electrical polarization induced by a high magnetic field have been studied.

On the peculiarities of the temperature dependence of the magnetoelectric susceptibility of chromium oxide

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. Its 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.