Elastic diffuse neutron scattering from weakly ferromagnetic Pt alloys

Abstract

Unpolarised neutron diffraction measurements on weakly ferromagnetic PtCo and PtFe alloys give elastic diffuse magnetic cross sections which exhibit marked forward peaks confirming the inhomogeneous distribution of magnetisation in these alloys. The local moments on the solute atoms are 2.00+or-0.06 mu B and 3.16+or-0.04 mu B for Co and Fe respectively. These are approximately independent of composition, but it is not possible to obtain a characteristic polarisation range for the Pt matrix algorithm indications are that this should be of the nearest-neighbour distance only. The forward cross sections are much larger than would reasonably be expected from the bulk magnetisation values of (d mu sat/dc), where mu sat is the saturation magnetisation. It is therefore argued that the Marshall model is inapplicable to unpolarised neutron data for weakly ferromagnetic alloys. Consequently an alternative model has been invoked wherein the cross sections have been interpreted as arising from the critical scattering of neutrons by concentration-driven critical fluctuations of the spontaneous magnetisation which occur near the percolation concentration for the onset of long-range ferromagnetic order.