Characterization of a Disordered above Room Temperature Skyrmion Material Co8Zn8Mn4

Abstract

Topologically nontrivial spin textures host great promise for future spintronic applications. Skyrmions in particular are of burgeoning interest owing to their nanometric size, topological protection, and high mobility via ultra-low current densities. It has been previously reported through magnetic susceptibility, microscopy, and scattering techniques that Co8Zn8Mn4 forms an above room temperature triangular skyrmion lattice. Here, we report the synthesis procedure and characterization of a polycrystalline Co8Zn8Mn4 disordered bulk sample. We employ powder X-ray diffraction and backscatter Laue diffraction as characterization tools of the crystallinity of the samples, while magnetic susceptibility and Small Angle Neutron Scattering (SANS) measurements are performed to study the skyrmion phase. Magnetic susceptibility measurements show a dip anomaly in the magnetization curves, which persists over a range of approximately 305 K-315 K. SANS measurements reveal a rotationally disordered polydomain skyrmion lattice. Applying a symmetry-breaking magnetic field sequence, we were able to orient and order the previously jammed state to yield the prototypical hexagonal diffraction patterns with secondary diffraction rings. This emergence of the skyrmion order serves as a unique demonstration of the fundamental interplay of structural disorder and anisotropy in stabilizing the thermal equilibrium phase.

Keywords: disordered; magnetism; skyrmion.

Figure 1

Polycrystalline Co${}_8$Zn${}_8$Mn${}_4$ sample mixed under argon of the dimensions 0.8 cm × 1.4 cm and a mass of 2 g. Each grid line corresponds to 1 mm.

Figure 2

The Rietveld refinement for the powder X-ray diffraction of Co${}_8$Zn${}_8$Mn${}_4$. The black curve is the predicted spectra, the red line is the data, and the blue is the difference between the two. The green vertical lines indicate the locations of the expected peaks. The refinement demonstrates the sample is phase pure with space-group $\beta$-Mn and lattice constant $6.37161\left(1\right)\AA$.

Figure 3

Laue image along (100) direction of the dominant grain on the front face of the cube. All peaks were indexed to the (100) direction, verifying the single grain portion of this material. The four-fold symmetry of the pattern is characteristic for the (100) direction of a cubic crystal.

Figure 4

Magnetic susceptibility per mol of Co${}_8$Zn${}_8$Mn${}_4$ after zero-field cooling (ZFC) and field cooling from 400 K in a magnetic field of 20 Oe. The bifurcation behaviour of the ZFC and FC curves at 7 K results from the path-dependent behaviour of the susceptibility (as indicated by the arrow in the ZFC curve), which is indicative of a spin glass transition.

Figure 5

Magnetic field-dependent magnetization upon increasing magnetic fields from 0 Oe to 400 Oe for a temperature range of 300 K–340 K in 5 K increments. Low–high temperature corresponds to blue–red curves. Note a substantial decrease in magnetization for temperatures greater then 320 K, consistent with a paramagnetic phase.

Figure 6

Temperature-dependent isothermal differential magnetic susceptibility upon increasing magnetic fields from 0 Oe to 400 Oe. Low–high temperature corresponds to dark blue–light blue curves. The dip structure (region contained within the rectangular dotted box) is most clearly pronounced for 310 K and presents at a field of ∼200 Oe, indicating the onset of the skyrmion phase.

Figure 7

The temperature dependence of AC magnetic susceptibility per mol of Co${}_8$Zn${}_8$Mn${}_4$ over a range of 305 K–317.5 K after increasing magnetic fields from 0 Oe to 500 Oe in a 100 Hz driving field, with an amplitude of 0.1 Oe. Low–high temperature corresponds to dark blue–lighter coloured curves. The skyrmion phase is most pronounced at a temperature of 310 K and is observed to persist in the dip anomaly between 100 Oe and 450 Oe (contained within the region bounded by the lines, as indicated by the arrow). The field value at the minimum of the dip determines the largest and most robust skyrmion phase; these temperature and field parameters are then used for SANS measurements on the material.

Figure 8

SANS images showing the disordered helical ground state at room temperature in zero field (a); the initial scattering ring for disordered skyrmion domains at 310 K in a magnetic field of 250 Oe (b); a schematic of the symmetry-breaking field rotation setup; and the SANS image after 30 rotations of the rotation sequence at 310 K in a field of 250 Oe (c). The increased intensity/preferential smearing of the peaks in the top right and bottom left diagonals of the four-fold helical image elucidate the anisotropy direction for the crystal. A schematic of the rotation setup illustrates the neutron propagation direction (n) is in the z-direction. For the symmetry-breaking rotation sequence, the sample is rotated symmetrically in the xz plane about $\theta$, with the magnetic field fixed in the z-direction.