Short- and long-range magnetic order in LaMnAsO

The magnetic properties of the layered oxypnictide LaMnAsO have been revisited using neutron scattering and magnetization measurements. The present measurements identify the Néel temperature $T_N=360\left(1\right)$ K. Below $T_N$ the critical exponent describing the magnetic order parameter is $\beta =0.33–0.35$, consistent with a three-dimensional Heisenberg model. Above this temperature, diffuse magnetic scattering indicative of short-range magnetic order is observed, and this scattering persists up to $T_{\text{SRO}}=650\left(10\right)$ K. The magnetic susceptibility shows a weak anomaly at $T_{\text{SRO}}$ and no anomaly at $T_N$. Analysis of the diffuse scattering data using a reverse Monte Carlo algorithm indicates that above $T_N$ nearly two-dimensional, short-range magnetic order is present with a correlation length of 9.3(3) Å within the Mn layers at 400 K. The inelastic scattering data reveal a spin gap of 3.5 meV in the long-range ordered state, and strong, low-energy (quasielastic) magnetic excitations emerging in the short-range ordered state. Comparison with other related compounds correlates the distortion of the Mn coordination tetrahedra to the sign of the magnetic exchange along the layer-stacking direction, and suggests that short-range order above $T_N$ is a common feature in the magnetic behavior of layered Mn-based pnictides and oxypnictides.

Figure 1

Temperature dependence of the lattice parameters of LaMnAsO. Neutron and x-ray diffraction results are shown for $T\ge 300$ K and $T\le 300$ K, respectively. Each data set was separately normalized to 300 K values. See also Table 1.

Figure 2

Results of magnetization measurements on LaMnAsO. (a) Isothermal magnetization curves showing the absence of any significant ferromagnetic signal. (b) Temperature dependence of the magnetic susceptibility, with the anomaly at 650 K indicated on the figure and the inset. Note the small cusp associated with the MnO impurity, and the absence of any magnetic anomaly near room temperature.

Figure 3

Neutron powder diffraction data from LaMnAsO collected with a neutron wavelength of 2.414 Å. (a) Rietveld refinement of data collected at 300 K. (b) Rietveld refinement of data collected at 730 K. Upper set of ticks locate nuclear reflections, lowest set of ticks at 300 K locate magnetic reflections, other sets indicate reflections from MnO and ${\mathrm{La}}_2{\mathrm{O}}_3$ impurities. (c) Comparison of diffraction patterns showing sharp peaks at 300 K indicating long-range magnetic order, a broad peak at 400 K indicating short-range order, and no magnetic scattering at 730 K. (d) A fit to the short-range order peak using a Lorentzian function. The two sharp reflections that appear in the $Q$ range of interest are modeled in the same way.

Figure 4

Temperature dependence of magnetic scattering from LaMnAsO. (a) Counts measured at the center of the 101, 100, and short-range order (SRO) peaks. (b) The behavior of the data shown in (a) near the Néel temperature.

Figure 5

(a) Measured diffuse magnetic scattering (circles) and calculated scattering (lines) from the spin configurations determined by reverse Monte Carlo using the program SPINVERT [49]. (b) The radial dependence of the spin-pair correlation function corresponding to the fits shown in (a). (c) Data from (b) grouped into sets corresponding to spins separated along to the indicated crystallographic directions. (d) Absolute value of the data in (b) for pairs of spins within a common layer (closed symbols) and in neighboring layers (open symbols). The vertical error bars on the spin-pair correlation values in (b), (c), and (d) are approximately the size of the data markers.

Figure 6

(a)–(c) Energy versus momentum transfer ($Q$) slices of the polarized inelastic neutron scattering measured in spin-flip configuration at 300, 370, and 385 K. Highly dispersive spin-wave excitation are located at momentum transfer $Q=1.6{\text{Å}}^{-1}$ near the 100 and 101 magnetic peak positions. The excitation spectrum at 300 K displays an anisotropy gap that vanishes with the long-range order. (d) Constant-$Q$ cuts near $Q=1.55\text{Å}$ revealing a spin gap of approximately 3.5 meV in the LRO state, at 300 K, and the gapless nature of the excitation spectrum in the SRO state at 370 K.