D. H. Jones

The behaviour of superparamagnetic small particles in applied magnetic fields: A Mössbauer spectroscopic study of ferritin and haemosiderin

Abstract Small magnetically ordered particles display characteristic temperature dependent superparamagnetic behaviour in their Mossbauer spectra, as a result of the interplay between the thermal energy and the magnetic anisotropy energy of each particle. In the presence of an applied magnetic field there is an additional energy term arising from the net magnetic moment of the particles. The resulting Mossbauer spectra depend on the size of the anisotropy energy relative to the energy of the magnetic moment of the particles in the applied field. The present applied field Mossbauer measurements on ferritin and haemosiderin indicate that in these materials the anisotropy energy dominates, although the spectra can still be analysed to give information on the magnetic moment of the particles.

A re-examination of models of superparamagnetic relaxation

Abstract Superparamagnetic relaxation is usually discussed in terms of the classical fluctuation models of Neel and of Brown. These models are re-examined and it is found that Browns approach, though more sophisticated, is fundamentally different from Neels only in that Brown equates the magnetic fluctuation energy to k B T , while Neel equates the vibrational fluctuation energy to k B T . In applications to magnetic structures other than the ferromagnet it is the older model, Neels, which gives the more physically reasonable results for particles of less than 10 6 atoms. Both classical fluctuation models fail to define the correlation time for the fluctuations in a consistent way. Consideration of this problem leads to the conclusion that small static anisotropy fields may play a role in the relaxation process. The effect of quantum fluctuations on the relaxation of small particles is discussed.

Mössbauer studies of site preference, relaxation and magnetic order in YBa2(Cu1-xFex)3O7-δ

Abstract The substitution of other transition metals for copper in YBa2Cu3O7-δ changes the properties of the superconductor in ways which, if understood, may lead to a greater understanding of the superconductivity. Questions concerning the site of the substitution, oxygen coordination and oxidation state arise. Fits to the Mossbauer spectra of YBa2(Cu1-xFex)3O7-δ with x = 0.01, 0.02 and 0.04 over a temperature range down to a few degrees Kelvin show that the dominant components of the spectra are quadrupole doublets having splittings of 2.0mms-1 and 1.3mms-1. The magnetically split spectra at temperatures of 4.2K and below show broadening which can be fitted to the standard relaxation model of Blume and Tjon. The evidence that such magnetic behaviour arises from iron atoms substituting on Cu(1) sites at twinning boundaries is discussed.

Mössbauer spectroscopy and the physics of superparamagnetism

It is argued that superparamagnetism, as well as being a useful diagnostic for small magnetic particles, contains interesting physics associated with the detailed nature of the relaxation process, its dependence on magnetic structure and also with spin fluctuations characteristic of small particles. The study of this physics will require more sophisticated treatment of experimental data than that which is conventionally used to derive an approximate size distribution.

The charge state of 57Fe doped into YBa2Cu3O7−x

Abstract Mossbauer spectra of YBa 2 Cu 2.97 57 Fe 0.03 O 7−x are interpreted to show that the dopant 57 Fe ions exist in a charge state well represented as Fe 4+ and that this is the charge state that should be expected on the basis of comparison with analogous materials.

A Mössbauer study of ferrihydrite and aluminium substituted ferrihydrites

Abstract Variable temperature Mossbauer measurements have been made on ferrihydrite and 8–92% aluminium substituted ferrihydrites prepared by rapid evaporation of concentrated nitrate solutions. Electron microscopy and X-ray diffraction have been used to characterise the materials more completely than is possible by Mossbauer spectroscopy alone. The Mossbauer data show a doublet to sextet transition at a temperature and of a character which depends on the aluminium concentration. This and other results are discussed in terms of the influence of particle size and interactions on the relaxation effects observed.

A study of the parameterisation of the uniaxial model of superparamagnetic relaxation

Mössbauer spectra of the iron storage protein ferritin taken over a range of temperatures exhibit the characteristic behaviour of superparamagnetic relaxation, and can be used to investigate a model for the superparamagnetic relaxation of a sample of small magnetic particles. A computer program is used to calculate a series of Mössbauer spectra based on the relaxation model, and these theoretical spectra are then fitted simultaneously to all the variable temperature experimental spectra, with the parameters of the uniaxial anisotropy model as the only variables.

Spin fluctuctuation phenomena in small particle magnetic systems: A Mössbauer spectroscopic study of collective excitations and superparamagnetic relaxation in haemosiderin

Mössbauer spectra obtained from a sample of the iron-storage protein haemosiderin have been analysed in both the collective excitation and superparamagnetic relaxation temperature regions. The results indicate that the low temperature spectra cannot be explained in terms of collective excitations, and that there must be other effects leading to the observed distribution of hyperfine fields.

Superparamagnetism in an aluminium substituted ferrihydrite

Mössbauer spectra of aluminium doped ferrihydrites prepared by rapid evaporation of a nitrate solution are shown to exhibit doublet/sextet coexistence characteristic of superparamagnetism. In undoped ferrihydrite an additional broad “transitional” component is observed over a considerable temperature range; this component is absent from the spectra of samples with >66% aluminium. Possible causes of this change in behaviour on aluminium doping, including superferromagnetism, non-collinearity, ionic spin relaxation in the pure material, and the isolation of small clusters beyond the percolation limit in the doped material, are discussed.

A Mossbauer investigation of critical fluctuations in antiferromagnets

As the magnetic ordering temperature is approached from below, Mossbauer spectra of the quasi-one-dimensional, two-dimensional and three-dimensional antiferromagnets K2FeF5, KFeF4 and Cs2FeCl5.H2O are found to show broadening and change in lineshape which are characteristic of a slowing of spin relaxation rates. This phenomenon is attributed to critical fluctuations similar to those found above the ordering temperature. The reduced temperature range showing these fluctuations is found to depend on the critical exponent beta .