Persistence of antiferromagnetic order upon La substitution in the $4d^4$ Mott insulator Ca${}_2$RuO${}_4$

Abstract

The chemical and magnetic structures of the series of compounds Ca${}_{2-x}$La${}_x$RuO${}_4$ [$x=0$, $0.05(1)$, $0.07(1)$, $0.12(1)$] have been investigated using neutron diffraction and resonant elastic x-ray scattering. Upon La doping, the low temperature S-Pbca space group of the parent compound is retained in all insulating samples [$x\le 0.07(1)$], but with significant changes to the atomic positions within the unit cell. These changes can be characterised in terms of the local RuO${}_6$ octahedral coordination: with increasing doping the structure, crudely speaking, evolves from an orthorhombic unit cell with compressed octahedra to a quasi-tetragonal unit cell with elongated ones. The magnetic structure on the other hand, is found to be robust, with the basic $k=(0,0,0)$, $b$-axis antiferromagnetic order of the parent compound preserved below the critical La doping concentration of $x\approx 0.11$. The only effects of La doping on the magnetic structure are to suppress the A-centred mode, favouring the B mode instead, and to reduce the Néel temperature somewhat. Our results are discussed with reference to previous experimental reports on the effects of cation substitution on the $d^4$ Mott insulator Ca${}_2$RuO${}_4$, as well as with regard to theoretical studies on the evolution of its electronic and magnetic structure. In particular, our results rule out the presence of a proposed ferromagnetic phase, and suggest that the structural effects associated with La substitution play an important role in the physics of the system.

arXiv comment10 pages, 9 figures

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• Correlated electron metal properties of the honeycomb ruthenate Na${}_2$RuO${}_3$
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• Competition of three-dimensional magnetic phases in Ca${}_2$Ru${}_{1-x}$Fe${}_x$O${}_4$: A structural perspective
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