Magnetocrystalline and Surface Anisotropy in CoFe2O4 Nanoparticles

Abstract

<jats:p>The effect of the annealing temperature Tann on the magnetic properties of cobalt ferrite nanoparticles embedded in an amorphous silica matrix (CoFe2O4/SiO2), synthesized by a sol-gel auto-combustion method, was investigated by magnetization and AC susceptibility measurements. For samples with 15% w/w nanoparticle concentration, the particle size increases from ~2.5 to ~7 nm, increasing Tann from 700 to 900 °C. The effective magnetic anisotropy constant (Keff) increases with decreasing Tann, due to the increase in the surface contribution. For a 5% w/w sample annealed at 900 °C, Keff is much larger (1.7 × 106 J/m3) than that of the 15% w/w sample (7.5 × 105 J/m3) annealed at 700 °C and showing comparable particle size. This indicates that the effect of the annealing temperature on the anisotropy is not only the control of the particle size but also on the core structure (i.e., cation distribution between the two spinel sublattices and degree of spin canting), strongly affecting the magnetocrystalline anisotropy. The results provide evidence that the magnetic anisotropy comes from a complex balance between core and surface contributions that can be controlled by thermal treatments.</jats:p>

Interplay between inter- and intraparticle interactions in bi-magnetic core/shell nanoparticles

“…The ZFC and FC magnetisation curves for both single phase and core/shell samples (Figure S4) showed typical behaviour of interacting mono-domain particles with different anisotropy (see details in the supporting information section). An estimation of the distribution of anisotropy energy (DAE) barriers can be obtained by the −dM FC-ZFC (T)/dT curve reported in Figure 4 (see supporting information for a detailed explanation of the method) [21,61]. In Figure 4 the filled areas under the −dM FC-ZFC (T)/dT curves represent the cumulative DAE (eq.…”

“…As it was reported by Song and Zhang [17], the linear trend is oversimplified and does not take into account magnetisation reversal processes which are expected to be different when the order of shell and core materials changes. Moreover, the significant contribution of the surface spins in the magnetic anisotropy of single magnetic phase CoFe 2 O 4 nanoparticles has been observed [21]. For core/shell systems, a phenomenological model considering both surface and interface effects was developed by Trohidou et al which estimated K eff for a two-phase exchange-coupled nanoparticle system [22]:…”

Magnetic Properties of Bi-Magnetic Core/Shell Nanoparticles: The Case of Thin Shells

“…The f (∆T B ) can be fitted by a (log-)normal function because T B is proportional to the particle volume, which is well-described by a (log-)normal function of the particle size distribution (Equations ( 2) and ( 3)) [22,25,26]; for the determination of T B in the case of unmerged ZFC/FC, the d(M FC −M ZFC )/dT is fitted by the normal function, then the average <T B > is determined as a temperature at which d 2 (M FC − M ZFC )/dT 2 = 0, to the left of the M ZFC peak position [24].…”

Multifunctional Fe3O4-Au Nanoparticles for the MRI Diagnosis and Potential Treatment of Liver Cancer

“…The reversibility of the ZFC/FC magnetization curves above 140 K confirms the superparamagnetic nature of the particles at room temperature. The irreversibility temperature (T irr ), determined as temperature when M FC and M ZFC curves merge [51,52], was found at about 140 K. Above T irr , all the particles are in the superparamagnetic state characterized by the presence of the thermal reversibility in the magnetic behavior (absence of remanent magnetization). With the decreasing of temperature below T irr , the particles are becoming "blocked" and present a ferro (i) magnetic-like behavior.…”