Combined Mössbauer and neutron scattering investigation of ball-milled FeSiB samples

Ferromagnetic amorphous alloys are very intriguing systems, combining a long range ferromagnetic (FM) order with the absence of long range crystalline order. In spite of that, the actual atoms disposition may have a remarkable effect on the alloy properties. Indeed, we observed that submitting Fe80Si7B13 ribbons prepared by melt spinning to mechanical milling, a transition at T ~ 50 K from a low temperature frozen collective state, similar to a cluster-glass, to a high temperature ferromagnetic regime (reentrant ferromagnet transition) was observed [1]. To investigate this result, we performed Mössbauer and inelastic neutron scattering measurements. We produced three samples, milled for 10, 20 and 70 hours in a shaker-type milling device under Ar atmosphere (ball to powder weight ratio 4:1); the samples, in form of micrometric powder, were labeled as M10h, M20h and M70h, respectively. Transmission Mössbauer spectra were collected at room temperature using a 57Co in Rh source, and the spectrometer was calibrated using an α-Fe foil. The neutron spectra were recorded at the same temperature and were integrated over all scattering angles, corrected for self-absorption coefficient and finally normalized to the sample mass. Mössbauer investigation as a function of milling time features the progressive precipitation of a minor fraction of bcc Fe nanocrystallites, also displayed by X-Ray diffraction measurements. The Mössbauer analysis indicates that, during this process, the averaged hyperfine field relative to the amorphous component does not change upon milling compared to the as-cast ribbon, suggesting that the value of the magnetic moment per Fe atom remains constant. In parallel, the comparison between the dynamic structure factor S(E) as a function of energy for the as-cast FeSiB ribbon and for the milled samples reveals a depletion of the energy region around 10 meV with increasing the milling time, corresponding to the suppression of vibrational modes proper of the amorphous FeSiB alloy. This behavior is consistent with the formation of bcc Fe nanocrystallites. The inelastic area of the S(E) spectra decreases upon milling over 10 hours; the elastic area decreases as well passing from 10 to 20 hours milling, in an amount larger than 3%. This reduction of the elastic scattering intensity may be accounted for considering that the milling process, prolonged for 20 hours, brings about a decrease in the magnetic cross section of the FeSiB powders, with respect to the as-cast ribbon. Measurements of the saturation magnetization of the samples by SQUID magnetometer fully support this description. As the Mössbauer analysis suggest that the magnetic moment per Fe atom remains constant, to discuss these results we resort to the peculiar magnetic properties of the ball milled ribbons. Indeed, the magnetic properties of the ball milled samples were explained in terms of the existence of a magnetic phase showing spin-glass like properties (speromagnetism), dispersed into the ferromagnetic FeSiB matrix and coexisting with the bcc Fe nanocrystallites. Hence, although ferromagnetism predominates at T = 300 K, we propose that antiferromagnetic interactions and imperfect magnetic moments alignment persist in the regions showing spin-glass like behavior at low temperature, which causes the decrease in the magnetic cross section. [1] L. Del Bianco, F. Spizzo, M. Tamisari, E. Bonetti, F. Ronconi, D. Fiorani, J. Phys.: Condens. Matter 22 (2010) 296010.