NEGF based tight-binding models for transport in magnetic heterostructures

Tehseen Zahra Raza, Purdue University

Abstract

Magnetic heterostructures, where a non-magnetic material is sandwiched between two ferromagnetic metals, are of great interest both for their intrinsic physics and for device applications. Recently, the use of MgO as the non-magnetic material has led to widespread applications due to high tunnel magnetoresistance (TMR) ratio which is believed to arise from a preferential orbital symmetry filtering, that the other insulators lack. It is thus crucial to capture this orbital symmetry filtering in any theoretical model, which calls for an atomistic treatment. Existing first principles models based on DFT (density functional theory) capture this effect but are resource intensive and often provide quantitatively wrong results for the TMR because they under-estimate or over-estimate the insulator band gap, which is a well known problem with DFT. The primary achievement in this thesis is to present what we believe is the first transport model for these heterostuctures using extended Hückel theory (EHT) which is a semi-empirical tight-binding model with a non-orthogonal basis - that has been successfully applied to describe the electronic structure in a variety of materials. It is computationally inexpensive with transferable parameters and gives the correct experimental band gap which should lead to tractable, versatile and accurate models. In addition, we present an independent-band tight-binding (IBTB) model with highly reduced computational complexity of the same order as that of the effective mass models. But unlike the effective mass models, it captures the orbital symmetry filtering mentioned earlier.

Degree

Ph.D.

Advisors

Datta, Purdue University.

Subject Area

Nanotechnology

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