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Measurements of heat capacity of systems with complex magnetic phase diagrams for magnetic field along hard magnetisation axes

Since the discovery of topologically non- trivial forms of order, e.g. the skyrmion lattice phase in MnSi in 2009 [1], the intensity of research in this field increased tremendously. The key experimental signatures are multiplek structures as probed by small- angle neutron scattering, while in the k = 0 space, e.g. bulk magnetic properties, a large topological Hall effect arises due to an emergent gauge field as theoretically found to be a manifestation of the Berry curvature of the excitations involved. The latter route was followed to identify new magnetic systems or compound families, possibly hosting non-trivial spin states by measuring a topological Hall effect signal. As one class the rare-earth intermetallics series RE- Cu were recently identified in our group as having multiple- k antiferromagnetic orderings and very large topological Hall resistivity two orders of magnitude higher than in the A-phase of MnSi. One key element to the energetic hierarchy of interactions is the magnetic anisotropy. To further probe possible signatures of spin textures by other bulk measurements, e.g. heat capacity, no option so far is available since strong magnetic torques in finite magnetic fields along hard magnetisation axes forbid detailed investigations by present state-of- the-art calorimetric techniques. Therefore essentially no information is available on the underlying changes in entropy across the phase boundaries. To overcome this issue an entirely new design to measure heat capacity was invented, constructed, characterised and optimised during the course of this master thesis to ensure mechanical stability against torques acting on the sample when applying magnetic fields along the hard axes. Two setups were designed, one a screw tightened Al-plate as sample platform and second a Kevlar setup which has a completely new structure, permitting to tune and adjust the setup for different materials, both fitting to a commercially available sample puck provided by the Physical Property Measurements System (PPMS) from Quantum Design. We started with a detailed characterization of the new setups in comparison to the conventional heat pulse calorimeter technique and concentrated on the heat capacity of ErCu along the three principal crystallographic directions in the temperature range from 2 K to 200 K up to magnetic fields of 14 T. It enabled us to shed light to the rich magnetic phase diagram in identifying a structural transition. A non-magnetic equivalent of the material - LuCu - was prepared and its specific heat was used to find the magnetic contribution in the compounds of interest.

heat capacity ; complex magnetic phase diagrams

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