  Spin textures with nontrivial topology, such as vortices and skyrmions, have
attracted attention as a source of unconventional magnetic, transport, and
optical phenomena. Recently, a new generation of topological spin textures has
been extensively studied in itinerant magnets; in contrast to the conventional
ones induced, e.g., by the Dzyaloshinskii-Moriya interaction in
noncentrosymmetric systems, they are characterized by extremely short magnetic
periods and stable even in centrosymmetric systems. Here we review such new
types of topological spin textures with particular emphasis on their
stabilization mechanism. Focusing on the interplay between charge and spin
degrees of freedom in itinerant electron systems, we show that itinerant
frustration, which is the competition among electron-mediated interactions,
plays a central role in stabilizing a variety of topological spin crystals
including a skyrmion crystal with unconventional high skyrmion number, meron
crystals, and hedgehog crystals. We also show that the essential ingredients in
the itinerant frustration are represented by bilinear and biquadratic spin
interactions in momentum space. This perspective not only provides a unified
understanding of the unconventional topological spin crystals but also
stimulates further exploration of exotic topological phenomena in itinerant
magnets.

Strongly Correlated Electrons