Probing the Behaviour of Antiferromagnets: Electric and Magnetic Measurements Across a Range of Length-scales

The spin Hall effect (SHE) describes the generation of a spin current orthogonal to an applied charge current within a heavy metal, whilst the spin Seebeck effect (SSE) refers to the generation of a spin current due to thermal gradients within a magnetic material. Both effects have attracted significant interest in recent years as means of generating spin currents and probing the behaviour of magnetic materials, with potential spintronic applications in data storage. The simultaneous action of the SHE, and its reciprocal effect, the inverse spin Hall effect (ISHE), is termed spin Hall magnetoresistance (SMR), and allows the magnetic behaviour of adjacent magnetic materials to be explored. The SSE provides information into the magnonic excitations within the magnetic materials themselves instead. These effects were measured for a number of antiferromagnets, and compared to magnetostatic measurements. SMR is primarily dependent on the interfacial properties of adjacent magnetic materials to the heavy metal, whereas the SSE probes deeper into the magnetic material, and so it was proposed that any differences between the magnetic behaviour over these different length scales could be determined. In general, these effects have been studied previously in relatively straightforward antiferromagnets, such as MnF₂. However, the complexity of behaviour displayed in other antiferromagnets could provide a rich playground with which to produce previously unexpected effects. Such complexity is observed in hematite within this thesis. Often oversimplified to a simple uniaxial antiferromagnet when spin transport is considered, by using a different orientation of crystal in this thesis to that typically considered, it was possible to investigate this complexity. It was then necessary to extend previous theories, particularly when considering the antiferromagnetic resonance of 𝑘 = 0 magnons. This was then subsequently applied to explain the SSE voltages observed, which would not have been predicted by simpler analyses. Two other classes of antiferromagnets were then investigated. KMF₃ compounds had previously shown interesting ultrafast properties, but their SMR and (DC) SSE properties had not previously been investigated. Here, the magnetostatics and SMR properties of KCoF₃ are obtained, but the reported theory of domain wall motion within this class of compounds could not explain the observed behaviour. It was instead necessary to consider a combination of spin flop transitions and domain wall motion, demonstrating a lack of homogeneity which had been ignored in previous experiments. Within the programme, a completely separate study into the rare earth orthoferrite, HoFeO₃, was then carried out, which has a complex magnetic phase diagram on account of the temperature-dependent Fe-Fe, Fe-Ho and Ho-Ho interactions. Little is known about the magnetic behaviour of this material, and even a complete description of its magnetostatics has not been reported previously. Since SMR primarily probes the Néel vector, a quantity inaccessible to standard macroscopic magnetic measurements, it was proposed that the combination of SMR and SSE measurements could actually be used as a diagnostic tool for magnetic behaviour. In spite of experimental difficulties, sufficient progress was made to justify this approach to the characterisation of magnetic behaviour. This study has demonstrated that the use of a simplified picture of antiferromagnets may miss significant aspects of magnetic behaviour, and as such it is necessary to introduce more complex models if the true potential of these materials is to be harnessed.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2023

Author dc:creator

• Bowen, Richard Denzil

Advisor dc:contributor.advisor

• Ciccarelli, Chiara

• Antiferromagnets
• Spintronics

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/ee513b0b-99b7-4b33-a95f-da9be55cf865/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.108647

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/368456