Transmission Electron Microscopy evidence of the growth of a magnetite layer acting as a spin filter in CoFe∕Al2O3∕CoFe magnetic tunnel junctions

Abstract

Magnetic tunnel junctions (MTJs) in which an iron oxide was deposited between the Al2O3 barrier and the pinned CoFe electrode were shown to exhibit a large tunneling magnetoresistance (TMR) value of 39% after annealing at 380°C. Local transmission electron microscopy (TEM) analyses were performed on the MTJs before and after the thermal treatments. In the as-deposited state, the high-resolution transmission electron microscopy (HRTEM) and the electron-energy-loss spectroscopy (EELS) demonstrate that the pure Fe exists at the Al2O3 interface, followed by the half-metallic ferromagnetic compound Fe3O4 that is formed when oxidizing the Fe layer. Similar structural studies show that the 380°C annealing achieves the oxidation of the iron into an extra Fe3O4 layer, now in contact with the Al2O3 barrier that leads to the high TMR values. These local TEM studies indicate that further annealing at higher temperature (400°C) induces the oxidation of the pinned CoFe electrode and the diffusion of Mn in the whole junction, resulting in a large decrease of the TMR.