Distinguishing magnetic blocking and surface spin-glass freezing in nickel ferrite nanoparticles

Abstract

Nickel ferrite nanoparticles dispersed in SiO2 matrix have been synthesized by sol-gel method. Structural analysis has been performed by using x-ray diffraction and transmission electron microscopy. Magnetic properties have been investigated by using superconducting quantum interference device magnetometry. In addition to the average blocking temperature peak at TB=120 K measured by a zero field cooled temperature scan of the dc susceptibility, an additional hump near 15 K is observed. Temperature dependent out-of-phase ac susceptibility shows the same features: one broad peak at high temperature and a second narrow peak at low temperature. The high temperature peak corresponds to magnetic blocking of individual nanoparticles, while the low temperature peak is attributed to surface spin-glass freezing which becomes dominant for decreasing particle diameter. To prove the dynamics of the spin (dis)order in both regimes of freezing and blocking, the frequency dependent ac susceptibility is investigated under a biasing dc field. The frequency shift in the “frozen” low-temperature ac susceptibility peak is fitted to a dynamic scaling law with a critical exponent zv=7.5, which indicates a spin-glass phase. Exchange bias is turned on at low temperature which signifies the existence of a strong core-shell interaction. Aging and memory effects are further unique fingerprints of a spin-glass freezing on the surface of isolated magnetic nanoparticles.

Spin dynamics and criteria for onset of exchange bias in superspin glass Fe/γ-Fe2O3core-shell nanoparticles

“…In the inset of Figure 3a, the frequency dependence of the real part of ac susceptibility (χ') is shown. The peak temperature (T P ) shifts to higher values as frequency increases, which is consistent with other reports 9,10 . A systematic investigation of the evolution of peak shift in χ' is carried out.…”

“…EB has been reported in thin films with a conventional ferromagnet (FM)-antiferromagnet (AFM) interface 8 ; and also in other magnetic nanostructures having ferrimagnetic (FIM) domains 9 , spin glasses 10 or surface spin disorder 11 . In the case of thin films, the origin of EB is well understood and is attributed to the unidirectional anisotropy developed at the FM-AFM interface due to pinning of magnetic moments.…”

Comparison of surface effects in SiO2 coated and uncoated nickel ferrite nanoparticles

“…6(a) and (b) shows the in-phase and out-ofphase AC susceptibility of both samples, respectively, at frequency f = 1000 Hz and AC field amplitude H ac = 2 Oe. In-phase part of coated nanoparticles shows a hump at low temperatures, while out-of-phase part exhibit two peaks; one at the higher temperatures corresponds to magnetic blocking of huge core spin and the other at lower temperatures due to disordered surface spins [35]. The low temperature peak in these type of nickel ferrite nanoparticles dispersed in SiO 2 matrix is already investigated by us in Ref.…”

Magnetic properties of glass-ceramics obtained by crystallization of iron-rich borosilicate glasses

“…) has been attributed to the spin-glass-like freezing of the disordered spins located on the surface shell of the nanoparticles [43]. However, the relaxation time obeys Eq.…”