Magnetic-ordering process in SrCr9(Ga-In)3O19

To investigate the anomalous magnetic properties of SrCr9–xGa3+xO19, we synthesized SrCr9–x(Ga0.95In0.05)3+xO19 compounds including the x=0 compounds and carried out magnetic and (mu)SR measurements. The results show clearly the insignificant role of the defects in the magnetic Cr lattice. Based on the results, a classical Heisenberg spin model, the plaquette-ordering model, is presented. Below 60 K, seven moments on a bi-tetrahedron plaquette correlate anti-ferromagnetically to give one moment, so that the Curie constant decreases to 1/9 at low temperatures. The surviving moments forming a triangular lattice do not exhibit long-range order due to frustration but the c-components of the moments are frozen randomly below Tf. The muoniums formed in the compound combine with the oxygen ions connecting Cr3+ ions and are subject to transferred hyperfine fields from Cr moments. This field is shielded by the electrons of the muonium so as to be hardly decoupled by external fields.