Neutron scattering study of correlated phase behavior in Sr${}_2$IrO${}_4$

Neutron diffraction measurements exploring the magnetic and structural phase behaviors of the candidate $J_{\mathrm{eff}}=1/2$ Mott insulating iridate Sr${}_2$IrO${}_4$ are presented. Comparisons are drawn between the correlated magnetism in this single-layer system and its bilayer analog Sr${}_3$Ir${}_2$O${}_7$, where both materials exhibit magnetic domains originating from crystallographic twinning and comparable moment sizes. Weakly temperature-dependent superlattice peaks violating the reported tetragonal space group of Sr${}_2$IrO${}_4$ are observed, supporting the notion of a lower structural symmetry arising from a high-temperature lattice distortion, and we use this to argue that moments orient along a unique in-plane axis demonstrating an orthorhombic symmetry in the resulting spin structure. Our results demonstrate that the correlated spin order and structural phase behaviors in both single-layer and bilayer Sr${}_{n+1}$Ir${}_n$O${}_{3n+1}$ systems are remarkably similar and suggest comparable correlation strengths in each system.

Figure 1

Radial $Q$ scans both above and below $T_N$ through magnetic Bragg peaks at (a) $\mathbf{Q}=(1,0,2)$, (b) $\mathbf{Q}=(1,0,4)$, (c) $\mathbf{Q}=(1,0,6)$, and (d) $\mathbf{Q}=(1,0,8)$ positions. Solid black lines are Gaussian fits to the data.

Figure 2

(a) Radially integrated intensities [filled (black) squares] of magnetic Bragg peaks are plotted as a function of (1,0,$L$). Expected intensities from the two-domain magnetic model discussed in the text are overplotted as filled (red) triangles. (b) Magnetic order parameters squared plotted as a function of reduced temperature for Sr-214 (squares) and Sr-327 (circles). (c) In-plane projections of the model of canted AF order utilized in the calculation of the Sr-214 moment. Relative $c$-axis locations of each plane within the unit cell are denoted to the left of each corresponding plane.

Figure 3

Radial scans at 3 and 260 K through potential nuclear superlattice peaks violating the $I{4}_1/acd$ space group at (a) $\mathbf{Q}=(1,0,1)$, (b) $\mathbf{Q}=(1,0,5)$, and (c) $\mathbf{Q}=(1,0,7)$. (d) Intensity of the (1,0,5) superlattice peak as a function of temperature. The dashed line shows the background level for the (1,0,5) peak.

Figure 4

(a) $Q$ scan along the (0,0,$L$) direction showing a diffuse rod of scattering at both 3 and 260 K. Peaks at (0,0,4) and (0,0,8) are primary Bragg peaks. (b) $H$ scan through the (0,0,3.3) position. The shaded (green) box masks background powder scattering from the sample mount. (c) $H$ scans through the $L=3$, 5.3, and 9.3 positions. The dashed line denotes the fit background, and solid lines are Gaussian fits to the data. For the ($H$,0,3) peak, two Gaussian line shapes were fit to the data.