Influence of annealing on the bias voltage dependence of tunneling magnetoresistance in MgO double-barrier magnetic tunnel junctions with CoFeB electrodes

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Low frequency noise has been measured in magnetic tunnel junctions with MgO barriers and magnetoresistance values up to 235%. The authors investigated the noise for different degrees of crystallization and CoFeB / MgO interface quality depending on the annealing temperature. The authors report an extremely low 1 / f noise, compared to magnetic junctions with Al 2 O 3 barriers. The origin of the low frequency noise is discussed and it is attributed to localized charge traps with the MgO barriers. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2749433͔The resistance of a magnetic tunnel junction ͑MTJ͒ depends on the relative magnetization of the ferromagnetic layers: it is usually low in the parallel configuration ͑PC͒ and high in the antiparallel configuration ͑APC͒. This effect is used for storage in magnetic random access memory and is now being adopted for read heads in place of giant magnetoresistance ͑GMR͒ sensors. The prediction 1,2 and recent observation 3 of tunnel magnetoresistance ͑TMR͒ ratios ͓͑R AP − R P ͒ / R AP ͔ as high as 1000% in MTJs with MgO barrier make those devices very attractive candidates to outclass GMR in high-resolution sensors. However, a high TMR ratio is not enough to make a good magnetoresistive sensor, since its performance is determined by the sensitivity which is the output voltage per unit field divided by the voltage noise.The noise level is as important as the magnitude of the signal itself. From earlier studies on MTJs with Al 2 O 3 barriers, the noise can be traced back to three sources. The first is the frequency-independent thermal noise, which is an intrinsic property of a tunnel junction that defines the ultimate noise limit. Here, we will focus on the two other contributions to the noise, which may be reduced or even eliminated provided the appropriate parameters are known. At low frequencies, noise is dominated by a 1 / f component, which is analogous to 1 / f noise in conductors. It is quantified by the Hooge-like parameter ␣ = AfS V ͑f͒ / V 2 , expressed in units of area; A is the surface area of the junction, S V ͑f͒ the noise power spectrum, and V the voltage drop across the tunnel barrier. Its origin is not fully understood, and, in particular, the influence of magnetization is still unclear although it is commonly observed that ␣ is higher in the APC than in the PC. 4,5 The quality of the metal-barrier interface may also play a role in the 1 / f noise. 4 The third component in MTJs is random telegraphic noise ͑RTN͒, namely, discrete fluctuators in the voltage-time traces associated with a Lorentzian-like spectrum. It is characterized by a strong sample-to-sample random of its amplitude and switching rate and it is interpreted either in terms of charge trapping or magnetic domain fluctuations. 6 In this letter, we discuss low frequency noise properties of MTJs with a MgO barrier which exhibit TMR ratios up to 235%. Our focus is on the annealing temperature, which is the crucial parameter for a high TMR ratio. We discuss the origins of the noise, based on c...

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Noise in MgO barrier magnetic tunnel junctions with CoFeB electrodes: Influence of annealing temperature

Scola

Polovy

Fermon

et al. 2007

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“…Well-oriented ͑001͒ MgO barrier layers were confirmed by x-ray diffraction. 9 After deposition of the stack, square-shaped junctions with an area from 12ϫ 12 to 24ϫ 24 m 2 were fabricated using conventional ultraviolet lithography. Typical resistance-area products were around 5 ϫ 10 6 ⍀ m 2 , which is close to that reported by Jiang et al 7 for barriers of the same thickness.…”

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High inverted tunneling magnetoresistance in MgO-based magnetic tunnel junctions

Feng

Coey

et al. 2007

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Inverted tunneling magnetoresistance, where resistance decreases as the free layer in a magnetic tunnel junction flips its direction of magnetization after saturation, has been observed at zero bias in magnetic tunnel junctions with a thin CoFeB layer in the pinned synthetic antiferromagnetic CoFe/ Ru/ CoFeB stack. Magnetoresistance values as high as −55% at room temperature are measured in MgO-based tunnel junctions when the thickness of the pinned CoFeB layer is 1.5 nm. The inverted magnetoresistance is associated with imbalance of the synthetic antiferromagnetic pinned layer. Asymmetric bias dependence with a magnetoresistance sign change is observed for a 0.5 nm pinned CoFeB layer. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2779241͔MgO-based magnetic tunnel junctions ͑MTJs͒ have transformed the prospects for spin electronic devices due to their remarkably high magnetoresistance at room temperature. [1][2][3][4] Tunneling magnetoresistance ͑TMR͒, defined as ͑R AP − R P ͒ / R P , can be as large as 360% in exchangebiased MTJs ͑Ref. 5͒ and 405% in unpinned pseudospinvalve MTJs. 6 Here R P and R AP are the resistances of the tunnel junctions when the magnetizations of the ferromagnetic electrodes in contact with tunnel barrier are parallel and antiparallel, respectively. High magnetoresistance in these devices is due to the spin filter effect of the crystalline MgO barrier, which is relatively transparent for majority spin electrons injected from an oriented bcc Fe or Fe-Co electrode but attenuates minority spin electrons, as result of the different symmetries of the ↑ and ↓ wave functions. Besides high TMR values, linear and hysteresis-free switching has also been observed in MgO-based MTJs when the thickness of the free CoFeB layer is less than 1.0 nm. 7 Moreover, an interfacial resonance state located in the minority band of Fe ͑001͒, which has been probed by spin-polarized tunneling in epitaxial Fe/ MgO / Fe MTJs, causes the TMR to change sign from positive to negative above a critical voltage. 8 Here we report the observation of a high inverted magnetoresistance when the free layer in MgO-based MTJs switches at zero bias. A switch in the sign of the resistance change near zero applied field occurs as a function of the thickness of the pinned CoFeB layer next to the MgO barrier in an MTJ stack with a synthetic antiferromagnet. The effect is not found when a single pinned layer is used.MTJs with a complete layer sequence Si/ SiO 2 ͑sub-strate͒ /Ta͑5͒ /Ru͑50͒ /Ta͑5͒ /Ni 81 Fe 19 ͑5͒ /Ir 22 Mn 78 ͑10͒ /Co 90 Fe 10 ͑2͒ / Ru͑0.85͒ / CoFeB͑t͒ / MgO͑2.5͒ / CoFeB͑3͒ /Ta͑5͒ / Ru͑5͒ were grown using a Shamrock sputtering tool, where the numbers in parentheses are layer thicknesses in nanometers. The thickness ͑t͒ of the pinned CoFeB ͑Co 40 Fe 40 B 20 ͒ layer was varied from 0 to 3.0 nm. The MgO was grown in a separate zone of our sputtering system, from a target-facingtarget source. Similar MTJs with AlO x barriers were also prepared for a comparative test. Well-oriented ͑001͒ MgO barrier layers were confir...

“…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.…”

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“…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.…”

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

Diao

Feng

et al. 2011

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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...