### Abstract

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 ${T}_{a1}=723K$, ${T}_{a2}=753K$, ${T}_{a3}=853K$ and ${T}_{a4}=903K$ for 0.5h have been studied. After the annealing at ${T}_{a1}$ and then at ${T}_{a2}$ the ribbons remain in the amorphous state, whereas after the final heat treatments at ${T}_{a3}$ and ${T}_{a4}$ they are partially crystallized. The grains of the hexagonal ε-Fe80Mn20 phase, 8 nm and 12 nm in size after the annealing at ${T}_{a3}$ and ${T}_{a4}$ for x=0 and 0.5, respectively, are paramagnetic at room temperature the first product of crystallization. After the final annealing at ${T}_{a4}$ 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 (${T}_{C}$) for x=0 decreases after the annealing at ${T}_{a1}$in comparison with the as-quenched state and then increases after the heat treatment at ${T}_{a2}$, whereas in the alloys containing Co atoms (x=0.25 and 0.5) only the increase of ${T}_{C}$ on annealing at ${T}_{a1}$ and at ${T}_{a2}$ 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 ${T}_{C}$ the larger peak value of the magnetic entropy change. An attempt to elucidate such behavior is undertaken.