M. El-Hilo

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.

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. >

Susceptibility phenomena in a fine particle system: II. Field dependence of the peak

Abstract The field dependence of the peak in the dc susceptibility has been studied for a weakly interacting fine particle system consisting of Fe 3 O 4 particles. Measurements of the peak show that T g moves to lower temperatures as the sample is warmed up in a large dc field. Also in this paper the field dependence of the mean blocking temperature 〈 T B H 〉 is calculated using the Neel superparamagnetic (blocking) model modified to take into account the effects of particle interactions.

The ordering temperature in fine particle systems

Abstract In this paper the origin of the ordering temperature in systems of fine particles in a solid matrix is examined. From theoretical and experimental results, the variation of the reciprocal of the initial susceptibility ( X i -1 ) with the temperature is found to be curved when a wide range of temperatures is examined. However if the variation of X i -1 vs. T is examined over a limited range of temperatures an apparent ordering temperature ( T O ) is predicted from a linear extrapolation of the data. Consequently, in interpreting experimental ordering temperature other contributions in addition to particle interactions must be considered.

The sweep rate dependence of coercivity in recording media

Abstract In this paper the sweep rate ( R = d H /d t ) dependence of coercivity is calculated, from these calculations the sweep rate dependence of H c ( R ) can be expressed as a power series of ln( R ) and the coefficients of the expansion give a useful method of determining the mean anisotropy field of the system.

The origin of non-linear ln(t) behaviour in the time dependence of magnetisation

Abstract Non-linear behaviour in the ln( t ) law of time depenence of magnetisation is found to be governed by the distribution function of the energy barriers f ( ΔE ) within the system. The variation of the magnetisation with ln( t ) is predicted to be concave downwards when f ( ΔE ( t )) is increasing with time, and concave upwards when f ( ΔE ( t )) is decreasing with time. A linear ln( t ) law is only the case when f ( ΔE ( t )) is approximately constant with time.

Time dependent magnetisation in systems with distributed energy barriers

Abstract In this paper the origin of non-linear behaviour in the In(t) law of time dependence of magnetisation is examined. We calculate the time dependence of magnetisation in a negative field after a previous saturation in a positive field for a system of fine particles. Also we derive an analytical expression based on a series expansion in In(t) for the change in magnetisation with time. The non-linear behaviour in the In(t) law is found to arise from the distribution of energy barriers ⨍(ΔE) within the system, and the magnitude of the non-linearity is governed by the width of the energy barrier distribution.

Magnetic properties of barium hexaferrite powders

Abstract In this paper, the magnetic properties of unsubstituted and substituted barium hexaferrite powders are examined. The powders were prepared by the glass crystallization method and characterized geometrically by electron microscopy, and magnetically by measurements of remanence curves. From an analysis of the energy barrier distributions determined from the differential of the primary remanence curves, it is shown that changing the substitution level influences the effective mean anisotropy field of the particles. Consequently this influences the magnetic microstructure as the coupling between the easy axis and the moment of the particle changes. Transverse susceptibility measurements were also made to obtain the values of the anisotropy field which are compared to VSM measurements.

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.