Ensemble Of Single-domain Particles Research Articles

Relaxation Mössbauer Spectroscopy of Iron Oxides Nanoparticles in Superferrimagnetic Regime

Film composites based on magnetite Fe3O4 nanoparticles in the polyvinyl alcohol (PVA) matrix and the dried ferrofluid ARA-250 (Chemicell GmbH, Germany) were studied at temperatures 78-300 K by Mössbauer spectroscopy and magnetic measurements in magnetic fields H < 5 kOe. The Mössbauer spectra taken in magnetic fields show essential restoration of the magnetic hyperfine structure characteristic for the nanoparticles. The experimental Mössbauer spectra have been analyzed within the recently developed three-level [ and multi-level [ relaxation models of magnetic dynamics in an ensemble of single-domain particles. These models take into account the precession and diffusion of particles uniform magnetization as well as the particles size distribution and interparticle interactions.

On the mechanism of the temperature evolution of the symmetric magnetic hyperfine structure of Mössbauer spectra of magnetic nanoparticles to the quadrupole doublet of lines

The multilayer relaxation model of Mossbauer spectra of an ensemble of single-domain particles has been generalized to the case of the presence of the electric field gradient on nuclei with a chaotic orientation of its principal axes. The generalized model makes it possible to take into account the physical mechanisms of the formation of the hyperfine structure of the spectra in the real situation and to numerically describe the qualitative features of the temperature evolution of the spectra from symmetric magnetic sextet to the quadrupole doublet of lines, which is observed in most experimental spectra of 57Fe nuclei in magnetic nanoparticles.

Super-ferrimagnetism of magnetite nanoparticles in a weak magnetic field

Film composites based on the magnetite Fe3O4 nanoparticles in the polyvinyl alcohol matrix were studied by means of the Mössbauer spectroscopy in a weak magnetic field, mainly at room temperature. The Mössbauer spectra in the absence of a magnetic field show a superposition of minor (poor-resolved) magnetic hyperfine structure and major collapsed spectrum typical for superparamagnetic particles. Such a pattern evidences for the mean particles size (<D> ≈ 4 nm) being close to the 'blocking' size for this material at a given temperature. The spectra taken in a weak (0.34 T) magnetic field show almost complete restoration of the magnetic hyperfine structure characteristic for the nanoparticles of the kind. The experimental Mössbauer spectra have been analyzed within the recently developed three-level relaxation model of magnetic dynamics in an ensemble of single-domain particles. This model takes into account the precession and diffusion of particle's uniform magnetization as well as the particles size distribution.

External field-induced superferrimagnetism in magnetite nanoparticles

Film composites based on magnetite nanoparticles with a mean diameter of 4 nm in a polyvinyl alcohol matrix were studied by means of Mossbauer spectroscopy in a magnetic field of up to 3.4 kOe. The spectra were analyzed within the three-level relaxation model of the magnetic dynamics of an ensemble of single-domain particles, taking into account the precession and diffusion of the homogeneous magnetization of particles and their size distribution.

Response from M.A. Chuev

A response is given to critical and erroneous comments on my results reported in Pis’ma Zh. Eksp. Teor. Fiz. 85, 744 (2007) [JETP Lett. 85, 611 (2007)]. A more accurate magnetic dynamic model has been proposed for the calculation of the magnetization curves of an ensemble of single-domain particles. The preliminary calculations confirm that the asymptotic behavior in weak magnetic fields differs from the Langevin limit.

Coercivity mechanism in CoPt alloy films with perpendicular magnetic anisotropy (abstract)

The coercivity is one of the key parameters for defining a suitable magneto-optic recording material. In order to get insight into coercivity mechanisms we have studied the magnetization reversal processes by means of magneto-optic methods in CoPt alloy films. These films were prepared in UHV by coevaporation with a typical composition ratio of Co28Pt72 and show excellent perpendicular magnetic anisotropy at elevated substrate temperatures during deposition. STM and x-ray measurements reveal characteristic grain size of about 15–20 nm and dispersion of crystallite axis of the order of 0.5°, respectively, which varies only slightly with total film thicknesses of 5–50 nm. The magnetization reversal processes are investigated by using polar Kerr microscopy and were found to be dominated by domain formation and domain wall propagation. A micromagnetic model was developed which describes the film as an ensemble of single domain particles. The size of these single domain particles was chosen to be of the order of the average grain size. In the numerical simulation we include dipolar interaction and domain wall energy contribution. The comparison of the results of the simulation with the experimental results clearly points out that the microstructure is responsible for the magnetization reversal behavior. A thermal as well as thermally activated magnetization processes contribute to the reversal mechanism. The coercivity is predominantly determined by pinning on grain boundaries.