The Magnon Spectrum in the Domain Ferromagnetic State of Antisite Disordered Double Perovskites

TL;DR

This paper investigates how antisite disorder in double perovskites creates complex magnetic states with ferromagnetic domains coupled antiferromagnetically, and analyzes the resulting magnon spectrum using a 1/S expansion.

Contribution

It provides a detailed theoretical analysis of magnon excitations in disordered double perovskites, highlighting the effects of antisite disorder on magnetic excitations.

Findings

Magnon modes are domain-confined with multiple energies at each wavevector.

Antisite disorder leads to a broadening of the magnon spectrum.

Correlated antisite disorder produces distinct spectral features compared to uncorrelated disorder.

Abstract

In their ideal structure, double perovskites like Sr_2FeMoO_6 have alternating Fe and Mo along each cubic axes, and a homogeneous ferromagnetic metallic ground state. Imperfect annealing leads to the formation of structural domains. The moments on mislocated Fe atoms that adjoin each other across the domain boundary have an antiferromagnetic coupling between them. This leads to a peculiar magnetic state, with ferromagnetic domains coupled antiferromagnetically. At short distance the system exhibits ferromagnetic correlation while at large lengthscales the net moment is strongly suppressed due to inter-domain cancellation. We provide a detailed description of the spin wave excitations of this complex magnetic state, obtained within a 1/S expansion, for progressively higher degree of mislocation, i.e., antisite disorder. At a given wavevector the magnons propagate at multiple energies, related, crudely, to `domain confined' modes with which they have large overlap. We provide a qualitative understanding of the trend observed with growing antisite disorder, and contrast these results to the much broader spectrum that one obtains for uncorrelated antisites.