Nd-induced Mn spin-reorientation transition in NdMnAsO

A combination of synchrotron x-ray, neutron powder-diffraction, magnetization, heat-capacity, and electrical-resistivity measurements reveals that NdMnAsO is an antiferromagnetic semiconductor with large Neel temperature $\left[T_{\text{N}}=359\left(2\right)\text{K}\right]$. At room temperature the magnetic propagation vector $\mathbf{k}=0$ and the Mn moments are directed along the crystallographic $c$ axis $\left[m_{\text{Mn}}=2.41\left(6\right){\mu }_{\text{B}}\right]$. Upon cooling a spin-reorientation (SR) transition of the Mn moments into the $ab$ plane occurs $\left(T_{\text{SR}}=23\text{K}\right)$. This coincides with the long-range ordering of the Nd moments, which are restricted to the basal plane. The magnetic propagation vector remains $\mathbf{k}=0$. At base temperature (1.6 K) the fitted moments are $m_{\text{ab},\text{Mn}}=3.72\left(1\right){\mu }_{\text{B}}$ and $m_{\text{ab},\text{Nd}}=1.94\left(1\right){\mu }_{\text{B}}$. The electrical resistivity is characterized by a broad maximum at 250 K, below which it has a metallic temperature dependence but semiconducting magnitude (${\rho }_{250\text{K}}=50\Omega \text{cm}$, residual resistivity $\text{ratio}=2$), and a slight upturn at the SR transition.

Figure 1

(Color online) Rietveld fit to 4 K synchrotron x-ray powder-diffraction data for NdMnAsO. Observed data are indicated by open circles, the fit by the solid line, and the difference curve is shown at the bottom. The Bragg markers are for NdMnAsO (top), MnAs (middle), and ${\text{Nd}}_2{\text{O}}_3$ (bottom).

Figure 2

(Color online) (a) Temperature dependence of the crystallographic $a$ and $c$ axes for NdMnAsO. (b) Temperature dependence of the unit-cell volume and the $c/a$ ratio for NdMnAsO.

Figure 3

(Color online) Temperature dependence of the ZFC magnetic $\left(\chi \right)$ and inverse magnetic $\left(1/\chi \right)$ susceptibilities for NdMnAsO. The solid line is a fit to the Curie-Weiss law. The insets show the field dependence of the magnetization at 2, 15, and 30 K, and a closeup of $\chi \left(T\right)$ at the Nd ordering transition.

Figure 4

(Color online) Temperature dependence of the electrical resistivity for NdMnAsO. The inset shows the field dependence of the normalized resistivity $R/R_0$ at 2, 15, 30, 100, and 250 K.

Figure 5

(Color online) Temperature dependence of the heat capacity $\left(C\right)$ in applied magnetic fields of 0 and 9 T. The inset shows a linear fit to $C/T$ versus $T^2$ at low temperatures.

Figure 6

(Color online) Rietveld fits to (a) 30 K and (b) 1.6 K neutron powder-diffraction data for NdMnAsO. The observed data are indicated by open circles, the fit by the solid line, and the difference curve is shown at the bottom. From top to bottom: Bragg markers are for NdMnAsO, the magnetic phase, ${\text{Nd}}_2{\text{O}}_3$ $\left[2.0\left(2\right)\text{wt}\mathrm{\%}\right]$ and MnAs $\left[1.0\left(2\right)\text{wt}\mathrm{\%}\right]$. (30 K: $R_{\text{wp}}=5.1\mathrm{\%}$, $R_{\text{p}}=3.2\mathrm{\%}$, and $R_{\text{F}}^2=3.4\mathrm{\%}$, 1.6 K: $R_{\text{wp}}=5.5\mathrm{\%}$, $R_{\text{p}}=3.5\mathrm{\%}$, and $R_{\text{F}}^2=5.1\mathrm{\%}$). The insets to (a) and (b) show graphic representations of the fitted magnetic structures. The temperature evolution of the fitted Nd and Mn moments between 1.6 and 50 K is given in (c). The inset shows the temperature evolution of the ordered Mn moment between 50–300 K. The solid line is a fit to $m=m_0{\left(1-T/T_{\text{N}}\right)}^{\beta }$ (see text). The unidentified reflections denoted by the asterisks do not change intensity between1.6 and 400 K.

Figure 7

(Color online) Schematic representation of the main magnetic interactions for NdMnAsO. Within the unit cell, the Nd and Mn ions lie on a diamond-shaped plane. The AF NN interactions $\left(J_1\right)$ between Mn ions are all satisfied, leading to G-type magnetic ordering. The symmetric Heisenberg-exchange interaction $\left(J_2\right)$ between the Nd and Mn sublattice is frustrated, leaving antisymmetric (Dzyaloshinsky-Moriya) exchange as the strongest magnetic interaction, and a postulated perpendicular orientation of the ordered Mn and Nd moments. Within the ${\text{Nd-O}}_2\text{-Nd}$ block the $J_3$ pathway is antiferromagnetic while $J_4$ is ferromagnetic. Magnetic interactions between the ${\text{As-Mn}}_2\text{-As}$ $\left({\text{Nd-O}}_2\text{-Nd}\right)$ blocks are expected to be much weaker due to the large separation between the blocks. The relative orientations of the ordered magnetic moments within each sublattice at 1.6 K are denoted by the $+/-$ signs. The labeling of the atoms corresponds to that used in Table .