Magnetic ordering of Dy super(3+) ions in RbDy(WO sub(4)) sub(2) single crystal

The specific heat C(T) of the monoclinic RbDy(WO sub(4)) sub(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 sub(N) identical with 0.818 plus or minus 0.005 K. The experimental value of the effective exchange parameter was obtained to be equal to J/k identical with -0.798 K. The C(T) dependence below Neel temperature (0.5 T sub(N) < T < 0.99T sub(N)) is well described by 2D Ising model, whereas in the temperature region close above T sub(N) (1.01 T sub(N) < T < 2T sub(N)) it can be described by neither 2D, nor 3D Ising model. The experimental and theoretical H-T sub(N) diagrams for field H parallel a are in a reasonable agreement for simple quadratic lattice.