Noncollinear phase of TbDy alloys

Abstract

TbxDy1−x is probably the simplest example of a magnetic system with competing anisotropy. Tb and Dy are both low-temperature easy-plane ferromagnets with well-characterized anisotropy energies, such that Tb moments prefer the b axis in the hcp structure, while Dy moments prefer the a axis. It has been predicted, by a two-subnetwork approximation, that the low-temperature phase of the alloys with x between 0.86 and 0.76 exhibits a nonsymmetry direction of magnetization, a noncollinear spin structure, and 12th-order anisotropy. However, the two-subnetwork approximation is suspect when used to describe magnetic structure in alloys near phase boundaries, such as those between the ferromagnetic and the noncollinear phase. Using a probability distribution for site occupation, we have calculated the fluctuations of the spin orientation and the spin-spin correlation function as a function of composition in the noncollinear phase. The mean-square fluctuation in spin orientation is proportional to (xb−x)1/2, where xb is the critical concentration which separates the b axis ferromagnet from the noncollinear magnet. The average orientation also varies as (xb−x)1/2. Therefore, the simple two-subnetwork model is not valid near the phase boundaries. In fact, we find that the fluctuations from the average site occupation increase the range of stability of the noncollinear phase.