LowTemperature Phase Transitions in Terbium
Abstract
Tb_{rm x}Dy _{1rm x} is probably the simplest example of a magnetic system with competing anisotropy. Tb and Dy are both easyplane ferromagnets at low temperature, with well characterized anisotropy energies, such that Tb moments prefer to lie along the vec b axis in the hcp structure, while Dy moments prefer the vec a axis. It has been predicted, using a twosubnetwork "average crystal" model, that the lowtemperature phase of the alloys with x between 0.86 and 0.76 exhibits a non symmetry magnetization direction, a noncollinear spin structure, and twelfthorder anisotropy. However, the approximation is suspect when used to describe magnetic structure in alloys near their phase boundaries, such as those between the ferromagnetic and the noncollinear phase. Using a more realistic probability distribution for site occupation, the fluctuations of the spin orientation and the spinspin correlation function have been calculated as a function of composition in the noncollinear phase. It was found that the mean square fluctuation in spin orientation is proportional to sqrt {rm x_{b}x} where x_{rm b} is the critical concentration which separates the vec b axis ferromagnet from the noncollinear magnet. The average orientation as calculated from the average crystal model also varies as sqrt {rm x_{b}x}. Therefore, the simple twosubnetwork model is not valid, at least near the phase boundaries. Upon including the fluctuations in the calculation of the phase boundaries, it has been found that the fluctuations from the average site occupation always increase the range of stability of the noncollinear phase. These characteristics, (1) a noncollinear phase space and (2) a widening of this phase space with the inclusion of fluctuations, should be valid for other similar alloy systems, and therefore the model should be useful in predicting the lowtemperature phase characteristics of a variety of binary alloys, and in modifying previous calculations based only on the average crystal model.
 Publication:

Ph.D. Thesis
 Pub Date:
 March 1990
 Bibcode:
 1990PhDT........66F
 Keywords:

 X)DYSPROSIUM(1 X) COMPETING ANISOTROPY ALLOYS (TERBIUM;
 DYSPROSIUM;
 Physics: Condensed Matter