A study on space charge dynamics in ultra thin aluminum-oxide barrier layer of TMR heads

With the increasing recording density and shrinking reader track width as well as shield-to-shield distance, the current-perpendicular-to-plane tunneling magnetoresistive (TMR) head is gaining ground in replacing the current-in-plane giant magneto-resistive (GMR) head for recording densities of 100 GB/in² and beyond. Typical TMRs for head application use ultra-thin Ti-oxide, Al-oxide, and MgO. This paper focuses on Al-oxide with a thickness of 5-10 Aring. Because of the ultra thin thickness of the barrier layer, barrier quality and reliability is of great interest. A simple experiment was conducted in which TMR head resistance was monitored at positive, negative, and alternating bias voltages. Polarity dependence was observed for some heads, but not all. Dynamic movement of space charge (accumulation/depletion) from defects such as under-oxidized Al-oxide, and the resulting non-uniform electric field distribution was proposed to explain the polarity dependence. The space charge density and its effective thickness were estimated from their effects on change of resistance under a simple model. It was found that the amount of space charge could be reduced by high temperature treatment (post-annealing) or during lapping stress-relaxation. This suggests the defects can be healed or confined to some extent, with less distortion from space charge, if Al is properly oxidized or if the barrier layer has less residual stress