Abstract
Tunnelling Anisotropic Magnetoresistance (TAMR) effect in Antiferromagnet (AFM) based Magnetic Tunnel Junctions (MTJs) was first demonstrated over a decade ago. TAMR results from the change in the energy of states near the Fermi level in a thin metallic AFM film, due to rotation of the NΓ©el vector with respect to the crystal lattice, leading to a change in magnetoresistance through the MgO tunnel barrier. The achievement of room temperature TAMR has important industrial potential, particularly for data storage applications. If successful, it would allow for the creation of a hard disk drive (HDD) reader with only one FM layer, thus dramatically improving its linear resolution, and HDD areal density capacity (ADC).
The main objective of this project is to explore the suitability of Ir20Mn80 based structures for room temperature TAMR applications. The project has investigated numerous fabrication parameters such as seed layers, composition, deposition power and Ar flow rates which impacts the magnetic and structural properties of the structures.
Coercivity enhancement is observed from standard magnetisation measurements in very thin Ir20Mn80 layers (2nm) despite no corresponding exchange bias at room temperature which means the Ir20Mn80 layer starts to reverse with the FM layer before exchange bias is observed at 4nm. The utilisation of X-ray Magnetic Linear Dichroism (XMLD) was essential as it is the only direct probe to determine whether the AFM lattice is rotating and thus determining if Co70Fe30/Ir20Mn80 is suitable for room temperature TAMR. Unfortunately, no reversal of the AFM sublattices for the thinnest Ir20Mn80 layer was observed upon performing XMLD along the hysteresis loop of the Mn L2,3 edges.
If a reversal of the AFM sublattices were observed, then there would have been a systematic change of the intensity of the XMLD signal when describing the hysteresis loop of the sample. As there was no correlation established between the XMLD intensity change and the hysteresis loop of the samples, this must be due to the polycrystallinity of the samples and the complex non-collinear magnetic structure of the disordered πΎ β πΌπππ3. This ultimately shows that this structure and/or the disordered πΎ β πΌπππ3 are not suitable for room temperature TAMR applications.
Thesis is embargoed until 31 July 2030.
| Date of Award | Jul 2025 |
|---|---|
| Original language | English |
| Awarding Institution |
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| Sponsors | Seagate Technology LLC & Engineering and Physical Sciences Research Council |
| Supervisor | Robert Bowman (Supervisor) & Robert Pollard (Supervisor) |
Keywords
- magnetism
- data storage
- neutron reflectometry
- dichorism
- antiferromagnetism
- material science
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