Field detection in MgO magnetic tunnel junctions with superparamagnetic free layer and magnetic flux concentrators

Magnetic tunnel junctions (MTJs) were deposited with CoFeB free layer thickness ranging from 1.3 to 3 nm. The samples were processed with areas of 10, 100, and 1000 μm2, presented RA products of ∼10 kΩ μm2, and tunneling magnetoresistance ratio values varying from ∼10% (t=1.3 nm) to ∼210% (t=3 nm). All the samples with t<1.5 nm presented linear responses (coercivity of <1 Oe) for all the studied areas; this behavior was associated with the CoFeB free layer transition from ferromagnetic to superparamagnetic. Noise measurements made in samples with superparamagnetic free layer showed negligible magnetic noise in the sensitive region. The sensitivity loss caused by the reduced free layer thickness was partially recovered using magnetic flux concentrators (MFCs). The MFC had a maximum gain of ∼3, a limited value to ensure a uniform gain for the MTJs with larger area (up to 1000 μm2). The MFC’s influence on the sensor’s noise behavior appears to increase with the sensor area, for both white noise background and frequency dependent noise.