(A) study on magnetic tunnel junctions with an amorphous CoFeB electrode

Abstract

The magnetoresistance effect of magnetic tunnel junctions has attracted significant interest in recent years for magneto-electronic applications such as magnetic read heads and magnetic random access memory elements. The tunneling current between two magnetic layers separated by a very thin insulating barrier depends on the relative orientation of the magnetization in the two adjacent magnetic layers. This is the origin of spin dependent tunneling. The tunnel probability of electrons through the insulating barrier depends on the product of the electron density of state of Fermi surface of both magnetic electrodes. When two magnetic layers have their magnetizations aligned parallel, the tunneling probability is high and the resistance of the system is low whereas when the magnetizations are antiferromagnetically aligned, the tunnel probability is low and the tunnel resistance is high. A current flow is perpendicular to the plane of the layers in magnetic tunnel junctions. This makes these junctions rather promising candidates for use as magnetic read heads and non-volatile memory elements. In this study, for high TMR ratio and thermal stability of magnetic tunnel junctions, amorphous CoFeB electrodes were used for the junctions. Their properties were investigated and compared with those of magnetic tunnel junctions with CoFe crystalline electrode. Also amorphous Al-oxide and crystalline MgO barrier were studied. The annealing effects on structural and magnetic properties of the magnetic tunnel junctions were investigated. In the first part, we have fabricated magnetic tunnel junctions, where the three kinds of bottom electrodes; amorphous CoFeB, CoFeB inserted CoFe/CoFeB/CoFe were used and these results were compared with that of crystalline CoFe. The tunnel barriers were formed by depositing 1.1 nm Al layers and oxidizing by a reactive method. The TMR of the plasma oxidized junctions increased with annealing temperature initially and decreased with a furth...