Spin transfer in asymmetric Co-Cu-Co bilayer magnetic nanopillars junctions has been studied at low temperature as a function of free-layer thickness. The phase diagram for current-induced magnetic excitations has been determined for magnetic fields up to 7.5 T applied perpendicular to the junction surface and freelayers thicknesses from 2 to 5 nm. The junction magnetoresistance is independent of thickness. The critical current for magnetic excitations decreases linearly with decreasing free-layer thickness, but extrapolates to a finite critical current in the limit of zero thickness. The limiting current is in quantitative agreement with that expected due to a spin-pumping contribution to the magnetization damping. It may also be indicative of a decrease in the spin-transfer torque efficiency in ultrathin magnetic layers.Spin transfer in magnetic nanopillar has become a major focus of experimental research 1-3 since Slonczewski and Berger's seminal theoretical work in 1996. 4,5 A spin current has been demonstrated to switch the magnetization direction of a small magnet at a specific current density, as well as to induce microwave excitations. There are applications of this effect to magnetic random access memory ͑MRAM͒ and high-frequency electronics. 1,6,7 It is of importance to determine the factors that control the critical current for magnetization dynamics for both the physics and technology of spin transfer. For instance, it is of interest to reduce the critical current for MRAM applications, and to increase it in magnetic sensor designs.In Slonczewski's theory, spin transfer is an interface effect: spin-angular momentum is transferred to the background magnetization when the spin current enters the ferromagnet-within the first few atomic layers. 8 For one polarity of the current, this generates a torque on the magnetization that is opposed by bulk damping. As a result, there is a threshold current to excite magnetization dynamics that is proportional to the volume of the magnet or, equivalently, the threshold current density is proportional to the thickness of the magnetic layer. There are alternative models in which the spin-transfer interaction occurs on a longer length scale, which predict a decrease in the efficiency of the torque in very thin magnetic layers. 9,10 It is also now widely appreciated that the magnetization damping in thin layers can be dominated by interfaces, in an effect known as "spin pumping." 11 For these reasons it is of importance to study spin transfer in samples in which the layer thicknesses are varied to gain insight into the factors that determine the strength and length scales of the spin-transfer interaction.Albert et al. 12 studied current-induced magnetization switching as a function of free-layer thickness at room temperature. Here thermal fluctuations are important and the intrinsic ͑zero temperature͒ critical current was determined by extrapolating from pulsed current measurements. The switching was between in-plane magnetized states, parallel and antiparallel to the f...
Current-induced excitations in bilayer magnetic nanopillars have been studied with large magnetic fields applied perpendicular to the layers at low temperatures. Junctions investigated all have Cu/Co/Cu/Co/Cu as core layer stacks. Two types of such junctions are compared, one with the core stack sandwiched between Pt layers (Type A), the other with Pt only on one side of the stack (Type B ). Transport measurements show these two types of junctions have similar magnetoresistances and slope of critical currents with respect to field, while A samples have higher resistance. The high field bipolar excitation, as was previously reported [ Özyilmaz et al. Phys. Rev. B, 71, 140403(R)] , is present in B samples only. This illustrates the importance of contact layers to spin-current-induced phenomena. This also confirms a recent prediction on such spin-wave excitations in bilayers.
When injected spin polarized electrons interact with the magnetic moment of a free layer, their angular momentum becomes transferred to the free layer [1,2]. If sufficient current is applied, the exerted torque switches the free layer either parallel or anti-parallel to the pinned layer depending on the direction of flow of the current. This type of localized current switching is attractive for an MRAM application because it does not have the half-select problems of conventional MRAM. Moreover, current requirements for spin transfer switching go down as devices scale to smaller sizes, so current-induced spin transfer is a good potential scaling path for the MRAM write operation. While interesting initial spin-transfer switching demonstrations were done with spin valve structures [3,4], these devices have impractically low output voltages. Recent advances in high tunneling magnetoresistance TMR devices with MgO barriers [5,6], including devices with low resistance-area (RA) products [7], have yielded a materials system capable of achieving sufficient output voltage for read operations in spin transfer devices. Indeed very recently, a first demonstration of a 4-Kbit spin-momentum transfer MRAM utilizing ~100x150nm2 MgO devices was reported [8]. This paper focuses on the fabrication and characterization of sub-100nm spin-transfer switching devices. The devices were fabricated utilizing extensions of a self-aligned lift off process[9], which was earlier utilized for first MTJ and MRAM demonstrations. Following deposition of the low RA magnetic stacks with MgO barriers, electron beam lithography with the negative resist (NEB) was used to pattern resist structures for ion milling masks to define the magnetic stack shapes. Following the ion mill, the resist mask also served as a self-aligned mask for lift-off of a deposited insulator. Finally a top electrode was fabricated utilizing a lift-off approach. These fabricated tunnel junctions were characterized at room temperature with a quasi-static electrical measurement setup. The RA of the junctions was measured to be ~2 Ohm-um2 and TMR of 50% was obtained from external field sweep measurements, as shown in Fig. 1(a). In an example current sweep measurement at an external field bias of -46 Oe shown in Figure 1(b), the junction switches from parallel to anti-parallel state at +0.65 mA while it switches back to the parallel state at -0.45 mA. This gives a switching current density of roughly 107A/cm2. We investigated the correlation between a spin-transfer device's structure characteristics and its transport behavior. We found that in well-formed junctions, such as shown in Fig 1, complete magnetic switching can be achieved by spin-current, as evidenced by the same amount of magnetoresistance change as inducible by the application of a magnetic field. On less well-formed devices, primarily involving non-uniform current flow through the free-layer, and in some occasions a nonuniform free-layer itself, we found that the spin-current-induced magnetic reversal tended to have pa...
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