W. P. Pratt

Oscillatory interlayer coupling and giant magnetoresistance in Co/Cu multilayers

Abstract We have studied the magnetic and magneto-transport properties of Co/Cu multilayers prepared by sputtering. We find oscillations of the interlayer coupling as a function of the Cu thickness with a period of about 12 A. A giant magnetoresistance is observed in the half-periods with antiferromagnetic interlayer coupling. Magnetoresistance as large as 78% at 4.2 K and 48% at room temperature have been observed.

Current-perpendicular (CPP) magnetoresistance in magnetic metallic multilayers

We present an experimentalists view of the theory and published data for the magnetoresistance (MR) of a multilayer composed of alternating ferromagnetic (F) and non-magnetic (N) metals measured with current flow perpendicular to the layer planes (CPP-MR). We explain the advantages of this geometry for determining the fundamental quantities underlying spin-polarized transport, describe the different techniques developed to measure the CPP-MR, summarize the salient features of the models used to analyze experimental data, and describe tests of those models. We then review what has been learned so far about spin-dependent scattering anisotropy and spin relaxation in F-metals and at F/N interfaces, specific resistances of F/N interfaces, the temperature dependence of spin-polarized transport parameters, and mixing of the spin-polarized electron currents. After a brief overview of some new directions, we conclude with a list of questions still to be answered.

Spin-diffusion lengths in metals and alloys, and spin-flipping at metal/metal interfaces: an experimentalist's critical review

In magnetoresistance (MR) studies of magnetic multilayers composed of combinations of ferromagnetic (F) and non-magnetic (N) metals, the magnetic moment (or related ‘spin’) of each conduction electron plays a crucial role, supplementary to that of its charge. While initial analyses of MR in such multilayers assumed that the direction of the spin of each electron stayed fixed as the electron transited the multilayer, we now know that this is true only in a certain limit. Generally, the spins ‘flip’ in a distance characteristic of the metal, its purity, and the temperature. They can also flip at F/N or N1/N2 interfaces. In this review we describe how to measure the lengths over which electron moments flip in pure metals and alloys, and the probability of spin-flipping at metallic interfaces. Spin-flipping within metals is described by a spindiffusion length, l M sf , where the metal M = F or N. Spin-diffusion lengths are the characteristic lengths in the current-perpendicular-to-plane (CPP) and lateral non-local (LNL) geometries that we focus upon in this review. In certain simple cases, l N sf sets the distance over which the CPP-MR and LNL-MR decrease as the N-layer thickness (CPP-MR) or N-film length (LNL) increases, and l F does the same for increase of the CPP-MR with increasing F-layer thickness. Spinflipping at M1/M2 interfaces can be described by a parameter, δM1/M2 ,w hich determines the spin-flipping probability, P = 1 − exp(−δ). Increasing δM1/M2 usually decreases the MR. We list measured values of these parameters and discuss the limitations on their determinations.

Current-driven magnetic excitations in permalloy-based multilayer nanopillars

We study current-driven magnetization switching in nanofabricated Ni(84)Fe(16)/Cu/Ni(84)Fe16 trilayers at 295 and 4.2 K. The shape of the hysteretic switching diagram at low magnetic field changes with temperature. The reversible behavior at higher fields involves two phenomena, a threshold current for magnetic excitations closely correlated with the switching current, and a peak in differential resistance characterized by telegraph noise, with an average period that decreases exponentially with current and shifts with temperature. We interpret both static and dynamic results at 295 and 4.2 K in terms of thermal activation over a potential barrier, with a current-dependent effective magnetic temperature.

Observation of spin-triplet superconductivity in co-based josephson junctions

We have measured a long-range supercurrent in Josephson junctions containing Co (a strong ferromagnetic material) when we insert thin layers of either PdNi or CuNi weakly ferromagnetic alloys between the Co and the two superconducting Nb electrodes. The critical current in such junctions hardly decays for Co thicknesses in the range of 12-28 nm, whereas it decays very steeply in similar junctions without the alloy layers. The long-range supercurrent is controllable by the thickness of the alloy layer, reaching a maximum for a thickness of a few nm. These experimental observations provide strong evidence for induced spin-triplet pair correlations, which have been predicted to occur in superconducting-ferromagnetic hybrid systems in the presence of certain types of magnetic inhomogeneity.

Two-channel analysis of CPP-MR data for Ag/Co and AgSn/Co multilayers

Abstract We present a 2-channel model for the magnetoresistance of a magnetic multilayer with the current flowing perpendicular to the layer planes (CPP-MR), and show that the model fits previous data on Ag/Co and new data on AgSn/Co multilayers better than our previous simple series resistance model. The 2-channel model supports the conclusions previously reached for Ag/Co, and permits a determination of the spin-orientation-dependent parameters α Co = 2.9(+0.5, -0.3) and α Ag / Co = 12(+5, -2).

Magnetization-dependent Tc shift in ferromagnet/superconductor/ferromagnet trilayers with a strong ferromagnet

We have measured the superconducting transition temperature

Giant magnetoresistance with current perpendicular to the multilayer planes

T_c

Perpendicular-current exchange-biased spin-valve evidence for a short spin-diffusion lenght in permalloy

of Ni/Nb/Ni trilayers when the magnetizations of the two outer Ni layers are parallel (P) and antiparallel (AP). The largest difference in

Spin-memory loss at 4.2 K in sputtered Pd and Pt and at Pd/Cu and Pt/Cu interfaces

T_c