Ultra-sensitive tunnel magneto-resistive sensor with nanoscale pinned layer on magnetic vortex free layer

Abstract We demonstrated extremely high sensitivity in a tunnel magneto-resistive (TMR) sensor utilizing a free layer with a magnetic vortex structure having a small aspect ratio, and a pinned layer with a small diameter below 100 nm. A high TMR ratio above 200% was achieved in the magnetic tunnel junctions using a CoFeSiB/Ta/CoFeB free layer. Moreover, by decreasing the thickness of the free layer disk, thereby its aspect ratio, the saturation magnetic field was lowered to approximately 100 Oe. In addition, by miniaturizing the size of the pinned layer, the effective saturation magnetic field H s , at which the magnetic vortex core moves to the outside of the pinned layer, was significantly reduced to 0.3 Oe. Thanks to the high TMR ratio and low H s , the maximum sensitivity of 268%/Oe within a magnetic field range of −0.3 Oe < H x < 0.3 Oe was successfully obtained in a vortex-type TMR sensor with a free layer with thickness of 70 nm and diameter of 10 μ m and a pinned layer of 80 nm. In addition, the small coercivity was observed in the minor magneto-conductance curves due to the reversible motion of the vortex core, reflecting the intrinsically hysteresis-free nature of the vortex structure. We realized both a substantial enhancement in sensitivity and a significant reduction in magnetic hysteresis for the vortex-type TMR sensors. The dramatic improvement in the performance of the TMR sensors will greatly contribute to magnetic sensing applications in biomedical and industrial fields.