Surface Effects in the Magnetic Textures of B20 MnGe Thin Films

Bulk MnGe hosts a three-dimensional skyrmion state with a period of only 3 nm. Using low-temperature (5K) spin-polarized scanning tunneling microscopy, we show that this magnetic texture is modified at the surface of MnGe thin films. We observe a stripe-like phase with a 6-8 nm period and a propagation vector that is not strongly pinned to any direction, but is influenced by step edges, surface termination, and atomic lattice strain. We also report the observation of target skyrmions with triangular shape that is set by the atomic lattice vectors, and a core size of 15 nm. We observe the target state is much more sensitive to magnetic fields than the stripe phase, and that pulses with the STM tip demonstrate the texture can be ‘switched’ between states with different topological charge. By analyzing the voltage and current applied during switching events we can determine their dependence on thermal energy of tunneling electrons, injection of spin-polarized current, or electric fields. Finally, we explore the magnetic texture in different film thicknesses and compare their larger scale film morphology to distinguish between thin film effects and true surface effects.