A study of head instabilities in TMR sensors using in high density perpendicular magnetic recoding systems

Abstract

We did an experimental study to examine head instabilities and noise of tunneling magnetoresistance (TMR) sensors using in high-density perpendicular recording systems. It found that the instability signatures on the transfer curves could be categorized into two groups; permanent magnet/free layer (PM/FL) failure and synthetic antiferromagnetic layers/antiferromagnetic layer (SAF/AFM) failure. The thermal-mechanical stress from the adaptive flying height (AFH) heater as well as head-disk interaction potentially degrades PM resulting in the reduction of the FL stabilizing field, therefore TMR ratio and signal increases. The non-uniformity of PM remnant magnetization and the switching of PM grains especially near the edge potentially degrade the biasing field. Scanning electron microscope images of these weak heads show rough surface and scratches close to the active area of read sensor. The deep scratches by particles in the head-disk interaction and shallow scratches by the slider lapping process is a potential cause to degrade PM. In another group, the thermal-mechanical stress from AFH heater potentially reduces the exchange-bias field and thermal stability of SAF/AFM layers allowing partial flipping of SAF edge magnetization, consequently it induces voltage fluctuations and hysteresis loops in the transfer curves.