Muons and magnets

Abstract:

We present the results of muon-spin relaxation $\left(\mu \mathrm{SR}\right)$ measurements of the static and dynamic magnetism of $M_{1/3}{\mathrm{NbS}}_2$ $(M=\mathrm{Fe}, \mathrm{V}, \mathrm{Mn})$ , three intercalated transition-metal dichalcogenides. Transitions to long-range magnetic order are observed in all three materials, and local magnetic fields at muon sites are compared to dipole field calculations. Measurements on ${\mathrm{Fe}}_{1/3}{\mathrm{NbS}}_2$ capture the evolution of two coexisting magnetic phases. In ${\mathrm{V}}_{1/3}{\mathrm{NbS}}_2$ , we observe a peak in the dynamic response at 9 K, coincident with previous reports of a possible low-temperature phase transition. The observation of high-frequency muon precession in ${\mathrm{Mn}}_{1/3}{\mathrm{NbS}}_2$ suggests the existence of an additional muon site that implies a difference in the electronic energy landscape compared to the other materials in the series. Taken together, this demonstrates that the change in intercalant species drives significant variations in magnetism, highlighting the $M_{1/3}{\mathrm{NbS}}_2$ series as an ideal group of materials for investigating a wide range of magnetic phenomena.

Abstract:

We present single-crystal neutron diffraction, powder muon spin rotation, and pulsed-field magnetometry measurements on the Heisenberg quantum chiral chain $\left[\mathrm{Cu}\left(\mathrm{pym}\right){\left({\mathrm{H}}_2\mathrm{O}\right)}_4\right]{\mathrm{SiF}}_6·{\mathrm{H}}_2\mathrm{O}$ (pym = pyrimidine), which displays a fourfold-periodic rotation of the local environment around the Cu() $S=1/2$ ions from site to site along the chain. Previous measurements on this material have shown the absence of magnetic order down to surprisingly low temperatures $\ge 20$  mK, as well as the presence of an energy gap for magnetic excitations that grows linearly with magnetic field. Here we find evidence at dilution refrigerator temperatures for a field-induced transition to long-range magnetic order above an applied magnetic field of 3 T. From the polarization of magnetic moments observed with magnetic fields applied in the $[-1,2,0]$ direction, we can identify the static magnetic structure that best accounts for the data. The proposed model is supported microscopically by the presence of an alternating component of the $g$ tensor, which produces an internal twofold staggered field that dictates both the direction of the ordered moments and the effective coupling between adjacent chains. The observed magnetic structure is contrary to previous proposals for the departure of the magnitude and field dependence of the energy gap from the predictions of the sine-Gordon model.