Abstract
Breaking the dilemma between small size and room-temperature stability is a necessary prerequisite for skyrmion-based information technology. Here we demonstrate by means of rate theory and an atomistic spin Hamiltonian that the stability of isolated skyrmions in ultrathin ferromagnetic films can be enhanced by the concerted variation of magnetic interactions while keeping the skyrmion size unchanged. We predict film systems where the lifetime of sub-10 nm skyrmions can reach years at ambient conditions. The long lifetime of such small skyrmions is due to exceptionally large Arrhenius pre-exponential factor and the stabilizing effect of the energy barrier is insignificant at room temperature. A dramatic increase in the pre-exponential factor is achieved thanks to the softening of magnon modes of the skyrmion, thereby increasing the entropy of the skyrmion with respect to the transition state for collapse. Increasing the number of skyrmion deformation modes should be a guiding principle for the realization of nanoscale, room-temperature stable skyrmions.
Highlights
Encoding data with metastable magnetic skyrmions[1,2,3] is an appealing solution for future information technology[4,5,6]
Thermal stability becomes an issue as thermal fluctuations can induce the spontaneous collapse of the skyrmion state and, corrupt the stored data
The qualitative difference in the behavior of the excitation spectra at the transition state and at the skyrmion state is the origin of large changes in the prefactor
Summary
Encoding data with metastable magnetic skyrmions[1,2,3] is an appealing solution for future information technology[4,5,6]. Thanks to the pronounced material dependence of the prefactor, it is possible to realize long-lived sub-10 nm skyrmions in FM films at room temperature and zero applied magnetic field This finding contrasts sharply with conclusions of previous studies where the skyrmion stability is assessed exclusively based on estimation of the energy barrier. It is unfeasible to reach energy barriers exceeding thermal energy by a factor of 40–50 at room temperature—a commonly used criterion for reliable information storage—while keeping the skyrmion size at nanoscale, the long lifetime of ultrasmall skyrmions can still be achieved due to the remarkably large value of the Arrhenius prefactor τ0, which is a unique phenomenon in magnetism This stabilization scenario is realized for skyrmions with a bubble-like profile providing a large number of skyrmion deformation modes and, thereby, high entropy barriers. Some of the contours of equal skyrmion radius R and lifetime τ are shown with blue and black solid lines, respectively
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