Magnetic multi-granule nanoclusters: A model system that exhibits universal size effect of magnetic coercivity

Abstract

It is well known that the coercivity of magnetic nanomaterials increases up to a maximum and then decreases to zero with decreasing particle size. However, until now, no single synthesis method has been able to produce magnetic nanoparticles with a wide range of sizes, i.e., from 10 to 500 nm, in order to uncover the coercivity evolution. Here we report the characterization of magnetite (Fe3O4) multi-granule nanoclusters (MGNCs) to demonstrate the transitional behaviour of coercivity. The M–H curves indicate that our samples had a relatively high saturation magnetization (MS) value of ~70 emu/g and that the coercivity (Hc) increased to the maximum value of ~48 Oe until the nanoclusters reached a size of ~120 nm; the coercivity then gradually decreased to zero.

Single-crystalline FeCo nanoparticle-filled carbon nanotubes: synthesis, structural characterization and magnetic properties

“…These values are approximately 200 times higher than for the bulk material at the same temperature ( H c (bulk) = 0.68 Oe) [22,54]. The increase in the coercive field as the particle size decreases can be attributed to the size dependence of coercivity in the vicinity of the critical size of domain formation in nanoparticles [55]. Briefly described, in this size regime, the coercivity of single domain (SD) magnets decreases upon size reduction while it increases in the multidomain (MD) state.…”

“…As known, these particles are subject to oxidation when they are exposed to air unless they are shielded by the carbon shell. To be specific, the presence of oxide layers would imply the presence of an antiferromagnetic shell around the ferromagnetic cores, i.e., the material would evolve the exchange bias effect where nanoparticles cooled under a magnetic field show a significant shift between the coercive field values at the positive ( H c+ ) and the negative ( H c− ) sides [30,55–56]. For the prepared samples, and within the experimental error, equal values of H c+ and H c− have been found, which is a further indication of the protective nature of the CNT shells.…”

Correlation between particle size/domain structure and magnetic properties of highly crystalline Fe3O4 nanoparticles

“…Much smaller critical sizes of 30–46 nm 15 – 18 have also been reported for cubic Fe 3 O 4 NPs. Multi-granule Fe 3 O 4 NPs with sizes of 16–512 nm showed a transitional size of approximately 120 nm 19 . Although the effects of size and shape on the behaviour of magnetic particles have been known for more than half a century 10 , the quantitative effects on Zn 0.4 Fe 2.6 O 4 NPs remained undiscovered in the size range of 20–140 nm until 2012 20 , when the critical size was found to be approximately 60 nm, and the saturation magnetisation ( Ms ) was found to be lower for spherical particles than for cubic ones.…”

Synthesis and ESR Study of Transition from Ferromagnetism to Superparamagnetism in La0.8Sr0.2MnO3 Nanomanganite

“…In this case, magnetization can randomly flip direction under the influence of temperature, causing the residual magnetization to be null (Figure 1) [14]. Superparamagnetic nanoparticles are preferred in biomedical applications because they have zero magnetization at room temperature and do not agglomerate [15,16].…”