A. Waknis

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.

‘‘Giant’’ magnetoresistance in sputtered Co–Ag alloy thin films

‘‘Giant’’ magnetoresistance ratios have been measured at room temperature in single layer Co–Ag alloy thin film. The effect has been observed over a broad range of Co concentrations with a maximum as‐deposited MR ratio of ≥22% (at x=0.38). The effects of two annealing sequences have also been studied. The MR ratio generally increases on annealing reaching a maximum of ∼28% (at x=0.38) after the second anneal.

Magnetic properties and structure of Al/Fe‐N periodic multilayer thin films

Periodic multilayer thin films of the form (xAl/yFe‐N)n were grown by sequential dc‐magnetron sputtering. The thicknesses of the individual Al and Fe‐N layers are given by x and y, respectively, and the total number of bilayer units is n. For this set of experiments, x was fixed at 3.5 nm and y was varied systematically from 3 to 135 nm. Magnetic properties were studied by vibrating sample magnetometry and crystal structure by x‐ray diffraction for both as‐deposited and annealed films. A strong enhancement of the saturation magnetization was found in multilayers containing the thinnest Fe‐N layers which was further strengthened by annealing. The coercivity was found to decline monotonically with decreasing Fe‐N layer thickness in the as‐deposited films. Annealing produced nonsystematic changes in coercivity. The evolution of magnetic properties with decreasing Fe‐N layer thickness was correlated with complex changes in iron‐nitride crystal structure. For the thinnest layers of Fe‐N (less than 12 nm) the m...

Magnetic properties and crystal structure of Ti/Ni superlattices (abstract)

A series of periodic multilayer Ti/Ni thin films of the form (xTi/yNi)n were grown at room temperature by alternate deposition of elemental Ti and Ni onto glass substrates using dc‐magnetron sputtering. The thicknesses of the individual Ti and Ni layers are given by x and y, respectively, and the total number of bilayer units is n. For this set of experiments, x was fixed at 2.8 nm while y was systematically varied from 2.4 to 154 nm. The product ny was held constant for all samples so that each film contained the same quantity of Ni. The growth rates of Ti and Ni were 0.14 and 0.32 nm/s, respectively. The multilayers were tested in the as‐deposited state for magnetic properties using a vibrating sample magnetometer. These superlattices showed a variety of magnetic characteristics which were dependent on the thickness of the Ni layers. The coercivity of the multilayers declined with Ni layer thickness from a maximum of 80 Oe to a minimum of 7 Oe. A systematic reduction in saturation magnetization (Ms) wit...

Giant and anisotropic magnetoresistance in single layer Ni66Fe16Co18–Ag films

Single layer Ni66Fe16Co18–Ag thin films were made by cosputtering from a composite Ni66Fe16Co18–Ag target. At the silver rich end of the composition range (≊57%–80 at. % Ag) we have found ‘‘giant’’ magnetoresistance (a maximum of ≊9% at room temperature at ≊72 at. % Ag after annealing at 300 °C for 1/2 h), while in the midcomposition range (≊42–≊57 at. % Ag), we have observed anisotropic magnetoresistance.

Modeling Giant Magnetoresistance and Relative Permeability in Granular Films

A Model for the field dependence of giant Magnetoresistance (GMR) in ‘granular’ co-sputtered alloy thin films (based on a relatively simple spin-dependent scattering concept appropriate to superparamagnetic and weakly ferromagnetic films) is applied to new experimental data from the Co 90 Fe 10 -Ag system. The Model and the experimental data can be shown to compare very well with the help of a single adjustable parameter related to spin correlation of adjacent Co-Fe clusters. A careful fit of field-dependent MR data and theory leads to a fairly reliable determination of spin-cluster radius. An analysis of the relative permeability of granular GMR films derived from the generalized form of the Clausius-Mossoti relationship is also presented. For a non-Magnetic Matrix the effective relative permeability is shown to be materials independent. The permeability model is applied to Co-Au granular films.

Electrical Resistivity And Crystal Structure Of Nickel-Based Multilayer Thin Films

The electrical resistivity and crystal structure of three Ni-based periodic multilayer thin film systems (Al/Ni, Ti/Ni, and Cu/Ni) have been investigated. In each series of films the Ni layer thickness was systematically varied while the thickness of the ‘spacer’ layer (Al, Ti, or Cu) was fixed. In the Al/Ni and Ti/Ni systems films with very thin Ni layers (and consequently large volume fractions of spacer and ‘interfacial’ material) yielded very high resistivities which dropped rapidly with increasing Ni thickness. By contrast, the resistivity of Cu/Ni multilayers continuously increased with Ni layer thickness due to the decline in volume fraction of high conductivity Cu. Both the Al/Ni and Ti/Ni systems exhibit Ni(111) texture in the thicker Ni layer samples. As the Ni layer thickness decreases the Ni(111) peak loses intensity and broadens due to finer grain size and increasing disorder. Al-Ni and Ti-Ni compounds are also noted. In the Cu/Ni system, however, the sharpness of the Ni(111) peak passes through a minimum as the Ni layer thickness decreases but then increases for the thinnest Ni layer samples.

Magnetic Properties of Sputtered Ni/Al Multilayers

Magnetic properties of sputtered Ni/Al multilayers have been investigated by vibrating sample magnetometry and ferromagnetic resonance. In these compositionally modulated films (CMFs) the Al ‘spacer’ layer thickness was fixed at 3.5 nm while the total Ni content of each film was held constant at 308 nm. The thickness of the individual Ni layers was varied from 4.8 to 154 nm. The CMFs showed a variety of magnetic characteristics which were dependent on the thickness of the Ni layers. CMFs with Ni layer thickness 30 nm and above showed clear evidence of perpendicular anisotropy. This anisotropy is characterized by low-remanence perpendicular hysteresis loops of the type commonly found in CoCr alloy films. As the Ni layer thickness diminishes the perpendicular anisotropy decreases and is eventually lost. Simultaneously, the CMFs show increasing in-plane remanence, rising to a peak squareness of greater than 0.5 at a Ni layer thickness of 11 nm. As the Ni thickness continues to decrease, the remanence again declines. At Ni thicknesses of a few nm the CMFs become quasi-superparamagnetic. These CMFs do not show a monotonic reduction in saturation magnetization, M s , with decreasing Ni layer thickness. Instead, both M s and the coercivity, Hc, pass through a maximum in the region of 40–80 nm Ni layer thickness. FMR measurements were also made on these films. A plot of the effective anisotropy field produces data of a similar form to the M s versus Ni layer thickness plot, again with a clear maximum. The FMR data also reveals interesting resonances in the films exhibiting perpendicular anisotropy. The presence of satellite resonances adjacent to the principal resonance peaks seems to suggest, in structural terms, a two-phase system as the basis of the observed anisotropy.