Separated CoFe₂O₄/CoFe nanoparticles by the SiO _x matrix: revealing the intrinsic origin for the small remanence magnetization

In order to clarify the intrinsic reason for the smaller remanence (M _r)-to-saturation (M _s) magnetization ratio M _r/M _s than that expected by the Stoner–Wohlfarth model in CoFe₂O₄/CoFe₂ nanoparticles in the previous report, we first prepared well-dispersed CoFe₂O₄ nanoparticles, and then they were diluted in the SiO₂ matrix followed by reduction in H₂ as far as possible to exclude or reduce disadvantageous variables (such as the growth and aggregation of particles and the exchange coupling between soft magnetic particles in the process of reducing) affecting magnetic properties. Such an idea has not been taken into account before to our knowledge. The analyses on the magnetic results indicate that the CoFe₂O₄/CoFe₂ nanoparticles herein reported are a pure dipolar system, in which the coercivity (H _c) and M _r/M _s ratio are very sensitive to the anisotropy and the strength of dipolar interaction. These results signify that it is important to maintain the CoFe₂O₄/CoFe₂ nanoparticles with higher anisotropy and weaker dipolar interaction for improving M _r/M _s and H _c. This suggestion was further confirmed by our another result wherein an M _r/M _s value of 0.64 was obtained even though no exchange coupling was observed in the CoFe₂O₄/CoFe₂ nanoparticles, and further work is in process.

Numerous efforts have devoted to improve the values of M _s and M _r/M _s by compositing hard CoFe₂O₄ (CFO) ferrite with soft CoFe₂ (CF) alloy, which unfortunately give the low M _r/M _s value (<0.5) even in presence of the exchange coupling. Key issues involve the preparation of CFO/CF composite. Previously the preparation of CFO/CF undergoes the synthesis of CFO and the subsequent reducing in the H₂ ambient, as shown in Figure (a), while in this work well dispersed CFO nanoparticles were first prepared , and then diluted in the SiO₂ matrix followed by reducing in H₂ to exclude or reduce disadvantageous variables, such as the growth and aggregation of particles and the exchange coupling between soft magnetic particles in the process of reducing, as shown in Figure (b). Our results suggest that higher anisotropy and weaker dipolar interaction favor the larger M _r/M _s value, as shown in Figure (c).