Dynamic susceptibility evidence of surface spin freezing in ultrafine NiFe₂O₄nanoparticles

Abstract

We investigated the dynamic behavior of ultrafine NiFe2O4 nanoparticles (average size D = 3.5 nm) that exhibit anomalous low temperature magnetic properties such as low saturation magnetization and high-field irreversibility in both M(H) and ZFC-FC processes. Besides the expected blocking of the superspin, observed at T1 approximately 45 K, the system undergoes a magnetic transition at T2 approximately 6 K. For the latter, frequency- and temperature-resolved dynamic susceptibility data reveal characteristics that are unambiguously related to collective spin freezing: the relative variation (per frequency decade) of the in-phase susceptibility peak temperature is approximately 0.025, critical dynamics analysis yields an exponent znu = 9.6 and a zero-field freezing temperature T(F) = 5.8 K, and, in a magnetic field, T(F)(H) is excellently described by the de Almeida-Thouless line delta T(F) = 1 - T(F)(H)/T(F) alpha H(2/3). Moreover, out-of-phase susceptibility versus temperature datasets collected at different frequencies collapse on a universal dynamic scaling curve. All these observations indicate the existence of a spin-glass-like surface layer that surrounds the superparamagnetic core and undergoes a transition to a frozen state upon cooling below 5.8 K.

Colloidal magnetic nanocrystal clusters: variable length-scale interaction mechanisms, synergetic functionalities and technological advantages

“…Similar characteristics have been also observed for nanocrystal assemblies with 35% [54] and 44% [56] magnetic material volume fractions. The memory effects at low temperature are accompanied by a spin glass freezing due to the surface defect environment of the incorporated nanocrystals [52,[121][122][123]. A canted surface spin structure has been found previously in antiferromagnetic NiO nanocrystals ( Figure 16A) [122,124] and also in ferrimagnetic ferrite nanocrystals ( Figure 16B) [123].…”

“…The memory effects at low temperature are accompanied by a spin glass freezing due to the surface defect environment of the incorporated nanocrystals [52,[121][122][123]. A canted surface spin structure has been found previously in antiferromagnetic NiO nanocrystals ( Figure 16A) [122,124] and also in ferrimagnetic ferrite nanocrystals ( Figure 16B) [123]. In addition, a minimal exchange bias field has been measured in maghemite nanoclusters stabilized with polyacrylic acid [21].…”

“…In this system though, the maximum χ″(T) at low temperature is attributed to superparamagnetism [90] and not to surface spin disorder like that established in polyacrylic acid-capped maghemite nanoclusters [21]. In such dendron-capped magnetite nanoclusters, the Vogel-Fulcher law has been utilized to fit the ac susceptibility data, and the resultant value of the attempt time was found to be comparable to that reported for nanoparticulate systems with intermediate-strength dipolar interactions [123]. The strength of the dipolar interactions among the particles and the absence of a spin glass freezing in the low temperature regime were also supported by the estimation of the relative variation of the χ″ peak temperature position per frequency decade, known as the Mydosh parameter, which is given from the equation: …”

Domain size correlated magnetic properties and electrical impedance of size dependent nickel ferrite nanoparticles

“…The decrease in M s originates from the surface effects, high surface to volume ratio, and canted spins on the particle surface. [4][5][6] Therefore synthesis and processing of nanoparticles have been a topic of great interest for large researchers in the field of nanoscience and nanotechnology. In order to obtain the homogeneous, fine and reproducible nanoparticles of high purity, the wet-chemical method like co-precipitation, hydrothermal processing, sol-gel, etc have been used.…”

“…Among these we followed the sol-gel method as a novel and easy route to synthesize magnetic nanoparticles in low temperature range between 300-600 o C. Ferrite nanoparticles show magnetic irreversibility at high fields, higher H c , slightly lower M s as compared to their bulk and shows a change in Neel temperature with altered magnetic moments due to lower grain size. The size effects of ferrite particles have been discussed by Ronald et al, 6 and Niu et al, 7 while studying the mechanism of ferrimagnetism in nano structured ferrites in terms of anisotropy induced by the internal elastic micro strain. The disordered and canted surface spins affect the magnetic, electrical, and dielectric properties due to the smaller grains and grain boundaries.…”