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

dipole intera~tion arises from the anisotropic exchange interaction which has original,ly been proposed by Van Vleck. Therefore, we derive the anisotropic exchange interaction on the basis of the perturbation method and estimate the anisotropy constants KI and K2 due to this interaction by use of the ex perimental 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 vatues of the same order with that of the experimental values. The anisotropy energy in an antiferromagnetic substance may be considered to consist of three parts: the first part arises from interactions of individual magnetic ions with their surrounding anisotropic crystalline field, the second part from magnetic dipole interactions between pairs of ions and the third part from the anisotropic exchange interactions between them. Magnetic dipole interactions do not produce any anisotropy in the first order ap proximation for crystals of cubic symmetry and the anisotropy arising from the crystalline field vanishes for crystals whose magnetic ions have a spin of 1/2. According to 'the calculation by Keffer,l)' the anisotropy energy of MnF2 having a crystal structure of rutile type arises mainly from the magnetic dipole interaction which makes spins point in the c-axis and those parts from the anisotropy of the crystalline field and the anisotropic exchange interaction amount only to ten percents of the experimental value. In the case of Mn02, however, which is similar to MnF2 in the crystal structure but different from it in the superstructure due to the antiferromagnetic spin orientation, the magnetic dipole interaction keeps the direction of spins in the plane perpendicular to the c-axis, but this interaction does not contribute to the anisotropy in this plane which may be considered to be arising from the crystalline field. 2) Therefore, both anisotropy from the magnetic dipole interaction and that from the anisotropy of the crystalline field are dominant in this substance. Thus, which is predominant among above mentioned three parts of anisotropy depends not only upon the crystal structure, but also upon the