Current-induced switching in single ferromagnetic layer nanopillar junctions

Current induced magnetization dynamics in asymmetric Cu/Co/Cu single magnetic layer nanopillars has been studied experimentally at room temperature and in low magnetic fields applied perpendicular to the thin film plane. In sub-100 nm junctions produced using a nanostencil process a bistable state with two distinct resistance values is observed. Current sweeps at fixed applied fields reveal hysteretic and abrupt transitions between these two resistance states. The current induced resistance change is 0.5%, a factor of 5 greater than the anisotropic magnetoresistance (AMR) effect. We present an experimentally obtained low field phase diagram of current induced magnetization dynamics in single ferromagnetic layer pillar junctions.Comment: 11 pages, 2 figure

Current Induced Excitations in Cu/Co/Cu Single Ferromagnetic Layer Nanopillars

Current-induced magnetic excitations in Cu/Co/Cu single layer nanopillars (~50 nm in diameter) have been studied experimentally as a function of Co layer thickness at low temperatures for large applied fields perpendicular to the layers. For asymmetric junctions current induced excitations are observed at high current densities for only one polarity of the current and are absent at the same current densities in symmetric junctions. These observations confirm recent predictions of spin-transfer torque induced spin wave excitations in single layer junctions with a strong asymmetry in the spin accumulation in the leads.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let

Focused-ion-beam milling based nanostencil mask fabrication for spin transfer torque studies

Focused-ion-beam milling is used to fabricate nanostencil masks suitable for the fabrication of magnetic nanostructures relevant for spin transfer torque studies. Nanostencil masks are used to define the device dimensions prior to the growth of the thin film stack. They consist of a wet etch resistant top layer and an insulator on top of a pre-patterned bottom electrode. The insulator supports a hard mask and gives rise to an undercut by its selective etching. The approach is demonstrated by fabricating current perpendicular to the plane Co/Cu/Co nanopillar junctions, which exhibit current-induced magnetization dynamics.Comment: 13 pages, 3 figures, submitted to AP

Gate controlled valley polarizer in bilayer graphene

Sign reversal of Berry curvature across two oppositely gated regions in bilayer graphene can give rise to counter-propagating 1D channels with opposite valley indices. Considering spin and sub-lattice degeneracy, there are four quantized conduction channels in each direction. Previous experimental work on gate-controlled valley polarizer achieved good contrast only in the presence of an external magnetic field. Yet, with increasing magnetic field the ungated regions of bilayer graphene will transit into the quantum Hall regime, limiting the applications of valley-polarized electrons. Here we present improved performance of a gate-controlled valley polarizer through optimized device geometry and stacking method. Electrical measurements show up to two orders of magnitude difference in conductance between the valley-polarized state and gapped states. The valley-polarized state displays conductance of nearly 4e2/h and produces contrast in a subsequent valley analyzer configuration. These results pave the way to further experiments on valley-polarized electrons in zero magnetic field

Current-driven excitations in magnetic multilayers: a brief review

In 1996, Berger and Slonczewski independently predicted that a large enough spin-polarized dc current density sent perpendicularly through a ferromagnetic layer could produce magnetic excitations (spin-waves) or reversal of magnetization (switching). In the past few years, both current-driven switching and current-driven excitation of spin-waves have been observed. The switching is of potential technological interest for direct 'writing' of magnetic random access memory (MRAM) or magnetic media. The spin-wave generation could provide a new source of dc generated microwave radiation. We describe what has been learned experimentally about these two related phenomena, and some models being tested to explain these observations.Comment: 5 pages, 7 figures, expected to appear in conf. proceeding

From One Electron to One Hole: Quasiparticle Counting in Graphene Quantum Dots Determined by Electrochemical and Plasma Etching

Graphene is considered to be a promising material for future electronics. The envisaged transistor applications often rely on precision cutting of graphene sheets with nanometer accuracy. In this letter we demonstrate graphene-based quantum dots created by using atomic force microscopy (AFM) with tip-assisted electrochemical etching. This lithography technique provides resolution of about 20 nm, which can probably be further improved by employing sharper tips and better humidity control. The behavior of our smallest dots in magnetic field has allowed us to identify the charge neutrality point and distinguish the states with one electron, no charge and one hole left inside the quantum dot

Current-Driven Magnetization Dynamics in Magnetic Multilayers

We develop a quantum analog of the classical spin-torque model for current-driven magnetic dynamics. The current-driven magnetic excitation at finite field becomes significantly incoherent. This excitation is described by an effective magnetic temperature rather than a coherent precession as in the spin-torque model. However, both the spin-torque and effective temperature approximations give qualitatively similar switching diagrams in the current-field coordinates, showing the need for detailed experiments to establish the proper physical model for current-driven dynamics.Comment: 5 pages, 2 figure