Strong competition between orbital-ordering and itinerancy in a   frustrated spinel vanadate
  The crossover from localized- to itinerant-electron behavior is associated with many intriguing phenomena in condensed-matter physics. In this paper, we investigate the crossover from localized to itinerant regimes in the spinel system Mn$_{1-x}$Co$_x$V$_2$O$_4$. At low Co doping, orbital order (OO) of the localized electrons on the V3+ ions suppresses magnetic frustration by triggering a tetragonal distortion. With Co doping, electronic itinerancy melts the OO and suppresses the structural phase transition while the reduced spin-lattice coupling produces magnetic frustration. Neutron scattering measurements and first-principles-guided spin models reveal that the non-collinear state at high Co doping is produced by weakened local anisotropy and enhanced Co-V spin interactions. 
J. Ma, J. H. Lee, S. E. Hahn, Tao Hong, H. B. Cao, A. A. Aczel, Z. L.   Dun, M. B. Stone, W. Tian, Y. Qiu, J. R. D. Copley, ...
cond-mat.str-el