Magnetic and tunneling magnetoresistive properties of an all-oxide Fe3 O4 - Al2 O3 granular system

Abstract

A systematic study on the structural, magnetic, and transport properties of an all-oxide granular system consisting of Fe3 O4 grains dispersed in an insulating Al2 O3 matrix is presented. X-ray diffraction and transmission electron microscopy analyses provide the structure of nanosized Fe3 O4 grains embedded in an amorphous Al2 O3 matrix and confirm that the Fe3 O4 grain size decreases with increasing Al2 O3 volume fraction x. Magnetic measurements further show that the coercivity of the granular films decreases with increasing x due to decreasing Fe3 O4 grain size, with the magnetization curves exhibiting typical superparamagnetic characteristics for x>0.19. Temperature dependence of resistance and nonlinearity in current-voltage characteristics confirms that the spin dependent tunneling of conduction electrons across grains dominates the transport properties. This tunneling conductance depends on the relative orientation of magnetization between Fe3 O4 grains, which results in an isotropic granular tunneling magnetoresistance (TMR) effect. We also observed that the granular TMR ratio has strong temperature dependence and decreases with increasing temperature for all the granular films. © 2007 The American Physical Society.