Spin current generation driven by skyrmion dynamics under magnetic anisotropy and polarized microwaves

Abstract We have investigated the spin-current pumped by the skyrmion-host material with the lack of inversion symmetry through the microwave resonance process. The effects of magnetic anisotropy and polarized microwaves are examined by micromagnetic simulations. Our results reveal two distinct skyrmion phases, designated as SkX type-I and II, which emerge at low ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>K</mml:mi> <mml:mi>z</mml:mi> </mml:msub> <mml:mstyle scriptlevel="0"/> <mml:mo>&lt;</mml:mo> <mml:mstyle scriptlevel="0"/> <mml:mn>0.1</mml:mn> </mml:mrow> </mml:math> meV) and high ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>K</mml:mi> <mml:mi>z</mml:mi> </mml:msub> <mml:mstyle scriptlevel="0"/> <mml:mo>&gt;</mml:mo> <mml:mstyle scriptlevel="0"/> <mml:mn>0.1</mml:mn> </mml:mrow> </mml:math> meV) magnetic anisotropy constants, respectively, having different characteristics of spin excitations. The SkX type-I exhibits spin dynamics where the resonant frequency of the breathing mode is lying in between the clockwise (CW) and counter CW (CCW) gyration modes of Bloch-type skyrmion at a very low anisotropy, and is crossing over the CCW mode at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>K</mml:mi> <mml:mi>z</mml:mi> </mml:msub> <mml:mo>∼</mml:mo> <mml:mn>0.04</mml:mn> </mml:mrow> </mml:math> meV. Meanwhile, the SkX type-II exhibits distinct spin excitations in which the CW mode is notably absent, while the CCW modes exist at both low and high resonant frequencies. This suggests that the magnetic anisotropy plays an essential role in the spin dynamics. Furthermore, the resulting spin excitations induce spin currents with exotic features under the polarized microwaves. The spin currents induced, for instance, by low-lying in-plane excitations are strongly enhanced under the left-handed circularly polarized microwaves, but quenched by the right-handed circularly polarized microwaves regardless of the sign of the Dzyaloshinskii–Moriya interaction. These results may pave the way for understanding the non-trivial interplay between magnetic anisotropy and polarized microwaves in the generation of spin currents by a resonant process.