K. O'Grady

Switching mechanisms in cobalt-phosphorus thin films

Studies have shown that although the interactions between grains in magnetic thin-film recording media can enhance the squareness, these interactions are also responsible for media noise. The authors made measurements on Co-P thin films that give a simple and practical way of investigating the role of interactions in the magnetic processes that occur in these films. All magnetic measurements were made on a PAR 155 vibrating sample magnetometer with a resolution of 10/sup -5/ EMU. The field measurements were made with a Bell-Gauss Hall probe calibrated with a proton magnetometer to give an accuracy of >

Spin-glass behavior in a fine particle system

A study of the magnetic behavior characteristic of a spin-glass, which has been examined for systems of ultrafine particles, is reported. The results obtained support the suggestion of E. P. Wohlfarth (1977) that spin-glass effects could arise from the clustering of impurity spins in the host material into ferromagnetic rich clusters. All the results are explained on the basis of the Neel superparamagnetic (blocking) model. The results obtained show that the behavior of fine particle systems is similar to that of certain spin-glass alloys (e.g., Au/Fe alloys). This result supports the idea that spin-glass alloys can be interpreted on the basis of a blocking model. >

Susceptibility phenomena in a fine particle system: I. Concentration dependence of the peak

Abstract The concentration dependence of the peak in the dc susceptibility of a weakly interacting fine particle system consisting of Fe 3 O 4 particle has been studied. Measurements of the initial susceptibility of the dispersion in the solid state show that the susceptibility values were depressed at low temperatures. The temperature of the peak ( T g ) in the susceptibility versus temperature curve was found to occur at higher temperatures as the concentration was increased. The variation of T g with concentration shows a simple power law relationship. The influence of dipolar interactions between the particles on the value of T g is discussed by considering the effects of particle interactions on the blocking temperature. In this study we develop an effective blocking temperature model, and apply it to our system.

Mechanisms of hyperthermia in magnetic nanoparticles

We report on a theoretical framework for magnetic hyperthermia where the amount of heat generated by nanoparticles can be understood when both the physical and hydrodynamic size distributions are known accurately. The model is validated by studying the magnetic, colloidal and heating properties of magnetite/maghemite nanoparticles of different sizes dispersed in solvents of varying viscosity. We show that heating arising due to susceptibility losses can be neglected with hysteresis loss being the dominant mechanism. We show that it is crucial to measure the specific absorption rate of samples only when embedded in a solid matrix to avoid heating by stirring. However the data shows that distributions of both size and anisotropy must be included in theoretical models.

Particle size analysis in ferrofluids

Abstract In this paper we examine the applicability of the Gaussian and lognormal probability functions to describe the distribution of particle sizes found in ferrofluids. Measurements have been made of the particle size distributions contained in a large number of ferrofluids prepared by different techniques. From these measurements we conclude that the form of the distribution may be associated with the technique of particle preparation.

Determination of f0 for fine magnetic particles

In this paper we have determined a value for the pre-exponential factor in the Neel-Arhennius equation for the iron oxyhydroxide particles found in the protein ferritin. The data were obtained using a combination of zero field magnetic and Mossbauer spectroscopy studies yielding a value for f0 of (9.5±2.7)X1011 Hz. This value is significantly different to that of 2.8 X 109 Hz commonly used and in closer agreement to that of 1013 Hz obtained for iron particles using an analogous technique. Using our experimental value for f0 gives a revised superparamagnetic criterion for DC magnetic measurements on a 100 second time-scale of KV < 32 kT and a Mossbauer spectroscopy criterion for a measurement time-scale of 10-9 s of KV < 8 kT. Our results together with other published data would suggest that a more appropriate estimate for the value of f0 would lie in the range 1012 to 1013 Hz.

The limits to magnetic recording — media considerations

At present longitudinal magnetic recording systems are the basis of all low cost high-density information storage systems. During the recent past the data density stored on rigid disk media which is the higher density format have increased at the rate of 60% per annum compound. However, very recently due to the introduction of new advanced GMR spin-valve heads this rate of advance has increased to 100% per annum in laboratory demonstrations. Hence, it is pertinent at this time to enquire as to where the fundamental physical limitations of longitudinal magnetic recording may lie. In this context there are two principle areas of interest: the first of these is limitations to data rate. These are concerned with the fundamental physics of the maximum rate at which a magnetic moment may reverse from one direction to the other. The theoretical calculation of these limits is complex and not well understood but the limits of our understanding will be reviewed in this paper. Secondly, and of principle concern is the limit to the density at which information can be stored in a magnetic thin film. This latter limitation is based around the signal to noise ratio and also the question of the stability of increasingly small written bits. Signal to noise considerations are extremely complex and derive from factors such as the shape of bits and cross-talk between neighbouring bits or even neighbouring tracks. In this article the fundamental origins of noise will be reviewed in terms of the basic physics that gives rise to variation in transition shapes. Cross-talk and cross-track interference will not be discussed as these are generally addressed through issues associated with the resolution of the servo-mechanism that positions the head above a track and is not associated with the fundamental physics of the medium itself. Thermal stability of a bit of information is of critical importance particularly as media is made ever thinner and will form a major aspect of the discussion in this work. Finally, possible material physics solutions to some of these limitations will be presented in terms of measurable parameters which to some limited degree may be controlled by process conditions.

The observation of multi-axial anisotropy in ultrafine cobalt ferrite particles used in magnetic fluids

It has been shown that nano-sized particles of cobalt ferrite produced by the coprecipitation method for use in magnetic fluids exhibit multiaxial anisotropy, which has not previously been reported for ferrite particles. The value of the magnetic anisotropy constant calculated from measurements of the decay of remanence is similar to that reported for bulk cobalt ferrite.

The characterisation of interaction effects in fine particle systems

Experimental methods and theoretical models that enable interaction effects in particulate systems to be examined are discussed. Two distinct regimes of behavior are considered. The weak interaction case is examined and it is shown that, in general, low-field susceptibility studies can be used to establish the effects of interactions only if care is taken to exclude other effects such as progressive blocking. The strong interaction regime is considered, and the use of remanence curves, chi /sub irr/ curves and the delta I(H) plot in the study of interaction effects is described. For both regimes, the use of numerical models in this area is reviewed. >

Magnetic measurement of interaction effects in CoNiCr and CoPtCr thin film media

Abstract In this paper we report the results of our investigations into interaction effects in Co metal alloy thin films. We have studied, through measurement and comparison of remanence curves, the effects of variable underlayer and magnetic layer thicknesses. Our results indicate a strong correlation between the magnitude of cooperative switching and measured signal-to-noise. Through analysis of the irreversible susceptibility, χ irr , we have also been able to distinguish the relative degree of quasi-particulate and exchange coupled behaviour within each film.