Superparamagnetism and the character of magnetic order in binary Cr–Fe alloys near the critical concentration

Abstract

Although the magnetic properties of Cr–Fe alloys have been widely studied, there are different interpretations of existing data in the literature, especially for alloys in the concentration range 10 to 25% Fe. The onset of ferromagnetism occurs somewhere in this concentration range, but there is little direct evidence of the precise critical concentration. Detailed small-angle scattering measurements have been carried out on alloys containing 16.7, 19.9, 20.9, 21.7, 24 and 25% Fe over a wide range of temperatures and magnetic fields. Small-angle scattering peaked in the forward direction develops at low temperatures, corresponding to the growth of magnetic correlations over distances of the order of 400 Å. The response of the scattering to the application of a magnetic field suggests that the scattering at low temperatures arises from superparamagnetic clusters. Curie temperatures were deduced from the critical scattering, and the variation of Tc with composition suggests that the critical concentration is close to 19% Fe.