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Abstract: The EPR spectrum of a KDy(WO4)2 monoclinic crystal is investigated. It is found that the EPR spectrum of magnetically concentrated materials at a low frequency (9.2 GHz) undergoes a substantial transformation in addition to the well-known broadening of the EPR lines. At low Dy3+ concentrations (x<10−2), the EPR spectrum of an isomorphic crystal, namely, KY(1−x)Dyx(WO4)2, is characterized by the parameters g x=0, g y=1.54, and g z=14.6. For a magnetically concentrated crystal KDy(WO4)2, the g values are as follows: g x=0, g y=0.82, and g z=2.52. It is demonstrated that the difference in the parameters is associated with the specific spin-spin interaction between Dy3+ ions, including the Dzyaloshinski interaction, which is not observed at high frequencies.

Abstract: The investigation of the specific heat of a RbDy(WO4)2 single crystal at temperatures 0.2–2.5 K and in magnetic fields up to 2 T are reported. The temperature dependence of the specific heat near T N=0.818 K is compared with the predictions for different models. The 2D Ising model describes satisfactorily C(T) below T N, while for T>T N none of the theoretical models agree with the behavior of C(T) of RbDy(WO4)2. The H-T phase diagram for H∥c is complicated and possesses a triple point, where regions of existence of three magnetic phases converge. The magnetic ordering is analyzed from the standpoint of the Jahn-Teller nature of the structural phase transitions occurring in RbDy(WO4)2 at higher temperatures. It is shown that the form of the phase diagram depends on the direction of the vector H, for the general case of an arbitrary direction of H, two phase transitions can occur with increasing field.

Abstract: The low-temperature thermal and magnetic-resonance properties of a monoclinic KDy(WO4)2 single crystal are investigated. It is established that a structural phase transition takes place at T c=6.38 K. The field dependence of the critical temperature is determined for a magnetic field oriented along the crystallographic a and c axes. The initial part of the H-T phase diagram is plotted for H∥a. The prominent features of the structural phase transition are typical of a second-order Jahn-Teller transition, which is not accompanied by any change in the symmetry of the crystal lattice in the low-temperature phase. The behavior of C(T) in a magnetic field shows that the transition goes to an antiferrodistortion phase. An anomalous increase in the relaxation time (by almost an order of magnitude) following a thermal pulse is observed at T>T c(H), owing to the structural instability of the lattice. A theoretical model is proposed for the structural phase transition in a magnetic field, and the magnetic-field dependence of T c is investigated for various directions of the field.