Room Temperature Spintronic Studies of Topological Weyl Antiferromagnet Mn3Ge

In recent times, spintronics has attracted a lot of attention among the research and the industrial communities. Improving the efficiency and adding functionalities to modern electronic devices by incorporating the spin degree of freedom of electrons is the central goal of spintronics. Key aspects to realize this goal include generation, propagation, processing and detection of spin-polarized currents in suitably chosen materials or heterostructures. Towards this end, a new interest has emerged to investigate conductive antiferromagnets. Mn3Ge, a room temperature noncollinear antiferromagnet, belongs to a class of materials commonly known as Weyl semimetals (WSMs). The WSMs are of particular interest not only because of their exotic Fermi-arc-type surface states, but also because of their appealing bulk chiral magneto-transport properties. Owing to the topological nature of the band structure of Mn3Ge, very large anomalous Hall effect (AHE) and spin Hall effect (SHE) have been found in the bulk Mn3Ge single crystals. We have synthesized hexagonal single phase, epitaxial, homogeneous and continuous films of Mn3Ge using the magnetron sputtering and the molecular beam epitaxy techniques. Anomalous charge transport properties were measured using Nernst measurement which shows very strong topological thermoelectric effects in the films. The Seebeck coefficient per unit saturation magnetization for this system is one of the largest experimentally measured figure of merit so far. Moreover, the topological nature of the band structure also results in a peculiar spin transport which we characterize using the spin-pumping ferromagnetic resonance (SP-FMR) and spin-torque ferromagnetic resonance (ST-FMR) techniques. Our analysis confirms that the Mn3Ge films show excellent spin-current generation and detection capabilities with a large spin diffusion length scale. The estimated figure of merit, the spin-Hall angle, is found to be an order of magnitude larger than the prototypical industry standard in Platinum. This promising result suggests that the power consumption for the data writing process in a magnetic data storage device could be reduced by two orders of magnitude. In addition, the spin-Hall angle shows a high degree of tunability with the applied magnetic field. In general, antiferromagnets have much faster spin reorientation dynamics which essentially lifts any limitation on the rate at which the data writing step could be carried out. Such large transport effects and tunability are highly desired for ultrafast and robust MRAM devices with larger areal density and reduced power consumption for data storage.

Host David Tanner