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

Abstract

Low frequency noise has been investigated in MgO double-barrier magnetic tunnel junctions ͑DMTJs͒ with tunneling magnetoresistance ͑TMR͒ ratios up to 250% at room temperature. The noise shows a 1 / f frequency spectrum and the minimum of the noise magnitude parameter is 1.2ϫ 10 −10 m 2 in the parallel state for DMTJs annealed at 375°C. The bias dependence of noise and TMR suggests that DMTJs with MgO barriers can be useful for magnetic field sensor applications. © 2011 American Institute of Physics. ͓doi:10.1063/1.3562951͔The large tunneling magnetoresistance ͑TMR͒ in MgO barrier magnetic tunnel junctions ͑MTJ͒ has greatly improved the performance of spintronic devices, such as magnetic random access memories ͑MRAMs͒, sensors, and logic devices. [1][2][3][4][5][6][7][8][9] The record room temperature TMR of 604% is found in single-barrier MTJs ͑SMTJs͒ with a pseudo spin valve stack, 6 which is close to the theoretical maximum. 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 10-13 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. 10 A high bias is needed to inject a sufficiently large critical current to facilitate spin transfer torque ͑STT͒ switching in magnetic nanopillars with MgO barriers, which is the basis for high-speed non-volatile STT-MRAM. 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. It has been previously reported that 1 / f noise dominates the low frequency response of MTJs. [15][16][17][18][19][20][21][22][23] The 1 / f noise can be characterized by a noise magnitude parameter ␣ = AfS V / V 2 , where A is the junction area, f is the frequency, S V is the noise power spectrum density, and V is the applied bias. 15 Recently, Guerrero et al. 24 suggested that 1 / f noise can be greatly reduced in field sensors, when a large number of MTJs are connected either in series or in parallel. However, devices constructed with a large number of MTJs may lack integrity and suffer from a much higher chance of failure. DMTJs may be able to halve the 1 / f noise without these drawbacks, opening up a way to reduce the noise.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. [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...

Tunable linear magnetoresistance in MgO magnetic tunnel junction sensors using two pinned CoFeB electrodes

“…Obviously, the noise level of the ferromagnetic layers in the antiparallel state is much higher compared to that in the parallel state due to the magnetic fluctuation contribution. 13,30 In a field of À400 mT, the device is almost in the parallel state, but the noise magnitude increases on increasing the field, which is related to the spin flop configuration adapted by the SAF in a range of field around À200 mT. 22,23 As shown in Fig.…”

“…This is different from ordinary MTJs, where the noise level is almost independent of field in the range from À200 to À50 mT. 13,24 Usually, equilibrium magnetic 1/f noise exhibits a linear dependence on the field sensitivity range 25,26 (1/l 0 R)(d R/d H). The linear response plotted in Fig.…”

“…11,12 Here NiFe is used to induce a well crystallized IrMn bottom layer. 13 All MTJs were patterned by UV lithography and Ar ion milling to give circular (4 lm in diameter) or rectangular-shaped (4 Â 12 lm 2 ) junctions. The experimental results show almost no difference between two shapes of MTJs because of the weak pinning of the top CoFeB, and all data shown below are for circular junctions.…”

Very low 1/f barrier noise in sputtered MgO magnetic tunnel junctions with high tunneling magnetoresistance

“…The lowest a P value for A-MTJs is 2.1 Â 10 À11 lm 2 at T a ¼ 400 C, which is almost one order of magnitude lower than previously reported in sputtered MgO MTJs. [13][14][15] Furthermore, the lowest a P value for B-MTJs is even lower compared with A-MTJs, reaching 2.5-3.3 Â 10 À12 lm 2 at T a ¼ 400 C, which only appears under a large magnetic field of either polarity. The highest TMR for B-MTJs reaches 330% at T a ¼ 425 C. Both the high TMR and low barrier noise may reflect the quality of the MgO barriers and the CoFeB/MgO interfaces in our sputtered MTJs.…”

“…12 Until now, the barrier noise level for sputtered MgO MTJs has been reported to be 1-3 Â 10 À10 lm 2 at room temperature. 10,[13][14][15] Improving the quality of the MgO can result in a lower barrier noise level in MTJs due to a lower density of defects and less disorder. The best barrier noise has been achieved in MTJs with MgO barriers grown by MBE 16 or electron-beam evaporation, 11 where it reaches 1-3 Â 10 À11 lm 2 at 300 K. Moreover, at 4 K, a barrier noise level as low as 2.7À9.6 Â 10 À13 lm 2 in sputtered CoFeB/MgO/CoFeB MTJs 10 and 1-1.3 Â 10 À12 lm 2 in epitaxial Fe/MgAl 2 O x /Fe MTJs 17 has been reported.…”