Tunable gigahertz dynamics of low-temperature skyrmion lattice in a chiral magnet

Abstract

Recently, it has been shown that the chiral magnetic insulator Cu2OSeO3 hosts skyrmions in two separated pockets in temperature and magnetic field phase space. It has also been shown that the predominant stabilization mechanism for the low-temperature skyrmions (LTS) phase is the crystalline anisotropy in contrast to temperature fluctuations, which stabilize the well established high-temperature skyrmion (HTS) lattice. Here, we report on the gigahertz dynamics in the LTS phase in Cu2OSeO3. The LTS phase is populated via a field cycling protocol with the static magnetic field applied parallel to the h100i crystalline direction of plate and cuboid-shaped bulk crystals. By analyzing temperature-dependent broadband spectroscopy data, clear evidence of low-temperature skyrmion excitations with clockwise (CW), counterclockwise (CCW), and breathing mode (BR) character at temperatures below T = 40 K are shown. We find that the modes’ intensities can be tuned with the number of field-cycles below the saturation field, and by tracking the resonance frequencies, the LTS phase diagram can be established. From our experiments, we conclude that the LTS phase is well separated from the high-temperature phase. Furthermore, by monitoring the strength of the observed hybridization between a dark CW mode and the BR as a function of temperature for the two differently shaped crystals, we unambiguously conclude that the magnetocrystalline anisotropy governs the hybridization.

Task-adaptive physical reservoir computing

“…It is clear that the MC metric shows similar behaviour to the performance of with H c , i.e., forecasting performance and MC increases then decreases with H c , suggesting that MC is a key property for a better performance in forecasting tasks. As discussed earlier, MC in the skyrmion phase stems from the history-dependent fading memory property generated by its gradual skyrmion nucleation with repeated field cycles 40 , 41 . Rich and complex spin-wave mode dispersion in the conical/ferrimagnetic phases provides the physical basis for the high NL and CP scores, offering a strong transformation task performance (see more detailed discussions in Supplementary Note 2 ).…”

“…Here, the Cu 2 OSeO 3 crystal was placed on a coplanar waveguide board, which sits on a copper cold finger of a closed-cycle helium cryostat. Here, we apply an external magnetic field H along the 〈100〉 crystallographic direction for the efficient generation of low-temperature skyrmions 40 , 41 . A VNA (ZNB40, Rohde & Schwarz) is employed to measure the spectral response of chiral magnetic crystals using ferromagnetic resonance (FMR) techniques.…”

“…Figure 2b displays the N dependence of the spectra for H c and temperature inside the skyrmion phase. For N = 100, a sharp peak around 4 GHz can be clearly observed, corresponding to low-energy spin-wave modes of the thermodynamically stable conical phase 39 – 41 . As we cycle further, the conical mode amplitude shrinks and the skyrmion modes appear around 2–3 GHz, as highlighted by the grey curves for N = 130–170.…”

“…Since different magnetic phases (skyrmion, helical and conical phases) exhibit distinct resonances, they offer broadly varying reservoir properties and computing performance, which can be reconfigurably tuned via the magnetic field and temperature. We use magnetic field cycling 40 , 41 to input data and measure the spin-wave spectra at each input step to efficiently achieve high-dimensional mapping. By quantitatively assessing each phase as a reservoir, we find that the thermodynamically metastable skyrmion phase has a strong memory capacity due to the magnetic field-driven gradual nucleation of skyrmions with an excellent performance in future prediction tasks.…”

“…As we cycle further, the conical mode amplitude shrinks and the skyrmion modes appear around 2–3 GHz, as highlighted by the grey curves for N = 130–170. These are the counterclockwise and breathing modes of the metastable low-temperature skyrmion phase generated by field cycling 40 , 41 . The mode frequencies move with our input magnetic fields, and as the cycling proceeds the skyrmions are continuously destroyed and renucleated, as evident from the peak amplitude.…”