Structural disorder versus chiral magnetism in ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_2$

The crystal structure of a disordered form of ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_2$ has been characterized using diffraction and inelastic scattering of synchrotron radiation. In contrast to the previously reported symmetry $\left(P{6}_{3}22\right)$, the crystal can be described by a regular twinning of an average $P{6}_3$ structure with three disordered positions of the Cr ions. Short-range correlations of the occupational disorder result in a quite intense and structured diffuse scattering; a static nature of the disorder was unambiguously attributed by the inelastic x-ray scattering. The diffuse scattering has been modeled using a reverse Monte-Carlo algorithm assuming a disorder of the Cr sublattice only. The observed correlated disorder of the Cr sublattice reduces the temperature of the magnetic ordering from 130 to 88 K and drastically modifies the field dependence of the magnetization as it is evidenced by the SQUID magnetometery. We conclude that, in contrast to the magnetic properties assumed for a helicoidal spin structure, the compound under study (which has the same chemical composition and an average structure very close to previous reports) shows the response of a ferromagnetically ordered anisotropic material.

Figure 1

Magnetization curves for $H\parallel ab$ and $H\parallel c$ at $T=6$ K.

Figure 2

Temperature dependent magnetization for (a) $H\parallel ab$ and (b) $H\parallel c$ for different magnetic fields.

Figure 3

Arrot plots for $H\parallel ab$ plane according to (a) a mean-field model $(\beta =0.5,\gamma =1)$ and (b) a 3D Heisenberg model $(\beta =0.365,\gamma =1.336)$.

Figure 4

Fragment of crystal structures for models I–III. The occupancy of disordered Cr sites is encoded as a colored sector.

Figure 5

Maps of the diffuse scattering for different layers in reciprocal space.

Figure 6

(a) The $hhl$ section of reciprocal space and (b) $(E,Q)$ intensity map for the region of interest indicated in (a).

Figure 7

Diffuse scattering patterns: (exp) symmetrized experimental, (calc) resulting from the RMC, and (diff) difference map. The same intensity scale is used.

Figure 8

Randomly chosen $xy$ sections of the model clusters with the given Cr content.