Giant magnetoresistance in AuFe assays: Evidence for the progressive unblocking of superparamagnetic particles

Measurements were performed of the magnetic-field dependence of the magnetization M(B) and the magnetoresistance MR(B) of a Au85Fe15 alloy. It was found that although MR(B) is proportional to the square of M(B) at higher temperatures (above 120 K), a term linear in M(B makes an important contribution to MR(B al lower temperatures. These results are interpreted as evidence for the presence in the alloy of a range of sizes for the: Fe particles which become progressively unblocked (superparamagnetic) as the temperature increases. A calculation based on this interpretation yields quantitative agreement with both the M(B) and MR(BI data at all temperatures.