Using thin film pillars ∼100 nm in diameter, containing two ferromagnetic Co layers of different thicknesses separated by a paramagnetic Cu spacer, we examine effects of torques due to spinpolarized currents flowing perpendicular to the layers. In accordance with spin-transfer theory, spin-polarized electrons flowing from the thin to the thick Co layer can switch the magnetic moments of the layers antiparallel, while a reversed electron flow causes switching to a parallel state. When large magnetic fields are applied, the current no longer fully reverses the magnetic moment, but instead stimulates spin-wave excitations.

The spin-transfer effect has been studied in magnetic tunnel junctions (PtMn/CoFe/Ru/CoFe/Al 2 O 3 /CoFe/NiFe) with dimensions down to 0.1x0.2 µm 2 and resistance-area product RA in the range of 0.5-10 Ωµm 2 (∆R/R=1-20%). Current-induced magnetization switching is observed with a critical current density of about 8x10 6 A/cm 2 . The attribution of the switching to the spintransfer effect is supported by a current-induced ∆R/R value identical to the one obtained from the R versus H measurements. Furthermore, the critical switching current density has clear dependence on the applied magnetic field, consistent with what has been observed previously in the case of spin-transfer induced switching in metallic multilayer systems.Magnetization switching induced by spin-polarized current has stimulated considerable interest in recent years due to its rich fundamental physics and potential for new magnetoelectronic applications. Low switching current density and high read signal are required for the application of the spin-transfer switching to non-volatile magnetic random access memory (MRAM). Most of the work to date, however, has focused on magnetic metallic multilayers, which require large currents applied in the currentperpendicular-to-plane direction but yield small resistance (R) and nominal magnetoresistance (∆R/R). 1 On the other hand, magnetic tunnel junctions (MTJ) have both high R and ∆R/R, resulting in high signal output. In order to utilize MTJs in spin transfer based MRAM, however, requires an understanding of the limits of both the spin transfer effect and the electron transport properties of tunnel barriers used in MTJs.We report the observation of the spin-transfer effect in low-resistance MTJs ( RA=0.5-10 Ωµm 2 ) with dimensions down to 0.1x 0.2 µm 2 . These deep submicron-sized MTJs minimize the Oersted (vortex) field contribution due to large vertical current through the MTJ pillars. 2,3 MTJ films Ta20/NiFeCr35/PtMn140/ CoFe20/Ru8 /CoFe22/ Al 2 O 3 / CoFe10/NiFe20/Ta50 (in Å) were deposited in a magnetron sputtering cluster system and annealed at 250-270 o C for 10 hours. A thin tunneling barrier was formed by two-step natural oxidation of the pre-deposited Al layer in a pure oxygen atmosphere. 4 The MTJ films were subsequently patterned into deep submicron ellipse-shaped pillars using DUV photolithography combined with resist trimming and ion milling. The pillar dimensions and microstructures have been characterized by high-resolution transmission electron microscope (TEM). The cross sectional TEM micrograph of an MTJ sample (0.12 x 0.23 µm 2 ellipse)

A thin film Co nanomagnet in the shape of an elongated hexagon has been incorporated in a vertical device structure consisting of the nanomagnet and a thin Cu spacer layer formed on top of a thick Co film. The spin-polarized current flowing between the nanomagnet and the Co film is used to abruptly switch the magnetic alignment of the nanomagnet relative to that of the thick Co layer by the transfer of spin angular momentum from the conduction electrons to the nanomagnet moment. The shape anisotropy in the nanomagnet promotes the single domain behavior required for nonvolatile memory applications.

We have measured the statistical properties of magnetic reversal in nanomagnets driven by a spin-polarized current. Like reversal induced by a magnetic field, spin-transfer-driven reversal near room temperature exhibits the properties of thermally activated escape over an effective barrier. However, the spin-transfer effect produces qualitatively different behaviors than an applied magnetic field. We discuss an effective current vs field stability diagram. If the current and field are tuned so that their effects oppose one another, the magnet can exhibit telegraph-noise switching.

We have studied magnetic switching by spin-polarized currents and also the magnetoresistance in sub-100-nm-diam thin-film Co/Cu/Co nanostructures, with the current flowing perpendicular to the plane of the films. By independently varying the thickness of all three layers and measuring the change of the switching currents, we test the theoretical models for spin-transfer switching. In addition, the changes in the switching current and magnetoresistance as a function of the Cu layer thickness give two independent measurements of the room-temperature spin-diffusion length in Cu.