A new effective method for thermal annealing of magnetic tunnel junctions in air with protective overlayers

Abstract

Thermal annealing is an important process to enhance greatly the tunneling magnetoresistance (TMR) of magnetic tunnel junctions (MTJs). MTJ annealing is conventionally carried out in vacuum below 10-5 Torr. However, this method involves the cost and complications of using a vacuum furnace. Pumping and venting a chamber are time-consuming; moreover, the sample temperature is difficult to measure accurately and therefore not easy to control. We have developed a method and an instrument to perform thermal annealing of MTJs in air. The method is based on protective overlayers, and the instrument has a simple structure composed of an air heat gun, thermocouple with feedback control, permanent magnets for magnetic field, and a sample holder. The influence of thermal annealing in air on MTJs properties was studied systematically on Al2 O3 MTJ samples. The samples are successfully protected from oxidation by using AuRu, or Al2 O3 films as protective overlayers. The Al2 O3 overlayer can be removed easily with NaOH solution. A MgO MTJ sample was annealed with this technique and its TMR increased from 17.5% to 141.3%. © 2008 U.S. Government.