Structural and magnetic properties of some Co(AlMn) alloys

Abstract

Magnetization, magnetic susceptibility, and neutron diffraction measurements have been made on the alloy series CoAl1−xMnx over a wide range of temperature for 0<x<0.4. In this range of concentration the alloys retain the singly ordered B2 (CsCl) structure and neutron diffraction results suggest that the Mn atoms predominantly enter Al sites. However, at concentrations less than x=0.15 about 1% of Mn atoms may possibly enter Co sites with consequent displacement of Co atoms to Al sites. All the alloys show ferromagnetic behavior with Curie temperatures up to 675 K. Evidence for the coexistence of superparamagnetism is found indicating some clusters may not be coupled to the infinite cluster. The superparamagnetism persists to a limited extent above the Curie temperatures for most of the alloys and conventional analysis suggests cluster moments of ∼1000μB for x=0.1 for example. At high temperatures Curie–Weiss behavior is consistent with a moment of ∼0.4μB per formula unit with a composition independent Curie constant suggesting an itinerant origin for the ferromagnetism. Low field (10 Oe or less) magnetization measurements have also been performed for x=0.1 and 0.2. There is a significant difference between zero-field cooled and field-cooled curves at low temperatures indicating a freezing at ∼80 K for x=0.1 and ∼15 K for x=0.2.