The crystal structures and magnetic properties of the quaternary lanthanide oxides Ba6Ln2Fe4O15 (Ln=Pr and Nd) are reported. They crystallize in a hexagonal structure with space group P63mc and have the "Fe4O15 cluster" consisting of one FeO6 octahedron and three FeO4 tetrahedra. Measurements of the magnetic susceptibility, specific heat, and powder neutron diffraction reveal that this cluster behaves as a spin tetramer with a ferrimagnetic ground state of ST=5 even at room temperature. The cluster moments show a long-range antiferromagnetic ordering at 23.2 K (Ln=Pr) and 17.8 K (Nd), and the magnetic moments of the Ln3+ ions also order cooperatively. By applying the magnetic field (∼2 T), this antiferromagnetic ordering of the clusters changes to a ferromagnetic one. This result indicates that there exists a competition in the magnetic interaction between the clusters.
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