Controlling domain configuration of the sensing layer for magnetic tunneling junctions by using exchange bias

The control of magnetic domain formation and fluctuations in the sensing layer is important to progress for low noise in magnetic tunnel junction sensors. We studied the effect of exchange bias on the domain structure in micro-patterned Permalloy (Py: Ni80Fe20) sensing layer. We deposited single Py films, and Pt48Mn52/Py films, where the latter showed exchange bias. By controlling the thickness of Py, Pt48Mn52 (15nm)/Py (t=235 nm) showed a small coercivity and exchange bias of 7 Oe. After micro-fabrication into circular pillars 80 µm in diameter, we measured the domain structure by Magneto Optical Kerr Effect (MOKE) microscopy. MOKE images showed that single Py pillars have a simple closure domain, where the domain wall at the center moved with the applied field. The exchange-biased Py pillars exhibited a more complicated structure, but with fixed domains at the center region due to the exchange bias overcoming the magnetostatic energy. The uniform rotation of magnetization at the center of the sample is promising for decreasing the domain hopping magnetic noise.