Antiferromagnetic ordering in the absence of structural distortion in $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Mn}}_x\right)}_2{\text{As}}_2$

Neutron and x-ray diffraction studies of $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Mn}}_x\right)}_2{\text{As}}_2$ for low doping concentrations $\left(x\le 0.176\right)$ reveal that at a critical concentration, $0.102<x<0.118$, the tetragonal-to-orthorhombic transition abruptly disappears whereas magnetic ordering with a propagation vector of $\left(\frac{1}{2}\frac{1}{2}1\right)$ persists. Among all of the iron arsenides this observation is unique to Mn doping, and unexpected because all models for “stripelike” antiferromagnetic order anticipate an attendant orthorhombic distortion due to magnetoelastic effects. We discuss these observations and their consequences in terms of previous studies of $\text{Ba}{\left({\text{Fe}}_{1-x}T{M}_x\right)}_2{\text{As}}_2$ compounds $\left(TM=\text{transition}\text{metal}\right)$, and models for magnetic ordering in the iron arsenide compounds.

Figure 1

(Color online) Resistance, normalized to the value at $T=300\text{K}$, and the temperature derivative of the resistance ratio for the $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Mn}}_x\right)}_2{\text{As}}_2$ samples with $x=0.074$, 0.102, and 0.118 samples. Lines are guides to the eyes.

Figure 2

(Color online) Neutron diffraction rocking scans through the (1/2 1/2 3) magnetic Bragg peak above (open squares) and below (filled circles) the AFM transition for (a) $\text{Ba}{\left({\text{Fe}}_{0.898}{\text{Mn}}_{0.102}\right)}_2{\text{As}}_2$ and (b) $\text{Ba}{\left({\text{Fe}}_{0.882}{\text{Mn}}_{0.118}\right)}_2{\text{As}}_2$. Panels (c) and (d) show scans along the [$\xi \xi 0$] direction through the (1 1 10) charge reflection above (open squares) and below (filled circles) the AFM transition for these samples. Note the splitting for the $x=0.102$ sample and its absence for $x=0.118$.

Figure 3

(Color online) Temperature dependence of the integrated intensities of the $\left(\frac{1}{2}\frac{1}{2}3\right)$ magnetic Bragg peak (filled circles) and the orthorhombic distortion (open circles) measured at the (1 1 10) charge peak positions $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Mn}}_x\right)}_2{\text{As}}_2$ with (a) $x=0.074$, (b) $x=0.102$, and (c) $x=0.118$. The insets to each panel show the temperature dependence of the broadening of the $\left(\frac{1}{2}\frac{1}{2}3\right)$ magnetic peak and the definition of $T^{\ast }$.

Figure 4

(Color online) (a) The compositional phase diagram for $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Mn}}_x\right)}_2{\text{As}}_2$ determined from neutron and x-ray diffraction measurements. Closed circles denote $T_N$ and open circles represent $T^{\ast }$ as described in the text. Crosses denote the temperature corresponding to minima of $\frac{dR}{dT}$ found in Fig. 1. The shaded region denotes the extent of the magnetic scattering above $T^{\ast }$. The vertical dashed line marks the approximate composition for the change from an orthorhombic to tetragonal structure. (b) The magnetic moment and structural distortion as a function of Mn-doping. The dashed line represents the value of the magnetic moment per Fe atom rather than Fe/Mn site as a function of Mn doping.