Ever-striving for smaller, faster and more energy-efficient computer components, the ICT sector has long-relied on magnetic technologies for data storage. Yielded from the field of Spintronics, Nobel-prize winning scientific developments are enabling new applications for nanoscopic magnetic devices, e.g. in non-volatile on-chip memories or in sources of microwave (THz) radiation for diagnostic/airport scanners, but also bringing closer the super-fast tele-communications of the future, Internet-of-things, etc. The non-volatile spin-transfer torque (STT)-MRAM is already adopted in ultralow-power processors and research efforts are devoted to novel materials for better MRAM energy-efficiency, reliability and cost. Low saturation magnetisation of the MRAM bits is desirable and new branches of anti-ferromagnetic (AFM) or ferri-magnetic (FiM) spintronics are quickly gaining momentum. The STT effect not only drives current-induced switching but also excites magnetisation dynamics accompanied with electromagnetic radiation. For AFM/FiM materials this can be into the THz range, the domain of numerous emerging imaging/communication applications.
We can solve the ballistic quantum transport problem for highly crystalline magnetic tunnel junctions (MTJs) and evaluate atomically-resolved STT from ab initio electronic structure theory based on the SMEAGOL code. A leading quantum transport platform, SMEAGOL is largely developed in TCD and combines the non-equilibrium Green’s function (NEGF) method with spin-density functional theory (SDFT) electronic structure (based on the SIESTA code). Resulting from the exchange interaction between the current-carrying quasi-particles and the local order parameter in non-collinearly spin-polarisd magnetic multilayers, the STTs have been studied and well-understood in conventional MTJs (e.g. Fe/MgO/Fe). We have recently demonstrated that in a chemically realistic MTJ based entirely on antiferromagnetic material (CuMnAs), STTs staggered over the two different magnetic sublattices and significant in magnitude arise. As such these can generate switching of the AFM layer in the MTJ. At the same time our calculations showed that the different magnetization states of the junction can be read by standard tunnelling magnetoresistance (TMR), a first such theoretical demonstration of the practicability of AFM-MTJs for magnetic memory applications.
