Poperezhai S. Spectral features of spin and phonon excitations in double alkali-rareearth molybdates and erbium alumoborate

The dissertation is devoted to identify the features of resonance and thermal properties, effects of short-range magnetic ordering in double potassium-thulium molybdate KTm(MoO4)2 and erbium aluminoborate ErAl3(BO3)4, to establish the origin of low-frequency vibrational spectrum for the potassium subfamily of double molybdates KR(MoO4)2 (R = Y, Dy, Er, Tm, Yb). The results of experimental study of the low-frequency vibrational spectrum in double alkali-rareearth molybdate crystals are presented and the mechanisms of its formation has been described. For describing the dispersion of low-frequency vibrational branches in these compounds the one-dimensional model of a chain with two masses and two elastic constants was developed. Within the framework of the proposed model using frequencies of the vibrational modes elastic constants, the values of the sound velosities and Raman frequencies were found, the vibrational spectrum in the Brillouin zone was numerically calculated and the parameters of damping of vibrations in crystals were qualitatively estimated. The calculated Raman frequencies and the sound velocities are in good agreement with the experimental data. Finally the IR transmission spectrum was calculated by choosing the values of the attenuation parameters. The calculated spectrum is consistent with the measured one. A triple EPR structure of the resonance absorption of the Tm3+ ion was found in KTm(MoO4)2 crystal. The triple structure is observed in the frequency range of 7-21 см-1 in magnetic fields 0.8-3.2 T. With increasing of the external magnetic field the interval between sattellites does not changes, but the sattellites amplitudes decrease. The obtained results are interpreted within the framework of the cluster resonance model. The coupling constant of neighbor Tm3+ ions in magnetic chains is estimated Jc ≈ 1.54 K. Manifestations of spin correlations between the Er3+ ions are revealed in the ErAl3(BO3)4 single crystal. An additional line with weak intensity was found in EPR spectra beside to the main line associated with transition inside the main doublet of the Er3+ ion. The g-factor of the additional line is approximately twice higher then g-factor of the main one. Moreover a triple structure of the main resonance line is observed when the external magnetic field is inclined by an angle from 2° to 8° direction from the c axis of the crystal. The temperature dependence of the specific heat capacity has a minimum at Т ≈ 4.5 K, which clearly indicates the existence of a magnetic contribution to the heat capacity of the sample at low temperatures. Within the framework of the proposed anisotropic model of paired clusters, the magnetic contribution to the heat capacity is described and the parameters of the interaction of the spins of Er3+ ions are estimated.