Ferromagnets with weak random anisotropy.
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We consider a continuous-symmetry ferromagnet for d=3 with random anisotropy which is weak compared to exchange (Hr Hex), in the presence of an external field H. At low fields the system is macroscopically disordered (the Imry-Ma or correlated spin-glass regime), and the external field may be treated as a perturbation for H Hr4/Hex3. For large fields the system is basically aligned, and the random anisotropy may be treated as a perturbation, thus leading to a unique state with a large but wandering magnetic moment (ferromagnet with wandering axis, or FWA). We show that the system retains its alignment only for H Hr2/Hex. Thus we deduce that there is an intermediate field regime which is well aligned but not accessible to perturbation theory. In addition, we consider a number of questions pertaining to the macroscopic collective modes of this system. We point out that the prediction of a longitudinal resonance depends upon a perturbation approach which may not be valid. Also, we observe that, even in H=0, samples small compared to the correlation length will have a spontaneous moment, therefore defeating the disordering effect of the random anisotropy. For well-aligned systems with random anisotropy (with the alignment produced either by an external field or by the fine particle effect, or both), the predicted longitudinal resonance, assumed to be valid, can be obtained solely in terms of the transverse resonance shift, the magnetization, and the differential susceptibility. For the FWA we explicitly calculate the anisotropy constants for uniform rotations, and we apply them to electron spin resonance. 1987 The American Physical Society.
