Inelastic neutron scattering measurements have been carried out at T = 300 K on FeSiB samples produced by mechanical milling from a ferromagnetic amorphous Fe80Si7B13 ribbon prepared by melt spinning. We have 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. The collected neutron spectra have been integrated over all scattering angles, corrected for self-absorption coefficient and finally normalized to the sample mass. The study of 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 X-ray diffraction and Mössbauer spectroscopy analyses on the same samples indicating that the milling process induces the formation of a minor fraction of bcc Fe nanocrystallites (in M70h, the resonant area ascribed to bcc Fe is about 7% of the total Mössbauer spectrum). 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 can 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. In M70h, the elastic region area is slightly larger than in M20h, likely because of the more relevant presence of bcc Fe nanocrystallites, but it remains definitely smaller than in the as-cast and M10h samples. Measurements of the saturation magnetization of the samples by SQUID magnetometer fully support this description. On the other hand, the Mössbauer analysis indicates that 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. We discuss these results by the light of previous magnetic studies revealing a peculiar magnetic behavior of milled FeSiB powder, featuring a transition at T ~ 50 K from a frozen collective state, similar to a cluster-glass, to a ferromagnetic regime (reentrant ferromagnet transition) [1]. The explanation for this effect was based on 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 et al., J. Phys.: Condens. Matter 22, 296010 (2010)
Inelastic neutron scattering investigation of ball-milled FeSiB samples
DEL BIANCO, Lucia;SPIZZO, Federico;
2013
Abstract
Inelastic neutron scattering measurements have been carried out at T = 300 K on FeSiB samples produced by mechanical milling from a ferromagnetic amorphous Fe80Si7B13 ribbon prepared by melt spinning. We have 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. The collected neutron spectra have been integrated over all scattering angles, corrected for self-absorption coefficient and finally normalized to the sample mass. The study of 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 X-ray diffraction and Mössbauer spectroscopy analyses on the same samples indicating that the milling process induces the formation of a minor fraction of bcc Fe nanocrystallites (in M70h, the resonant area ascribed to bcc Fe is about 7% of the total Mössbauer spectrum). 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 can 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. In M70h, the elastic region area is slightly larger than in M20h, likely because of the more relevant presence of bcc Fe nanocrystallites, but it remains definitely smaller than in the as-cast and M10h samples. Measurements of the saturation magnetization of the samples by SQUID magnetometer fully support this description. On the other hand, the Mössbauer analysis indicates that 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. We discuss these results by the light of previous magnetic studies revealing a peculiar magnetic behavior of milled FeSiB powder, featuring a transition at T ~ 50 K from a frozen collective state, similar to a cluster-glass, to a ferromagnetic regime (reentrant ferromagnet transition) [1]. The explanation for this effect was based on 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 et al., J. Phys.: Condens. Matter 22, 296010 (2010)I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
