Low Temperature Magnetic Properties of FCC FeRh Obtained by Ball Milling

Abstract

Starting from four different FeRh compositions, a nanocrystalline fcc phase, paramagnetic at room temperature, has been obtained, for each case, through mechanical alloying by ball-milling. The starting selected compositions were: ferromagnetic and bcc-ordered Fe 65 Rh 35 , antiferromagnetic and also bcc-ordered Fe 50 Rh 50 , paramagnetic and fcc Fe 28 Rh 72 and Fe 26 Rh 74 . The average grain size of the resulting fcc phase has been found to vary in the 6-12 nm range, depending on the Rh content of the precursor. At low temperatures, some characteristic behaviors typical of the spin-glass or superparamagnetic state have been observed in all the compositions: a maximum in the AC susceptibility, at a temperature (Tm) which increases with the grain size, a maximum in the measurements of the thermal variation of the ZFC and FC magnetization and a wide distribution of hyperfine fields in the Mossbauer spectra at T<Tm. The possibility that this behavior could be determined by the presence of iron impurities, deriving from the stainless steel vial and balls used for the milling procedure, has been definitely removed since similar effects have been also observed in samples milled in a vial and with balls of agate. However, the analysis of the dc field-cooled magnetization measurements, revealing the absence of any critical behavior of the non-linear susceptibility first coefficient in the vicinity of Tm, rules out the possibility of a true spin-glass state. Actually, all the results seem to indicate that the observed behavior is intrinsic to the nanocrystalline nature of fcc FeRh.