A Study of Anisotropy in Antiferromagnets

Spintronics or spin electronics is an emerging field of applied physics which studies the intrinsic spin of an electron and its associated magnetic moment. The most successful spintronic device to date is based on the Giant Magneto Resistance effect and has been used in the read head sensor of hard disk drives for over 20 years. Until recently, most studies were based on ferromagnetic materials. In 2014 it was announced that significant spintronic phenomena occurred in metallic antiferromagnetic (AF) materials. These phenomena are an unexpected spin-Hall effect and, critically, the ability to manipulate the orientation of the AF axes using a spin polarised current pulse. This latter phenomenon is of critical importance as it has been shown that an AF material can relax 1000 times faster than a typical ferromagnetic device. Hence, in principle, an AF based storage or switching device would be capable of being many times faster than a conventional Magnetic Random Access Memory (MRAM) device which itself switches faster than a Complementary Metal-Oxide Semiconductor (CMOS) device. Such system would require significantly lower power and, importantly, any resulting orientation would not be subject to the normal demagnetising field effect that limits the performance of a device based on a ferromagnet. Because of these potential major advantages and the new physics involved in manipulating an AF material the level of worldwide research in AF materials has burgeoned. Hence this is a new emerging field of endeavour which is rapidly becoming dominant for both scientific and technological reasons. For both potential storage and switching applications the anisotropy of AF materials will be a critical parameter whose origin(s) are not yet understood.

We aim to address this via an integrated programme of ab-initio modelling and experimental measurements. The former will be undertaken via Density Functional Theory (DFT) modelling which requires no assumptions regarding, for example, crystal structure. This is important for the widely used system IrMn(x) where the AF anisotropy increases for the non-stoichiometric (x>3) composition. The experimental programme will focus on the measurement of the anisotropy. The technique to measure this property was developed by our group and is based on what is known as the exchange bias effect.