Magnetic, magnetoresistive and low-frequency noise properties of tunnel magnetoresistance sensor devices with amorphous CoFeBTa soft magnetic layers

Magnetic field sensors using the tunnel magnetoresistance (TMR) effect require linear resistance–magnetic field (R–H) response curves with small hysteresis, for which the soft magnetic property of the free layer (FL) is critical. In this work, we investigated amorphous CoFeBTa (CFBT) as a soft magnetic layer of the FL of CoFeB/MgO/CoFeB-based magnetic tunnel junctions in view of magnetic, TMR, and low-frequency noise properties. A two-step annealing process enabled an orthogonal magnetization configuration between the FL and the reference layer, by which linear R–H curves with small hysteresis were realized. The change in the shape of the R–H curve depending on annealing temperature is explained by the Stoner–Wohlfarth model. The highest TMR ratio of ∼160% and sensitivity of ∼70%/mT were obtained with a CFBT (20 nm)/Ta (0.3 nm)/CoFeB (3 nm) FL. The noise of the TMR devices are dominated by 1/f noise below ∼10 kHz in frequency, which limits the detectivity (D) of the magnetic field of the sensor. The sensor devices patterned to 50 µm diameter circular shapes showed a minimum D of ∼2 nT/$\sqrt{\text{Hz}}$ at 10 Hz, which is superior to the previously reported values for the TMR sensors with NiFe soft magnetic layers. (paper)