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Polarized absorption spectra and magnetic circular dichroism (MCD) spectra of Nd0.5Gd0.5Fe3(BO3)4 single crystal were measured in the range of 10000 - 21000 cm-1. The absorption spectra were studied as a function of temperature in the range of 2 - 293 K and as a function of magnetic field 0 - 65 kOe at 2 K. The excited 4f states of the Nd3+ ion were identified in terms of the irreducible representations and in terms of |J,+-MJ> wave functions of the free atom. The changes of the Lande factor during f-f transitions and the exchange splitting of the ground and excited states were determined. It was found out that the selection rules for electron transitions substantially changed in the magnetically ordered state of the crystal. They are different for the different transitions and they strongly depend on the orientation of the Fe sublattice magnetic moment relative to the light polarization. It was revealed that the local magnetic properties in the vicinity of the excited Nd3+ ion substantially depended on its state. In particular, in some excited states a weak ferromagnetic moment appears, the field induced reorientation magnetic transitions occur and the changes of the local magnetic anisotropy type take place. In some excited states the nonequivalent Nd3+ centers were found out., Comment: 24 pages, 41 figures

Polarized absorption spectra of single-crystal Nd0.5Gd0.5Fe3(BO3)4 were studied in the region of transition 4I9/2-(4G5/2+2G7/2) in Nd3+ ion as a function of temperature (2-34 K) and magnetic field (0-65 kOe). Spectra of natural circular dichroism were measured in the range of 5-40 K. It was found out that the local magnetic properties in vicinity of the excited ion substantially depend on its state. In particular, a weak ferromagnetic moment appears in some excited states. It was found out that selection rules for electron transitions in magnetically ordered state substantially deviate from those in paramagnetic state of the crystal. They are different for different transitions and they are very sensitive to orientation of the sublattice magnetic moment relative to the light polarization. In the spectrum of the natural circular dichroism, the transition is revealed which is not observed in the absorption spectrum., Comment: 17 pages, 25 figures

The influence of uniform pressures $P$ up to 5 kbar on the superconducting transition temperature $T_c$ was studied for the FeSe$_{1-x}$Te$_x$ ($x=0$, 0.85, 0.88 and 0.9) system. For the first time, we observed a change in sign of the pressure effect on $T_c$ when going from FeSe to tellurium rich alloys. This has allowed to specify the pressure derivative d$T_c$/d$P$ for the system as a function of composition. The observed dependence was compared with results of the {\em ab initio} calculations of electronic structure and magnetism of FeSe, FeTe and FeSe$_{0.5}$Te$_{0.5}$, and also with our recent experimental data on pressure effects on magnetic susceptibilities of FeSe and FeTe compounds in the normal state. This comparison demonstrates a competing interplay between superconductivity and magnetism in tellurium rich FeSe$_{1-x}$Te$_x$ compounds, Comment: 7 pages, 6 figures

A detailed magnetization study for the novel FeSe superconductor is carried out to investigate the behavior of the intrinsic magnetic susceptibility $\chi$ in the normal state with temperature and under hydrostatic pressure. The temperature dependencies of $\chi$ and its anisotropy $\Delta \chi=\chi_{|}-\chi_{\bot}$ are measured for FeSe single crystals in the temperature range 4.2-300 K, and a substantial growth of susceptibility with temperature is revealed. The observed anisotropy $\Delta \chi$ is very large and comparable with the averaged susceptibility at low temperatures. For a polycrystalline sample of FeSe, a significant pressure effect on $\chi$ is determined to be essentially dependent on temperature. Ab initio calculations of the pressure dependent electronic structure and magnetic susceptibility indicate that FeSe is close to magnetic instability with dominating enhanced spin paramagnetism. The calculated paramagnetic susceptibility exhibits a strong dependence on the unit cell volume and especially on the height $Z$ of chalcogen species from the Fe plane. The change of $Z$ under pressure determines a large positive pressure effect on $\chi$ which is observed at low temperatures. It is shown that the literature experimental data on the strong and nonmonotonic pressure dependence of the superconducting transition temperature in FeSe correlate qualitatively with calculated behavior of the density of electronic states at the Fermi level., Comment: 8 pages, 10 figures

The magnetic susceptibility and magnetization of high quality single crystal beta-TeVO4 are reported. We show that this compound, made of weakly coupled infinite chains of VO5 pyramids sharing corners, behaves as a S = 1/2 one-dimensional Heisenberg antiferromagnet. From magnetic experiments we deduce the intrachain antiferromagnetic coupling constant J/kB = 21.4 $\pm$ 0.2 K. Below 5 K a series of three phase transitions at 2.26, 3.28 and 4.65 K is observed., Comment: Accepted for publication in Physical Review B

The AFMR spectra of the NdFe$_3$(BO$_3$)$_4$ crystal are measured in a wide range of frequencies and temperatures. It is found that by the type of magnetic anisotropy the compound is an "easy-plane" antiferromagnet with a weak anisotropy in the basal plane. The effective magnetic parameters are determined: anisotropy fields $H_{a1}$=1.14 kOe and $H_{a2}$=60 kOe and magnetic excitation gaps $\Delta\nu_1$=101.9 GHz and $\Delta \nu_2$=23.8 GHz. It is shown that commensurate-incommensurate phase transition causes a shift in resonance field and a considerable change in absorption line width. At temperatures below 4.2 K nonlinear regimes of AFMR excitation at low microwave power levels are observed.

Long-lived photoinduced absorption and dichroism in the Ca_3Ga_{2-x}Mn_xGe_3O_{12} garnets with x < 0.06 were examined versus temperature and pumping intensity. Unusual features of the kinetics of photoinduced phenomena are indicative of the underlying electronic processes. The comparison with the case of Ca_3Mn_2Ge_3O_{12}, explored earlier by the authors, permits one to finally establish the main common mechanisms of photoinduced absorption and dichroism caused by random electric fields of photoproduced charges (hole polarons). The rate of their diffusion and relaxation through recombination is strongly influenced by the same fields, whose large statistical straggling is responsible for a broad continuous set of relaxation components (observed in the relaxation time range from 1 to about 1000 min). For Ca_3Ga_{2-x}Mn_xGe_3O_{12}, the time and temperature dependences of photoinduced absorption and dichroism bear a strong imprint of structure imperfection increasing with x., Comment: 20 pages, 10 figures