Deepanjan Datta

Proposal for an all-spin logic device with built-in memory.

The possible use of spin rather than charge as a state variable in devices for processing and storing information has been widely discussed, because it could allow low-power operation and might also have applications in quantum computing. However, spin-based experiments and proposals for logic applications typically use spin only as an internal variable, the terminal quantities for each individual logic gate still being charge-based. This requires repeated spin-to-charge conversion, using extra hardware that offsets any possible advantage. Here we propose a spintronic device that uses spin at every stage of its operation. Input and output information are represented by the magnetization of nanomagnets that communicate through spin-coherent channels. Based on simulations with an experimentally benchmarked model, we argue that the device is both feasible and shows the five essential characteristics for logic applications: concatenability, nonlinearity, feedback elimination, gain and a complete set of Boolean operations.

Voltage Asymmetry of Spin-Transfer Torques

Experimentally, it is seen that the free magnetic layer of a spin torque transfer (STT) device experiences a larger in-plane torque when a negative (rather than positive) voltage is applied to the fixed layer. This is surprising because magnets do not have any intrinsic asymmetry. In this paper, we 1) provide a simple physical explanation, based on the polarization of fixed layer in the energy range of transport; 2) extend it to explain the asymmetric bias dependence of out-of-plane torque as observed in some of the experiments; and 3) propose an asymmetric STT structure that can lead to a significant difference in the in-plane torques exerted on two contacts, even if they are identical. This effect 3 has not been observed to our knowledge and if demonstrated can find important applications.

Quantum Transport Simulation of Tunneling Based Spin Torque Transfer (STT) Devices: Design Trade offs and Torque Efficiency

We present a quantum transport simulation of tunneling based spin torque transfer (STT) devices using the non equilibrium Greens function (NEGF) formalism in the ballistic regime. We show that it is possible to achieve a quantitative agreement for both the tunneling magneto resistance (TMR) and the amplitude of the switching current with the same set of parameters. We also analyze the device performance as a function of parameter variations. Comparison with experiment shows that there is a lot of scope for improving the interface between the magnetic contact and the tunneling oxide, which, in turn, will improve the device performance significantly.

Quantitative model for TMR and spin-transfer torque in MTJ devices

We present a Non-Equilibrium Greens Function (NEGF)-based model for spin torque transfer (STT) devices which provides qualitative as well as quantitative agreement with experimentally measured (1) differential resistances, (2) Magnetoresistance (MR), (3) In-plane torque (τ∥) and (4) out-of-plane torque (τ⊥) over a range of bias voltages, using a single set of three adjustable parameters. We believe our model is able to cover this diverse range of experiments.

Physics-based factorization of Magnetic Tunnel Junctions for modeling and circuit simulation

We present a physics-based factorization of Magnetic Tunnel Junctions (MTJ) in terms of a minimal number of experimentally and theoretically accessible parameters that can be used to optimize existing MTJ designs as well as to probe emerging MTJ devices. Our model fully captures angular/voltage dependence of state-of-the-art MTJs and Spin Valves (SV) and is compatible with existing circuit simulation frameworks such as Verilog-A and SPICE.

Key Role of Non Equilibrium Spin Density in Determining Spin Torque

In summary, starting from the NEGF formalism, we have showed that the current induced torque is proportional to the spin density at the ferromagnetic surface. This automatically gives the well known Slonczewski and field terms in the current torque thus showing the relation between their relative amplitudes without the phenomenology commonly used in literature. We obtain a reasonable quantitative agreement with experimental data that validates our model. From the same simulation, we show that when the spin injection efficiency is 100%, the torque transfer efficiency saturates at 50%. This result is significant considering that the existing devices are not far from this limit and hence novel device designs [S. Salahuddin and S. Datta, 2006] going beyond the conventional three layer structure may be necessary to reduce the switching current to technologically viable levels (~105 A/cm2).

Low band-to-band tunnelling and gate tunnelling current in novel nanoscale double-gate architecture: simulations and investigation

Large band-to-band tunnelling (BTBT) and gate leakage current can limit scalability of nanoscale devices. In this paper, we have proposed a novel nanoscale parallel connected heteromaterial double gate (PCHEM-DG) architecture with triple metal gate which significantly suppress BTBT leakage, making it efficient for low power design in the sub-10 nm regime. We have also proposed a triple gate device with p+ poly–n+ poly–p+ poly gate which has substantially low gate leakage over symmetric DG MOSFET. Simulations are performed using a 2D Poisson–Schrodinger simulator and verified with a 2D device simulator ATLAS. We conclude that, due to intrinsic body doping, negligible gate leakage, suppressed BTBT over symmetric DG devices, metal gate (MG) PCHEM-DG MOSFET is efficient for low power circuit design in the nanometre regime.