New Characterisation Techniques for Thin Film Antiferromagnetic Materials

Poole, S F (2023) New Characterisation Techniques for Thin Film Antiferromagnetic Materials. PhD thesis, University of Nottingham.

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Antiferromagnets (AFMs), with their rise in prominence and the discovery of new effects, have led to growing interest in antiferromagnetic spintronics and their possible applications. Since AFMs have little or no stray fields and very low magnetic susceptibility, one of the biggest challenges faced by the field of AFM spintronics is the difficulty to manipulate and measure the magnetic structure of these systems. Most techniques capable of unambiguous detection of magnetic changes or domain structure require large facilities, and so the aim of the contained thesis was to support the development of new benchtop techniques capable of making such observations in AFMs.

Spin flop is a reliable and well understood method to create a repeatable reorientation of moments in an AFM. The extents to which spin flop reorients the moments in a uniaxial thin film sample of CuMnAs was determined using neutron diffraction and was found to be up to 98~\% at 8~T. This study also demonstrated the capability to detect AFM ordering in films as thin as 20~nm using neutron diffraction. Accompanying transport measurements, using anisotropic magnetoresistance (AMR) as the readout mechanism, were performed and show good agreement between the techniques in both the shape of the signals and the strength of the spin flop threshold field.

Asynchronous optical sampling was used to develop a rapid technique capable of characterising CuMnAs films grown on GaAs(001) and GaP(001) substrates in a matter of hours, without the need for high magnetic fields, cryogens or facilities. This utilised the changes in the Voigt effect to measure the difference in polarisation of light during and after a heating pump pulse partially demagnetises a CuMnAs film. The same polarisation changes were used to successfully detect the reorientation of moments undergoing spin flop using delay line pump probe.

While working on these optical setups, some of the elastic constants of CuMnAs were measured to be used in simulations of domain formation, with the values being included in magnetostriction and shape anisotropy terms. The measured constants are c33 = 150 +- 10 GPa, c11 = 37 +- 1 GPa and c44= 73 +- 7 GPa, where the final constant determinable in these studies was found to have two different possible values of c13 = -40 +- 6 GPa or c13' = -104 +- 7 GPa.

A room temperature vacuum system was developed that was demonstrated to be capable of keeping a sample between -8 and +60 °C, with a stability below 5 mK at all temperatures in that range, where the ambient temperature was approximately 22 °C. The vacuum chamber can be evacuated down to 10e-8 hPa using a readily available turbo pump.

The room temperature system was used to develop a high power probing technique for transport measurements in which the applied measurement currents are up to twenty times higher than accessible using DC probing. In the attempts to measure the deflection as a result of Néel order spin-orbit torques, the combination of high currents and high fields led to the discovery of a new, large AMR signal which generates a large difference in measured AMR when electrical pulses are applied in opposite directions in CuMnAs devices, which would normally be symmetric under this change in sign of the current. This large signal is promising, but attempts to determine its origin were inconclusive.

Overall, several developments were made toward the aim of accessible benchtop techniques for measuring the magnetic structure of AFM materials.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Wadley, Peter
Edmonds, K W
Akimov, Andrey
Keywords: Spintronics Antiferromagnets CuMnAs Spin Flop Elastic Constants Voigt Effect
Subjects: Q Science > QC Physics > QC350 Optics. Light, including spectroscopy
T Technology > TK Electrical engineering. Electronics Nuclear engineering > TK7800 Electronics
Faculties/Schools: UK Campuses > Faculty of Science > School of Physics and Astronomy
Item ID: 76335
Depositing User: Poole, Stuart
Date Deposited: 06 Jun 2024 10:10
Last Modified: 06 Jun 2024 10:10

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