Correlation of superparamagnetic relaxation with magnetic dipole interaction in capped iron-oxide nanoparticles

Abstract

Six nanometer sized iron-oxide nanoparticles capped with an organic surfactant and/or silica shell of various thicknesses have been synthesized by a microemulsion method to enable controllable contributions of interparticle magnetic dipole interaction via tunable interparticle distances. Bare particles with direct surface contact were used as a reference to distinguish between interparticle interaction and surface effects by use of Mössbauer spectroscopy. Superparamagnetic relaxation behaviour was analyzed by SQUID-magnetometry techniques, showing a decrease of the blocking temperature with decreasing interparticle interaction energies kBT0 obtained by AC susceptibility. A many-state relaxation model enabled us to describe experimental Mössbauer spectra, leading to an effective anisotropy constant Keff ≈ 45 kJm(-3) in case of weakly interacting particles, consistent with results from ferromagnetic resonance. Our unique multi-technique approach, spanning a huge regime of characteristic time windows from about 10 s to 5 ns, provides a concise picture of the correlation of superparamagnetic relaxation with interparticle magnetic dipole interaction.

Annealing-dependent structural and magnetic properties of nickel oxide (NiO) nanoparticles in a silica matrix

“…The temperature at which such a maximum in the ZFC curve occurs is usually referred to as the blocking temperature, below which the magnetic moments are frozen in fixed directions or "blocked" [53,54]. The blocking of a superparamagnetic particle is related to its magnetic anisotropy energy E a [55]. Moreover, the blocking temperature is defined as the temperature at which the relaxation time of the magnetic moments is equal to the timescale of the measurement technique ( m ) [56].…”

On the interpretation of Mössbauer spectra of magnetic nanoparticles

“…Landers et al [ 40] A three-level model was used (ܵ = 1ሻ. ‫ܥ‬ was therefore calculated using ‫ܥ‬ = ሺܴ େ ሻ ିଵ ߪ ଵ/ଶ .…”

“…Rather, they can be described as sextet components with asymmetrically broadened lines that are increasingly broadened with increasing temperature. This has been seen in, for example, several Mössbauer studies of hematite, maghemite and goethite nanoparticles, see, for example [ 31,[33][34][35][36][37][38][39][40][41][42][43][44][45]. Two competing explanations for the line shape have been made in the literature: (1) that the relaxation is influenced by interparticle interactions; (2) that the particles are non-interacting, but the intrinsic value of ‫ܥ‬ is relatively large.…”

Magnetic Properties of Poly(trimethylene terephthalate‐block‐Poly(tetramethylene oxide) Copolymer Nanocomposites Reinforced by Graphene Oxide–Fe3O4 Hybrid Nanoparticles

“…Two resonance techniques-ferromagnetic resonance (FMR) and Mössbauer spectroscopy usually deliver complementary magnetic characteristics. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] Recent intense research on graphene and its derivatives follows the trend of exploration of novel low-dimensional systems and is driven by possible promising applications of graphene-based magnets in spintronics. However, due to delocalized π bonding network, graphene lacks localized magnetic moments and is intrinsically nonmagnetic.…”