Derek Waldron

First principles modeling of tunnel magnetoresistance of Fe/MgO/Fe trilayers

We report ab initio calculations of nonequilibrium quantum transport properties of Fe/MgO/Fe trilayer structures. The zero bias tunnel magnetoresistance is found to be several thousand percent, and it is reduced to about 1000% when the Fe/MgO interface is oxidized. The tunnel magnetoresistance for devices without oxidization reduces monotonically to zero with a voltage scale of about 0.5-1 V, consistent with experimental observations. We present an understanding of the nonequilibrium transport by investigating microscopic details of the scattering states and the Bloch bands of the Fe leads.

Ab initio simulation of magnetic tunnel junctions

In this paper, we present the mathematical and implementation details of an ab initio method for calculating spin-polarized quantum transport properties of atomic scale spintronic devices under external bias potential. The method is based on carrying out density functional theory (DFT) within the Keldysh non-equilibrium Greens function (NEGF) formalism to calculate the self-consistent spin densities. We apply this method to investigate nonlinear and non-equilibrium spin-polarized transport in a Fe/MgO/Fe trilayer structure as a function of external bias voltage.

Ab initio giant magnetoresistance and current-induced torques in Cr/Au/Cr multilayers

We report on an ab-initio study of giant magnetoresistance (GMR) and current-induced-torques (CITs) in Cr/Au multilayers that is based on non-equilibrium Green’s functions and spin density functional theory. We find substantial GMR due primarily to a spin-dependent resonance centered at the Cr/Au interface and predict that the CITs are strong enough to switch the antiferromagnetic order parameter at current-densities � 100 times smaller than typical ferromagnetic metal circuit switching densities. Magnetic metals are often well described using the effective mean-field description provided by the KohnSham equations of spin-density functional theory. In this description, the Kohn-Sham quasiparticles experience exchange-correlation potentials with a spin dependence that is comparable in strength to band widths and other characteristic electronic energy scales. The spindependent part of the Kohn-Sham quasiparticle potential acts like an effective magnetic field that is locally aligned with the electron spin-density. Because of these strong spin-dependent potentials, the resistance of a ferromagnetic metal circuit will change substantially when the magnetization orientation in any part of the circuit is altered, an effect known as giant magnetoresistance (GMR). Conversely, transport currents can destabalize magnetization configurations that are metastable in the absence of a current and change collective magnetization dynamics. In the case of circuits containing ferromagnetic elements the influence of transport currents on the magnetization can be understood as following from the conservation of total spin angular momentum; the torques that reorient quasiparticle spins as they traverse a non-collinear magnetic circuit are accompanied by current-induced reaction torques (CITs) that act on the magnetic condensate. This type of phenomenon is not by any means limited to magnetic systems. For example, there have been recent studies of the interaction between transport and charge density waves (CDW), which find that the CDW order parameter configuration influences transport and conversely that transport can alter the CDW. 5

Current-induced order parameter dynamics: Microscopic theory applied to Co ∕ Cu ∕ Co spin valves

Transport currents can alter alter order parameter dynamics and change steady states in superconductors, in ferromagnets, and in hybrid systems. In this article we present a scheme for fully microscopic evaluation of order parameter dynamics that is intended for application to nanoscale systems. The approach relies on time-dependent mean-field-theory, on an adiabatic approximation, and on the use of non-equilibrium Greens function (NEGF) theory to calculate the influence of a bias voltage across a system on its steady-state density matrix. We apply this scheme to examine the spin-transfer torques which drive magnetization dynamics in Co/Cu/Co spin-valve structures. Our microscopic torques are peaked near Co/Cu interfaces, in agreement with most previous pictures, but suprisingly act mainly on Co transition metal d-orbitals rather than on s-orbitals as generally supposed.

Automistic modeling of direct tunnelling in metal-oxide-semiconductor nanostructures

The authors report an atomistic modeling formalism and its associated software tool for simulating nonlinear and nonequilibrium charge transport in nanostructures from quantum mechanical first principles. This formalism is based on carrying out density functional theory (DFT) within the Keldysh nonequilibrium Greens function (NEGF) framework. The authors have calculated tunnelling current in metal-SiO 2-nSi MOS structures from atomic point of view using the NEGF-DFT approach and compare our results to those obtained by traditional tunnelling formula and identify a number of important issues to be resolved toward establishing parameter-free atomistic modeling of semiconductor nanoelectronics