Dagoberto Oyola Lozano

Mössbauer study of alloy Fe67.5Ni32.5, prepared by mechanical alloying

We present the study of effect of the particle size on the structural and magnetic properties of the Fe67.5Ni32.5 alloy, prepared by mechanical alloying (MA). After milling the powders during 10 hours they were separated by sieving using different meshes. The refinement of the X-ray patterns showed the coexistence of the BCC (Body Centered Cubic) and the FCC (Face Centered Cubic) phases in all samples with lattice parameters and crystallite sizes independent of the mean particle size. However, big particles presented bigger volumetric fraction of BCC grains. The Mossbauer spectra were fitted with a broad sextet corresponding to the ferromagnetic BCC phase, a hyperfine magnetic field distribution and a broad singlet which correspond to the ferromagnetic and paramagnetic sites of the FCC phase, respectively. Hysteresis loops showed a magnetically, soft behavior for all the samples, however, the saturation magnetization values are smaller for the original powder and for the powders with small, mean, particle size due to the dipolar magnetic interaction and the smaller mean magnetic moment, respectively. These effects were proved by Henkel plots that were made to the samples.

Evidence of Dipolar Magnetic Interaction in Melted Fe50Al50 Samples

Powders of the Fe50Al50 melted and heat treated samples were separated at different mean particle sizes as well as characterized based on their structural and magnetic properties. All the samples, except that with biggest particle size, showed Fe-Al BCC and α-Fe phases as well as the same nanostructure. The powder with the biggest particles, with particle size bigger than 105 μm, presents the additional minority presence of FeO and Al2O phases. The powders were found to be ferromagnetic at room temperature with a mean hyperfine magnetic field (MHMF) that increases with increasing particle size. The origin of the increase of the MHMF is attributed to the increment of the magnetic dipolar interactions between the magnetic moment of each particle and the field produced by other magnetic moments of their surrounded particles. This field also increases as the particle size increases. Besides the dipolar magnetic interaction between particles, the Fe-Al and α-Fe grains present a ferromagnetic coupling between them.