Magnetic phase tuning in Ca$_2$Ru$_{1-x}$Fe$_x$O$_4$: A structural perspective

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 scattering and X-ray diffraction. The Fe-doping reduces the Ru-O bond length in both apical and planar directions. The smaller Ru(Fe)O$_6$ octahedra leads to its reduced distortion. The $Pbca$ space group is maintained and so is the apical flattening. Warming has similar effect as Fe-doping in releasing the distorted octahedra except an abrupt change of flattening across the Neel temperature in x=0.08. Two types of antiferromagnetic orders, $A$- and $B$-centered phases with the same inplane spin arrangement but different stack sequences, have been found. The Fe induced structure with relaxed octahedra seems to prefer the $B$-phase to the $A$-centered one, which is systematically overwhelmed and replaced as the Fe-doping increases. The nature of the magnetic phase competition and its connection to its host structure is discussed.