The Effect of Mn and B on the Magnetic and Structural Properties of Nanostructured Fe₆₀Al₄₀ Alloys Produced by Mechanical Alloying

The effect of Mn and B on the magnetic and structural properties of nanostructured samples of the Fe₆₀Al₄₀ system, prepared by mechanical alloying, was studied by ⁵⁷Fe Mössbauer spectrometry, X-ray diffraction and magnetic measurements. In the case of the Fe_60–x Mn _x Al₄₀ system, 24 h milling time is required to achieve the BCC ternary phase. Different magnetic structures are observed according to the temperature and the Mn content for alloys milled during 48 h: ferromagnetic, anti-ferromagnetic, spin-glass, reentrant spin-glass and superparamagnetic behavior. They result from the bond randomness behaviour induced by the atomic disorder introduced by the MA process and from the competitive interactions of the Fe–Fe ferromagnetic interactions and the Mn–Mn and Fe–Mn antiferromagnetic interactions and finally the presence of Al atoms acting as dilutors. When B is added in the Fe₆₀Al₄₀ alloy and milled for 12 and 24 hours, two crystalline phases were found: a prevailing FeAl BCC phase and a Fe₂B phase type. In addition, one observes an additional contribution attributed to grain boundaries which increases when both milling time and boron composition increase. Finally Mn and B were added to samples of the Fe₆₀Al₄₀ system prepared by mechanical alloying during 12 and 24 hours. Mn content was fixed to 10 at.% and B content varied between 0 and 20 at.%, substituting Al. X-ray patterns show two crystalline phases, the ternary FeMnAl BCC phase, and a (Fe,Mn)₂B phase type. The relative proportion of the last phase increases when the B content increases, in addition to changes of the grain size and the lattice parameter. Such behavior was observed for both milling periods. On the other hand, the magnetic hyperfine field distributions show that both phases exhibit chemical disorder, and that the contribution attributed to the grain boundaries is less important when the B content increases. Coercive field values of about 10² Oe slightly increase with boron content. Comparison with previous results on FeAlB alloys shows that Mn promotes the structural stability of the nanostructured powders.