Communicating skyrmions as the main mechanism underlying skyrmionium (meta)stability in quasi-two-dimensional chiral magnets
arxiv(2024)
摘要
We re-examine the internal structure of skyrmioniums stabilized in
quasi-two-dimensional chiral magnets with easy-axis uniaxial anisotropy.
Skyrmioniums are particle-like states of two nested skyrmions with opposite
polarities contributing to zero topological charge. The physical principles of
skyrmionium stability are drawn from both the analytical analysis with a trial
function and from numerical simulations within the framework of micromagnetism.
We deduce that the radii of the internal skyrmion with the positive polarity
and the ring-shaped external skyrmion with the negative polarity are mutually
dependent, which constitutes the paradigm of communicating skyrmions. For large
central skyrmions, the skyrmionium transforms into a narrow circular domain
wall, whereas for small internal radii, the ring expands, which occurs at the
verge of collapsing into an ordinary isolated skyrmion. We show that
skyrmioniums may form lattices of two varieties depending on the polarity of
the internal skyrmion. At the phase diagram (magnetic field)-(uniaxial
anisotropy), both skyrmionium lattices share the same area with one-dimensional
spiral states and remain metastable solutions for the whole range of control
parameters. By expanding at the critical line, skyrmionium lattices do not
release isolated skyrmioniums. Isolated skyrmioniums of just one type exist
apart from the corresponding lattice in a narrow field region restricted by the
critical line of expansion from below and by the line of collapse above.
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