Suppression of the commensurate magnetic phase in nanosized hübnerite $\mathrm{MnW}{\mathrm{O}}_4$

Magnetic structures of nanosized (20 to 70 nm) powders of $\mathrm{MnW}{\mathrm{O}}_4$ and $\mathrm{MnW}{\mathrm{O}}_4$:Mo were studied using neutron powder diffraction (NPD). Sizes and shapes of the crystallites calculated from anisotropic peak broadening of diffraction peaks were found to be orthogonal parallelepipedlike with the longest edge along the $c$ axis and the shortest along the $b$ axis. SQUID measurements indicate the presence of two magnetic transitions around 8 and 12 K. Rietveld refinement of the NPD data below the magnetic transition was consistent with the presence of an incommensurate spiral-like ($ac\text{−}\mathrm{AF}2$) phase. A commensurate phase AF1 was not observed down to 2.5 K for all of the samples.

Figure 1

Crystal structure of $\mathrm{MnW}{\mathrm{O}}_4$ with one of the (010) planes highlighted.

Figure 2

(a) Magnetization curves reveal a broad transition at around 6 K and small hump around 14 K. (b) Comparing the derivative to a digitization of the bulk powder shows a smearing of the ordering feature and a change in the $T1$ transition character. The transition near 8 K, $T^{*}$, is found to be related to a change in spiral ellipticity that is seen in the neutron diffraction.

Figure 3

Merged temperature scans of S1 show the robustness of the incommensurate phase down to the lowest measured temperature and the disappearance of the long range magnetic order at around 10 K.

Figure 4

Peak intensities (nuclear N green and magnetic peaks M1 red, M2 blue) shown are from fits to Fig. 3.

Figure 5

Merged temperature scans of sample 2 revealed lack of commensurate phase AF1 and disappearance of the long range magnetic order at 12 K.

Figure 6

Extracted peak intensities (nuclear N green and magnetic peaks M1 red, M2 blue) presented with fits of the order parameter in the 8.5–15 K region.

Figure 7

Merged temperature scans of S3 revealed a lack of the commensurate phase AF1 and disappearance of the long range magnetic order at 12 K.

Figure 8

Nuclear (N green) and magnetic peaks (M1 red, M2 blue) intensities obtained from fits for S3. Order parameter fits to magnetic peaks in the 8–14 K range give similar critical temperature $T_N=13.6\left(7\right)\mathrm{K}$.

Figure 9

Rietveld refinement of BT-1 data for S1. Blue vertical bars indicate the nuclear structure. Red verticals bars mark magnetic reflections.

Figure 10

Plot of the Rietveld refinement of the S2 at $T=2.8\mathrm{K}$.

Figure 11

Dependence of the agreement factor $R_{\mathrm{mag}}$ on the $C2$ coefficient that is proportional to the moment along the $b$ axis. The inset shows a dependence of $C1$ (red) and $C3$ (blue) vs $C2$.

Figure 12

A plot of the Rietveld refinement of the S3 at $T=5\mathrm{K}$.

Figure 13

Maps of $\log \left(R_{\mathrm{mag}}\right)$ around the ${\chi }^2$ minimum from reported Rietveld refinements. The values of the minima are denoted by open blue circles.