Competing magnetic ground states in nonsuperconducting Ba(${\mathrm{Fe}}_{1-x}{\mathrm{Cr}}_x$)${}_2$${\mathrm{As}}_2$ as seen via neutron diffraction

We present neutron diffraction measurements on single-crystal samples of nonsuperconducting Ba(${\mathrm{Fe}}_{1-x}{\mathrm{Cr}}_x$)${}_2$${\mathrm{As}}_2$ as a function of Cr doping for $0⩽x⩽0.47$. The average spin-density-wave moment is independent of concentration for $x⩽0.2$ and decreases rapidly for $x⩾0.3$. For concentrations in excess of 30% chromium, we find a new competing magnetic phase consistent with G-type antiferromagnetism which rapidly becomes the dominant magnetic ground state. Strong magnetism is observed for all concentrations measured, naturally explaining the absence of superconductivity in the Cr-doped materials.

Figure 1

Ba(${\mathrm{Fe}}_{1-x}{\mathrm{Cr}}_x$)${}_2$${\mathrm{As}}_2$: $x=0.025$ [(a)–(c)], $x=0.125$ [(d)–(f)], $x=0.2$ [(g)–(i)], and $x=0.335$ [(j)–(l)]; and Ba(Fe${}_{0.96}$Co${}_{0.04}$)${}_2$${\mathrm{As}}_2$ [(m),(n)]. Top row: intensity of the (1/2,1/2,3)${}_T$ magnetic Bragg peak indicating the magnetic transition temperature $T_{\mathrm{SDW}}$. Middle row: TAS measurements of the intensity of the nuclear Bragg peak (2,2,0)${}_T$. The inflection point indicates the tetragonal to orthorhombic structural transition $T_S$. Bottom row: Gaussian full width at half maximum of the (2,2,0)${}_T$ nuclear Bragg peak measured on HB-3A, showing the structural transition $T_S$. Ba(Fe${}_{0.96}$Co${}_{0.04}){}_2$${\mathrm{As}}_2$ was not measured on HB-3A.

Figure 2

(a) $T$-$x$ phase diagram of Ba(${\mathrm{Fe}}_{1-x}{\mathrm{Cr}}_x$)${}_2$${\mathrm{As}}_2$ indicating the SDW magnetic and structural phase transitions. In addition, the phase boundary representing the transition from paramagnetic to G-type AFM order is shown. (b) The average ordered Fe/Cr moment as a function of $x$ for the SDW state. The dotted line represents the average moment for $x⩽0.2$.

Figure 3

Measurements of the (1,0,1)${}_T$ magnetic reflection representative of G-type magnetic order. Radial scans for temperatures above and below $T_N$ are shown for (a) $x=0.47$, (c) $x=0.335$, and (e) $x=0.305$. The temperature dependence of the integrated intensity of rocking curves through the (1,0,1)${}_T$ reflection is shown for (b) $x=0.47$, (d) $x=0.335$, and (f) $x=0.305$. Polarized neutron diffraction measurements of rocking curves through the (1,0,1)${}_T$ reflection for a crystal with $x=0.335$ is shown in both (g) non-spin-flip and (h) spin-flip configurations for temperatures above and below $T_N$. This demonstrates the magnetic nature of the additional intensity.