Single-bilayer $E$-type antiferromagnetism in Mn-substituted Sr${}_3$Ru${}_2$O${}_7$: Neutron scattering study

Neutron scattering studies reveal an unusual $E$-type antiferromagnetic (AFM) structure in Mn-substituted single crystals of ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ ($x=0.16$). The material behaves as quasi-two-dimensional (2D) antiferromagnet with in-plane $\left(ab\right)$ long-range ordering and exhibits only single-bilayer (5–6 $\AA$) ferromagnetic correlations along the $c$ direction below $T_N=78$ K. However, the critical behavior of the staggered magnetization, the AFM order parameter, does not reflect the expected behavior of a 2D magnetic phase transition. Such an unusual magnetic structure deviates from what would be expected from the conventional spin-lattice coupling scenario in this class of materials.

Figure 1

(a) A schematic representation of the latest reported phase diagram for various substitution concentrations ($x$) of the Sr${}_3$(Ru${}_{1-x}$Mn${}_x$)${}_2$O${}_7$ series (unit cell representation shown in the inset) (Ref. 13). Region I is a paramagnetic metallic (PM-M) phase; region II is a paramagnetic insulating (PM-I) phase; region III is a metallic phase with AFM correlation (AFMC-M); and region IV is a long-range AFM insulating phase (LR-AFM-I). Neutron scattering studies were carried out in region IV. (b) $T$ dependence of integrated magnetic scattering intensity at chosen AFM Bragg peaks for the Sr${}_3$(Ru${}_{0.84}$Mn${}_{0.16}$)${}_2$O${}_7$ series.

Figure 2

Neutron diffraction patterns of SRMO16 in the $(H,K,0)$ and $(H,0,L)$ reciprocal planes for temperatures above and below $T_N$. The color bars represent the intensities in counts per 5 min and the powder rings observed are those of aluminum. The raw data reported on the top panel (on a logarithmic scale) show (a) the structural peaks at $T>T_N$ and (b) the combination of both structural and magnetic peaks at $T<T_N$. Peaks are indexed using the orthorhombic notation (see text). The differential diffraction patterns at $T=10$ K from $T=100$ K is shown in (c) and (d), where the studied magnetic Bragg peaks observed at the base $T$ are zoomed in for clarity.

Figure 3

Neutron diffraction profiles at magnetic Bragg peaks (a) $(0.5,0,0)$ in the $(H,K,0)$ and (b) $(1.5,0,0)$ in the $(H,0,L)$ scattering planes, respectively. The narrow (resolution limited) linewidth of the $H$ scans is indicative of long-range AFM ordering in the basal plane while the very broad Lorentzian profiles of $L$ scans indicate short-range AFM odering [see the inset in (b)] along the $c$ axis. The solid lines are a guide to the eye.

Figure 4

Schematic AFM spin configuration in the ground state of SRMO16 resulting from a magnetic refinement (see text). The single-bilayer magnetic structure of the (Ru/Mn)O${}_6$ octahedra is displayed where the zigzag chains of the $E$-type AFM order are marked with solid lines. Two inequivalent spins in the in-plane magnetic unit cell and associated octahedra are illustrated, while the top and bottom (Ru/Mn)O${}_6$ layer have an identical spin ordering structure.