Detection of pinholes in magnetic tunnel junctions by magnetic coupling

Pinholes in tunnel barriers are detrimental to the performance of magnetic tunnel junctions (MTJs) since they create direct magnetic exchange coupling between the free and pinned magnetic films and may act as current short circuits. A simple and straightforward technique which enables observation of pinholes and distinguishes pinhole coupling from orange-peel coupling would aid greatly in optimizing the performance of MTJs. However, the existing methods for this determination are quite complex and destructive and do not work on complete structures. We have developed a simpler, nondestructive method that works on full MTJ structures which is able to identify whether an observed coupling arises primarily from magnetic exchange coupling through pinholes or from orange-peel coupling. The method is based on the shift in the free layer hysteresis loop at low temperatures. It is well known that the shift in the pinned layer loop at low temperatures is due to the sharp increase of the IrMn pinning strength. If pinholes exist, the free layer loop will also exhibit a shift due to direct exchange coupling. If there are no pinholes, no shift will be observed since orange-peel coupling is magnetostatic and cobalt has essentially no increase in magnetization below 300 K. In this way, a quick diagnosis can be made of whether or not pinholes exist in the MTJ.