Microstructure and some thermomagnetic properties of amorphous Fe-(Co)-Mn-Mo-B alloys

X-ray diffractometry, high resolution transmission electron microscopy (HREM) and transmission Mössbauer spectroscopy are used to study the microstructure and magnetic ordering in rapidly quenched amorphous Fe70-xCoxMn10Mo5B15 alloys with x = 0, 0.25, 0.5, 1 and 2. HREM images reveal the existence of medium range ordered (MRO) regions 2-4 nm in size in the as-cast alloys. The best fitting of the experimental, transmission Mössbauer spectra at 300 K is achieved applying the spectra decomposition into a set of symmetric quadrupole doublets and a single line for the paramagnetic at room temperature alloy (x = 0) or into a set of sextets and the single line for ferromagnetic alloys (x = 0.25, 0.5,1 and 2). The single line is described to the MRO regions which remain in paramagnetic state in the 77-300 K temperature range with small volume fraction in the ribbons. Within the experimental error the isomer shift of the single line for all alloys is the same indicating that MRO regions form similar paramagnetic phase independently of Co content. The magnetic properties of the studied alloys are determined mainly by the amorphous phases presented in the specimens. The Curie temperature rises with the increase of Co content and exhibits almost linear dependence on the average value of the hyperfine magnetic field induction. The maximum values of the magnetic entropy change occur near the ferromagnetic-paramagnetic transition temperature of the amorphous phase and are related to the Curie temperatures. An attempt to explain such feature is undertaken.