Periodic Reporting for period 1 - VOLTEMAG (Voltage-Controlled Electronic and Magnetic Phase Transitions in Nano-Devices based on Correlated Mott Materials)
Reporting period: 2022-11-01 to 2024-10-31
Materials with intriguing magnetic and electronic properties have always been at the forefront of research, capturing the attention of the scientific community not only for their fascinating underlying physical mechanisms but also for their potential applications in future technologies. One such field is spintronics, where, in addition to the electronic charge, the spin degree of freedom plays a crucial role in data storage and computing. In these materials, the interplay between electron-electron interactions, spin-orbit coupling, and spin degrees of freedom is highly complex, giving rise to unique properties. Electronic transport in such systems depends on this intricate interplay, enabling spin manipulation. When integrated into nanoscale devices, the electronic and magnetic properties of these materials can undergo significant transitions under the influence of an applied bias voltage. A material undergoes such transitions resembles the ON and OFF state of a transistor, therefore enabling logic computing. Magnetic tunnel junctions (MTJs) exploits such two states of electrical conductivity to store information in form of bits. MTJs are building blocks in commercial hard disk drives and magnetic random-access memory (MRAM) revolutionized memory storage by enabling low power consumption and fast switching. An MTJ consists of two magnetic material layers separated by a thin insulating barrier. The charge current in these systems depends on the relative orientation of the spins in the two magnetic layers and can result in high tunnel magnetoresistance (TMR), arising from the electronic band structures and quantum tunnelling across the insulating layer. The application of an external bias voltage introduces additional complexity, altering the electronic and magnetic properties and thereby influencing TMR. In today’s data-driven era, there is an exponentially increasing demand for maximizing storage density sustainably and at low cost. This demand drives the need for device miniaturization using defect-free, atomically thin materials. However, the exfoliation of bulk materials into nanoscale devices faces practical challenges. Two-dimensional (2D) materials hold great promise in this context, as they can be easily exfoliated down to a few layers or even monolayers, owing to their weak van der Waals (vdW) gaps. Despite their promise, magnetism in 2D materials is rare and is primarily limited by the Mermin-Wagner theorem. However, recent breakthroughs in achieving magnetism in 2D systems, have opened new avenues for designing all-2D MTJs. In this context, addressing the fundamental physics of quantum transport in these materials within nanoscale devices is crucial, especially with regard to their electronic and magnetic properties. Additionally, understanding how voltage-induced changes in electronic and magnetic properties modulate quantum transport is vital for exploring their technological applications. In our project, VOLTEMAG, we employed a robust theoretical framework combining the Non-Equilibrium Green’s Function (NEGF) formalism, Density Functional Theory (DFT), and Dynamical Mean-Field Theory (DMFT) to investigate the fundamental physics of nanoscale devices composed of 2D ferromagnetic materials. Our study focuses on understanding how external bias voltage affects the quantum transport properties of these materials, paving the way for advancements in spintronic device technologies.
VOLTEMAG employed DFT-NEGF transport calculations, as implemented in the Smeagol code, to investigate the spin-dependent transport properties of a family of 2D ferromagnetic compounds with the formula FenGeTe2 and Fe3GaTe2, where n varies from 3 to 5. Detailed calculations on Fe4GeTe2 revealed that its coherent transport exhibits a unique half-metallic property along the transport direction, despite the material being metallic overall. This property persists even in the monolayer limit under significant finite external bias, making Fe4GeTe2 behave almost like a spin filter. The spin polarization of the material remains robust in the presence of spin-orbit coupling and dynamic electron-electron correlations. The half-metallic transport behavior of Fe4GeTe2 can be leveraged in designing MTJs. VOLTEMAG predicted a large TMR at equilibrium for MTJs comprising two layers of Fe4GeTe2 separated by an insulating vdW layer. The TMR remained substantial even under finite external bias, suggesting potential applications in future all-2D-vdW MTJs. VOLTEMAG extended the study to other compounds in this family, identifying additional candidates with similar half-metallic transport properties. Furthermore, it proposed the possibility of designing MTJs with two different vdW materials in the pinned and free layers, enabling independent switching of their relative magnetization by exploiting differences in coercive fields.
Although the primary aim of VOLTEMAG was to investigate the physical phenomena occurring when a bias voltage is applied across a nano-junction comprising transition metal compounds, it went a step further by highlighting the potential of 2D materials in future spintronics applications. VOLTEMAG began with a fundamental understanding of the electronic structure of 2D metallic ferromagnets, identifying the origin of their near half-metallic transmission along the out-of-plane direction through a detailed theoretical and computational framework. The study explored this unique property of 2D materials for MTJs, calculating their TMR at equilibrium and examining its robustness under out-of-equilibrium conditions. Additionally, VOLTEMAG addressed a key challenge in MTJ design: enabling independent rotation of the magnetization direction in the free layer by utilizing two different 2D materials in the pinned and free layers. This approach offers a more practical path for designing devices, paving the way for experimental investigations. Looking ahead, there is significant potential to discover new 2D magnetic materials with high transition temperatures and out-of-plane anisotropy. Their relevance to spintronics can be explored through a multidisciplinary pipeline that combines machine learning, DFT and NEGF formalism.