Synthetic antiferromagnets to model the predicted behavour of antiferromagnets in spintronic applications

Since the prediction of Slonczewski and Berger that a spin current, where electrons with opposite spins travel in opposite directions inside a material, could torque a ferromagnet's magnetisation, the field of spintronics has mainly focused on phenomena in ferromagnetic systems. From an experimental standpoint, ferromagnets have a large net magnetic moment that can be manipulated and measured easily. Antiferromagnets (AFs) by contrast, which are made up of two alternating sublattices with opposite magnetic moments, have no net magnetisation that can be used to read out the state of the system, and are also impermeable to manipulation by an external magnetic field.
Despite these challenges, AFs now represent one of the most avenues for spintronics. Their main advantage over traditional ferromagnetic devices is the strong exchange interaction between anti-parallel sublattices that enables magnetic phenomena to occur on ultrafast, high frequency, timescales compared to ferromagnets, and renders them insensitive to stray magnetic fields. This is advantageous for applications such as fast magnetic memories, and the generation of high frequency electrical signals. Although theoretical predictions of their properties are numerous, the aforementioned lack of an easily detectable magnetisation, coupled with their high frequency dynamics that presently only optical techniques are capable of detecting, has limited their application in experimental spintronics.
In this project, we propose and evaluate the suitability of synthetic antiferromagnets (SAFs) as a model system for modelling spintronic phenomenon in AFs. SAFs are composed of two ferromagnetic layers, separated by a non-magnetic spacer layer, which, if tuned correctly, causes the magnetic moments of each layer to align anti-parallel at zero magnetic field, like the sublattices in an AF.
Unlike in an AF, the 'exchange interaction,' which causes the magnetic moments to align anti-parallel, is much weaker, enabling dynamics hundreds to thousands of times slower. This makes performing experiments with SAFs experimentally feasible with current technologies. To evaluate their suitability as a model system, we decided to 'simulate' an AF undergoing an experimental procedure known as spin torque ferromagnetic resonance (ST-FMR), using a SAF.
ST-FMR involves an oscillating spin-current torqueing the magnetisation of a device at a certain frequency. When this frequency hits the resonance frequency of the device, it causes large scale oscillations in the magnetic moments. This is a widely used technique in ferromagnets for characterising certain magnetic properties of the device.
Recent reports have shown that spin-currents are able to couple to the anti-parallel magnetic moments in an AF and are capable of exhibiting large magnetoresistance (where the electrical resistance depends on the magnetisation direction) necessary to detect these oscillations. Combined, these reports suggest a similar technique should be possible in AFs, but the large frequencies required at even extremely small magnetic fields present a considerable experimental barrier.
We show that the behaviour of a SAF under the influence of an incident microwave frequency spin-current, generated in each layer by an adjacent Pt layer, is comparable to that predicted in a real AFs. This demonstrates that SAFs are a reliable model system to test the feasibility of many of the proposed spintronic applications and mechanism for real AFs, whose experimental realisations may be far off at the present time, and to discover new physics mediated by an antiferromagnetic exchange interaction. For the remainder of the project, we have begun exploring new ST related physics in SAFs, largely related to the 'anti-damping' torque, which can result in an effectively dissipationless device. This has been suggested as a promising, but unrealised, route towards AF magnetic memories, and the generation of high frequency electric signals