Properties of incommensurate spin density waves in iron aluminides (invited)

Abstract

Neutron diffraction in Fe(Al) reveals incommensurate spin density waves (SDWs) in alloys known to be spin glasses. The wave vectors for crystals of Fe(34Al), Fe(40Al) and Fe(43Al) show n varying from 11 to 6 for q=2π(h±1/n,k±1/n,l±1/n)/ao, where (h,k,l) and ao characterize the parent bcc lattice of the CsCl structure. The magnetic reflections are present far above the spin-glass freezing temperatures. These SDWs keep the spins on nearest-neighbor Fe atoms close to parallel, in contrast with SDWs in Cr, which keep nearest-neighbor spins close to antiparallel. The competition between near-neighbor Fe–Fe ferromagnetism and 180° superexchange through the Al site has been used to explain the spin-glass behavior, but the appearance of the SDWs calls for a more fundamental source of the periodicity. The phase shift mechanism for SDW interactions with magnetic moments is invoked to explain the breadth of the peaks, which resemble the results for Cu(Mn), Pd(Mn), and Pd(Cr). The data are interpreted using cubic symmetry, but it has yet to be established whether the wave vectors all occur in a single domain or whether there are multiple domains. There are 48 wave vectors of magnitude almost equal to the {110} wave vectors of the bcc lattice, which could stabilize the SDWs by spanning the Fermi surface. These unanticipated results should have pervasive ramifications for the theory of metallic magnetism.