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

TL;DR

This study investigates how La substitution affects the structural and magnetic properties of Ca$_2$RuO$_4$, revealing that antiferromagnetic order persists despite structural changes, with La doping mainly influencing magnetic modes and transition temperatures.

Contribution

It provides detailed experimental insights into the robustness of antiferromagnetic order in Ca$_2$RuO$_4$ upon La doping and clarifies the role of structural effects in its magnetic behavior.

Findings

Antiferromagnetic order persists up to x≈0.11 La doping.

La doping suppresses the A-centred magnetic mode.

Néel temperature decreases with La substitution.

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 [$x\leq0.07(1)$], 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 $x\approx0.11$. 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 N\'{e}el 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.