Simultaneous effects of surface spins: rarely large coercivity, high remanence magnetization and jumps in the hysteresis loops observed in CoFe2O4 nanoparticles

Abstract

Well-dispersed uniform cobalt ferrite nanoparticles were synthesized by thermal decomposition of a metal–organic salt in organic solvent with a high boiling point. Some of the nanoparticles were diluted in a SiO₂ matrix and then the undiluted and diluted samples were characterized and their magnetic behavior explored. The undiluted and diluted samples exhibited maximum coercivity H_c of 23 817 and 15 056 Oe at 10 K, respectively, which are the highest values reported to date, and the corresponding ratios of remanence (M_r) to saturation (M_s) magnetization (M_r/M_s) were as high as 0.85 and 0.76, respectively. Interestingly, the magnetic properties of the samples changed at 200 K, which was observed in magnetic hysteresis M(H) loops and zero-field cooling curves as well as the temperature dependence of H_c, M_r/M_s, anisotropy, dipolar field, and the magnetic grain size. Below 200 K, both samples have large effective anisotropy, which arises from the surface spins, resulting in large H_c and M_r/M_s. Above 200 K, the effective anisotropy decreases because there is no contribution from surface spins, while the dipolar interaction increases, resulting in small H_c and M_r/M_s. Our results indicate that strong anisotropy and weak dipolar interaction tend to increase H_c and M_r/M_s, and also clarify that the jumps around H = 0 in M(H) loops can be attributed to the reorientation of surface spins. This work exposes the underlying mechanism in nanoscale magnetic systems, which should lead to improved magnetic performance.