Surface-size and shape dependencies of change in chiral soliton number in submillimeter-scale crystals of chiral magnet CrNb₃S₆

We examine the surface size- and shape-effects of soliton annihilation and soliton nucleation in chiral magnet CrNb₃S₆. We measure magnetization (M) curves of submillimeter-sized single crystals with an equal length along the c-axis (L_c = 10 μm) but with different cross sections in the ab-plane (S_ab = 0.120-0.014 mm²). We find a ferromagnetic type of magnetizing (FMM) with a convex curve (d²M/dH² < 0) near zero field (H = 0) and a major jump in M near the forced ferromagnetic state, which are more conspicuous, compared with earlier samples with submillimeter L_c [K. Tsuruta et al. J. Phys. Soc. Jpn. 85, 013707 (2016)]. A new finding is that the major jump in M occurs at lower fields in samples with the smaller S_ab. We further perform numerical simulation of the magnetization process with the Landau-Lifshitz-Gilbert equation of the Langevin-type. Based on the numerical results, we attribute the FMM at small fields to rapid annihilation of soliton assisted by the reduction of Dzyaloshinskii-Moriya interaction near the surfaces. We also discuss possible penetration processes of chiral soliton through the ac-(bc-)plane as well as ab-plane, and its relation to the major jump in M. Our experimental and calculated results will contribute to understanding of the effects of topological metastability in chiral magnets.