Fine particle magnets also occur inside of us, such as in the hydrated iron oxyhydroxide cores ... These factors include such things as inter- particle interactions ...
Hyperfine Interactions 90(1994)201-214
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The magnetism of fine particle iron oxides and oxyhydroxides in applied fields Q.A. Pankhurst 1 Department of Physics, University of Liverpool, Liverpool L69 3BX, UK
The magnetic properties of a series of fine particle iron oxides, oxyhydroxides and hydrated oxyhydroxides are discussed with reference to the data obtained from lowtemperature, high-field MOssbauer spectroscopy experiments, using a two-sublattice atomic spin Hamiltonian model to help in the interpretation of the results.
1.
Introduction
The study of fine particle magnetism is a fascinating, lively and important pursuit. Naturally occurring fine particle magnets surround us, in rocks, soils and coals, as well as in living organisms: bacteria, molluscs, plants, fish, birds and mammals. Fine particle magnets also occur inside of us, such as in the hydrated iron oxyhydroxide cores of iron storage proteins found in the liver and the spleen, and as agglomerates of iron oxide particles in the brain. In addition to these forms, manufactured fine particles proliferate, as in magnetic fluid bearings and seals, and in particulate magnetic recording media. As such, they form an intrinsic and vital part of the everyday technology of the industrialised world. It is in this broad context that the potential ramifications of a new or improved understanding of the physics of fine particle magnetism become apparent. For example, in palaeomagnetism the remanent magnetization of magnetic minerals (mostly fine particle iron oxides) in rocks can be used to study the historic excursions and reversals in the direction of the earth's magnetic field. Recent work on domain stability as a function of grain size, shape and composition may lead to improved methods of interpreting the remanent magnetization in terms of the events contributing to its formation, and therefore to a better understanding of geomagnetism [1]. Magnetic particles also have many medical applications of potential significance. For instance, one method of treating cancer is to direct fine particle magnetite (Fe304) to tumour sites using a liquid suspension of magnetite and dextran (a bloodplasma substitute), and to subsequently use a radio-frequency generator to locally heat the magnetite and kill the targeted cells [2]. In another application, antibodies labelled with magnetite particles may be used to detect very low concentrations of viral antigens. The magnetised antibodies attach themselves to the target viruses, IPresent address: Dept. of Physics and Astronomy, University College London, London WC1E 68T, UK.
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Q.A. Pankhurst / The magnetism of fine particle iron oxides
and a magnetic field is then applied; the magnetic particles are attracted to the surface of the specimen, resulting in a lifting up of a small surface area, which may be detected and measured using a laser interferometer. Using this method, concentrations as low as 0.1 pg/ml of HIV (human immunodeficiency virus) antigen can be detected in human blood-serum [3]. Very recently, a novel magnetic protein, "magnetoferritin", has been produced by the controlled reconstitution of maghemite (T-Fe203) into 8 nm diameter spherical cavities inside the iron-storage protein ferritin [4,5]. The development of a substance that is tantamount to a biocompatible ferrofluid may lead to the development of many as yet unforeseen applications. In industrial applications of magnetic fine particles, there are many areas where a better comprehension of the physical properties of the particles could lead to significant advances in product development. An obvious example is the continuing world-wide research effort into increasing the information storage capacity of particulate magnetic recording media. Other examples include research into more stable and durable magnetic fluids for use in leak-tight O-ring seals (as in lasers, sputtering systems and computer disk drives) and more sensitive magnetic fluids for use in Bitter-method magnetic imaging (as in non-destructive testing for structural defects in massive steel structures). In the light of potential benefits such as these, it is not surprising that there are currently a large number of research programs, in both academic and commercial laboratories, that in one way or another have as an objective the improved understanding of the fundamental physical properties of fine particle magnetism. The problem is a difficult one. There are a host of complicating factors which, while also present in bulk systems, only become significant in fine particle systems - particles with size dimensions on the mircron or nanometre scale. These factors include such things as interparticle interactions, impurities, structural disorder, surface effects, sublattice occupancy imbalances, shape anisotropy, superparamagnetism and incoherent magnetization reversal mechanisms. It is research into the origins and properties of phenomena such as these that forms the basis for current studies of fine particle magnetism [6]. In this paper, one aspect of current research into the properties of fine particle magnets will be reviewed, namely the identification and characterisation of the magnetic ground state (i.e. the magnetic structure at zero or low temperature) of a particulate system. The discussion will centre on a selection of commonly encountered fine particle iron oxides, oxyhydroxides and hydrated oxyhydroxides. The experimental technique used for this purpose is low-temperature, high-field M/Sssbauer spectroscopy, and the data analysis technique is based on a two-sublattice spin Hamiltonian model. Both these techniques are introduced in the following sections.
2.
Applied field Mfissbauer spectroscopy of fine particle magnets
M/Sssbauer spectroscopy is a popular experimental technique for studying the magnetic properties of fine particle systems. The reasons for this are quite straightforward.
Q.A. Pankhurst / The magnetism of fine particle iron oxides
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To begin with, the majority of the fine particle magnets that are of scientific interest both from the basic and applied viewpoints contain iron as the magnetic element. This is immediately advantageous with regard to the M/Sssbauer technique, since 57Fe M/Sssbauer spectroscopy is well known as a cheap, reliable and sensitive method for studying the electronic and magnetic states of iron. The fact that the specimens to be studied are polycrystalline in nature means that absorbers of optimum thickness many be prepared, maximising the observable signal-to-noise ratio. Homogeneous samples are also readily prepared, which is a factor that may simplify the subsequent analysis of the data - this is particularly relevant when the only available alternative specimens are poorly characterised mosaic-like single crystals. The use of low-temperature, high-field M/Sssbauer spectroscopy to establish the magnetic ground state of a fine particle system is a less common practise, but nonetheless is a logical and effective approach. In establishing the ground state of a magnetic material, it is pertinent to reduce the sample temperature as much as is conveniently possible. In most cases, a liquid helium bath cryostat, offering sample temperatures from 4.2 K in ambient conditions to 1.3 K in reduced vapour pressure mode, provides sufficiently low temperatures. The use of large applied fields, typically in the range 6 - 1 2 T, is essential in enabling the accurate identification of the magnetic ground state. This is because the true magnetic state is often only distinguished by observing the response of the system to an applied field. This is illustrated in fig. 1, where the 4.2 K zero-field and high-field (mostly 9 T) M/Sssbauer spectra of several fine particle iron oxyhydroxides and hydrated oxyhydroxides are shown. It is clear on inspection of fig. 1 that the zero-field spectra are all very similar. In contrast, the high-field spectra are markedly different, exhibiting distinctive responses to the applied field. These distinctive responses contain the information that allows the interpretation and identification of the magnetic ground states of the different materials. There is one other advantage of the applied-field M6ssbauer technique which deserves a mention. It is that the M/Sssbauer spectra contain signals from every magnetic (i.e. iron) atom in the particle, irrespective of whether they are in the core of the particle, or on the surface. This may be an important consideration. For example, a bulk magnetization measurement of a fine particle may reveal a small net magnetic moment. It is not always clear from the bulk measurement whether or not such a net moment reflects an inherent ferrimagnetic magnetic ground state, or alternatively an antiferromagnet with some unpaired moments. In the latter case, the magnetization measurement does not see any signal from the bulk of the antiferromagnetically coupled moments, whereas the applied-field M6ssbauer measurement does. Such an effect has been observed in fine particle goethite (ct-FeOOH), where remanent magnetization was observed in particles of size of order 10 nm x 30 nm x 100 nm [7], while the applied-field MiSssbauer spectra of the same sample revealed a predominantly antiferromagnetic response [8].
Q.A. Pankhurst I The magnetism of fine particle iron oxides
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