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)

We present spin transfer switching results for MgO based magnetic tunneling junctions (MTJs) with large tunneling magnetoresistance (TMR) ratio of up to 150 % and low intrinsic switching current density of 2-3 x 10 6 A/cm 2 . The switching data are compared to those obtained on similar MTJ nanostructures with AlO x barrier. It is observed that the switching current density for MgO based MTJs is 3-4 times smaller than that for AlO x based MTJs, and that can be attributed to higher tunneling spin polarization (TSP) in MgO based MTJs. In addition, we report a qualitative study of TSP for a set of samples, ranging from 0.22 for AlO x to 0.46 for MgO based MTJs, and that shows the TSP (at finite bias) responsible for the current-driven magnetization switching is suppressed as compared to zero-bias tunneling spin polarization determined from TMR.

Dual magnetic tunnel junction (MTJ) structures consisting of two MgO insulating barriers of different resistances, two pinned reference layers aligned antiparallel to one another, and a free layer embedded between the two insulating barriers have been developed. The electron transport and spin dependent resistances in the dual MTJ structures are accounted for by sequential tunneling with some spin-flip relaxation in the central electrode (the free layer). With a tunneling magnetoresistance ratio of 70%, a switching current density Jc (at 30ms) of 0.52MA∕cm2 is obtained, corresponding to an intrinsic value of Jc0 (at 1ns) of 1.0MA∕cm2. This value of Jc0 is 2–3 times smaller than that of a single MgO insulating barrier MTJ structure and results from improvements in the spin-transfer torque efficiency. The asymmetry between JcAP→P and JcP→AP is significantly improved, which widens the read-write margin for memory device design. In addition, the experimental results show that the switching current density can be further reduced when an external field is applied along the hard axis of the free layer.

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Spin-transfer-driven magnetization switching was studied in single magnetic tunneling junctions (MTJ: Ta∕PtMn∕CoFe∕Ru∕CoFeB∕Al2O3∕CoFeB∕Ta) and dual spin filters (DSF: Ta∕PtMn∕CoFe∕Ru∕CoFeB∕Al2O3∕CoFeB∕spacer∕CoFe∕PtMn∕Ta) having resistance-area (RA) product in the range of 10–30Ωμm2 and tunnel magnetoresistance (TMR) of 15%–30%. The intrinsic critical current density (Jc0) was estimated by extrapolating experimentally obtained critical current density (Jc) versus pulse width (τ) data to a pulse width of 1ns. Jc, extrapolated to τ of 1ns (∼Jc0), was 7×106 and 2.2×106A∕cm2, respectively, for the MTJ and improved DSF samples having identical free layers. Thus, a significant enhancement of the spin transfer switching efficiency is seen for DSF structures compared to the single MTJ case.