Non-collinear spin textures in oxides for spintronics and superconducting spintronics
This PhD focuses on the basic physics of epitaxial transition metal oxides for the development of new materials for spintronics and superconducting spintronics. The starting point of the project was to investigate the possibility of using the magnetic non-collinearity hosted in SrRuO3 to enhance conversion between spin-0 singlet and spin-1 triplet Cooper pairs in an epitaxial oxide ferromagnetic Josephson junction. This starting point has led to four main results:
- Experiment: anti-ferromagnetic coupling in SrRuO3/La0.7Ca0.3MnO3. These heterostructures are grown on different substrates with different epitaxial strains. By measuring magnetic minor loops as a function of field-cool strength, I see an effective field acting on the softer La0.7Ca0.3MnO3 layer consistent with an anti-ferromagnetic exchange coupling from the SrRuO3. The strength of this coupling varies with the substrate used and essentially disappears when a non-magnetic spacer layer is added. The coupling combined with differences in magnetic anisotropy between layers is thought to create a non-collinear spin texture in the SrRuO3.
- Review: the ambiguity between topological and inhomogeneous Hall effects. Topological Hall effect measurements are commonly used in the spintronics community as a signature of chiral spin textures. In this comprehensive literature review I argue this is not a reliable method since similar Hall signals can be created by experimental problems or an inhomogeneous anomalous Hall effect.
- Experiment: two-channel anomalous Hall effect in SrRuO3. Using transport measurements, volume magnetometry, and magnetic force microscopy, I show that the Hall effect anomaly often attributed to skyrmions in ultra-thin SrRuO3 is instead likely caused by single unit cell thickness variations in films resulting from substrate miscut creating an inhomogeneous anomalous Hall effect.
- Theory: numerical modelling of non-collinear ferromagnetic Josephson junctions. I develop a numerical model of magnetic Josephson junctions using Matsubara Green functions in the (clean) tunnelling limit. In agreement with prior literature, I find magnetic non-collinearity in junctions gives finite triplet currents which can show 0-π transitions versus ferromagnet strength or angle, and chiral junctions give a Josephson current with zero phase difference between superconducting layers. This model allows the exploration the junction properties as a function of temperature, ferromagnet strength, angle, and Rashba spin-orbit coupling strength, using more realistic electronic dispersion relations and using superconductors with s- or d-wave symmetry.
Engineering and Physical Sciences Research Council (1937217)