Nathan C. Emley

Simulation Studies of Nanomagnet-Based Logic Architecture

We report a simulation study on interacting ensembles of Co nanomagnets that can perform basic logic operations and propagate logic signals where the state variable is the magnetization direction. Dipole field coupling between individual nanomagnets drives the logic functionality of the ensemble, and coordinated arrangements of the nanomagnets allow for the logic signal to propagate in a predictable way. Problems with the integrity of the logic signal arising from instabilities in the constituent magnetizations are solved by introducing a biaxial anisotropy term to the Gibbs magnetic free energy of each nanomagnet. The enhanced stability allows for more complex components of a logic architecture capable of random combinatorial logic, including horizontal wires, vertical wires, junctions, fanout nodes, and a novel universal logic gate. Our simulations define the focus of scaling trends in nanomagnet-based logic and provide estimates of the energy dissipation and time per nanomagnet reversal.

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.

Nanomagnetic cascade logic

Interacting Nano-scale magnetic islands show promise for low-power logic applications. Using anisotropy engineering we demonstrate through micromagnetic simulations a novel universal logic gate capable of signal fanout.