Field detection in single and double barrier MgO magnetic tunnel junction sensors

Abstract

Single and double barrier MgO based magnetic tunnel junction sensors were processed with area of 200μm2 and small aspect ratios (<2). These sensors present resistance area products ranging from 10to70kΩμm2 and tunneling magnetoresistance (TMR) values of up to 160%. Sensor linearization was mostly obtained by thinning the CoFeB free layer to 15.5A, close to the free layer transition from ferromagnetic to superparamagnetic behavior. Three different thicknesses were studied for the free layer CoFeB: 1.55, 1.8, and 3nm. The CoFeB thickness decrease required for the linearization implies a loss in TMR. Field detection range of ∼80pT∕Hz0.5 was obtained for both double and single barrier sensors (at 500KHz). Out of the 1∕f regime, the detection range decreases to ∼2pT∕Hz0.5 for the single barrier sensor. The field detection dependence on bias voltage was also studied at 500KHz and outside the 1∕f dominated regime.

1 / f noise in MgO double-barrier magnetic tunnel junctions

“…Until now, a few reports have been published on the low frequency noise in DMTJs with a thick free layer 12,20 but their TMR ratio of around 80%-120% is considerably less than we report here. Earlier studies of MgO DMTJs did not produce high TMR because the middle CoFeB layer remains amorphous after annealing.…”

“…1,2 However, the TMR ratio in an MTJ falls off with increasing bias. 10 Double barrier MTJs ͑DMTJs͒ offer TMR of 105%-212% at room temperature [10][11][12][13] but bias dependence of TMR is reduced because the applied voltage is divided over two single barriers. This helps to preserve the high TMR ratio at high bias.…”

“…14 In conventional DMTJs with a thick free layer, TMR is often lower than that in SMTJs. [10][11][12][13] However, a DMTJ can potentially improve the signal-to-noise ratio of a magnetic field sensor due to the increase in output voltage compared to an SMTJ. Hence, the low frequency noise of DMTJs is worth exploring, in comparison with that of SMTJs.…”

“…Earlier studies of MgO DMTJs did not produce high TMR because the middle CoFeB layer remains amorphous after annealing. [10][11][12]20 The adjacent MgO barriers cannot absorb boron, which maintains the amorphous nature of CoFeB. 25 Here we investigate the low frequency noise in DMTJs, with TMR ratios as high as 222% for symmetric MgO layers and 250% for asymmetric MgO layers at room temperature.…”

“…22 Importantly, V 1/2 in both ␣ and TMR for DMTJs is approximately double that for SMTJs. 10,12 The noise-bias dependence was also measured for the DMTJ at -45 mT, where only the magnetization of the bottom pinned CoFeB layer switches. The ␣ value at -45 mT has a very similar variation with bias compared to that for SMTJs ͓Fig.…”