Persistence of local-moment antiferromagnetic order in Ba${}_{1-x}$K${}_x$Mn${}_2$As${}_2$

BaMn${}_2$As${}_2$ is a local-moment antiferromagnetic insulator with a Néel temperature $T_{\mathrm{N}}$ of 625 K and a large ordered moment of 3.9 ${\mu }_{\mathrm{B}}$/Mn. Remarkably, this compound can be driven metallic by the substitution of as little as 1.6$\%$ K for Ba while retaining essentially the same ordered magnetic moment and Néel temperature, as previously reported. Here, using both powder and single crystal neutron diffraction we show that the local moment antiferromagnetic order in Ba${}_{1-x}$K${}_x$Mn${}_2$As${}_2$ remains robust up to $x=0.4$. The ordered moment is nearly independent of $x$ for 0 $\le x\le$ 0.4 and $T_{\mathrm{N}}$ decreases to 480 K at $x=0.4$.

Figure 1

Neutron diffraction profile fits from Rietveld analysis of nuclear and magnetic Bragg reflections from Ba${}_{0.75}$K${}_{0.25}$Mn${}_2$As${}_2$ at 14 and 600 K. The observed data points are given by red circles, the calculated intensity patterns by black solid lines, and the corresponding residuals (i.e., the difference between the observed and calculated patterns) by the blue curves at the bottom of each panel. Small angular regions that contain peaks from the MnO second phase were excluded in the refinement. The green vertical upper tick marks reflect the Bragg positions for nuclear reflections, and the lower green tick marks give the Bragg positions for magnetic reflections.

Figure 2

Neutron diffraction data collected in the first segment of the powder diffraction pattern from Ba${}_{1-x}$K${}_x$Mn${}_2$As${}_2$ ($x=0.05$ and 0.25) in the high-temperature furnace. The background from the empty furnace, measured at 300 K, has been subtracted from the data at each temperature.

Figure 3

Integrated intensity of the (101) magnetic Bragg reflection measured for Ba${}_{1-x}$K${}_x$Mn${}_2$As${}_2$ ($x=0.05$ and 0.25). Note that there is a low intensity nuclear Bragg peak at (101) present at all temperatures for both samples. The insets show expanded plots near $T_{\mathrm{N}}$ for both samples.

Figure 4

Integrated intensity of the (101) Bragg peak from the Ba${}_{0.6}$K${}_{0.4}$Mn${}_2$As${}_2$ single-crystal sample. The dashed horizontal red line represents the contribution of the nuclear scattering to the Bragg peak intensity that is present at all temperatures.

Figure 5

Summary of results for $T_{\mathrm{N}}$ (open circles) and the ordered moment (solid triangles) for Ba${}_{1-x}$K${}_x$Mn${}_2$As${}_2$ as a function of K content. The solid (dashed) line represents a fit (extrapolation) of the data using a second-order polynomial in $x$. The parent compound ($x=0$) is semiconducting, whereas all other samples, with $x\ge$ 0.016, are metallic as described in Ref. 11.