Keywords: Heusler alloys, nano Co2FeSi, mechanical alloying, magnetic properties, 57Fe Mossbauer, nanotechnology, milling, superparamagnetic doublets
Magnetic properties of nano–sized Co2FeSi
Nano–sized samples of Co
2FeSi have been synthesised by mechanical alloying using a high energy ball mill. The average crystallite size shows a systematic decrease with milling time. M?ssbauer and DC magnetisation studies show that the Co
2FeSi phase is formed at 9 h of milling and is optimised after 12 h of milling. Further milling results in the formation of a superparamagnetic phase with a corresponding deterioration in magnetic properties. A distribution in crystallite size is evident from the co–existence of superparamagnetic doublets with the magnetic hyperfine field in Mossbauer spectra. The presence of a large magnetically dead layer at the surface of the particles makes the value of saturation magnetisation less in the case of the 12 h sample as compared to the bulk. After 24 h of milling, the alloying process is complete but most of the particles of Co
2FeSi are now superparamagnetic.