Academics and actors do not usually mix with one another, but Liverpool University is fortunate in having a long-standing relationship with the city's Everyman Theatre. Two years ago these links led me to organize a project called GetSET97 as part of the UK's national week of science, engineering and technology The idea was to produce a revue consisting of short performances by groups from local schools, based round scientific and technological themes.

Biological systems can be used both to explore the properties of and to provide novel examples of nanophase magnetic materials. The Mössbauer spectroscopic measurements of the iron-storage protein ferritin and its derivatives, discussed in this paper, provide examples of both these aspects.

Mössbauer spectroscopy has been used to investigate metal particle powders and tapes. The results indicate a significant non-magnetic fraction in the powders and substantial out-of-plane orientation of the magnetic moments in the tape.

The effects of milling time and heat treatment on local structural changes during mechanical alloying Fe40Co10Cu50 have been investigated by means of x-ray diffraction (XRD), neutron diffraction technique, vibrating sample magnetometer, and Mössbauer spectroscopy. Mössbauer spectroscopy showed that two kinds of magnetic phases coexist in the mechanically alloyed powder, which is in agreement with XRD and neutron diffraction study. One is corresponding to α-Fe and the other is a new magnetic phase formed during mechanical alloying. With increasing processing time, the new magnetic phase becomes more ordered and dominant. Mössbauer spectrum from the annealed sample consists of ferromagnetic component with a hyperfine field of 36.2 T and a paramagnetic component.

The relationship between crystallinity and magnetocrystalline anisotropy has been studied in a series of ultrafine barium ferrite samples prepared using the citrate precursor method. Mössbauer data have been collected at 4.2 K in an applied field of 9 T, and effective anisotropies BA have been estimated using a simple two sublattice model. For relatively well-crystallised particles (as evidenced by sharp XRD and Mössbauer lines) a correlation is observed between particle size and anisotropy, with BA ≈ 2.5 T in 50 nm particles, and BA ≈ 1.3 T in 100 m, particles. A non-crystalline phase (evidenced by strongly broadened Mössbauer lines) seen in some of the smallest samples had a larger anisotropy, BA ≈ 7.3 T.

Polarised Mössbauer spectroscopy was used to measure the in-plane and out-of-plane angles of the magnetic moments in tape and powder samples of Co: γ-Fe2O3 in an applied field of 250 mT. The results indicate better alignment in the case of the tape sample, and demonstrates the usefulness of this technique.

Mössbauer spectroscopic and magnetic measurements have been made on a novel magnetic protein produced by the controlled reconstitution of ferritin. The data indicate that the predominant mineral form in the iron-containing cores is maghemite (γ-Fe2O3) rather than magnetite (Fe3O4).

Applied field Mössbauer spectra obtained from ferritin and haemosiderin have been fitted using a spin-Hamiltonian model, to determine the effect of the applied field on the direction of the iron magnetic moments. The results of these fits indicate that the ordering of magnetic moments within the ferrihydrite core of these proteins is antiferromagnetic. The values of the anisotropy field obtained from these fits correspond to much higher values of the superparamagnetic blocking temperatures than those actually observed. This anomaly has implications for our understanding of the superparamagnetic relaxation processes in these materials.

Ultrafine particles of barium ferrite in the size range 5–100 nm have been synthesized by thermal decomposition of a citrate precursor. The precursor decomposed at 425°C is amorphous, but crystalline barium ferrite starts forming at temperatures of 550°C and above. Barium ferrite which shows a monophase X-ray diffraction pattern and well-resolved Mössbauer spectra is obtained at 700°C. The Mössbauer spectra at both liquid helium and room temperature of samples annealed at 700, 750 and 800°C (with average particle sizes in electron micrographs of 60, 80 and 100 nm, respectively) could be satisfactorily resolved into five components corresponding to the five sublattice sites in barium ferrite. The Mössbauer parameters: magnetic hyperfine field, quadrupole splitting and isomer shift, all show particle size dependence. The magnetic hyperfine field and quadrupole splitting are smaller for smaller particles (compared to bulk barium ferrite) and they increase with increasing particle size and annealing temperature for all the five sublattice sites. Isomer shifts also differ from bulk values but the variation with particle size is also dependent on the sublattice sites.

Mössbauer spectra were recorded of tissue from β-thalassaemia/haemoglobin E spleen, liver, pancreas and heart and of crude haemosiderins (insoluble iron fractions) isolated from the organs. Iron in the crude haemosiderins from the spleen and heart remains paramagnetic below 4.2K indicating that the iron is in a non-crystalline form. Superparamagnetic behaviour of the crude haemosiderins from the pancreas and liver indicate the presence of ferrihydrite cores with some cores with a structure based on defect-goethite.

Two examples of biological magnetic fine particles are considered: iron-storage proteins and magnetotactic bacteria. These cover many aspects of this field, including the use of well-defined biological systems for testing theoretical models, using magnetic properties to distinguish between different biological materials, producing magnetic materials by biological processes, and using optimised biological magnetic systems as a guide to the production of synthetic magnetic materials.

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.

Mössbauer spectroscopy has shown that the iron-containing cores of the biological iron storage material haemosiderin produced under normal and various pathological conditions are significantly different in their magnetic properties. The differences have been correlated with information on the particle size, morphology, crystallinity and mineral form of the haemosiderin cores obtained by complementary electron microscopy and electron diffraction studies. These results have important implications for the use of Mössbauer spectroscopy in determining the properties of small particle magnetic systems and also considerable relevance for the improved understanding and treatment of iron overload disease.

Mössbauer spectra of the microcrystalline cores of thalassaemic human spleen ferritin and haemosiderin in applied magnetic fields have shown that the magnetic anisotropy dominates over the energy term arising from the net magnetic moments of the microcrystals in the applied field. The spectra have been analysed to give information on the net magnetic moments of the protein cores.

Mössbauer spectra obtained from the haemosiderin isolated from ironoverloaded horse and reindeer spleen and liver show superparamagnetic behaviour with a much lower transition temperature than haemosiderin from iron-overloaded human spleen. These data clearly indicate the existence of different forms of haemosiderin, which are primarily distinguishable by Mössbauer spectroscopy.

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.

This paper discusses recent work showing the application of Mössbauer spectroscopy to the study of the properties of the magnetically ordered materials which occur in a variety of biological systems. These materials display a diversity of behaviour which provides good examples of the various possibilities which can arise with iron-containing particles of different compositions and sizes.