Magnetic and thermomagnetic characterization of Co2MnGa thin films

The study of materials with non-trivial topological phase boomed in the recent years, with many effects that are peculiar to these phases, and record-breaking magnitudes, which are intimately connected to the complex physics underlying. Examples can be found concerning the Anomalous Hall Effect, the Spin Hall Effect, or the emergence of superconductivity as well. However, one aspect has remained unexplored up to date, which is the charge-to-heat conversion, which could lead to remarkable applications for quantum computers or photon detectors, both having to operate at cryogenic temperatures. Magnetic Weyl semimetals are one of the newest topological phases, discovered only in 2015 in TaAs, with the key signature being the chirality-resolved cones, composed of the valence and conduction bands dispersing linearly and touching at the Weyl point. At this peculiar point, the gap between the bands closes, and the Berry phase becomes singular. For this reason, it has been predicted theoretically that the charge-to-heat and heat-to-charge conversion effects should be significantly large. In Co2MnGa, a full-Heusler compound, this has been repeatedly proven for the Nernst Effect, which entails a heat-to-charge conversion, but not at all for its counterpart, the Ettingshausen Effect, describing an electric current converted into a temperature gradient. In this thesis work, I study indeed the presence of the Ettingshausen Effect in Co2MnGa, trying to relate it with its magnetic structure, in the same way it has been done many times for the Nernst Effect. The samples I received came from Simon Granville’s group in Welington, New Zealand, and were composed of 50 nm thin films of Co2MnGa grown on a substrate of MgO. In particular, one set of film was uncapped, and another was coated with a Ta capping layer: they have been successively patterned into a Hall bar by collaborators at imec and characterized by Rikkie Joris and Lino da Costa Pereira at KU Leuven. I firstly performed MFM on the samples, to gain insight on the domain structure. They have been magnetized at negative field, and then progressively saturated with positive magnetic field. A peculiar square domain structure has been observed for the uncapped Hall bar, whereas the maps obtained on the capped film and Hall bar were featureless. Successively, SThM was employed to study the Ettingshausen Effect, by injecting an electric current into the devices, applying an external magnetic field, and scanning the tip to observe the induced temperature difference. The signal was demodulated at the frequency of the current to separate the effect of interest from the Joule effect. Line features in the thermal maps have been observed: their interpretation is not straightforward, even if calculations performed reveal that the effect should be large enough to be detected by the setup. The experimental work has been complemented with COMSOL simulation to help in the interpretation of the data, and estimate the magnitude of the effects of interest. The results presented in the work are promising, even if other experimental proofs are required to confirm to have observed the effect in Co2MnGa.