In the past decade, the role of topological properties in condensed matter physics has been uncovered in many different systems: 2D electron gas in the quantum Hall regime, topological insulators, Weyl or Dirac semi-metals, and topological superconductors. Topological spin textures have also been investigated, stimulated by the possibility of discovering unusual physical phenomena owing to the interplay between magnetism and topology. A most prominent example is the magnetic skyrmion, a non-collinear spin structure with particle-like topologically protected states weakly affected by perturbations or material defects which makes it a potential carrier of information in future data storage devices, such as racetrack nano devices.
In most systems experimentally investigated to date, skyrmions emerge as classical objects forming a skyrmion lattice. However, the recent discovery at very low temperature of skyrmions with nanometer length scales, spins wound over a few lattice spacings only, where quantum effects cannot be ignored has sparked interest in their quantum properties, introducing the notion of quantum skyrmions, such as quantum tunneling and energy-level quantization.
Our research activity is oriented towards the understanding of the exotic physics of these fascinating objects through different types of measurements from transport (resistivity, Hall effect, thermoelectricity, thermal conductivity) and thermodynamic (specific heat, magnetization) measurements towards microscopic analysis (muon and neutron techniques). It is a great challenge with fundamental science issues together with potential outcomes for future data storage application.
Here we present results obtained for the model system MnSi in which a lattice of skyrmions was first identified and for EuPtSi that was recently shown to host extremely small skyrmions. In the former, we have quantitatively derived the microscopic interaction parameters including the Heisenberg exchange and Dzyaloshinskii-Moriya interactions
[1, 2] and their dependence with hydrostatic pressure. In the latter, our measurements have unveiled a magnetic field – temperature phase diagram much more complex than in the traditional skyrmionic systems and the presence of metastable states extending over a large portion of it.
Magnitude of the Heisenberg exchange interaction in MnSi and its evolution with the volume of the crystal unit cell or the hydrostatic pressure. (Collaboration with the Paul Scherrer Institute in Switzerland and the Babes-Bolyai university in Romania).
Magnetic field-temperature phase diagram of EuPtSi derived from resistivity measurements with the presence of a skyrmion phase (Collaboration with D. Aoki, Tohoku University and Y. Onuki, RIKEN Center for Emergent Matter Science, in Japan).