Simultaneous effects of surface spins: rarely large coercivity, high remanence magnetization and jumps in the hysteresis loops observed in CoFe₂O₄nanoparticles

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 Hc 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 (Mr) to saturation (Ms) magnetization (Mr/Ms) 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 Hc, Mr/Ms, 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 Hc and Mr/Ms. Above 200 K, the effective anisotropy decreases because there is no contribution from surface spins, while the dipolar interaction increases, resulting in small Hc and Mr/Ms. Our results indicate that strong anisotropy and weak dipolar interaction tend to increase Hc and Mr/Ms, 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.

Role of SiO2 coating in multiferroic CoCr2O4 nanoparticles

“…The surface to volume ratio becomes also high with the decrease in particle size. 27,28 It is also observed that the M s value of bare (y = 0) nanoparticles is greater than coated nanoparticles which is due to their large average crystallite size of uncoated nanoparticles as compared to coated nanoparticles. 29 Similar phenomenon was also reported for NiFe 2 O 4 and CoFe 2 O 4 ferrite nanoparticles.…”

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

“…The undiluted 9- and 13-nm NPs, i.e., H9 and H13, as well as the 16-nm NPs reduced at 300 °C (LT16) have the H c values of 20.0, 18.8, and 20.5 kOe at 10 K, smaller than 23.8 kOe (10 K) for our previously reported 10.7-nm CoFe 2 O 4 NPs [32] which particle size is close to the critical size of a single domain [33]. These H c values are much larger than that for the bulk CoFe 2 O 4 (6.8 kOe at 10 K) [34], because on the one hand, the particles are approximately single-domain sized, and on the other hand, the interaction between surface spins induces extra anisotropy and hence enhances H c , which will be further discussed below.…”

Ethanol-assisted synthesis and adsorption property of flake-like NiFe2O4 nanoparticles

“…It is indicated that, in this temperature region, the dipolar interaction between the nanoparticles is prevailing [25]. Whereas, the FC curves present a small increase below about 16K, which is caused by the orientation of surface spins in the direction of the applied field [26]. Table 1 lists the S BET of the as-synthesized samples.…”