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In magnetoresistive (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. Spinflipping within metals is described by a spin-diffusion 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 sf does the same for increase of the CPP-MR with increasing F-layer thickness. Spin-flipping at M1/M2 interfaces can be described by a parameter, δ M1/M2 , which determines the spin-flipping probability, P = 1 -exp(-δ). Increasing δ M1/M2 usually decreases the MR. We list measured values of these parameters and discuss the limitations on their determinations. OrganizationThis review is organized as follows. Section I provides a brief history and overview, defines the lengths of interest, briefly explains the physics underlying the spin-diffusion lengths that are the focus of the review, and discusses caveats on theoretical analysis and limitations on the measurements of the parameters of interest: the transport mean-free-path, λ t ; the spin-diffusion length in non-magnetic (N) metals or alloys, l N sf ; the spin-diffusion length in ferromagnetic (F) metals or alloys, l F sf ; and the interfacial spin-flip parameter, δ, where the spin-flipping probability is P = 1 -exp(-δ). Section II describes the different ways in which these parameters have been measured, and gives more specifics of their limitations. Section III contains four tables. Table I should be unique to each sample at 4.2K, but should be intrinsic in sufficiently high purity samples at 293K. Table III lists values of l F sf in ferromagnetic metals and alloys, mostly at 4.2K. Table IV lists values of δ N1/N2 for several N1/N2 metal pairs at 4.2K. Each table is preceded by some comments about the results. Section IV contains a brief summary and our conclusions. I. History, Overview, Definitions, and Caveats and Limitations. IA. History and Overview.The discovery in 1988 of Giant Magnetoresistance (GMR) in ferromagnetic/non-magnetic (F/N) metallic multilayers [1] [2] stimulated the growth of a new subfield of transport stud...

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.

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).

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