Quantum Transport in Antiferromagnetic Spintronics

Abstract

Antiferromagnetic spintronics represents a promising evolution beyond conventional ferromagnet-based technologies. By utilizing materials with zero net magnetization, antiferromagnets offer intrinsic advantages such as robustness against magnetic perturbations, the absence of stray fields for ultra-dense memory integration, and terahertz-scale dynamics for ultrafast operation. Despite this potential, their compensated magnetic order has been very difficult to study with simple electrical probes, posing significant challenges for reading and writing information. Altermagnetism, a third theoretically predicted magnetic phase, presents a new frontier for spintronics by combining the large, non-relativistic spin-splitting of ferromagnets with the vanishing net magnetization of antiferromagnets.

This thesis explores the fundamental physics and device potential of this new magnetic class, centred on its prototypical yet highly controversial material candidate, Ruthenium Dioxide (RuO2). The research directly confronts the primary obstacle in the field: the ongoing debate over whether RuO2 possesses an intrinsic magnetic ground state. This work navigates the landscape of conflicting experimental reports by providing a complete computational overview and establishing a theoretical benchmark for the performance of an ideal altermagnetic RuO2 device.

The thesis delivers three primary contributions:

First, it presents a comprehensive computational characterization of RuO2, demonstrating that a parameter-free approach using the Atomic Self-Interaction Correction (ASIC) method provides a physically sound model for its hypothesized altermagnetic phase.

Second, it establishes a general "golden rule" framework for the systematic design of novel, experimentally viable altermagnetic tunnel junctions (ALMTJs). This design recipe, based on screening for metallic character, high Neel temperature, and structural compatibility, successfully identifies promising new heterostructures, such as CrSb/NiTe, demonstrating its utility beyond a single material.

Third, and most centrally, it provides the first-ever theoretical investigation of finite-bias quantum transport in a prototypical RuO2/TiO2/RuO2 junction, performed using a combination of Density Functional Theory and Non-Equilibrium Green's Function (NEGF) formalism.

These first-principles simulations predict a giant tunnelling magnetoresistance (TMR) of approximately 427% at zero bias. Critically, the calculations reveal novel non-equilibrium physics, discovering a remarkable inversion of the TMR from large positive to large negative values at a finite bias of around 0.35 V. This effect is traced to a bias-induced spin-symmetry filtering mechanism. By delivering a robust computational model for RuO2, a tangible framework for future materials discovery, and the discovery of novel transport phenomena, this thesis provides a critical theoretical foundation for the advancement of altermagnetic spintronics.