Magnetic structural change of Sr${}_2$IrO${}_4$ upon Mn doping

The layered $5d$ transition-metal oxide Sr${}_2$IrO${}_4$ has been shown to host a novel $J_{\mathrm{eff}}=\frac{1}{2}$ Mott spin-orbit insulating state with antiferromagnetic ordering, leading to comparisons with the layered cuprates. Here we study the effect of substituting Mn for Ir in single crystals of Sr${}_2$Ir${}_{0.9}$Mn${}_{0.1}$O${}_4$ through an investigation involving bulk measurements and resonant x-ray and neutron scattering. We observe a new long-range magnetic structure emerge upon doping through a reordering of the spins from the basal plane to the $c$ axis with a reduced ordering temperature compared to Sr${}_2$IrO${}_4$. The strong enhancement of the magnetic x-ray scattering intensity at the $L_3$ edge relative to the $L_2$ edge indicates that the $J_{\mathrm{eff}}=\frac{1}{2}$ state is robust and capable of hosting a variety of ground states.

Figure 1

Field-cooled magnetization measurements on single crystals of Sr${}_2$Ir${}_{0.9}$Mn${}_{0.1}$O${}_4$ for applied fields parallel to the (a) $ab$ plane and (b) $c$ axis. Inset shows the isothermal magnetization from $-$7 to 7 T at 2 K. Zero-field resistivity measurements along the (c) $ab$ plane and (d) $c$ axis.

Figure 2

Elastic RXS of Sr${}_2$Ir${}_{0.9}$Mn${}_{0.1}$O${}_4$ at the Ir $L_3$ edge in the $\sigma -\pi$ mode. (a) and (b) $L$ scans at constant ($H,K$) reveal long-range magnetic order at ($10\text{odd}$) and ($01\text{odd}$) at 5 K. The same reflections are present at 80 and 120 K. (c) No magnetic reflections are observed at ($00L$), ($11L$), or ($0.50.5L$) positions at 5, 80, 120, 160, or 170 K. (d) The change in integrated intensity with temperature for two magnetic reflections.

Figure 3

Single-crystal neutron scattering measurements. (a) Rocking scans of the (103) magnetic reflection above and below the magnetic transition temperature. Similar scans were performed at the (101), (105), (107), and (109) magnetic reflections. (b) The normalized experimental intensity for the measured reflections is compared to instrument resolution and absorption corrected calculated scattering for both magnetic moments in the $ab$ plane and along the $c$ axis. (c) The long-ranged magnetic structure for Sr${}_2$Ir${}_{0.9}$Mn${}_{0.1}$O${}_4$.

Figure 4

RXS energy dependence of the iridium $L_3$ edge at 11.22 keV and the $L_2$ edge at 12.82 keV. These correspond to the inflection point at the low-energy side of the enhancement in the fluorescence scans. Measurements at both edges of Sr${}_2$Ir${}_{0.9}$Mn${}_{0.1}$O${}_4$ only yield appreciable resonant enhancement at the $L_3$ edge for $\sigma -\pi$ magnetic scattering. This behavior is a signature of the $J_{\mathrm{eff}}=\frac{1}{2}$ state.