MAGNETIC-PROPERTIES OF AMORPHOUS FE-CR-B NANOPARTICLES EMBEDDED IN ANALUMINA MATRIX

Amorphous nanoparticles of Fe80-xCrxB20 (5 less than or equal to x les s than or equal to 25) embedded in an alumina matrix have been studied . The samples have been prepared by the cosputtering technique and by using a mixed target consisting of amorphous ribbons and alumina. Seve ral area ratios have been used leading to different average diameters of particles. The samples have been characterized by means of an elect ron microprobe and transmission electron microscopy with ETX analysis. Thermal variation of the magnetic properties have been determined mai nly from zero-field-cooled magnetization, magnetization versus applied field, AC susceptibility and Mossbauer spectroscopy measurements. The average diameters of the particles obtained range between 1.9 and 5.3 nm. The same kind of random packing of atoms occurs in particles as i n bulk (ribbons) though the preparation process and the size scale are different. The hyperfine parameters deduced from Mossbauer spectrosco py are similar to those obtained from the ribbons, but show a size dep endence which is only appreciable for the smallest particles. The magn etic structure and the magnetic properties are similar to those of the corresponding ribbons except for x above 20-25 where a canted spin st ructure probably appears. favoured by the magnetic defects existing in the particle surface. The dynamical properties of particles are more intricate. Interparticle interactions are present, but the model [Dorm ann et al., Adv. Chem. Phys. 98 (1997) 283] accounting for the dipolar interactions is not sufficient for explaining the properties. It is s uggested thats due to the magnetic disorder coming from the Cr substit ution, the effective anisotropy acting on the reversal process of the magnetic moments of particles is temperature dependent. A good agreeme nt is obtained for the variation of the blocking temperature with the measuring time. (C) 1998 Elsevier Science B.V. All rights reserved.