Cobalt ferrite nanoparticles: Achieving the superparamagnetic limit by chemical reduction

Abstract

An unanticipated superparamagnetic response has been observed in cobalt ferrite materials after thermal treatment under inert atmosphere. Cobalt ferrite particles were prepared via normal micelle precipitation that typically yields Co x Fe 3−x O 4 nanoparticles ͑x = 0.6− 1.0͒. While samples thermally treated under oxygen show majority spinel phase formation, annealing in nitrogen gas yields materials consisting of Co-Fe alloy, FeS, and CoFe 2 O 4 spinel. After thermal treatment, thermomagnetic studies reveal composition-insensitive, but highly treatment-sensitive, saturation magnetization, coercivity, blocking temperature, and Verwey transition temperature dependence. Extremely high saturation magnetization ͑159 emu/g͒ with low coercivity ͑31 Oe͒ was observed for one of the treated compositions, which drastically deviates from prototypical cobalt ferrite with large magnetocrystalline anisotropy. We attribute such unique magnetic response to Co-Fe alloy coexisting with FeS and CoFe 2 O 4 spinel where the diameter of the metallic phase is below the superparamagnetic limit. While thermal treatment in nitrogen was not anticipated to yield Co-Fe alloy, chemisorbed surfactant molecules ͑i.e., sodium dodecylsulfate͒ are postulated to act as reducing agents in the present scenario.

CoFe2O4 and CoFe2O4‐SiO2 Nanoparticle Thin Films with Perpendicular Magnetic Anisotropy for Magnetic and Magneto‐Optical Applications

“…2016, 26, 1954-1963 www.afm-journal.de www.MaterialsViews.com measured magnetization at saturation, M s , is 35 emu g −1 , which is in the reported range for nanoparticle CoFe 2 O 4 fi lms, [ 14 ] but lower than the bulk value of 80 emu g −1 . [ 31 ] Many factors contribute to a lowering in M s for nanocrystalline samples in comparison to the bulk, such as the difference in inversion parameters or the presence of a dead layer. [ 32 ] XAS and XMCD measurements give insights into the origins for the lower M s , Figure 7 b,c.…”

Inverse magnetocaloric effect in sol–gel derived nanosized cobalt ferrite

“…Cobalt ferrites, with particle size less than 10 nm, are of interest because they exhibit interesting magnetic and structural properties [17]. At sizes <5 nm they are found to be exhibiting superparamagnetic properties [18].…”

Single-Particle Phenomena in Magnetic Nanostructures

“…This effect was attributed by some groups [127][128][129] to the unquenched orbital contribution that would stem from the breaking of symmetry at the surface of metallic particles. In addition, XMCD and FMR measurements have been performed to evaluate the spin and orbital contributions to the magnetic moment (see end of Sections 3.5 and 3.6 for the latest experimental results).…”