Giant topological Hall effect in strained Fe$_{0.7}$Co$_{0.3}$Si epilayers

The coupling of electron spin to real-space magnetic textures leads to a variety of interesting magnetotransport effects. The skyrmionic spin textures often found in chiral B20-lattice magnets give rise, via real-space Berry phases, to the topological Hall effect, but it is typically rather small. Here, B20-ordered Fe$_{0.7}$Co$_{0.3}$Si epilayers display a giant topological Hall effect due to the combination of three favourable properties: they have a high spin-polarisation, a large ordinary Hall coefficient, and dense chiral spin textures. The topological Hall resistivity is as large as 820 n$Omega$cm at helium temperatures. Moreover, we observed a drop in the longitudinal resistivity of 100 n$Omega$cm at low temperatures in the same field range, suggesting that it is also of topological origin. That such strong effects can be found in material grown in thin film form on commercial silicon wafer bodes well for skyrmion-based spintronics.