On the Origin of the Anisotropy Energy of CuCl2-2H2O

Abstract

The anisotropy energy of CuCl2-2H2O having an orthorhombic symmetry can be expressed by K1β2 + K2γ2, where β and γ are the direction cosines of the magnetic moment with respect to the b-axis and the c-axis. The calculated values of K2 arising from the magnetic dipole interaction nearly accords with the experimental value, but that of K1 becomes much smaller. Since the magnitude of the spin of CuCl2-2H2O is equal to 1/2, the crystalline field can not produce any anisotropy energy, so that we must consider that the other part of the anisotropy energy than that from the magnetic dipole interaction arises from the anisotropic exchange interaction which has originally been proposed by Van Vleck. Therefore, we derive the anisotropic exchange interaction on the basis of the perturbation method and estimate the anisotropy constants K1 and K2 due to this interaction by use of the experimental g-values which are relating to the energy differences between the ground state and the excited states of Cu2+ ions. Thus it is shown that the sum of theoretical anisotropy constants due to both origins gives the values of the same order with that of the experimental values.