Anomalous $E-$type Antiferromagnetism in the ground state of Mn-substituted Sr$_{3}$Ru$_{2}$O$_{7}$*

The bi-layer perovskite, Sr$_{3}$Ru$_{2}$O$_{7}$, has sparked a lot of interest because of the quantum critical behavior---related to a metamagnetic (magnetic field-tuned) phase transition. One of the key issues is related to the magnetism in the system. Here we report an investigation of the effects on magnetism resulting from chemical substitution in this compound. Our neutron scattering investigation reveals an unusual $E-$type antiferromagnetic (AFM) structure induced by Mn-substitution in the ground state of Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$ ($x$ = 0.16). The AFM structure exhibits a long-range order in \textit{ab}-plane but almost only a single bilayer-thickness correlation along the $c$-direction, thus characterizing the system as a quasi-two-dimensional antiferromagnet while the AFM order parameter shows almost three-dimensional-like scaling character as $T$ approaches $T_{N}$ ($\sim $ 82 K). The magnetic moments are aligned along the c-axis with an upper limit of $\sim $ 0.70 \textit{$\mu $}$_{B}$/Ru site. The induced AFM order most likely results from the enhancement of super-exchange interactions rather than from structural distortions or from freezing of electronic instabilities due to the nesting character of Fermi surface in the parent compound.