Comparative study between melted and mechanically alloyed samples of the Fe50Mn10Al40 nanostructured system

Abstract

Nanostructured powders of melted and 48 h-mechanically alloyed (MA) samples of the Fe50Mn10Al40 system were studied by XRD, SEM and Mossbauer spectroscopy (MS), and their properties were compared. The samples present BCC structure with similar mean lattice parameter (near 2.92 A) and grain size (around 22 nm). The MA sample presents additionally the α-Fe phase. Mossbauer spectra of the samples present a hyperfine field distribution (HFD), a broad paramagnetic (P) site and a sextet showing that the BCC ternary phase is disordered with Fe sites in environments rich in Fe (HFD) and rich in Al and Mn (P), respectively. The mean hyperfine magnetic field vs. temperature curve of melted sample presents two kinks, one at 28 K and other at 210 K and a Curie temperature at 340 K. A similar curve is observed for the milled sample but the kinks occur at 65 and 265K. Mossbauer spectra at different temperatures with and without applied field permit to conclude that the low temperature anomaly corresponds to the freezing temperature of a re-entrant spin-glass phase (RSG) and that of the second one corresponds to a blocking temperature of a superparamagnetic (SP) phase. These phases are possible in the samples due to their disordered character induced by the preparation conditions and the competitive interactions of the Fe and Mn atoms. The enhancement of the magnetic behaviour of the MA sample is due its larger disorder induced by the preparation method that can also explain the increase of the RSG and SP transition temperatures.