Increased Tunneling Magnetoresistance Using Normally bcc CoFe Alloy Electrodes Made Amorphous without Glass Forming Additives

Abstract

Using cross-section transmission electron microscopy we show that films of CoFe alloys, sandwiched between two conventional amorphous materials, are amorphous when less than approximately 25-30 A thick. When these amorphous layers are integrated into magnetic tunnel junctions with amorphous alumina tunnel barriers, significantly higher tunneling magnetoresistance is found compared to when these layers are made crystalline (e.g., by heating or by thickening them). We postulate that this is likely due to changes in interfacial bonding at the alumina-CoFe interface. Indeed, x-ray emission spectroscopy shows a significant increase in the Fe, but not the Co, 3d density of states at the Fermi energy for thin amorphous CoFe layers.