Geometric magnetic frustration in the hexagonal perovskite (La,Sr)_4−zRuO_7+δ

The hexagonal perovskite (La,Sr)_4−zRuO_7+δ exhibits a strong magnetic frustration due to the geometric arrangement of the paramagnetic Ru⁵⁺ ions, which form a network of edge-sharing elongated tetrahedra. In this magnetic sublattice, frustration may either be restricted to the crystallographic ab plane, in which the ruthenium ions form a triangle lattice, or concern the entire tetrahedra. Crystals of (La,Sr)_4−zRuO_7+δ were grown by the floating zone method, and their magnetic properties were measured with respect to the crystallographic axes. Magnetic ordering was only detected for the external magnetic field parallel to the ab plane, while for B‖c the samples showed no magnetic transition down to 2 K. Heat capacity measurements revealed a magnetic entropy corresponding to a spin of 1/2, a value much smaller than the S=3∕2 expected for the (t_2g)³ configuration of Ru⁵⁺ ions. From symmetry considerations, we conclude that only the orbitals corresponding to the irreducible representation A_1g mediate the cooperative interaction between the magnetic moments, giving rise to an orbitally ordered state.