Andrew P. Chen

Effect of 3d, 4d, and 5d transition metal doping on damping in permalloy thin films

The effect of doping on the magnetic damping parameter of Ni80Fe20 is measured for 21 transition metal dopants: Ti, V, Cr, Mn, Co, Cu, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, Pt, and Au. For most of the dopants, the damping parameter increases linearly with dopant concentration. The strongest effects are observed for the 5d transition metal dopants, with a maximum of 7.7×10−3 per atomic percent osmium.

Ferromagnetic resonance mode interactions in periodically perturbed films

Ferromagnetic resonance linewidth measurements are reported for thin films of Ni80Fe20 deposited on substrates with 320 nm period grooves or parallel aperiodic scratches. These films are model systems for studying the effects of inhomogeneity on ferromagnetic resonance. With the grooved substrate, the ferromagnetic resonance spectra show that interactions split the resonant frequencies of interacting modes leading to two resonances for strong interactions and broadened resonances for weak interactions. The results obtained with the scratched substrate are in agreement with the two-magnon model of linewidth, showing sharply increased linewidth when the scratches are oriented to allow coupling to degenerate spinwave modes. The results are not in agreement with a simple model of linewidth as a direct measure of the spread in local resonance fields.

Detection of pinholes in magnetic tunnel junctions by magnetic coupling

Pinholes in tunnel barriers are detrimental to the performance of magnetic tunnel junctions (MTJs) since they create direct magnetic exchange coupling between the free and pinned magnetic films and may act as current short circuits. A simple and straightforward technique which enables observation of pinholes and distinguishes pinhole coupling from orange-peel coupling would aid greatly in optimizing the performance of MTJs. However, the existing methods for this determination are quite complex and destructive and do not work on complete structures. We have developed a simpler, nondestructive method that works on full MTJ structures which is able to identify whether an observed coupling arises primarily from magnetic exchange coupling through pinholes or from orange-peel coupling. The method is based on the shift in the free layer hysteresis loop at low temperatures. It is well known that the shift in the pinned layer loop at low temperatures is due to the sharp increase of the IrMn pinning strength. If pi...

Structural, magnetic, and thermal stability of IrMn exchange biased layers

The magnetic domain structure, magnetic properties, and thermal stability of IrMn biased top spin valves containing Co and NiFe have been investigated. The magneto-optical indicator film pictures of Ni–Fe biased IrMn spin valve show the formation and motion of domain walls that separate large domains in the free film unlike the Co biased IrMn spin valve structure with many small microdomains. From the angular dependence of the ferromagnetic resonance linewidth (ΔH) and resonance field (Hres) it was found that the IrMn biased Co film is very different from the behavior of a NiO biased Permalloy film which exhibits two-magnon scattering. To investigate the thermal stability two kinds of magnetoresistance measurements at anneal temperature (TAnn) and room temperature (TRT) following anneals were accomplished.

Effect of conformal roughness on ferromagnetic resonance linewidth in thin permalloy films

The ferromagnetic resonance linewidth is a result of both intrinsic damping and contributions from inhomogeneities, which in thin films can be dominated by the roughness. Microstructural measurements and magnetization dynamics are reported here for 50-nm films of Permalloy deposited on substrates with controlled roughness. Ferromagnetic resonance measurements reveal a strong linewidth broadening that peaks when the magnetization is approximately 30° from the in-plane direction. These results are compared with a recently developed two-magnon theory of resonance broadening due to inhomogeneous demagnetizing fields in a rough film.

Fluctuating field calculations for a bimodal medium

Bimodal magnetic behavior has been observed in a (0.3-nm Co/1.2-nm Pt)15 multilayer film. In that film, the Kerr rotation and ellipticity exhibit magneto-optic hysteresis loops with two distinct field-dependent steps, so that there are four stable magnetization states. The magnetic aftereffects of the two distinct types of decay at the two sets of holding fields have been analyzed. Thus, it is possible to measure and calculate the two types of decay individually. We show that the shift in log time of the aftereffect curves for both the sets are linear. The fluctuation fields for the two states are calculated using these methods. We also confirm experimentally the conjecture of the Preisach–Arrhenius model that the shape of the aftereffect curve is the same as the major hysteresis curve. This helps in finding the chemical potential.