Granular Magnetic Systems Research Articles

Interparticle interactions and time effects in very thin Fe–SiO2 granular films

Time decay and remanence measurements have been made on a series of very thin Fe–SiO2 granular thin films of varying composition. The susceptibility and the magnetic viscosity decreases drastically when the film thickness is thin enough to reach a two-dimensional arrangement of the granules. The measured values of the magnetic viscosity are very low (2% loss per decade) in the thinnest films so that further investigations in granular magnetic systems with higher anisotropy promise greater advantages for recording media.

Influence of conduction electrons and dipolar interactions on the susceptibility of granular materials

We present an analysis of the effect of intergranular interactions on magnetic susceptibility of granular magnetic systems. The RKKY-type interaction between magnetic granules, via the conduction electrons, is found non-oscillatory antiferromagnetic, unlike in multilayer systems. The demagnetizing effect of dipolar interaction is shown to produce deviations from simple superparamagnetic behavior predicted by the Langevin model.

Microscopic theory of magnetoresistance in granular materials

Based on a simple model of spin-dependent scattering, an improved microscopic approach is proposed to describe the magnetoresistance in granular magnetic systems. The feature is the explicit account of correlated scattering of conduction electrons between granule pairs and of transitions between spin-up and spin-down states. Besides the usual term, proportional to the square of the magnetization, an extra conribution to the magnetoresistance ratio $\ensuremath{\Delta}\ensuremath{\rho}/\ensuremath{\rho}$ is obtained, proportional to the correlator $〈\mathrm{cos}{\ensuremath{\theta}}_{12}〉$, where ${\ensuremath{\theta}}_{12}$ is the angle between magnetic moments of neighbor granules. The field dependence of $\ensuremath{\Delta}\ensuremath{\rho}/\ensuremath{\rho}$, together with its dependencies on granule concentration, granule mean size, granule size distribution, temperature and annealing effects, are considered, leading to a reasonable agreement with available experimental data.

Nanoparticle morphology in a granular Cu–Co alloy with giant magnetoresistance

The morphology of nanometer-sized cobalt granules in a granular Cu88Co12 alloy was directly determined utilizing an atom probe-field ion microscope. The granules are spherical in shape, and exhibit a size distribution. Giant magnetoresistance (GMR) was observed in alloys with an average granule size ranging from 1.5 to 6 nm in diameter. A well-known theoretical model of general GMR behavior in magnetic granular systems was confirmed based on measurement of size distribution of the granules.

A formal theory of the conductivity and application to the giant magnetoresistance

The transport in a system with inhomogeneous elastic scattering is described in terms of a probabilityconserved Boltzmann equation. We demonstrate that the spatially varied current density depends only on the voltage drop between the ends of the sample. This fact enables us to develop a formal and general theory for the conductivity without determining the actual electric field inside the sample. The theory is first applied to multilayer systems and shown to recover the previous theory. By including the spin-dependent interface scattering and bulk scattering, we employ our theory to account for the giant magnetoresistance (MR) in magnetic granular systems with both spherical and cylindrical granules. The results obtained reproduce the experimental dependence of the MR on annealing temperature.

Magnetic properties and giant magnetoresistance of melt-spun granular Cu100-x-Cox alloys.

Room-temperature measurements of magnetization and giant magnetoresistance were performed on rapidly solidified granular ${\mathrm{Cu}}_{100\ensuremath{-}x}{\mathrm{Co}}_{x}$ systems ($x=5, 10, 15$). The magnetoresistance of melt-spun ${\mathrm{Cu}}_{100\ensuremath{-}x}{\mathrm{Co}}_{x}$ ribbons was enhanced either by suitable furnace annealings or by exploiting the dc Joule-heating technique in the attempt of precipitating smaller magnetic particles. The particle-size distribution, the particle density, and mean distance are obtained for all compositions and heat treatments through a suitable analysis of the magnetic behavior of samples. The magnetoresistance is plotted as a function of the reduced magnetization, and a significant deviation from the quadratic behavior predicted by the independent-moment approach is observed at low fields. A simple theory taking explicitly into account the correlation existing among the magnetic particles is proposed. A general expression for the magnetoresistance in granular magnetic systems is obtained, and shown to accurately fit all the experimental curves, indicating that this effect is basically determined by the ratios between two distinct correlation ranges for the magnetic-moment fluctuations and the electronic mean free path.

Theory of giant magnetoresistance in magnetic granular systems.

We present an explanation of the giant magnetoresistance observed in some magnetic granular solids as well as magnetic multilayers by considering spin-dependent s-s and/or s-d scattering. We obtain an explicit expression for the magnetoresistance of magnetic granular solids. This expression relates the giant magnetoresistance to the microscopic quantities and geometric parameters, and is in agreement with the experimental data. We also address a few relevant aspects of experiments.

Granular Magnetism

ABSTRACTRecent studies of granular magnetic solids involving Fe and its alloys embedded in either an insulating medium (e.g. SiO2 A12O3) or a metallic medium are described. Below the percolation threshold, isolated magnetic granular exhibit single-domain properties and superparamagnetic relaxation. Giant coercivity has been uncovered in granular Fe-SiO2, with Hc- as high as 3 kOe at 5 K and 1.1 kOe at 300 K. Some recent results on granular Fe in a metallic medium, and other granular magnetic systems will also be discussed.

Magnetic relaxation in Fe-(SiO2) granular films.

Magnetic relaxation of granular Fe-(${\mathrm{SiO}}_{2}$) solids with low metal-volume fraction has been studied by employing dc SQUID (superconducting quantum interference device) magnetometry and M\ossbauer spectroscopy with the characteristic measuring time differing by nine orders of magnitude. The blocking temperatures (${T}_{B}$), extrapolated to zero external field as measured by the two techniques maintain a constant ratio of 0.35 independent of samples. Arrhenius law was found adequate in describing the relaxation process, with a well-defined relaxation-time constant of ${\ensuremath{\tau}}_{0}$\ensuremath{\simeq}${10}^{\mathrm{\ensuremath{-}}13}$ sec. A cusp in the susceptibility is observed at ${T}_{B}$, below which irreversible difference between the field-cooled and zero-field-cooled magnetizations occurs. Despite this spin-glass-like behavior, granular magnetic systems are found to be fundamentally different from spin glasses.