Structure and some magnetic properties of amorphous and partially crystallized Fe-(Co)-Mn-Mo-B alloys


Microstructure, revealed by X-ray diffraction and transmission Mössbuaer spectroscopy, and some thermomagnetic properties of the amorphous Fe70-xCoxMn10Mo5B15 (x = 0, 0.25 and 0.5) alloys in the as-quenched state and after accumulative annealing at Ta1=723K, Ta2=753K, Ta3=853K and Ta4=903K for 0.5h have been studied. After the annealing at Ta1 and then at Ta2 the ribbons remain in the amorphous state, whereas after the final heat treatments at Ta3 and Ta4 they are partially crystallized. The grains of the hexagonal ε-Fe80Mn20 phase, 8 nm and 12 nm in size after the annealing at Ta3 and Ta4 for x=0 and 0.5, respectively, are paramagnetic at room temperature the first product of crystallization. After the final annealing at Ta4 in samples containing Co (x=0.25 and x=0.5) α-Fe grains are also detected. X-ray diffraction studies are qualitatively very consistent with transmission Mössbauer spectra measurements. The Curie temperature (TC) for x=0 decreases after the annealing at Ta1in comparison with the as-quenched state and then increases after the heat treatment at Ta2, whereas in the alloys containing Co atoms (x=0.25 and 0.5) only the increase of TC on annealing at Ta1 and at Ta2 is observed as compared to the as-quenched state. Isothermal magnetic entropy change is rather modest and reaches its peak value in the vicinity of the Curie temperature of the amorphous phase. The higher TC the larger peak value of the magnetic entropy change. An attempt to elucidate such behavior is undertaken.

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