B.Q. Geng

The effects of surface spin on magnetic properties of weak magnetic ZnLa0.02Fe1.98O4 nanoparticles

In order to prominently investigate the effects of the surface spin on the magnetic properties, the weak magnetic ZnLa0.02Fe1.98O4 nanoparticles were chosen as studying objects which benefit to reduce as possibly the effects of interparticle dipolar interaction and crystalline anisotropy energies. By annealing the undiluted and diluted ZnLa0.02Fe1.98O4 nanoparticles at different temperatures, we observed the rich variations of magnetic ordering states (superparamagnetism, weak ferromagnetism, and paramagnetism). The magnetic properties can be well understood by considering the effects of the surface spin of the magnetic nanoparticles. Our results indicate that in the nano-sized magnets with weak magnetism, the surface spin plays a crucial rule in the magnetic properties.

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

In order to clarify the intrinsic reason for the smaller remanence (Mr)-to-saturation (Ms) magnetization ratio Mr/Ms 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 (Hc) and Mr/Ms 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 Mr/Ms and Hc. This suggestion was further confirmed by our another result wherein an Mr/Ms value of 0.64 was obtained even though no exchange coupling was observed in the CoFe2O4/CoFe2 nanoparticles, and further work is in process.Graphical abstractNumerous efforts have devoted to improve the values of Ms and Mr/Ms by compositing hard CoFe2O4 (CFO) ferrite with soft CoFe2 (CF) alloy, which unfortunately give the low Mr/Ms 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 Mr/Ms value, as shown in Figure (c).

The remanence ratio in CoFe2O4 nanoparticles with approximate single-domain sizes

AbstractApproximately single-domain-sized 9-, 13-, and 16-nm CoFe2O4 nanoparticles are synthesized using the thermal decomposition of a metal-organic salt. By means of dilution and reduction, the concentration, moment, and anisotropy of nanoparticles are changed and their influence on the magnetic properties is investigated. The relation of Mr/Ms ∝ 1/lgHdip is observed, where Mr/Ms is the remanence ratio and Hdip is the maximum dipolar field. Especially, such relation is more accurate for the nanoparticle systems with higher concentration and higher moment, i.e., larger Hdip. The deviation from Mr/Ms ∝ 1/lgHdip appearing at low temperatures can be attributed to the effects of surface spins for the single-phase CoFe2O4 nanoparticles and to the pinning effect of CoFe2O4 on CoFe2 for the slightly reduced nanoparticles. Graphical AbstractApproximately single-domain-sized 9-, 13-, and 16-nm CoFe2O4 nanoparticles were synthesized and then the concentration, moment, and anisotropy of these NPs were changed. The correlation of Mr/Ms ∝ 1/lgHdip was observed, independent of the size, concentration, moment, and anisotropy, and especially, such correlation is more accurate for the nanoparticle systems with higher concentration or moment, i.e., stronger dipolar interaction, which has not been reported before as far as we know.