The Role of Grain Boundaries in Determining the Transport Properties in   Magnetite
  We present magnetoresistance and Hall-effect measurements on single crystal magnetite Fe3O4 close to the Verwey transition Tv = 123.8 K. We show that the formation of grain boundaries accompanying the reduction in crystal symmetry plays a significant role in the electron scattering mechanism, and that grain boundaries can account for the apparent change in sign of the charge carriers below Tv. 
David C. Mertens (1), W. Montfrooij (1), R. J. McQueeney (2), M.   Yethiraj (3), J. M. Honig (4) ((1) Department of Phhysics...