Complex magnetic structure of YBaCo4O7

The new series of mixed-valent oxides RBaCo4O7 (R=Yb,Tb,Y) show complex structural and magnetic behavior. We have recently revealed, for the Yb analog, that a stuctural phase transition occurs in response to an extremely underbounded Ba2+ site. The symmetry lowering from orthorhombic to tetragonal, releases the frustration and allows the system to order magnetically below ~80K. Here we present our neutron diffraction study of the analog compound YBaCo4O7, that shows the same structural phase transition at high temperature and a magnetic transition at around 110K, where the system orders antiferromagnetically with propagation vector k=0. The magnetic structure, solved by global optimization algorithms, shows a non colinear Co-spins arrangement that results from the unique topology of the Co interactions. The magnetic structure is found to be strongly temperature dependent between 1.6K and 110K, which provide crucial information about the relative strengths of competing interactions.