Neutron scattering studies of spin-glass alloys

Abstract

Neutron diffraction investigations of binary alloys such as Cu‐Mn, beginning with the earliest reported measurements in 1957, show no evidence of any long range magnetic order except for the broad short range order peaks at the (1 1/2 0) type positions, attributed to both atomic and magnetic short range ordering in alloys containing more than about 10–15 at.% Mn. The current interest in spin glasses is focused on the theoretical possibility of a magnetic phase transition to the spin glass state in the dilute random substitutional alloys. Recently neutron scattering measurements have been attempted to study the dynamics of spins in the alloys with a view to understanding the nature of the processes leading to the spin glass state. Small angle neutron scattering measurements on Au‐Fe alloys with Fe concentrations below the percolation threshold for ferromagnetism show a series of q‐dependent maxima in the total scattering intensity suggesting a continuum of freezing temperatures. Other measurements employing energy analysis by the time‐of‐flight technique at somewhat larger q’s on the same and other alloys such as Cu‐Mn, Pd‐Mn show a broad quasi‐elastic paramagnetic spectrum at high temperatures whose energy width decreases with decreasing temperature, apparently goes through a minimum especially at the higher q’s but always remains finite as T→0 K. The intensity of the quasi‐elastic scattering, however, diminishes progressively below some temperature with a simultaneous continuous increase of the elastic peak intensity. Although the implications of the recent phase transition theories of spin glasses to the dynamics of spins at finite scattering vectors are not entirely clear, the present neutron scattering results appear not inconsistent with the alternative picture of a continuous process of freezing of spins occurring at least over a finite temperature range.