Patrick M. Braganca

A Three-Terminal Approach to Developing Spin-Torque Written Magnetic Random Access Memory Cells

Using a self-aligned fabrication process together with multiple-step aligned electron beam lithography, we have developed a nanopillar structure where a third contact can be made to any point within a thin-film multilayer stack. This substantially enhances the versatility of the device by providing the means to apply independent electrical biases to two separate parts of the structure. Here, we demonstrate a joint magnetic spin-valve (SV)/tunnel junction structure sharing a common free layer nanomagnet contacted by this third electrode. A spatially nonuniform spin-polarized current flowing into the free layer via the low-resistance SV path can reverse the magnetic orientation of the free layer as a consequence of the spin-torque (ST) effect, by nucleating a reversal domain at the spin injection site that propagates across the free layer. The free layer magnetic state can then be read out separately via the higher resistance magnetic tunnel junction (MTJ). This three-terminal structure provides a strategy for developing high-performance ST magnetic random access memory (ST-MRAM) cells, which avoids the need to apply a large voltage across a MTJ during the writing step, thereby enhancing device reliability, while retaining the benefits of a high-impedance MTJ for read-out.

Mechanisms limiting the coherence time of spontaneous magnetic oscillations driven by dc spin-polarized currents

The spin-transfer torque from a DC spin-polarized current can generate highly-coherent magnetic precession in nanoscale magnetic-multilayer devices. By measuring linewidths of spectra from the resulting resistance oscillations, we argue that the coherence time can be limited at low temperature by thermal deflections about the equilibrium magnetic trajectory, and at high temperature by thermally-activated transitions between dynamical modes. Surprisingly, the coherence time can be longer than predicted by simple macrospin simulations.

Graphene Magnetic Field Sensors

Graphene extraordinary magnetoresistance (EMR) devices have been fabricated and characterized in varying magnetic fields at room temperature. The atomic thickness, high carrier mobility and high current carrying capabilities of graphene are ideally suited for the detection of nanoscale sized magnetic domains. The device sensitivity can reach 10 mV/Oe, larger than state of the art InAs 2DEG devices of comparable size and can be tuned by the electric field effect via a back gate or by imposing a biasing magnetic field.

Tunable Nanoscale Graphene Magnetometers

The detection of magnetic fields with nanoscale resolution is a fundamental challenge for scanning probe magnetometry, biosensing, and magnetic storage. Current technologies based on giant magnetoresistance and tunneling magnetoresistance are limited at small sizes by thermal magnetic noise and spin-torque instability. These limitations do not affect Hall sensors consisting of high mobility semiconductors or metal thin films, but the loss of magnetic flux throughout the sensors thickness greatly limits spatial resolution and sensitivity. Here we demonstrate graphene extraordinary magnetoresistance devices that combine the Hall effect and enhanced geometric magnetoresistance, yielding sensitivities rivaling that of state of the art sensors but do so with subnanometer sense layer thickness at the sensor surface. Back-gating provides the ability to control sensor characteristics, which can mitigate both inherent variations in material properties and fabrication-induced device-to-device variability that is unavoidable at the nanoscale.

Zero field high frequency oscillations in dual free layer spin torque oscillators

We observe microwave oscillations in relatively simple spin valve spin torque oscillators consisting of two in-plane free layers without spin polarizing layers. These devices exhibit two distinct modes which can reach frequencies >25 GHz in the absence of an applied magnetic field. Macrospin simulations identify these two modes as optical and acoustic modes excited by the coupling of the two layers through dipole field and spin torque effects. These results demonstrate the potential of this system as a large output power, ultrahigh frequency signal generator that can operate without magnetic field.

Coherent and incoherent spin torque oscillations in a nanopillar magnetic spin-valve

We report enhanced spin-torque oscillator results obtained in spin-valve nanopillars. When biased within the optimal range of a moderate, ≤600 Oe, hard axis field, the spin-torque-driven oscillations exhibit a sharp increase in power and a sharply narrowed linewidth, ≤10 MHz, which, based on micromagnetic simulations, we ascribe to a transition from incoherent to coherent dynamics. The simulations indicate that the coherent dynamics are enabled by the combination of strong coupling between the two oscillator end modes of the magnetic free layer and strong non-linear damping arising from a non-uniform magnetization that leads to a spatially varying anti-damping spin torque.

Highly Textured IrMn 3 (111) Thin Films Grown by Magnetron Sputtering

Non-collinear spin ground states in metallic antiferromagnets can give rise to anomalous Hall effect, which enables magnetic state readout in antiferromagnetic spintronic devices. Here we report growth of highly textured films of a non-collinear antiferromagnet IrMn3 on MgO(111) substrates by magnetron sputtering. The films consist of epitaxial (111) twin domains rotated by 60 degrees within the film plane. The films exhibit partial L12 ordering that supports the non-collinear T1 spin structure predicted to give rise to anomalous Hall effect. An MgO buffer layer evaporated onto the substrate increases the L12 order parameter from 0.4 to 0.75 but destroys the (111) crystallographic texture.