Magnetic and structural properties of Fe-Mn-Al alloys produced by mechanical alloying

The effects of concentration and preparation conditions on the magnetic and structural properties of samples of the Fe-Mn-Al system prepared by mechanical alloying were studied by Mossbauer spectroscopy, X-ray diffraction and susceptibility techniques. X-ray diffraction showed that samples with high Al content (40 at.%) and prepared with low milling times (lest than 12 hours) present the peaks of Mn, Fe-Al alloy and broad peaks which correspond to the ternary bee alloy. For 12 hours the Mn peak disappears and for 24 h or more the bee ternary phase is totally formed. For t greater than or equal to 24 h Mossbauer spectroscopy studies proved that: samples rich in Fe are ferromagnetic (F) at room temperature (RT) and present a reentrant spin-glass (RSG) behavior for low T; samples with intermediate Fe values are paramagnetic (P) at RT and SG for low T; and those with low Fe content are P at RT, antiferromagnetic (AF) at low T and RSG at very low T. By ac magnetic susceptibility it was possible to detect an additional magnetic phase in the F and P regions which was attributed to a superparamagnetic (SP)-like behavior. XRD and Mossbauer spectroscopy of samples with low Al content (7.5 at.%) showed that they present a fcc AF phase and a Fe-Al F phase for low milling times (less than or equal to 10 h). For bigger times the only detected phase is the fcc, which is AF for low Fe contents and P for high Fe contents. The effect of using small balls is to enhance the alloyed process.