Effects of field cooling direction on magnetoresistance of exchange-biased magnetic tunnel junction rings

We report an effect of field cooling direction on the magnetoresistance of ring-shaped magnetic tunnel junctions consisting of Ta 5/Cu 20/ Ta 5/NiFe 2/Cu 5/MnIr 10/CoFe 4/Al-N 1.5/CoFe 4/NiFe 20/Ta 5-cap (thickness in nm). The magnetoresistances were both enhanced, in comparison to the as-fabricated ring device, with field-cooling directions applied in the film plane as well as perpendicular to the film plane but with greater increase in the case of perpendicular-field-cooled. Many cycles of planar- and perpendicular-field- cooled alternately gave the same results. The greater enhancement of magnetoresistance is shown to be due to the formation of an onion like magnetization configuration in the pinned layer after perpendicular-field- cooled, resulting in a better relative magnetization alignment between the pinned layer and the free layer in the minor loop region. Magnetic force microscopy (MFM) was undertaken to reveal the uniform and onion like magnetization configurations in the pinned layer of ring devices with the same dimensions, but with layer structures of Ta 5/NiFe 2/Cu 5/MnIr 10/CoFe 4/Cu 1/Ta 1 (thickness in nm) for the planar- and perpendicular-field-cooled, respectively. In addition, size-dependent behavior was explored and the results show that the formation of onion like magnetization after perpendicularfield- cooled is associated with the strong shape anisotropy in the narrower linewidths of ring devices.