Magnetic properties of the spin tetramer compound Ba₆la ₂Fe₄O₁₅

The crystal structure and magnetic properties for quaternary oxide Ba ₆La₂Fe₄O₁₅ have been investigated. This compound has a hexagonal structure in which four Fe ions per formula unit occupy the sites in one FeO₆ octahedron and three FeO₄ tetrahedra. These polyhedra form a larger unit Fe₄O₁₅ cluster by corner sharing. This cluster shows behavior characteristic of a magnetic tetramer with a ferrimagnetic ground state of the total spin S _T = 5 by the antiferromagnetic interaction between Fe³⁺ ions. From the magnetic susceptibility and specific heat measurements, we observed an antiferromagnetic transition at 12.8 K. The magnetic structure was determined by power neutron diffraction at 2.3 K, which indicates that the transition is due to the long-range antiferromagnetic ordering of the Fe ₄O₁₅ clusters. The ordered magnetic moment for each Fe³⁺ ion is determined to be ∼4μ_B. In addition, it is found that the antiferromagnetic structure changes to a ferromagnetic structure of clusters in an applied field of ∼2 T (at 1.8 K).