Investigation of collinear and non-collinear antiferromagnets for spintronics

In 2021, roughly 1.72 trillion photos were taken every day, 6 million tweets were posted on twitter every minute, 143 billion mobile apps were downloaded. If just social media and smartphone users generate such staggering numbers, it is not hard to imagine why 90% of the world’s data was created just in the last two years. The technological solution to this ever expanding thirst for data storage is to make data storage devices more energy efficient, reliable, faster and smaller. A novel method to write data in magnetic materials with electric currents is to use spin-orbit torques (SOTs), which were pioneered in heavy metal/ferromagnet bilayer systems. Recent studies show sub-nanosecond timescale writing of data in such devices and large reading voltages by tunneling magnetoresistance (TMR). Although ferromagnets remain one of the most reliable non-volatile memory storage materials, antiferromagnets have recently been investigated for their potential to be much faster, more energy efficient, highly resilient to external stimuli and even exhibit multi-level memory, which could empower analog-like brain inspired computing. However, information in antiferromagnets is hard to detect and antiferromagnetic ordering is hard to reorient with external fields making current induced changes to antiferromagnetism important. In this thesis we explore the use of antiferromagnets both as a data storage material and as a source of spin currents instead of heavy metals. In the first part of this thesis, we explore the use of the collinear antiferromagnets for field-free current-induced switching of a perpendicular ferromagnet. The need for an external magnetic field is a hurdle for SOT-based memories. PtMn being an antiferromagnet can induce an exchange bias when in contact with a ferromagnet such as a [Co/Ni] multilayer while simultaneously functioning as a spin current source. The exchange bias couples the PtMn and [Co/Ni] at the interface, thus emulating an effective magnetic field, which facilitates memristive field free switching. X-ray photoemission electron microscopy reveals that the switching occurs by gradual expansion and sequential switching of pre-existing blocked magnetic domains as a result of current pulses. The PtMn layer has a granular texture, with average domains sizes smaller than 100 nm, whereas the ferromagnetic domains in Co/Ni are typically larger than 200 nm.This implies that, although the AFM critically affects the switching characteristics through the exchange bias, there are no spatial correlations between the AFM and FM domains. The blocked domains and the granular distribution of exchange bias constrain the origin as well as the displacement of the domain walls, thus leading to highly reproducible switching patterns as a function of the applied current pulses. Secondly, we explore the non collinear topological Weyl antiferromagnet Mn3Sn as an information storage medium and elucidate the mechanism of current induced switching in this antiferromagnet. Despite it’s small magnetic moment of 2kA/m this material exhibits a large anomalous Hall effect (AHE) of 1μΩcm, making the detection of the antiferromagnetic order viable. Extensive optimization efforts were undertaken to realise polycrystalline and epitaxial samples of Mn3Sn suitable for magnetotransport. We demonstrate SOT switching of the magnetic octupole vector in Mn3Sn as a function of pulse shape, magnetic field, temperature, and time. We discover that the switching behavior can be either bistable or tristable depending on the temporal structure of the current pulses. Time-resolved Hall effect measurements performed during the current pulsing reveal that Mn3Sn switching proceeds via a two-step demagnetization-remagnetization process caused by self- heating over a timescale of tens of ns followed by cooling in the presence of spin-orbit torques. Our results shed light on the switching dynamics of Mn3Sn and prove the existence of extrinsic limits on its switching speed. This sets a precendent for the first time-resolved measurement of switching in a topological antiferromagnet. Additionally, epitaxial growth of Mn3Sn in different orientations sets the stage for future experiments that explore the exotic nature of the material and as a source of spin-currents. The third topic is dedicated to the exploration of the non-collinear ferromagnet FeMnGe, which is known to have an incommensurate spin structure. Magnetometry shows a large magnetic moment of 300 kA/m below an ordering temperature of 220 K and 30 kA/m at room temperature. X-ray magnetic circular dichroism reveals ferromagnetic coupling of the Fe and Mn sublattices at low temperature and the Mn exhibits weakly ferromagnetic properties at room temperature. AHE and magnetoresistance measurement correlate well with the magnetometry up to room temperature but show that a second phase transition exists at 465 K which is only reached by Joule heating. A large linear negative magnetoresistance due to magnon-magnetoresistance is maximised at the two transition temperatures indicative of magnetic ordering. We discover a higher order anomalous Hall effect which can be separated based on angular symmetry and exhibits a field dependence hinting at the presence of unconventional magnetic ordering and spin textures in the FeMnGe thin films