K. O’Grady

Structural and magnetic properties of uniform magnetite nanoparticles prepared by high temperature decomposition of organic precursors

Magnetite nanoparticles (Fe3O4) of three different sizes below the limit for single domain magnetic behaviour have been obtained by thermal decomposition of an iron precursor in an organic medium in the presence of a surfactant. Good agreement between mean particle size obtained by TEM, crystal size calculated from x-ray diffraction and magnetic diameter calculated from magnetization curves measured at room temperature shows that the samples consist of uniform, crystalline and isolated magnetite nanoparticles with sizes between 5 and 11 nm. High saturation magnetization and high initial susceptibility values have been found, the latter decreasing as the particle size decreases. The main contribution to the anisotropy is magnetocrystalline and shape anisotropy, since surface anisotropy is suppressed by the oleic acid molecules which are covalently bonded to the nanoparticle surface.

A magnetic evaluation of interaction and noise characteristics of CoNiCr thin films

The nature of media recording noise in metallic, quasiparticulate thin films is principally related to the grain size, crystallographic orientation, and intergranular exchange and magnetostatic coupling in the films. In this study the results of a magnetic evaluation of magnetostatic interactions in CoNiCr thin films of varying Cr underlayer thickness are reported. The evaluation is undertaken through the measurement and comparison of remanence curves. The results presented here indicate enhanced cooperative switching as Cr underlayer thickness is reduced from 2000 to 100 A, with a strong correlation between signal‐to‐noise measurements. In addition, the transition from principally exchange‐coupled to quasiparticulate thin films, as Cr underlayer thickness increases, has been established.

Measurement of the anisotropy constant of antiferromagnets in metallic polycrystalline exchange biased systems

A method for the measurement of the anisotropy constant of the antiferromagnet (AF) KAF in exchange biased systems has been developed. This has been achieved by measurement of the median blocking temperature ⟨TB⟩ of a CoFe∕IrMn bilayer. In thermal activation-free conditions, this is the temperature at which equal volumes of the AF are oriented in opposite senses. Hence, for a grain size dependent model, the critical volume for thermal activation at this point is equal to the median volume of the grain size distribution. A value of (5.5±0.5)×106erg∕cc has been obtained at room temperature for a 4nm thick IrMn layer.

Structural and magnetic transformation of monodispersed iron oxide particles in a reducing atmosphere

Uniform metal iron ellipsoidal particles of around 200 nm in length were obtained by reduction and passivation of alumina-coated α–Fe2O3 (hematite) particles under different conditions of temperature and hydrogen flow rate. The monodispersed hematite particles were prepared by the controlled hydrolysis of ferric sulfate and further coated with a homogeneous thin layer of Al2O3 by careful selection of the experimental conditions, mainly pH and aluminum salt concentration. The reduction mechanism of α–Fe2O3 into α–Fe was followed by x-ray and electron diffraction, and also by the measurements of the irreversible magnetic susceptibility. The transformation was found to be topotactic with the [001] direction of hematite particles, which lies along the long axis of the particles, becoming the [111] direction of magnetite and finally the [111] direction of metal iron. Temperature and hydrogen flow rate during the reduction have been found to be important parameters, which determine not only the degree of reduct...

Preparation of high moment CoFe films with controlled grain size and coercivity

In this paper a preparation method for high moment CoFe thin films with soft magnetic properties is reported. A full control of coercivity in a series of 20-nm-thick CoFe films has been achieved without using seed layers, additives, or thermal annealing. The films were sputtered directly onto Si substrates and the coercivity was varied by changing the mean grain size in the sputtered films. The mean grain size was in turn controlled via the sputtering rate. A reduction in the coercivity has been observed from 120Oe for samples with a mean grain size larger than 17nm down to 12Oe for a sample with a mean grain size of 7.2nm. The results are in good agreement with the “random anisotropy model” relating the coercivity to the mean grain size in polycrystalline ferromagnetic films.

Fluctuation fields and reversal mechanisms in granular magnetic systems

Abstract In this paper, the behaviour of the fluctuation of the fluctuation field ( H f ) and its relationship with the activation energy for reversal in magnetic systems is examined. At a constant level of magnetisation, the link between thermal energy and field induced changes in the magnetisation of the system using different forms of the equation of state are found to give the same value of H f . Accordingly, the former S / χ irr and the ∂ H/ ∂ln ( M irr )| M irr techniques for determining H f are found to be consistent so long as the analysis of H f is made at the same constant level of irreversible magnetisation ( M irr ). Both techniques show that the fluctuation field can be determined using H f =Δ H /Δln( t )| M irr which is more accessible to experiment. Hence, one can define H f as the change in field required to maintain the magnetisation of the system unchanged during a time interval of Δln( t )=1. The behaviour of the fluctuation field in systems that contain a distribution of activation energies ( f (Δ E )) is found to be governed by the nature of f (Δ E ). For the case where this distribution arises from distributions of both particle volumes and anisotropy fields, the variation of H f with M irr is predicted to be approximately constant when both distributions have comparable standard deviations. Also H f is predicted to be constant with M irr is the case where both distributions become extremely narrow (i.e. the case of a single activation energy). Hence H f remaining constant with M irr cannot be used to show that the activation energy in the system is constant. In addition if the system contains a single activation energy, a constant value of H f cannot be used to distinguish between reversal mechanisms such as coherent rotation or weak domain wall pinning.

Thermal phenomena in IrMn exchange biased systems

In this paper, we report on a detailed study of reversal in the antiferromagnetic layer in IrMn-based exchange-biased systems. By a process of careful field cycling and heating with the antiferromagnet aligned in the opposite direction to the original set direction, we are able to study thermal activation over the energy barriers in the antiferromagnetic material. We find that the peak in the coercivity is observed, but in no way correlates to the blocking temperature. We conclude that the origin of the coercivity enhancement in exchange-biased systems lies in effects at the interface.

The effect of interactions on GMR in granular solids

In this article the effects of dipolar and exchange interactions on the magnetoresistance curves in granular solids has been calculated using a generalized 3‐d Monte Carlo model. This model allows the final configuration of magnetic moments to be calculated as a function of concentration, temperature, and magnetic field. The results show that interaction effects give rise to a finite resistivity in small magnetic fields. The magnitude of this finite magnetoresistance is found to increase with increasing concentration. Thus the maximum change in MR with field is found to be lowered by the interaction effects.

A model of the magnetic properties of coupled ferromagnetic∕antiferromagnetic bilayers

A granular level model which is capable of predicting the bulk magnetic properties of coupled ferromagnetic and antiferromagnetic layers is described. The model is used in an extensive investigation of the effect of the thermal instability of the antiferromagnetic layer as a function of the layer thickness, grain diameter, temperature, and the grain size distribution σ. The calculations give good qualitative agreement with experiment and provide an understanding of the role of the antiferromagnetic layer in determining the exchange bias field and the coercivity.

The Magnetic Properties of Fine Particles

Magnetic fine particles find a large number of applications, perhaps most notably as particulate recording media. In addition, the ideas of fine particle magnetism are applicable in a number of areas as diverse as geomagnetism and biomagnetism. The purpose of this chapter is to introduce the fundamental aspects of fine particle magnets, including magnetisation reversal phenomena, interaction effects and time dependence, and to describe applications to geomagnetism and recording media.