A. L. Chernyshev, M. E. Zhitomirsky, N. Martin, L. -P. Regnault
We demonstrate that local modulations of magnetic couplings have a profound effect on temperature dependence of the relaxation rate of optical magnons in a wide class of antiferromagnets in which gapped excitations coexist with acoustic spin waves. In a two-dimensional collinear antiferromagnet with an easy-plane anisotropy, disorder-induced relaxation rate of the gapped mode, Gamma_imp=Gamma_0+A(TlnT)^2, greatly exceeds magnon-magnon damping, Gamma_m-m=BT^5, negligible at low temperatures. We measure the lifetime of gapped magnons in a prototype XY antiferromagnet BaNi2(PO4)2 using high-resolution neutron-resonance spin-echo technique and find experimental data in a close accord with the theoretical prediction. Similarly strong effects of disorder in three-dimensional case and in non-collinear antiferromagnets are discussed.
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http://arxiv.org/abs/1206.4690
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