W. F. Egelhoff

Magnetoresistance values exceeding 21% in symmetric spin valves

We report values of the giant magnetoresistance (GMR) effect exceeding 21% in symmetric spin valves, the highest values ever reported for such structures. The key elements in this achievement are the use of a Co/Cu/Co/Cu/Co multilayer in which the center Co layer is substantially thicker than the outer Co layers and the use of the antiferromagnetic insulator NiO at the top and bottom to pin the adjacent Co layers magnetically. The relative Co layer thicknesses suggest that some specular scattering of conduction electrons may occur at the metal/insulator interfaces and may enhance the GMR.

Optimizing the giant magnetoresistance of symmetric and bottom spin valves (invited)

We have attempted to optimize the values of the giant magnetoresistance in symmetric spin valves of the type NiO/Co/Cu/Co/Cu/Co/NiO (achieving 23.4%) and in bottom spin valves of the type Co/Cu/Co/NiO (achieving 17.0%), the largest values ever reported for such structures. The key elements in this achievement are improved vacuum conditions and careful attention to the film thicknesses.

Large surface anisotropies in ultrathin films of bcc and fcc Fe(001) (invited)

Large uniaxial anisotropies associated with interfaces are observed for ultrathin films (3‐28 ML) of bcc Fe(001) grown epitaxially on Ag(001) single‐crystal substrates and for epitaxial sandwiches of fcc Fe(001) grown with three layers of Fe using Cu as substrate and coverlayers. The uniaxial anisotropy is well described by a pseudosurface anisotropy term as theoretically predicted, yet that theory also predicts large in‐plane anisotropies that are not observed. Adequate treatment of spin‐orbit coupling in magnetic theories remains a challenge. Comparisons of ultrathin films of bcc Fe(001) on Ag(001) with different coverlayers of Ag or Au show subtle differences in magnetic behavior as studied by ferromagnetic resonance (FMR) and Brillouin light scattering (BLS). The FMR measurements were carried out at 9.6, 36.6, and 73 GHz microwave frequencies. The BLS measurements were performed using a six‐pass Fabry–Perot interferometer. The power of the techniques of molecular‐beam epitaxy (MBE) for producing well‐...

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.

Development of magnetic anisotropies in ultrathin epitaxial films of Fe(001) and Ni(001)

Ultrathin films, bcc Fe(001) on Ag(001), fcc Fe(001) on Cu(001) and Fe/Ni(001) bilayers on Ag, were grown by molecular beam epitaxy. A wide range of surface science tools were employed to establish the quality of epitaxial growth. Ferromagnetic resonance and Brillouin light scattering were used to extract the magnetic properties. Emphasis was placed on the study of magnetic anisotropies. Large uniaxial anisotropies with easy axis perpendicular to the film surface were observed in all ultrathin structures studied. These anisotropies were particularly strong in fcc Fe and bcc Fe films. In sufficiently thin samples the saturation magnetization was oriented perpendicularly to the film surface in the absence of an applied field. It has been demonstrated that in bcc Fe films the uniaxial perpendicular anisotropy originates at the film interfaces. In situ measurements indentified the strength of the uniaxial perpendicular anisotropy constant at the Fe/vacuum, Fe/Ag and Fe/Au interfaces asKus = 0.96, 0.63, and 0.3 ergs/cm2 respectively. The surface anisotropies deduced for [bulk Fe/noble metal] interfaces are in good agreement with the values obtained from ultrathin films. Hence the perpendicular surface ansiotropies originate in the broken symmetry at abrupt interfaces. An observed decrease in the cubic anisotropy in bcc Fe ultrathin films has been explained by the presence of a weak 4th order in-plane surface anisotropy,K1∥S=0.012 ergs/cm2. Fe/Ni bilayers were also investigated. Ni grew in the pure bcc structure for the first 3–6 ML and then transformed to a new structure which exhibited unique magnetic properties. Transformed ultrathin bilayers possessed large inplane 4th order anisotropies far surpassing those observed in bulk Fe and Ni. The large 4th order anisotropies originate in crystallographic defects formed during the Ni lattice transformation.

Magnetic properties of sandwiches and superlattices of fcc Fe(001) grown on Cu(001) substrates

The magnetic properties of Cu/Fe epitaxial sandwiches and superlattices have been measured using Brillouin light scattering and ferromagnetic resonance. All of the samples are perpendicularly magnetized at room temperature, due to the presence of large uniaxial anisotropies with an easy axis perpendicular to the sample surface. Inclusion of a second‐order uniaxial anisotropy term is particularly important for one of the superlattice samples.

Growth of giant magnetoresistance spin valves using indium as a surfactant

We have investigated the use of In as a surfactant to achieve smoother interfaces in spin‐valve multilayers of the general type: FeMn/Ni80Fe20/Co/Cu/Co/Ni80Fe20/glass. The coupling field is reduced from ∼0.8 to ∼0.3 mT, presumably by suppressing roughness at the Co/Cu/Co interfaces, when 0.5–1.0 nm In is deposited on the first Co film just prior to Cu deposition or on the Cu film just prior to deposition of the second Co film. The In has a strong tendency to float‐out to the surface during deposition of the spin valve leaving the spin‐valve layers largely intact. The exchange bias at the FeMn/Ni80Fe20 interface can be increased from 12 to 25 mT by the use of thicker In (1.4 nm).

X‐ray photoelectron and Auger electron forward‐scattering studies of lattice expansions and contractions in epitaxial films

X‐ray photoelectrons and Auger electrons emitted by atoms in a lattice undergo forward scattering by neighboring atoms during their propagation through the lattice. This forward scattering produces enhancements in the emission intensity along nearest‐neighbor directions. In the present work, the directions of these enhancements are used to infer lattice expansions and contractions in epitaxial films. Epitaxial films of Mn grown on Cu(100) expand outwards and on Ag(100) contract inwards, as compared to a simple extension of the face‐centered‐cubic substrate lattice. Thus, in both cases, the Mn lattice is body‐centered‐tetragonal. This expansion and contraction is reasonable since the Mn atom is intermediate in atomic volume between Cu and Ag, so that it must by compressed laterally to match the Cu(100) surface and stretched laterally to match the Ag(100) surface. However, it is of interest that the combined magnitude of these effects yield, in both cases, Mn with a larger atomic volume than that found in e...

Specular electron scattering in metallic thin films

Specular electron scattering at the surface of metallic thin films is an important phenomenon for a class of magnetic multilayers known as giant magnetoresistance (GMR) spin valves. In the very best GMR spin valves, a significant part of the GMR effect is attributable to specular electron scattering. We have investigated the importance of specular electron scattering by developing surface-modification techniques that produce diffuse electron scattering. We have used these techniques to investigate specular electron scattering in GMR spin valves and in pure metals. Some of the largest effects are found in Au films. It is noted that specular electron scattering will be a highly desirable property in future generations of microelectronic interconnects as the feature size approaches the mean free path of the conduction electrons. In order to meet the Semiconductor Industry Association Roadmap requirements for interconnect resistivity, interconnects with highly specular surfaces must be developed.

Validation of the microelectromechanical system flux concentrator concept for minimizing the effect of 1/f noise

With the microelectromechanical system (MEMS) flux concentrator, we have been able to increase the operating frequency of small magnetic sensors above the region where 1/f noise dominates. The device accomplished this by modulating the field via the oscillatory motion of flux concentrators on MEMS flaps. Electrostatic comb drives were used to drive the MEMS flaps. We have demonstrated an increase in the signal to noise ratio at 1 Hz, that the power signal correctly depends on V4 where V is the amplitude of the voltage energizing the comb drives, and that the signal increases dramatically with vacuum packaging.