In-Field Orientation and Dynamics of Ferrofluids Studied by Mössbauer Spectroscopy

Abstract

By studying the response behavior of ferrofluids of 6–22 nm maghemite nanoparticles in glycerol solution exposed to external magnetic fields, we demonstrate the ability of Mössbauer spectroscopy to access a variety of particle dynamics and static magnetic particle characteristics at the same time, offering an extensive characterization of ferrofluids for in-field applications; field-dependent particle alignment and particle mobility in terms of Brownian motion have been extracted simultaneously from a series of Mössbauer spectra for single-core particles as well as for particle agglomerates. Additionally, information on Néel superspin relaxation and surface spin frustration could be directly inferred from this analysis. Parameters regarding Brownian particle dynamics, as well as Néel-type relaxation behavior, obtained via Mössbauer spectroscopy, have been verified by complementary AC-susceptometry experiments, modulating the AC-field amplitude, and using an extended frequency range of 10–1 to 106 Hz, while field-dependent particle alignment has been cross-checked via magnetometry.

Mechanism of magnetization reduction in iron oxide nanoparticles

“…This results in a magnetic anisotropy energy of 27 kJ m −3 , which is comparable to previously reported values for similar particles 51,52 and significantly larger than the maghemite magnetocrystalline anisotropy of K = 4.7 kJ m −3 obtained from measurements on single crystal films and powders. 53 The close proximity of the splitting of the curves to the maximum of the ZFC curve points to a narrow particle size distribution 50 which is consistent with the results obtained from SAXS experiments.…”

Ferrofluids and bio-ferrofluids: looking back and stepping forward

“…Different methodologies have been developed in this respect in the last years, according to reports. 242–244…”

“…Different methodologies have been developed in this respect in the last years, according to reports. [242][243][244] 3.4 Magneto-optics: light scattering Transmission electron microscopy investigations are limited to highly diluted and dried/vitrified samples; therefore, these refer mainly to particle size distributions, without details concerning structuring processes themselves, which require the use of various scattering techniques (Fig. 13).…”

Magnetic response of CoFe2O4 nanoparticles confined in a PNIPAM microgel network

“…This will strongly reduce the microgel particle net magnetic moment m MG $ ffiffiffiffi N p Á m MNP À Á , as is sometimes observed in magnetic nanoparticle agglomerates. 57,58 Thereby, the distribution extracted from the phenomenological model cannot be converted easily into a number distribution of MNPs in a specific state of confinement. For this, further knowledge about the microgels' magnetic structure including nanoparticle interaction effects and nanoparticle rheology would be required.…”