M. L. Watson

The effect of the microstructure on the magnetic interactions in CoFe–AgCu granular films: From demagnetizing to magnetizing interactions

The sign and strength of the dominant magnetic interactions in Co34Fe8Ag54Cu4 granular films were modified by changing the microstructure through annealing. Magnetic force micrographs showed that in the as-cast sample the magnetic moments of neighboring grains tended to be arranged parallel along a direction out of the film plane, forming elongated magnetic clusters that were themselves aligned antiparallel, with dominant demagnetizing interactions. This is a direct evidence that an uncompensated antiferromagneticlike microstructure is stabilized below the volume percolation threshold. However, in the sample annealed at 750 °C the particle growth led to large in-plane ferromagneticlike clusters with dominant magnetizing interactions. Thus, in this letter we present direct correlation of interactions effects with magnetic measurements and show that ΔM plots correlate with changes in the magnetic microstructure in these systems.

Magnetotransport properties of NiFe–Ag granular alloys: Origin of the thermal behavior

The effect of the temperature and magnetic field on the giant magnetoresistivity (GMR) of two FeNi–Ag granular alloys of composition Fe11.4Ni6.4Ag82.2 and Fe7.6Ni16.4Ag76.0 is discussed. Both samples were prepared by rf magnetron sputtering. Parts of them were rapidly annealed at 600, 650, and 750 °C. All samples displayed giant magnetoresistivity which decays from its maximum value with a Tm behavior, with m≈0.8–0.9, suggesting that the decrease in the maximum magnetoresistivity is due to the reduction in the particle magnetization associated with the spin wave excitation, which is a different mechanism to the electron-magnon interaction responsible for the T dependence of GMR in magnetic multilayers. Magnetoresistivity ρM decreases with temperature sharing essentially the same temperature decrease as the square of the macroscopic magnetization M in the whole magnetic field range studied, which is due to the reduction in the particle magnetization and to superparamagnetic effects. The effect of the width...

From demagnetizing to magnetizing interactions in CoFe–AgCu granular films

CoFe–AgCu granular films of compositions ranging from 0.17–0.44 ferromagnetic atomic concentration were prepared by rf sputtering. The microstructure and the transport and magnetic properties suggested that this family of samples can be classified into two groups with a crossover concentration at about 32 at. %. The experimental results for samples Co34Fe8Ag54Cu4 and Co18Fe8Ag70Cu4, which are representative of both different behaviors, are discussed. For the as-prepared sample with higher CoFe content, an uncompensated out-of-plane antiferromagneticlike microstructure with dominant demagnetizing interactions was observed. The particle growth through the annealing led to large in-plane ferromagneticlike clusters with dominant magnetizing interactions. The thermal dependence of the remanence-to-saturation ratio of the as-prepared and annealed samples indicated the existence of a high degree of magnetic correlations leading to a very low magnetoresistivity: In none of the cases was a Stoner–Wohlfarth behavio...

Magnetic microstructures from magnetic force microscopy and Monte Carlo simulation in CoFe-Ag-Cu granular films

CoFe-Ag-Cu granular films, prepared by rf sputtering, displayed magnetic domain microstructures for ferromagnetic concentrations above about 32% at, and below the percolation threshold. All samples have a fcc structure with an (111) texture perpendicular to the film plane. Magnetic force microscopy (MFM) showed a variety of magnetic domain microstructures, extremely sensitive to the magnetic history of the sample, which arise from the balance of the ferromagnetic exchange, the dipolar interactions and perpendicular magnetocrystalline anisotropy, MFM images indicate that in virgin samples, magnetic bubble domains with an out-of-plane component of the magnetization are surrounded by a quasicontinuous background of opposite magnetization domains. The application of a magnetic field in different geometries drastically modifies the microstructure of the system in the remanent state: i) for an in-plane field, the MFM images show that most of the magnetic moments are aligned along the film plane, ii) for an out-of-plane field, the MFM signal increases about one order of magnitude, and out-of-plane striped domains with alternating up and down magnetization are stabilized. Numerical simulations show that a variety of metastable domain structures (similar to those observed experimentally) can be reached, depending on magnetic history, in systems with competing perpendicular anisotropy, exchange and dipolar interactions.

THICKNESS DEPENDENCE OF GIANT MAGNETORESISTANCE OF AGNIFE HETEROGENEOUS ALLOYS FILMS

We have investigated the thickness dependence of the giant magnetoresistance (GMR) observed in AgNiFe heterogeneous alloy films. The films were sputtered from a mosaic target at the system ambient temperature onto glass substrates. In order to maintain interfacial uniformity, and to control the spin‐dependent transmission of electrons at the AgNiFe film interfaces, they were covered by 200‐A‐thick NiFe under‐ and overlayers. The samples have total thicknesses in the range 400–3000 A. The film resistivity, magnetoresistivity, and magnetization were measured in the temperature range 4–300 K and in fields of up to 1.1 T and the structure examined using both large‐ and small‐angle x‐ray diffraction. The results indicate that the observed thickness dependence of the GMR of the AgNiFe films is within the experimental error It is postulated that this is due to coherent spin transmission of conduction electrons across the sandwich interfaces which does not degrade the GMR by mixing of the spin currents.

Interaction effects and magnetic ordering in GMR alloys

In this paper AgNiFe alloy films were examined both before and after annealing. Characterization of the samples and examination of interaction effects was carried out by measuring the temperature decay of remanence, initial susceptibility, and magnetoresistance. The temperature decay of remanence reveals that annealing widens the distribution of energy barriers, which is indicative of grain growth. The behavior of the initial susceptibility as a function of temperature is analogous to that found in spin glasses. From these measurements, it is believed that interaction effects in these systems are small.

Giant magnetoresistance in NiFe‐Ag granular alloys

Some FeNi‐Ag granular films of composition Fe11.43Ni6.35Ag82.22 (sample A) and Fe7.62Ni16.4Ag75.98 (B) were prepared by using rf magnetron sputtering, and once deposited were rapidly annealed at 600, 650, and 750 °C. All samples displayed giant magnetoresistance. The zero‐field‐cooled and field‐cooled processes evidence the segregation of ferromagnetic particles with a broad size distribution. The temperature and magnetic field dependence of the resistance is analyzed. The magnetoresistance follows a Hn law at high fields and it decays from its maximum value with a Tm behavior, with m approaching 1 at high fields.

Giant magnetoresistance in Ag1−xNix−yFey heterogeneous alloy films

We have investigated the effects of phase segregation and compositional variation on the giant magnetoresistance (GMR) observed in heterogeneous AgNiFe alloy films. The films were sputtered from a mosaic Ag/NiFe/Fe target onto glass substrates at the system ambient temperature. They had a thickness of 200–300 nm, and were rapidly thermally annealed (RTA) under vacuum in order to promote phase segregation. By altering the amount of Fe in the target mosaic, the Ni:Fe ratio was systematically varied from 80:20 to 34:66 atomic percent. The Ag concentration for all films was fixed at 78±5 atomic percent. The GMR was maximized using a RTA temperature of 650 °C. Annealing at higher temperatures decreased the saturation field of the GMR, but also lowered its size. Changing the film composition had little effect on the saturation field of the GMR, but for both as‐deposited and optimally annealed samples the magnetoresistance was found to decrease as the Ni concentration decreased.

Modeling giant magnetoresistance and magnetization of Ag1−xNix−yFey heterogeneous alloy films (abstract)

We have investigated the effects of the particle size distribution on the giant magnetoresistance (GMR) and magnetization of Ag1−xNix−yFey heterogeneous alloy films both experimentally and through computer simulation. The samples were prepared by sputtering from a mosaic target onto glass substrates at the system ambient temperature. They have thicknesses in the range of 100–300 nm and were rapidly thermally annealed at up to 750 °C to promote phase segregation. The resistivity and magnetoresistivity have been measured in the temperature range 4–300 K in fields of up to 12 kOe and the magnetization by vibrating sample magnetometer at fields of up to 6 T. We have modeled the magnetization and GMR of the system using an ensemble of superparamagnetic particles which have a log normal distribution of diameters. We obtain an excellent fit to the experimental magnetization data at room temperature. This is true for all annealing strategies used. Conversely, we have found a marked difference between the modeled ...

Jitterbug spin channel mixing in heterogeneous giant magnetoresistive material

A mechanism is described which considers the effect of small magnetic particles on the spin diffusion length in a granular giant magnetoresistive material. Spin depolarization occurs by precession of the spin orientation of the carrier due to the s–d exchange interaction within a magnetic particle. Numerical simulation of this jitterbug effect is found to generate a temperature and field dependence of the distance a carrier may travel within the sample without losing its spin memory.