Mechanism of grain-boundary magnetoresistance in Fe_3O_4 films

摘要

The magnetotransport properties of magnetite films with different microstructures were investigated in order to identify prerequisites for the attainment of a large tunnelling magnetoresistance in polycrystalline samples. Epitaxial films on MgAl_2O_4, polycrystalline films on Al_2O_2 and rough MgAl_2O_4 substrates and a polycrystalline La_(0.7)Ca_(0.3)MnO_3 film on MgO were compared. Although grain boundaries induce a large high-field magnetoresistance in magnetite films, the low-field magnetoresistance characteristic for spin-polarized tunnelling was virtually absent in these samples. Two factors might be responsible for this behaviour: (1) grain boundaries in magnetite are conducting and do not form tunnelling barriers and (2) the spin-polarization near grain boundaries is suppressed due to non-stoichiometry.