Magnetic anisotropy Ising-type

If the Hund rule of maximum spin is fulfilled, the above-mentioned 3d-levels are consistent with the experimental fact that the magnetic anisotropy of Fe ions is so large that they can be regarded, along the c-axis, as Ising spins. On the other hand, the Co ions do not seem so anisotropic magnetically (see sect. 6.2.4). Another evidence of the level scheme is the absence of the (1, l)-type... 

An important aspect of one-dimensional magnetism is the nature of the magnetic interaction between the spins. It can be isotropic, as described by the spin Hamiltonian Equation (6), or anisotropic. In the axial limit the anisotropy can be of the easy-axis type (Ising anisotropy) or of the easy-plane type (XY anisotropy). The former means that the spins tend to orient parallel to a preferential direction while for the latter they orient in a plane. The physical properties of the chains are strongly influenced by the nature of the anisotropy. ... 

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