Topological Phase Transitions and Critical Phenomena Associated with Unwinding of Spin Crystals by High Magnetic Fields

A variety of magnetic particles with unique topology have been explored since the experimental discovery of magnetic skyrmions. Those topologically-nontrivial spin-twisting knots cannot be transformed continuously into other conventional magnetic structures, e.g., the ferromagnetic state, due to the geometrical restriction. Such topological protection leads to the high stability of magnetic particles, opening up their possible technological applications. On the other hand, there remain many fundamental challenges for the topological stability, such as the observation of unwinding of topological spin configurations and the understanding of the influence of topology on critical phenomena. This review introduces the experimental examples of topological phase transitions and critical phenomena in the magnetic-particles condensates, i.e., the topological spin crystals, of the chiral magnets MnSi_1−xGe_x. The dramatic transformation of topological spin textures are manifested as giant emergent transport properties at high magnetic fields, such as topological Hall effect, magnetoresistance, thermoelectric effects and so on. Their unconventional responses to magnetic fields highlight the central role of critical spin fluctuations pronounced at the topological phase transitions.