Microscopic Model for Reorientation of the Easy Axis of Magnetization

We present a microscopic model to describe the reorientation of the easy axis of magnetization from one symmetry axis of the crystal to another, which is known to occur in a number of magnetic insulators. We find that the easy axis either rotates continuously with temperature, beginning at T₁ and ending at a higher temperature T₂, or jumps discontinuously with associated hysteresis effects. In the former case, we have calculated the temperature dependence of the easy direction of magnetization, and the behavior of the specific heat and correlation lengths in the vicinity of T₁ and T₂. Measurement of T₁ and T₂ allows the evaluation of the ratios K₁K₄ and K₂K₄, where K₁, K₂, and K₄ give the strength of the pseudodipolar spin interaction, and the second- and fourth-order single-ion anisotropy terms in the spin Hamiltonian, respectively.