Long wavelength spin dynamics in diluted magnetic systems: Scaling of magnon lifetime

Abstract

Abstract Spin wave excitations in disordered magnetic systems have been one of the most widely studied fields in condensed matter physics for several decades. However, a careful and extensive search reveals a longstanding controversy on one important aspect, which is the wave-vector dependence of the spin wave intrinsic linewidth. We theoretically investigate the low-temperature spin wave excitations in disordered (diluted) ferromagnetic systems with a particular focus on the linewidth behavior in the long wavelength limit ( q → 0 ). The linewidth is extracted from a proper finite size analysis of the dynamical spectral functions, taking into account the effects of disorder and spin fluctuations treated within self-consistent local RPA. We obtain an unambiguous q5 scaling of the intrinsic linewidth, which is attributed to the disorder induced damping of the spin waves. This is in agreement with some previous theoretical studies on the Heisenberg ferromagnets, although the exchange interactions were mostly restricted to nearest neighbors unlike in our case. We also demonstrate the difficulties in extracting the correct scaling of the linewidth as it is sensitive to the q values considered, and one can obtain an incorrect q-dependence if the q's are not sufficiently small. Finally, our findings are discussed in the light of prospective spintronics applications.