Crystal Structure and Magnetic Properties of SrCaMnGaO5+delta

The room-temperature crystal structure of the brownmillerite SrCaMnGaO5+delta(delta = 0.035) has been refined from neutron powder diffraction data; space group Ima2, a = 15.7817(6); b = 5.4925(2), c = 5.3196(2) angstroms. Mn and Ga occupy 99.0(2)% of the 6- and 4-coordinate sites, respectively. A combination of magnetometry, neutron diffraction and muSR spectroscopy has shown that the compound orders magnetically at 180 K, and that the low-temperature phase has a G-type antiferromagnetic structure, with an ordered magnetic moment of 3.30(2) muB per Mn at 2K. Displaced hysteresis loops provide evidence that the atomic moment has an additional, glassy component. Magnetometry shows that significant short-range magnetic interactions persist above 180 K, and muSR that the spin fluctuations are thermally activated in this temperature region. The compound is an electrical insulator which at 159K shows an unusually large magnetoresistance of 85% in 6T, increasing to 90% in 13 T.