Magnetic ordering of Dy3+ ions in RbDy(WO4)2 single crystal

The specific heat C(T) of the monoclinic RbDy(WO4)(2) crystal has been studied at very low temperatures 0.2 less than or equal to T less than or equal to 1.9 K and in magnetic fields 0 less than or equal to H less than or equal to 0.38 T. The Neel temperature was shown to be equal to T-N = 0.818 +/- 0. 005 K. The experimental value of the effective exchange parameter was obtained to be equal to J/k = -0.798 K. The C(T) dependence below Neel temperature (0.5T(N) < T < 0.99T(N)) is well described by 2D Ising model, whereas in the temperature region close above T-N (1.01T(N) < T < 2T(N)) it can be described by neither 2D, nor 3D Ising model. The experimental and theoretical H-T-N diagrams for field H parallel to a are in a reasonable agreement for simple quadratic lattice.