Dennis Richard Wilhoit

Magnetotransport properties of magnetically soft spin‐valve structures (invited)

The magnetic and magnetotransport properties of several series of sandwiches consisting of two ferromagnetic layers (Ni, Co, Ni80Fe20) separated by a noble metal (Cu, Ag, Au) are described. In order to vary the relative orientation of the magnetizations of the two ferromagnets, one of them was constrained by exchange anisotropy (e.g., NiFe/Fe50Mn50). The ferromagnetic layers are magnetically soft and not coupled antiparallel, giving very large changes of resistance at low fields. At room temperature relative changes ΔR/R of 4.1% in 10 Oe for Si/Ta 50 A/NiFe 62 A/Cu 22 A/NiFe 40 A/FeMn 70 A/Ta 50 A and 8.7% in 20 Oe has been obtained for a structure based on Co/Cu/Co layers. The magnetoresistance versus the thickness of the ferromagnetic layer shows a broad peak near 80 A for Ni, Co and NiFe, demonstrating the importance of bulk rather than interfacial spin‐dependent scattering, in contrast to Fe/Cr multilayers. The magnetoresistance decreases exponentially with increasing interlayer (Cu and Au) thickness,...

Spin-valve effect in soft ferromagnetic sandwiches

Abstract We demonstrated in a variety of systems that the in-plane resistivity of sandwiches of soft ferromagnetic layers separated by nonmagnetic metallic layers depends on the relative angle between their magnetizations. We observe this phenomenon, which we term the spin-valve effect, in sandwiches where we are able to control the relative angle between the magnetizations of two ferromagnetic layers either by constraining one layer through exchange anisotropy or by fabricating layers with different coercivities. In the first case, for example Si/50A Ta/60A NiFe/25A Cu/40A NiFe/50A FeMn/50A Ta we have seen relative changes in resistance of more than 4% at room temperature in a range of in-plane field of 0 to 15 Oe. In a system where the layers have different coercivities, Si/8 × (30A Fe/60A Ag/30A Co/60A Ag), we observed a relative change of 1.6% at room temperature for fields between 0 and 50 Oe. Since the ferromagnetic layers are essentially decoupled and have high squareness, one can rule out any mechanism requiring scattering by domain walls. The usual anisotropic magnetoresistance in these structures is much smaller than the spin-valve effect. In contrast to noble metals, when using Ta, Al, Cr or Pd spacers of similar thickness (20 to 150A) between layers of permalloy, only the anisotropic magnetoresistance is observed. We believe the spin-valve effect to be related to spin-dependent scattering at the interface and within the ferromagnetic layers, in balance with spin-dependent relaxation within the layers. We also report the observation of a weak exchange-like coupling between the ferromagnetic layers.

The role of spin‐dependent impurity scattering in Fe/Cr giant magnetoresistance multilayers

To probe the mechanism of giant magnetoresistance (GMR) observed in Fe/Cr multilayers, we have sputter deposited at the interfaces of Fe(15 A)/Cr(12 A) multilayers an additional, ultrathin (0–4 A) layer of a variety of third elements (V, Mn, Ge, Ir, and Al). When alloyed with Fe in dilute concentrations, the elements chosen have known resistivities for spin‐up (ρ↑) and spin‐down (ρ↓) currents arising from spin‐dependent impurity scattering. The results show a clear correlation between α=ρ↓/ρ↑ of the respective element and the way in which GMR varies with the ultrathin layer thickness. In addition, little difference in GMR is observed between multilayers where the ultrathin layer thickness t/2 is deposited on every Fe/Cr interface and those with a thickness t deposited on alternate interfaces. This investigation demonstrates the importance of the type and total number of scattering centers per multilayer period to the GMR effect.

High sensitivity spin-valve strain sensor

A technique for detecting strain has been demonstrated based on a spin-valve sensor. The 400 A thick sensor has been integrated onto an atomic force microscope cantilever. An applied strain caused by bending of the cantilever changes the orientation of the free-layer magnetization due to magnetostriction. This in turn results in a change in the electrical resistance because of the giant magnetoresistance effect. With the proper magnetic bias, a base-line strain sensitivity of 10−10/Hz1/2 has been achieved. The corresponding gauge factor of 150 is roughly 1.6× that of similar silicon piezoresistive cantilevers. In the future, one might be able to enhance the sensitivity by another factor of 3–5.

Magnetostriction in spin valves

We report an accurate method for the measurement of the saturation magnetostriction λs of thin conducting ferromagnetic films using spin valve structures. We describe an alternative to the cantilever beam method, called the bending method, which utilizes the inverse magnetostrictive effect. Typical soft ferromagnetic films exhibit hysteretic magnetization loops, which make the measurements of the magnetoelastic energy (hence λs) imprecise or erroneous. We show how to resolve the hysteresis problem by application of a transverse field. We also demonstrate the quantitative connection between the cantilever and bending method in a comparative study of films spanning a large magnetostriction range. The sensitivity of measurement of magnetostriction using the bending method is better than 1×10−7.

CAN SPIN VALVES BE RELIABLY DEPOSITED FOR MAGNETIC RECORDING APPLICATIONS?(INVITED)

The tolerance of the expected read-back signal of spin valve giant magnetoresistance based structures to varying deposition and process conditions are described. We determine if spin valves can be produced reliably, and evaluate which thicknesses and properties are most critical. First, the dependence of spin valve properties on layer thickness are experimentally determined. Next, the variation of read-back signal and transfer curve characteristics with spin valve properties is calculated from micromagnetic modeling. Finally, these are convolved with the expected reproducibility of layer thickness to obtain an effective “yield” of structures within 10% of the mean amplitude. We find that spin valves can be reliably deposited, with “yields” well in excess of 90% likely.

Change in conductance is the fundamental measure of spin‐valve magnetoresistance

The absolute change of sheet conductance (ΔG) of spin‐valve multilayers is shown analytically and experimentally to be the macroscopic observable most directly related to the physical mechanism of giant magnetoresistance. Unlike the changes in resistance ΔR/R or ΔR, ΔG is directly connected to the changes in the Fermi spheres of the ferromagnetic layers induced by the variation in magnetic alignment. In structures comprising Si/Co 80 A/Cu 25 A/NiFe 50 A/FeMn 90 A/Ta 50 A/Cu tCu/Ta 50 A, in which only the thickness tCu is varied, the values and the thermal variations of ΔR/R(T) and ΔR(T) are strongly influenced by tCu through its shunting effect. In contrast ΔG is found to be independent of shunting.

Influence of Au and Ag at the interface of sputtered giant magnetoresistance Fe/Cr multilayers

Multilayer films consisting of 20-AA Fe and 15-AA Cr with amounts of Au and Ag varying from 0 to 4 AA deposited at alternate Fe/Cr interfaces have been RF-diode-sputtered, and their magnetic and magnetotransport properties have been measured. When no noble metal is included, the films exhibit strong antiparallel coupling between adjacent Fe layers, with saturation fields as large as 7 kOe, and giant magnetoresistance (GMR) of 6% at 300 K, similar to that of MBE (molecular beam epitaxy)-grown films reported by others. The interruption of intimate contact between Fe and Cr at the interface resulting from noble-metal deposition gives rise to a substantial decrease in the magnitude of the GMR and the interfacial coupling. This suggests that the mechanism of GMR is associated with Fe and Cr in contact, as is required by models invoking spin-dependent impurity scattering. The results also suggest that the antiparallel coupling arises from direct exchange. >

Giant magnetoresistance of Fe/Cr multilayers: Impurity scattering model of the influence of third elements deposited at the interfaces

We describe the least complex model of giant magnetoresistance that maintains the essential features of spin‐dependent impurity scattering; it pictures the multilayer as a homogeneous distribution of scattering centers whose scattering properties depend only on the impurity element and local magnetization direction of the layers into which the centers are imbedded. Model predictions are compared with results of sputtered Fe/Cr multilayer films where small amounts of third elements (V, Cr, Mn, Al, Ir, and Ge), with known spin‐dependent scattering properties in dilute alloys, are deposited at the interfaces. The model is in agreement with the observed behavior.

Media noise in periodic multilayered magnetic films with perpendicular anisotropy

Periodic, finely multilayered thin‐film magnetic structures (≤200 A) with high in‐plane coercivity suitable for longitudinal recording investigations have been fabricated. CoPt20/Pd and CoPt12Cr17/Pd periodic multilayers can exhibit a perpendicular orientation, originating from surface anisotropy effects, with a magnitude depending on the thickness of the component layers. The perpendicular orientation influences the transition noise, but for the particular systems studied, the results suggest that for a given value of coercivity squareness, ≲0.8, the greatest improvements in media noise performance can be achieved through reduced exchange in films with a well‐defined, in‐plane orientation.