Mykola F. Kharchenko

The article presents the results of experimental studies of the anisotropic properties of two unconventional magneto-optical effects: the quadratic magnetic rotation of the plane of polarization of light (quadratic Faraday effect) and the linear magneto-optical effect (magnetic analogue of the electro-optical Pockels effect) in the antiferromagnetic tetragonal garnet crystal Ca3Mn2Ge3O12. The results obtained indicate that the symmetry of the spontaneously emerging antiferromagnetic phase differs from that recognized nowadays. In addition, it was found that when cooling the crystal in a magnetic field applied in a certain direction, it is possible to create a metastable antiferromagnetic state that can exist even in the absence of the field.

Journal of Nano- and Electronic Physics. Scientific journal. ISSN: 2077-6772. Journal abbreviation: J. Nano- Electron. Phys. No page charges. All articles are freely available on-line. Issued 4 times per year. Publisher: Sumy State University. (Sumy, Ukraine)

A study of the differential magnetic susceptibility and electric polarization of an antiferromagnetic LiCoPO₄ crystal in a pulsed magnetic field with an intensity of up to 290 kOe, directed along the antiferromagnetism vector of the spin-ordering main mode (Hǁb), at initial sample temperatures between 1.6 and 20.8 K. An adiabatic (H, T_i)-diagram of magnetic phase transitions is constructed. In addition to the three transitions that were discovered earlier at helium temperatures, higher temperatures revealed singularities that indicate the existence of a first-order phase transition line in the high-field phase II, which ends at a critical point. The coordinates of the critical point at which the lines of the phase transitions bordering the region of existence of high-field phase III converge with the line of the phase transitions from phase II to the saturated paramagnetic phase, are also determined.

The acoustoelectric transformation in single-crystal LiCoPO₄ is studied. At temperatures corresponding to the paramagnetic phase, a piezoresponse to longitudinal elastic deformation parallel to the crystallographic a axis is observed. Arguments are introduced in support of identifying this compound as pyroelectric, as opposed to the generally accepted viewpoint which assigns it to the class of centrally symmetric crystals.

The magnetoelectric effect in single-crystal antiferromagnetic LiNiPO₄ (T_N = 29.8 K) is studied in pulsed magnetic fields up to 28 T. The electric polarization along the crystallographic axis a induced by a magnetic field H||c is measured. It is found that electric polarization develops in a low-field phase I (fields from 0 to 13 T) and a high-field phase V (20–21.5 T). In the low-field phase, the electric polarization has both linear and pronounced cubic dependences on the field.

The magnetoelectric effect in single-crystal LiCoPO4 (TN = 21.8 K) is studied in strong pulsed magnetic fields which destroy the antiferromagnetic structure of the crystal spin ordering. The electric polarization along the crystallographic a axis induced by a magnetic field H||b is measured. New features of the electric polarization in a magnetic field are found, including a spike in the polarization near the field for the first magnetic transition at H1 = 123 kOe, a recovery of the electric polarization at the second magnetic transition H2 = 210 kOe, and a gradual reduction to zero on approaching the third transition at H3 = 263 kOe. Various possible magnetic structures in the high field phases are examined which are consistent with their magnetization and electric polarization. The observed linear dependence of the polarization on field strength in the initial antiferromagnetic phase and the vanishing of the polarization in the first high-field phase are in good agreement with previous studies of the magnetoelectric effect in LiCoPO4 [Wiegelmann et al., Ferroelectrics 161, 147 (1994); H. Wiegelman, Ph.D. thesis (University of Konstanz, Konstanz, 1995)].

The low-temperature behavior of optical and magneto-optical properties of microcavity 1D-MPCs with the Bi, Gd, Al-substituted YIG cavity layers of different compositions including compositions with a magnetic compensation temperatures were investigated. The optical transmittance and position of the resonant peaks in optical transmittance and the Faraday rotation spectra were found to be independent of temperature in all types of 1D-MPCs. The temperature dependence of changing the sign of Faraday rotation experienced the hysteresis with a value 8 K in one of two investigated MPCs with magnetic compensation temperature, while in the another MPC the hysteresis did not exceed 1 K. The possible origins of these properties are discussed.