Competition of three-dimensional magnetic phases in Ca$_2$Ru$_{1-x}$Fe$_x$O$_4$: A structural perspective

TL;DR

This study investigates how Fe-doping influences the structural and magnetic phases of Ca$_2$RuO$_4$, revealing a transition from A- to B-centered antiferromagnetic order driven by orbital degrees of freedom.

Contribution

It provides a detailed structural and magnetic phase analysis of Fe-doped Ca$_2$RuO$_4$, highlighting the role of orbital degrees of freedom in magnetic phase competition.

Findings

Fe-doping reduces Ru-O bond lengths and octahedral distortion.

B-centered magnetic phase replaces A-centered phase with increased doping.

Orbital degrees of freedom significantly influence magnetic phase transitions.

Abstract

The crystalline and magnetic structures of Ca$_2$Ru$_{1-x}$Fe$_x$O$_4$ (x=0.02, 0.05, 0.08 and 0.12) have been studied using neutron and X-ray diffraction. The Fe-doping reduces the Ru-O bond length in both apical and planar directions. The smaller Ru(Fe)O$_6$ octahedron leads to its reduced distortion. The $Pbca$ space group is maintained in all the Fe-dopings, so is the octahedral flattening. Warming has a similar effect on the lattice to that of the Fe-doping in releasing the distorted octahedra but precipitates an abrupt octahedral elongation near the N$\acute{e}$el temperature. Two competing antiferromagnetic orders, $A$- and $B$-centered phases have been observed. The Fe-doping-relaxed crystal structure prefers the latter to the former. As the doping increases, the $B$-centered phase continuously grows at the cost of the $A$-centered one and eventually replaces it at x=0.12. The absence of the two-dimensional antiferromagnetic critical fluctuations above the magnetic transition temperature and the three-dimensional magnetic correlation below the transition, together with the anomalous lattice response, point to an important role of orbital degree of freedom in driving the magnetic phase competition.