MICROSCOPIC MODEL FOR REORIENTATION OF 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. © 1969 The American Physical Society.