Investigation of phase transitions in multiferroics HоFe3-xGax(BO3)4 and TbFe3-xGax(BO3)4 solid solution with huntite structure

RFBR funded the reported study according to the research project № 18-02-00754

Magnetic Phase Transitions in the NdFe_3(BO_3)_4 multiferroic

Low temperature studies of the behavior of the sound velocity and attenuation of acoustic modes have been performed on a single crystal NdFe_3(BO_3)_4. Transitions of the magnetic subsystem to the antiferromagnetically ordered state at T_N \approx 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 \parallel a and H \parallel b have been constructed. The phenomenological theory, which explains observed features, has been developed

Magnetization and specific heat of TbFe3(BO3)4: Experiment and crystal field calculations

We have studied the thermodynamic properties of single-crystalline TbFe3(BO3)4. Magnetization measurements have been carried out as a function of magnetic field (up to 50 T) and temperature up to 350K with the magnetic field both parallel and perpendicular to the trigonal c-axis of the crystal. The specific heat has been measured in the temperature range 2-300K with a magnetic field up to 9 T applied parallel to the c-axis. The data indicate a structural phase transition at 192 K and antiferromagnetic spin ordering at 40 K. A Schottky anomaly is present in the specific heat data around 20 K, arising due to two low-lying energy levels of the Tb3+ ions being split by f-d coupling. Below TN magnetic fields parallel to the c-axis drive a spin-flop phase transition, which is associated with a large magnetization jump. The highly anisotropic character of the magnetic susceptibility is ascribed mainly to the Ising-like behavior of the Tb3+ ions in the trigonal crystal field. We describe our results in the framework of an unified approach which is based on mean-field approximation and crystal-field calculations.Comment: 10 pages, 10 figures, 20 references, accepted by Phys. Rev.

Magnetoelectricity in the system RAl3(BO3)4RAl_3(BO_3)_4 (RR = Tb, Ho, Er, Tm)

The magnetoelectric effect in the system $RAl_3(BO_3)_4$ ($R$ = Tb, Ho, Er, Tm) is investigated between 3 K and room temperature and at magnetic fields up to 70 kOe. We show a systematic increase of the magnetoelectric effect with decreasing magnetic anisotropy of the rare earth moment. A giant magnetoelectric polarization is found in the magnetically (nearly) isotropic $HoAl_3(BO_3)_4$. The polarization value in transverse field geometry at 70 kOe reaches 3600 $\mu C/m^2$ which is significantly higher than reported values for the field-induced polarization of linear magnetoelectric or even multiferroic compounds. The results indicate a very strong coupling of the f-moments to the lattice. They further indicate the importance of the field-induced ionic displacements in the unit cell resulting in a polar distortion and a change in symmetry on a microscopic scale. The system $RAl_3(BO_3)_4$ could be interesting for the technological utilization of the high-field magnetoelectric effect.Comment: 6 pages, 4 figure

Soft modes in HoFe2.5Ga0.5(BO3)4 solid solution

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.The condensation of two soft modes has been found when studying the Raman spectra of the solid solution HoFe2.5Ga0.5(BO3)(4) in the temperature range from 7 to 350 K. The first high-temperature soft mode is associated with the structural phase transition from the R32 phase to the P3(1)21 phase. The second soft mode is related to the reveal of the phonon-magnon interaction during magnetic ordering in the crystal. The temperatures of the structural phase transition T-1 = 266 K and the magnetic phase transition T-2 = 28 K are established. Experimentally interaction between the structural phase transition order parameter fluctuations and the magnetic order parameter fluctuations was found