Magnetoresistance In Granular Alloys Research Articles

Magnetoresistive effects in Co/Pd multilayers on self-assembled nanoparticles (invited)

The deposition of Co/Pd multilayers onto self-assembled spherical particles provides a system with unique magnetic properties. The magnetic caps have high perpendicular magnetic anisotropy, are single-domain, and strongly exchange decoupled, but in electrical contact with each other, thus enabling magnetotransport measurements. By applying an external magnetic field, the caps can be switched individually. We systematically studied the magnetoresistance on a two-dimensional cap array consisting of Co/Pd multilayers deposited on particles with a diameter of 200 nm. In the vicinity of the coercive field, a hysteretic resistance peak occurs. It can be explained with the random magnetization configuration of the magnetic caps leading to an increased spin-dependent scattering of the conduction electrons. The underlying mechanism might be comparable to the one causing giant magnetoresistance in granular alloys. For temperatures above 77 K, additional resistivity contributions with high saturation fields are observed, which are tentatively explained by the decreasing size of magnetically ordered parts of the caps with increasing temperature, resulting finally in superparamagnetic behavior in the contact area between neighboring caps.

Theory of giant magnetoresistance in granular alloys

A theory of giant magnetoresistance in granular alloys was developed by considering the spin-dependent scattering within the magnetic granules and at their boundaries and the continuous distributions of granule sizes and directions of magnetization. The CPP formalism was recovered in the limit when the effective conductivities in the granules are much larger than the matrix conductivity. When this condition is not satisfied, the transport properties of the granular alloys derive from the crossover of the parallel and perpendicular current configurations. @S0163-1829~99!10309-6#

Investigating Giant Magnetoresistance in Granular Alloys by Experiments and Simulations

Investigating Giant Magnetoresistance in Granular Alloys by Experiments and Simulations

Influence of the distribution of magnetic moments on the magnetization and magnetoresistance in granular alloys

In granular solids, the magnetoresistance is directly related to the macroscopic magnetization, but this relationship is extremelly complex due to the distribution of grain sizes and the intergranular magnetic interactions. The dependence of the magnetoresistance on the magnetization is here investigated by means of a theoretical model that is developed taking explicitly into account the magnetic moment distribution and the spin-dependent electron-impurity scattering within magnetic grains and at the interface between the grains and the metallic matrix. Using this model, one can explain large experimental deviations from the parabolic behavior of the magnetoresistance vs magnetization curves that are typically expected for equal noninteracting superparamagnetic grains. The expressions for the magnetization and magnetoresistance, obtained for general distribution funtions, are tested considering a log-normal-type distribution function by fitting on data obtained from melt-spun Cu${}_{90}$Co${}_{10}$ ribbons after annealing by dc Joule heating. The experimental data are well traced using just three parameters that determine the particle size distribution, the particle density, and the ratio of the scattering cross section at the boundaries of the grains to the scattering cross section within the grains.

Spin-disorder and spin-valve magnetoresistance in granular alloys

Two main contributions to the giant magnetoresistance (MR) in granular alloys have been identified: Spin-valve MR (as in magnetic multilayers) and spin-disorder MR (as in bulk ferromagnets close to their Curie temperature or as in spin-glasses). Both are due to spatial inhomogeneities in the magnetization but on different scales. They have different thermal variations.

Numerical study of magnetoresistance in granular alloys

Abstract Numerical simulation of giant magnetoresistance in magnetic granular alloysis performed by using the recursive Green's function method and the Kubo formula. It is shown that the magnetoresistance depends on the shape of magnetic clusters and that classical cross-section of clusters is crucial factor for the magnetoresistance.

Giant magnetoresistance in magnetic granular alloys.

Numerical simulation on resistance and giant magnetoresistance in magnetic granular alloys is performed for finite-size systems by making use of the real-space Green's-function method based on the Kubo formalism. Spin-dependent scattering causes the giant magnetoresistance in the magnetic granular alloys as in the magnetic multilayers. It is shown by using the microscopic theory that the magnetoresistance increases with decreasing the size of magnetic grains, in agreement with experiment. The theoretical results indicate that scattering at the surfaces of magnetic grains governs the resistance and magnetoresistance in granular alloys. Difference between the giant magnetoresistances in the granular alloys and multilayers is discussed.