S. S. Krotov

Magnetoelectric effects in gadolinium iron borate GdFe3(BO3)4

Magnetoelectric interactions have been investigated in a single crystal of gadolinium iron borate GdFe3(BO3)4, whose macroscopic symmetry is characterized by the crystal class 32. Using the results of this study, the interplay of magnetic and electric orderings occurring in the system has been experimentally revealed and theoretically substantiated. The electric polarization and magnetostriction of this material that arise in spin-reorientation transitions induced by a magnetic field have been investigated experimentally. For H ‖ c and H ⊥ c, H-T phase diagrams have been constructed, and a strict correlation between the changes in the magnetoelectric and magnetoelastic properties in the observed phase transitions has been ascertained. A mechanism of specific noncollinear antiferroelectric ordering at the structural phase transition point was proposed to interpret the magnetoelectric behavior of the system within the framework of the symmetry approach in the entire temperature range. This ordering provides the conservation of the crystal class of the system when the temperature decreases to the antiferroelectric ordering point. The expressions that have been obtained for the magnetoelectric and magnetoelastic energy describe reasonably well the behavior of gadolinium iron borate observed experimentally.

Magnetoelectric and magnetoelastic properties of rare-earth ferroborates

The magnetic, electric, magnetoelectric, and magnetoelastic properties of rare-earth ferroborates RFe3(BO3)4 (R=Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er) as well as yttrium ferroborate YFe3(BO3)4 have been studied comprehensively. A strong dependence not only of the magnetic but also magnetoelectric properties on the type of rare-earth ion, specifically, on its anisotropy, which determines the magnetic structure and the large contribution to the electric polarization, has been found. This is manifested in the strong temperature dependence of the polarization below the Neel point TN and its specific field dependence, which is determined by the competition between the external and exchange f-d fields. A close correlation has been found between the magnetoelastic properties of ferroborates and the magnetoelastic and magnetic anomalies at magnetic-field induced phase transitions. It is found that in easy-plane ferroborates, together with magnetic-field induced electric polarization spontaneous polarization also ari...

Features of the magnetoelectric properties of BiFeO3 in high magnetic fields

It is shown that the destruction of the cycloidal structure of the magnetic ferroelectric BiFeO3 by a high magnetic field (Hn≈200 kOe) leads to the onset of a linear magnetoelectric effect and the appearance of a toroidal moment. The proof of the existence of a toroidal moment T in a high magnetic field (H>Hn) is based on the experimental observation that the off-diagonal components of the linear ME effect tensor are asymmetric (α12=−α21 for L‖c, where L is the antiferromagnetic vector), inasmuch as Tz∼α12−α21.

Features of the magnetoelectric behavior of the family of multiferroics RMn2O5 at high magnetic fields (Review)

The family of multiferroics comprised of the orthorhombic manganates RMn2O5 (R=Eu,Gd,Er,Y), in which the coexistence of antiferromagnetism and ferroelectricity has been reported previously, is investigated at high magnetic fields. These compounds, unlike the members of the family RMnO3 (where R=Eu,Ge,Tb,Dy) have two subsystems of magnetic mixed-valence d ions Mn3+ and Mn4+, the direct and indirect interactions between which, being of the ferro- or antiferromagnetic type, depending on the particulars of the environment and properties of the rare-earth ions, enhance substantially the role of the frustrations observed in RMnO3 compounds. These systems, as a consequence of the specific combination of the additional magnetic degeneracy realized in them (due to competition between nearest- and next-nearest-neighbor interactions of nearly equal magnitude) and their strong magnetoelastic coupling, display a cascade of magnetic phase transitions, with the appearance/disappearance of incommensurate (modulated) magn...

Magnetic and structural phase transitions in YMn2O5 ferromagnetoelectric crystals induced by a strong magnetic field

Magnetic, magnetoelectric, and magnetoelastic properties of YMn2O5 ferromagnetoelectric single-crystals are investigated in strong pulsed magnetic fields of up to 250 kOe and in static magnetic fields of up to 12 kOe. It is found that, in YMn2O5 at T < TN=42 K, a transverse weakly ferromagnetic moment of σ 0=0.8 G cm3/g exists that is oriented along axis a and is attributed to the magnetoelectric interaction. When a magnetic field is directed along axis b, which is likely to be the axis of antiferromagnetism, a spin-flop transition is observed that is accompanied by jumps in magnetostriction and electric polarization. When a magnetic field is directed along axis a, the temperature of ferroelectric transition shifts from 20 to 25 K at H≈200 kOe. A theoretical analysis of the experimental results is given within phenomenological theory with regard to the fact that a YMn2O5 compound belongs to noncollinear antiferromagnetic crystals even in the exchange approximation.

Attraction of inclined vortices in magnetic superconductors

Abstract It is shown that in magnetic superconductors with easy axis magnetic anisotropy the flux lines attract each other at large distances if they are inclined with respect to the anisotropy axis. The lower critical field H C1 corresponds to the penetration of a vortex chain into the sample rather than a single vortex.

Investigation of the anomalies of the magnetoelectric and magnetoelastic properties of single crystals of the ferroborate GdFe3(BO3)(4) at phase transitions

The transformation of various properties of gadolinium ferroborate single crystals at phase transitions, both spontaneous and induced by magnetic fields up to 200 kOe, is investigated theoretically and experimentally. Particular attention is paid to elucidating the role of magnetoelectric interactions and the change in them at spin-reorientation transitions accompanied by a change of magnetic symmetry. With that goal the magnetoelastic and magnetoelectric properties of the system are investigated over a wide range of temperatures for two orientations of the magnetic field, H∥c and H⊥c, and a fundamental difference of the character of the field dependences of the magnetostriction and electric polarization is found. In the framework of a symmetry approach a description of the magnetic structures and their transformations in the system GdFe3(BO3)4 is proposed, and an interpretation of the experimentally observed properties is given.

Effect of Gd-Mn exchange on phase transitions in GdMn2O5 induced by a strong magnetic field

Complex studies of the magnetic, magnetoelectric, and magnetoelastic properties of GdMn2O5 single crystals in strong pulsed magnetic fields are carried out in order to obtain additional indirect information on the character of the rare-earth and manganese spin ordering. It is shown that magnetic ordering of Gd3+ spins affects the manganese sublattice spin orientation and initiates new magnetic phase transitions. The observed magnetoelectric properties of the GdMn2O5 system are interpreted in terms of the theory of phase transitions.

The possible mechanisms of phase transitions in RMn2O5 oxide family

The magnetic, magnetoelectric, and magnetoelastic properties of ErMn2O5 single crystals have been studied at low temperatures and strong magnetic fields (up to 250 kOe) and compared to the analogous results obtained previously for YMn2O5. Based on these data, the possible mechanisms of various spontaneous and magnetic-field-induced phase transitions in these compounds are considered within the framework of the theory of representations of the Pbam-D2h9 space group. It is shown that a biquadratic exchange plays an important role in the formation (and mutual transformation) of magnetic structures revealed by neutron diffraction in the RMn2O5 oxide family.

Influence of metamagnetic transition on the vortex phase in antiferromagnetic superconductors

Abstract The metamagnetic (or spin-flop) transition which is experimentally observed in certain antiferromagnetic superconductors results in the formation of magnetic domain structure if the demagnetization factor of the sample is nonzero. It is demonstrated that such structure leads to the modulation of vortex lattice density which is maximal in the domains of spin-flop phase. The structure of a vortex with a ferrimagnetic domain near its core is also investigated. The results of calculations show that under appropriate conditions the interplay between superconductivity and metamagnetism makes the formation of peculiar two-quanta vortices energetically favourable.