H.-D. Pfannes

Mössbauer spectroscopy, superparamagnetism and ferrofluids

Some fundamentals of Mossbauer spectroscopy and of fluctuating magnetic hyperfine interactions are reviewed. An expression for calculation of spin-phonon interaction transition probabilities on the ground of a Debye model is given. Basic ideas of superparamagnetism and classical results for low-temperature relaxation rates are presented. A theory of superparamagnetism based on spin-phonon interaction is introduced. It is shown how to calculate from this Mossbauer spectra taking all spin-levels into account. Experimental spectra of ferrofluids are compared with simulated spectra and found in good accordance.

Study of ferrofluids by Mössbauer spectroscopy

The parameters of the log‐normal size distribution of a MnFe2O4 ferrofluid powder sample have been determined by X‐ray diffraction. The mean blocking temperature was determined from the maximum of Xi. Mössbauer spectra at 4.2–300 K are interpreted by a new simple theory of superparamagnetism and taking a reduction of the internal magnetic field for small particles, a size dependence of the anisotropy constant, the size distribution and collective excitations into account.

Methods of Simulating Superparamagnetic Relaxation Mössbauer Spectra and Attempts of Foundation of a Microscopic Theory of Superparamagnetism

A spin-Hamiltonian that describes the orientational energy of nanoscopic magnetic particles is presented. A few short observations on spin tunneling are made. Basic ideas on superparamagnetism and classical results of calculations of relaxation times are presented. A model for spin–phonon interaction induced spin reversals is proposed and the relaxation time for spin jumps between next nearest spin levels (ΔSz=±2) is calculated. Mössbauer patterns are simulated by means of a multi-level method for spin transitions between next nearest and nearest (ΔSz=±1) spin levels. The simulations with (ΔSz=±1) show better concordance with experimental spectra and allow the use of a reduced spin, which speeds up considerably the calculations and allows to include particle size distribution.

Investigation of the Magnetic Behavior in Fe3O4 Ferrofluid Functionalized by Carapa Guianensis Oil

A ferrofluid based on Fe3O4 has been synthesized using the condensation method by coprecipitating aqueous solutions of FeSO4 and FeCl3 mixtures in NH4OH and treated further in order to obtain colloidal sols by creating a charge density on their surface and functionalized by carapa guianensis (andiroba oil). Aqueous sample with an average particle diameter ∼7 nm were studied by Mossbauer spectroscopy and dc magnetization measurements in the range of 4.2–250 K. The saturation magnetization (Ms) at 4.2 K was determined from M vs 1/H plots by extrapolating the value of magnetizations to infinite fields, to 5.6 emu/g and coercivity to 344 Oe. The low saturation magnetization value was attributed to spin noncollinearity predominantly at the surface. From the magnetization measurements a magnetic anisotropy energy constant (K) of 1×104 J/m3 was calculated. Fe3O4 spectra at room temperature showed a singlet due to superparamagnetic relaxation and a sextet at low temperature.

Calculation of Mössbauer spectra of superparamagnets based on a spin–phonon interaction like model

Abstract Mossbauer spectra with superparamagnetic relaxation under the presence of an external magnetic field are calculated using diagonal hyperfine interaction and a multilevel model. The magnetization is described by a large spin S and spin–phonon interaction spin–transition probabilities are calculated from a Debye model using S =3222, Δ S z =±2 and realistic parameters of the constants involved. It is shown that the calculated spectra exhibit the typical shape of experimental spectra. Spectra calculated with a reduced S =322 or 32 demonstrate almost identical patterns when the temperature is adequately reduced.

Ultra-Thin FexMn1−x Alloy Films on Ag(100)

Fe x Mn1−x ultra-thin alloy films were grown epitaxially on Ag(100) under ultra-high vacuum conditions by co-evaporation of 57Fe and Mn. Structural and magnetic characteristics of the alloy films were investigated by in situ Conversion Electron Mossbauer Spectroscopy as a function of Fe content (0.4 l) at two thicknesses (3 and 6 monolayers). X-ray photoelectron spectra and Mossbauer spectra were characteristic of Fe-Mn alloying. As indicated by Mossbauer spectra of the 6 monolayers thick films the ferromagnetic α-Fe x Mn1−x phase could be stabilized up to 28% of Mn content, higher than the maximum value of 20% reported previously for epitaxially grown films on GaAs(001) (C. Jing et al., J. Magn. Magn. Mater., 198–199 (1999) 270.). For higher Mn contents (x<0.5) the films exhibit typical γ-Fe x Mn1−x (and eventually ɛ-phase) spectra. However in strained alloy films a complete distinction between the bcc and fee phase is not possible. Most probably the films suffer an increasing tetragonal deformation with increasing Mn content, which could be responsible for the magnetic transition observed in spectra.

Simulation of Mössbauer Spectra of Superparamagnets under External Fields Using a Many-State Model

AbstractThe magnetization in a uniaxial superparamagnetic particle is modelled by spin-phonon relaxation of a giant spin. A multi-level formalism is used to simulate Mössbauer superparamagnetic relaxation spectra of spatially uniformly oriented particles. A longitudinal (parallel to anisotropy direction) external magnetic field

Influence of particle size on superparamagnetic relaxation of MnFe2O4 particles in ferrofluids

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