M.R. Parker

‘Giant’ magnetoresistance observed in single layer Co-Ag alloy films

Abstract ‘Giant’ magnetoresistance ratios have been measured at room temperature in as-deposited, single layer Co-Ag alloy thin films sputtered from a composite target. The effect has been observed over a broad range of Co concentrations (from 0.25 to 0.56 atom fraction Co, x) with a maximum magnetoresistance ratio of #62; 17% found at x = 0.43. In the as-deposited state these Co-Ag films are very fine grained single phase fcc polycrystalline alloys which exhibit essentially superparamagnetic M - H loops.

GMR multilayers and head design for ultrahigh density magnetic recording

This paper describes materials development, device modeling, and initial test results for a novel GMR reproduce head for ultrahigh density recoding (/spl sim/ 10 Gbit/in/sup 2/). It consists simply of a 20-bilayer GMR multilayer, self-stabilized by intrinsic antiferromagnetic interlayer coupling, and self-biased by sense current into a bipolar, field-gradient-sensing bias state. Analogous to a dual magnetoresistive (DMR) such a GMR-DMR head is capable of achieving very high linear resolution without shields. Tape-test results using evaporated NiFe/Cu and sputtered NiFeCo/Cu agree reasonably well with theoretical expectations. 1/f noise in these GMR multilayers will also be discussed.

Temperature dependence of the pinning field and coercivity of NiFe layers coupled with an antiferromagnetic FeMn layer

The pinning field Hp (the amount of the shift of the hysteresis loops) and the coercivity Hc of the samples of the form glass/Ta 120 A/(Cu 100 A)/NiFe 75 A/FeMn 150 A/Ta 50 A increase almost linearly with decreasing temperature down to 20 K, below which Hc increases sharply. The observed strong positive correlation between Hp and Hc, seems to be reasonably explained by a combination of a newly developed model in which a directional distribution of the pinning field caused by a random distribution of the crystalline orientations in the antiferromagnetic FeMn layer is taken into account and Hoffmann’s ripple theory in which the local anisotropy is assumed to be proportional to Hp, although the sharp increase in Hc at very low temperatures remains to be explained.

Analytical model of giant MR in multilayers with biquadratic coupling

Abstract A simple analytical model is described for the magnetic and attendant giant magnetoresistive properties of thin film multilayers antiferromagnetically coupled through conducting spacers by both Heisenberg and biquadratic exchange. Although fully analytical, the model describes in detail the GMR response in both easy and hard directions of a multilayer stack comprising 2 N layers, where N is any finite number.

Hysteresis and interaction between the magnetic layers in spin valves

Spin valve characteristics have been investigated in the samples of glass /Ta(X /spl Aring/)/NiFe(Y/sub f/ /spl Aring/)/Cu(Z /spl Aring/)/NiFe(Y/sub p/ /spl Aring/)/FeMn(150 /spl Aring/)/Ta(50 /spl Aring/) with various X, Y/sub f/, Y/sub p/ and Z values. The pinning field of FeMn increases and saturates X, Y/sub f/ and Y/sub p/, followed by an improvement of (111) texturing of the NiFe/Cu/NiFe/FeMn layers. GMR vs. field curves in the easy magnetization direction were studied for various Cu thicknesses, Z. With decreasing Z, the coercivity of the free layer begins to decrease, followed by a reduction of the GMR effect which finally disappears. This change can be explained semi-quantitatively by theoretical analysis which assumes some dispersion in the direction of the pinning field.

Modeling field dependence of giant magnetoresistance in ‘‘granular’’ and quasi‐granular magnetic films

Giant magnetoresistance in granular alloy thin films prepared by co‐sputtering as well as in quasi‐granular disordered multilayers has been modeled with relative ease using a simple scattering concept appropriate to superparamagnetic films and involving the Langevin function. The model has been applied to both as‐deposited and annealed samples with the result that approximate sizes of spin clusters may be determined.

Observation of giant magnetoresistance in Ag‐Ni‐Fe alloy films

We report the observation of a large magnetoresistance in as‐deposited co‐sputtered Ag‐Ni‐Fe films. The size of the magnetoresistance has been measured as a function of the film composition. Measurement of the samples’ hysteresis indicates that smaller Ag concentrations result in regions of the films becoming ferromagnetically coupled while greater concentrations result in the formation of a superparamagnetic state. Crystallographic evidence obtained using x‐ray diffraction shows the samples to have the crystal structure of a contracted Ag lattice possibly caused by the inclusion of Ni and Fe.

Low-field giant magnetoresistance in Co/Cu, CoFe/Cu and CoNiFe/Cu multilayer systems

In this paper, methods of increasing the sensitivity of the giant magnetoresistive effect are investigated. These include (111) texturing, topologically reduced coupling and appropriate alloying of Ni, Fe and Co in the magnetic layer. >

Magnetic filtration of ferrofluids

We have used a modified HGMS apparatus to selectively filter aggregated particles from samples of ferrofluid. We have identified the reduced initial susceptibility \bar{X_{i}} (=[dI/dH]/I_{S}) as a quality control parameter for fluid stability to field gradients. We show that \bar{X_{i}} is a far more sensitive measure than particle sizing techniques which are insensitive to the presence of aggregates.

Hysteresis reduction in NiFeCo/Cu multilayers exhibiting large low‐field giant magnetoresistance

Giant magnetoresistance (GMR) has been observed in Ni66Fe16Co18/Cu multilayered uniaxial magnetic thin films prepared by dc magnetron sputtering. Both easy and hard axis loops saturate at very low applied fields. The MR ratio reaches ∼80% of its saturation value in less than 30 Oe. A maximum MR ratio of ∼10% has been observed in the as‐deposited state in a structure containing only ten bilayers. Typical hysteresis values for magnetoresistance loops are ∼4 Oe in the as‐deposited state. A gradual decrease of this hysteresis has been observed with annealing. An increase in the saturation field is also observed with annealing. Short‐term annealing increases the magnitude of the magnetoresistance ratio; prolonged annealing causes a decrease. A field sensitivity of about 0.3% per Oe has been observed and dc magnetoresistance minor loops exhibit no measurable decrease in sensitivity.