Microscopic approach to macroscopic dynamics in XY spin glasses.
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Recently it has become possible to perform experimental studies on spin glasses with strong anisotropy forcing the spins into a plane. We have undertaken a theoretical investigation of the macroscopic dynamical behavior of such systems, beginning with a determination (at the microscopic level) of the appropriate macroscopic variables. Classical spins are considered at T=0. In the absence of weak random anisotropy in the plane, we predict spin waves with a linear dispersion and derive an expression for their velocity. When weak random anisotropy in the plane is included, a microscopic calculation shows a gap at zero wave vector. This uniform mode has no field dependence for a static field normal to the plane, and therefore would be difficult to observe using electron-spin resonance; however, the following paper shows that, in the presence of a remanence induced by a cooling field, the resonance frequency becomes field dependent for a static field in the plane. A formal expression for the in-plane macroscopic anisotropy constant is derived, employing both microscopic and macroscopic considerations. Assuming bilinear microscopic anisotropy, there is only uniaxial macroscopic in-plane anisotropy (to which Dzyaloshinsky-Moriya anisotropy does not contribute). A macroscopic study is made of the normal modes including both exchange and random anisotropy. 1986 The American Physical Society.
