W.-C. Chiang

Generation and detection of phase-coherent current-driven magnons in magnetic multilayers

The magnetic state of a ferromagnet can affect the electrical transport properties of the material; for example, the relative orientation of the magnetic moments in magnetic multilayers underlies the phenomenon of giant magnetoresistance. The inverse effect—in which a large electrical current density can perturb the magnetic state of a multilayer—has been predicted and observed experimentally with point contacts and lithographically patterned samples. Some of these observations were taken as indirect evidence for current-induced excitation of spin waves, or ‘magnons’. Here we probe directly the high-frequency behaviour and partial phase coherence of such current-induced excitations, by externally irradiating a point contact with microwaves. We determine the magnon spectrum and investigate how the magnon frequency and amplitude vary with the exciting current. Our observations support the feasibility of a spin-wave maser or ‘SWASER’ (spin-wave amplification by stimulated emission of radiation).

Perpendicular-current transport in exchange-biased spin-valves

We have measured giant magnetoresistance (GMR) with current perpendicular to the layer planes (CPP-MR) in exchange-biased (EB) spin-valves-a controllable spin-switch. We will present CPP data on EB-spin valves of Permalloy (Py) and Cu, and explain how the data are analyzed. Interpreted with the theory of Valet and Fert, the data yield an unexpectedly short value for the length over which electrons lose spin-memory in Py, the spin-diffusion length, l/sub sf//sup Py/=5.5/spl plusmn/1 nm. We will show that the CPP parameters derived with the aid of single EB-spin-valves predict rather well the CPP-MRs of double EB spin-valves.

MAGNETORESISTANCE OF NIMNSB-BASED MULTILAYERS AND SPIN VALVES

We report magnetic and magnetoresistance (MR) measurements in the current-in-plane (CIP) and current-perpendicular-to-the-plane (CPP) geometries of multilayers and spin-valve structures based on the predicted half-metallic (100% spin-polarized) ferromagnetic Heusler alloy NiMnSb. Multilayers of the form [NiMnSb/Cu]×10, as well as a trilayer spin valve of the form [NiMnSb/Cu/Y] (Y=Ni80Fe20 and NiMnSb/FeMn) have been grown using a low-rate, low-pressure sputtering deposition technique. The multilayers do not show evidence of antiferromagnetic coupling between the NiMnSb layers, and the measured CPP magnetoresistance is 4.5% at 4.2 K. NiMnSb/Cu/NiFe spin valves and NiMnSb/Cu/NiMnSb/FeMn quasispin-valves adopt more closely antiparallel alignments of adjacent magnetic layer magnetizations, and CPP MR ratios as high as 7.2% are measured at 4.2 K. Although these CPP MRs are larger than the CIP MRs for similar samples, the largest CPP MR observed is still smaller than expected for a 100% spin-polarized material.

How predictable is the current perpendicular to plane magnetoresistance? (invited)

Prior measurements of the current perpendicular to the layer planes (CPP) resistances, taken on Co/Cu/Py/Cu (Py=Ni84Fe16) multilayers with a single pair of Co and Py thicknesses, are extended to three additional pairs of thicknesses. The same parameters, obtained from independent measurements on Co/Cu and Py/Cu multilayers that fit the original pair reasonably well, fit the three new pairs almost as well, from which we conclude that there is substantial predictability in the CPP magnetoresistance (MR). Because the predictability is not perfect, we examine the extent to which we can improve the fits to the Co/Cu/Py/Cu data by varying the Py/Cu and Co/Cu parameters within their uncertainties, without substantially weakening the fits to the original Co/Cu and Py/Cu data. We conclude by presenting the first CPP‐MR measurements on Co/Ag/Py/Ag multilayers. The data are similar to those for Co/Cu/Py/Cu multilayers, but the Co/Ag/Py/Ag MRs are noticeably larger.

Giant magnetoresistance of current-perpendicular exchange-biased spin-valves of Co/Cu

We present the first measurements of the specific resistance change, A/spl Delta/R, with increasing Co layer thickness, t/sub Co/, of current-perpendicular (CPP) exchange biased spin-valves (EBSVs) of Co and Cu. A/spl Delta/R grows about as predicted for a long spin diffusion length in Co until about t/sub Co/=20 nm, after which it grows more slowly than predicted. Possible reasons for this deviation from prediction are considered, including a short spin diffusion length in Co.

Variation of multilayer magnetoresistance with ferromagnetic layer sequence: Spin-memory loss effects

We find the magnetoresistance (MR) with current flow perpendicular to the layer planes (CPP-MR) of multilayers made of the ferromagnetic metals Co and Py (permalloy=Ni84Fe16) and the nonmagnetic metal Ag to vary with the Co and Py layer sequencing. That is, [Co/Ag/Py/Ag]N (type I) multilayers have larger MRs than [Co/Ag]N[Py/Ag]N (type II) ones, and the difference grows with the number of repeats N. We ascribe the differences to an apparent short spin-diffusion length in Py. Interestingly, similar differences appear also in the current in plane MR, probably due to finite mean-free paths in all three metals, plus interface scattering.

Spin-memory loss and current-perpendicular-to-plane-magnetoresistance in sputtered multilayers with Au

We derive a spin-diffusion length at 4.2 K in sputtered Au, lsfAu=35−5+65 nm, spin-memory-loss at Au/Cu interfaces, δAu/Cu=0.13−0.02+0.08, and Au/Cu interface specific resistance, 2ARAu/Cu=0.35−0.05+0.10 fΩ m2. We also show that exchange biased spin valves with Au sandwiched between Co layers produce changes in specific resistance, AΔR, comparable to those for Cu and Ag.

Pd/Ag and Pd/Au interface specific resistances and interfacial spin flipping

We measured the specific resistances, 2ARPd∕Ag and 2ARPd∕Au (sample area A times resistance R), and spin-flip probabilities δPd∕Ag and δPd∕Au for Pd/Ag and Pd/Au interfaces with current perpendicular to the interfaces. 2ARPd∕Ag=0.7±0.15fΩm2 and 2ARPd∕Au=0.45±0.15fΩm2 are smaller than our revised estimate of 2ARPd∕Cu=0.85±0.15fΩm2, and lie further from no-free-parameter calculations. Our estimates of δPd∕Ag=0.15±0.08 and δPd∕Ag=0.08±0.08 are less than our prior estimate of δPd∕Cu∼0.24.

Resistivities of sputtered Ag(3.3 nm)/Cu(tCu), Ag(3.3 nm)/Au(tAu), and Ni(4.2 nm)/Co(tCo) multilayers at 4–5 and 295 K

To help clarify the sources of the 67% periodic oscillations in the in‐plane saturation resistivity, ρ S (ρ S varied from 2.5 to 5 μΩ cm at 4.2 K) of molecular beam epitaxy‐grown epitaxial Ni/Co multilayers of total thickness, t T ≊100 nm recently reported by Gallego et al. [Phys. Rev. Lett. 74, 4515 (1995)] we have measured the in‐plane resistivities at 4–5 and 295 K of Ag/Cu, Ag/Au, and Ni/Co multilayers with t T ≊100 nm, dc sputtered nonepitaxially onto (001) Si. In no case do we see reproducible periodic oscillations, and any variations are always similar at 4–5 and 295 K. Our results show that oscillations of the size seen by Gallego are not simply a consequence of an average multilayer resistivity as low as 3.5 μΩ cm.