I. V. Bilych

Low-temperature behavior of the magnetoelastic characteristics of praseodymium ferroborate

The behavior of the elastic moduli and sound absorption in a PrFe3(BO3)4 single crystal at low temperatures is studied. A transition of the magnetic subsystem into an antiferromagnetically ordered state is manifested in the temperature behavior of the sound velocities and absorption. The characteristic behavior of the elastic properties of PrFe3(BO3)4 in an external magnetic field is observed. A phenomenological theory that gives a qualitative description of the observed features is constructed. It is proposed that a weak magnetic moment exists in the crystal.

Magnetic phase transitions in the NdFe3(BO3)4 multiferroic

Low-temperature studies of the behavior of the sound velocity and attenuation of acoustic modes have been performed on a single crystal NdFe3(BO3)4 Transitions of the magnetic subsystem to the antiferromagnetically ordered state at TN ≈ 30.6 K have been revealed in the temperature behavior of the elastic characteristics. The features in the temperature behavior of elastic characteristics of the neodymium ferroborate and its behavior in the external magnetic field, applied in the basic plane of the crystal, permit us to suppose that the transition to an incommensurate spiral phase is realized in the system. This phase transition behaves as the first order one. H–T phase diagrams for the cases H ∥ a and H ∥ b have been constructed. The phenomenological theory, which explains observed features, has been developed.

Low-temperature phase transitions in the rare-earth ferroborate Nd0.75Dy0.25Fe3(BO3)4

The low-temperature behavior of the elastic characteristics of a Nd0.75Dy0.25Fe3(BO3)4 single crystal has been studied. Features are found in the temperature and magnetic-field dependences of the velocity and absorption of transverse sound. These features are interpreted as being a manifestation of magnetic phase transitions in the compound studied. The H‐T phase diagram is constructed.

Magnetoelastic effects in terbium ferroborate

The behavior of the elastic moduli and sound absorption in a terbium orthoborate single crystal at low temperatures is studied. The components of the tensor of the elastic moduli of this system are determined. A structural phase transition and a transition of the magnetic subsystem into an antiferromagnetically ordered state appear in the temperature dependences of the sound velocities and absorption. The magnetic field dependences of the velocities of transverse sound exhibit singularities in the form of jumps at a magnetic field equal to the field of the spin-flop transition of the antiferromagnetic subsystem. Theoretical analysis shows that the observed behavior of the acoustic characteristics are associated not with the rare-earth subsystem of ferroborate but rather with the renormalization of the exchange interaction between iron ions as a result of the magnetoelastic coupling.

Elastic and piezoelectric moduli of Nd and Sm ferroborates

The sound speeds are measured and the elastic and piezoelectric moduli are calculated for single crystal NdFe3(BO3)4 and SmFe3(BO3)4. These compounds are characterized by enhanced rigidity in the base plane with respect to stress-strain deformations and by a rather strong piezoelectric effect.

Magnetic anisotropy in the basal plane of the rare-earth ferroborate Nd0.75Dy0.25Fe3(BO3)4

The magneto-acoustic behavior of the rare-earth multiferroic Nd0.75Dy0.25Fe3(BO3)4 in external magnetic fields lying in the basal plane of the crystal is studied. The data confirm the existence of a magnetic anisotropy in this plane and of magnetic phase transitions in the crystal caused by this anisotropy. H-T phase diagrams are constructed for the cases H||C2 and H⊥C2.

Magnetic field-induced phase transitions in the antiferromagnet Nd0.6Dy0.4Fe3(BO3)4

Low-temperature studies of elastic and magnetic characteristics of the single crystal Nd0.6Dy0.4Fe3(BO3)4 have been performed. A transition to the antiferromagnetically ordered state of the magnetic subsystem has been manifested in the temperature behavior of the velocity and attenuation of acoustic modes and magnetization. Spin-reorientation phase transitions, which reveal themselves as anomalies in the behavior of elastic and magnetic characteristics of the crystal in the external magnetic field applied along the axis of the trigonal symmetry of the crystal have been discovered. The phase H–T diagram for H ‖ C3 has been constructed.

Reentrant low-temperature phase transition in an “orbital nematic”

The influence of an external magnetic field on the Jahn–Teller phase transition at Tc ≈ 14 K was studied in a KDy(MoO4)2 single crystal. At temperatures below Tc a significant change in the velocity and attenuation of the acoustic modes of the crystal was observed for the magnetic field oriented along the [110] direction. This behavior of the elastic characteristics occurs due to the destruction of the antiferrodistortive ordering, which is, in turn, coupled with the ordering of the quadrupole orbital moments, characteristic for an “orbital nematic.” The low-temperature region of the H–T phase diagram for this direction of the field was reconstructed. It was found that the magnetic field dependence of the critical temperature is nonmonotonic, i.e., the phase transition has the reentrant character. Theoretical description of the observed anomalies of the temperature and magnetic field behavior of the acoustic characteristics was given.

Piezoelectric response in SmFe3(BO3)4, a non-piezoactive configuration. The surface piezoelectric effect

An investigation of the mechanisms responsible for the manifestation of the piezoelectric effect in configurations that should not allow for piezoelectric response in the paramagnetic state due to their symmetry. In spite of these symmetry constraints the existence of such a piezoelectric response is detected in this study. It is assumed that these results are associated with the surface on which symmetry constraints are absent. In the magnetically ordered state an indirect piezoelectric effect is both symmetrically permissible and actually observable in these configurations, and it consists of the combined effects of magnetoelectric and magnetoelastic mechanisms.

Magnetodielectrical and magnetopiezoelectrical effects in NdFe3(BO3)4

Magnetodielectric (magnetocapacitance) and magnetopiezoelectric effects in neodymium ferroborate are systematically investigated. The existence of a previously observed magnetopiezoelectric effect in samarium ferroborate is confirmed. This effect consists in indirect renormalization of the piezoelectric modulus as a result of the combined action of the magnetoelectric and magnetoelastic mechanisms. The manifestation of a new type of phenomena—direct renormalization of the piezoelectric interaction in a magnetically ordered phase—is observed. The ratio of the contributions of the rare-earth and iron systems to the magnetoelectric interaction is estimated. It is shown that the latter plays a ballast role, impeding orientation fluctuations of the magnetic structure as a whole. The boundaries of existence of the helicoidal phase on the H-T plane are determined. The phenomenological interpretation of the studied effects is given.