J. Bass

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.

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).

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.

Deviations from Matthiessen's Rule

Abstract We present a comprehensive review of the subject of Deviations from Matthiessenss Rule, including a historical survey and detailed critical evaluations of both theory and experimental data. We discuss deviations occurring: in substitutional alloys (both magnetic and non-magnetic); after quenching; after radiation damage; after plastic deformation; and in thin samples. The only topic omitted is that of anomalous negative deviations occurring at low temperatures (e.g. the Kondo effect).

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).

Changing Exchange Bias in Spin Valves with an Electric Current

An electrical current can transfer spin angular momentum to a ferromagnet. This novel physical phenomenon, called spin transfer, offers unprecedented spatial and temporal control over the magnetic state of a ferromagnet and has tremendous potential in a broad range of technologies, including magnetic memory and recording. Recently, it has been predicted that spin transfer is not limited to ferromagnets, but can also occur in antiferromagnetic materials and even be stronger under some conditions. In this paper we demonstrate transfer of spin angular momentum across an interface between ferromagnetic and antiferromagnetic metals. The spin transfer is mediated by an electrical current of high density (~10^12 A/m^2) and revealed by variation in the exchange bias at the ferromagnet/antiferromagnet interface. We find that, depending on the polarity of the electrical current flowing across the interface, the strength of the exchange bias can either increase or decrease. This finding is explained by the theoretical prediction that a spin polarized current generates a torque on magnetic moments in the antiferromagnet. Current-mediated variation of exchange bias can be used to control the magnetic state of spin-valve devices, e.g., in magnetic memory applications.

Giant magnetoresistance with current perpendicular to the multilayer planes

Abstract We review recent progress in measuring giant magnetoresistance in magnetic multilayers with the current flowing perpendicular to the multilayer planes (CPP-MR), and present new data for Co/Cu and Co/Ag multilayers.

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

Abstract We present the first measurements of giant magnetoresistance with current perpendicular to the layer planes (CPP-MR) of an exchange-biased spin-valve (EBSV) - a controllable spin-switch. Interpreted with the theory of Valet and Fert, data on EBSVs of permalloy (Py) with Cu yield an unexpectedly short spin-diffusion length in Py − l sf Py =5.5± nm. Such a short length has implications for the interpretation of Johnsons data on a Py/Au/Py spin-switch.

The formation and motion energies of vacancies in aluminium

Abstract The resistance quenched into pure aluminium wires has been measured as a function of quench temperature for four quench speeds, varying by a factor of 200. The resulting data are consistent with the theory of Flynn et al. (1965), which assumes that the vacancy loss during a quench occurs by migration of single vacancies to fixed sinks. Although the data determine vacancy formation and motion energies of about 0·71 ev and 0·77 ev respectively for Q = 1·48 ev, an examination of previous experiments and of possible sources of random and systematic error in the present experiment suggests that the actual formation energy might be slightly higher than this value. The values Ef =0·73±0·03 ev and Em = 0·75±0·08 ev are proposed as the best presently available for aluminium.