Competition of three-dimensional magnetic phases in Ca2Ru1−xFexO4 : A structural perspective

The crystalline and magnetic structures of ${\mathrm{Ca}}_2{\mathrm{Ru}}_{1-x}{\mathrm{Fe}}_x{\mathrm{O}}_4$ ($x=0.02$, 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($\mathrm{Fe}){\mathrm{O}}_6$ 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 $\mathrm{N}\acute{e}\mathrm{el}$ 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.