The low DC field magnetic susceptibility of dilute PtMn alloys

Abstract

Using an RF SQUID based magnetometer, the temperature-dependent magnetic susceptibility of the dilute PtMn spin-glass alloys containing 0.26-3.83 at.% Mn has been measured in the temperature range 1-4.2K and in DC fields between 3 and 40G. The field-cooled susceptibility when measured as a function of temperature shows a very small but almost cusp-like peak at a well defined magnetic 'freezing' or ordering temperature T_f, and it obeys a Curie-Weiss law at temperature T>T_f. At a temperature of roughly 0.7 T_f, a shallow minimum has been observed. The Curie-Weiss temperature theta is negative, and both T_f and mod theta mod increase linearly with concentration below 1.67% Mn and faster than linearly above this value. The spin values per Mn atom decrease from 2.64 to 2 as the concentration increases. These observations indicate that, in PtMn, the nearest-neighbour direct Mn-Mn antiferromagnetic interactions are important as well as the dominant RKKY interactions. The behaviour of the susceptibility above and around the spin-glass freezing temperature can be explained by means of the mean random molecular field model of the spin glasses.