MgO-based Tunnel Junctions Research Articles

Extrinsic tunnel Hall effect in MgO-based tunnel junctions

The Hall effect that occurs when current flows through a CoFeB/MgO/Pt tunnel junction is investigated. It is shown that the transverse voltage in Pt electrode is nonlinear on a DC voltage applied to the tunnel junction. It has both linear (odd) and quadratic (even) parts. The linear part contains well-known contributions of the anomalous Hall effect in the ferromagnetic electrode, inverse spin-hall effect in platinum and others. The quadratic part is a phenomenon caused by the spin-orbit scattering of electrons in an external electric field induced by a voltage applied to the barrier. This field reaches values of $10^9$ V/m which is close to internal atomic fields. The magnitude of both effects decreases as thickness of Pt electrode is increased due to shunting effects.

Giant magnetoresistance in antiferromagnetic Mn2Au-based tunnel junction

Recent studies on the electrical switching of tetragonal antiferromagnet (AFM) via Neel spin-orbit torque have paved the way for the economic use of antiferromagnetic materials. The most difficult obstacle that presently limits the application of antiferromagnetic materials in spintronics, especially in memory storage applications, could be the small and fragile magnetoresistance (MR) in the AFM-based nanostructure. In this study, we investigated the spin transports in Mn$_2$Au-based tunnel junctions based on the first-principle scattering theory. Giant MRs more than $1000%$ are predicted in some Fe/MgO/Ag/Mn$_2$Au/Ta junctions that are about the same order as that in an MgO-based ferromagnetic tunnel junction with same barrier thickness. The interplay of the spin filtering effect, the quantum well resonant states, and the interfacial resonant states could be responsible for the unusual giant and robust MRs observed in these Mn$_2$Au-based junctions.

Seeking large thermoelectric effects in MgO-based tunnel junctions

There is much controversy concerning thermoelectric effects in the MgO-based magnetic tunnel junctions (MTJs) as reported in some recent publications. To clarify the problem, we give calculations from atomic first-principles systemically. Large Seebeck coefficient (S) and up-limit of figure of merit (ZT) are predicted in double- and multi-barrier MTJs, with those in single-barrier MTJs being relatively small. By restraining the phonon thermal conductance through the introduction of one vacuum barrier or numbers of MgO barriers, the up-limit of ZT can be obtained. Room temperature and ZT are predicted in an asymmetric Fe MgO Fe Vaccum Fe MTJs. The resonant quantum-well states are suggested to be responsible for the enhanced thermoelectric effects in the MTJs with double- and multi-barrier.

Enhanced thermal spin transfer in MgO-based double-barrier tunnel junctions

Based on atomic first principles, we predict enhanced thermal spin transfer (TST) effects and small switching temperature gradient in Fe MgO Fe MgO Fe double-barrier magnetic tunnel junctions (MTJs). At room temperature, temperature gradient ∼10 with ∼10 K nm−1 across barriers would be sufficient to switch the magnetic configurations circularly in a junction with 3 MgO atomic layers (L), which is about one order smaller than that in Fe MgO(3L) Fe MTJs. This temperature gradient is under the current experimental capability. The resonant quantum-well states in companion with resonant interfacial states are responsible for the enhancement. Moreover, a thermal induced ‘off’ state is found in a double-barrier MTJ.

Large Voltage-Induced Changes in the Perpendicular Magnetic Anisotropy of an MgO-Based Tunnel Junction with an Ultrathin Fe Layer

Voltage control of magnetic anisotropy (VCMA) in an ultrathin ferromagnet metal layer is a promising approach for ultralow-power spin manipulation, but interface effects spoil its efficiency for applications such as memory devices. The authors study VCMA in the fully epitaxial ultrathin Fe layer of a magnetic tunnel junction, and achieve an efficiency of almost 300 fJ V${}^{-1}$ m${}^{-1}$ with high perpendicular anisotropy, which satisfies the requirements for gigabit-class spintronic memory.

Thin Co/Ni-based bottom pinned spin-transfer torque magnetic random access memory stacks with high annealing tolerance

Spin-transfer torque magnetic random access memory (STT-MRAM) is considered as a replacement for next generation embedded and stand-alone memory applications. One of the main challenges in the STT-MRAM stack development is the compatibility of the stack with CMOS process flows in which thermal budgets up to 400 °C are applied. In this letter, we report on a perpendicularly magnetized MgO-based tunnel junction (p-MTJ) on a thin Co/Ni perpendicular synthetic antiferromagnetic layer with high annealing tolerance. Tunnel magneto resistance (TMR) loss after annealing occurs when the reference layer loses its perpendicular magnetic anisotropy due to reduction of the CoFeB/MgO interfacial anisotropy. A stable Co/Ni based p-MTJ stack with TMR values of 130% at resistance-area products of 9 Ω μm2 after 400 °C anneal is achieved via moment control of the Co/Ta/CoFeB reference layer. Thinning of the CoFeB polarizing layer down to 0.8 nm is the key enabler to achieve 400 °C compatibility with limited TMR loss. Thinning the Co below 0.6 nm leads to a loss of the antiferromagnetic interlayer exchange coupling strength through Ru. Insight into the thickness and moment engineering of the reference layer is displayed to obtain the best magnetic properties and high thermal stability for thin Co/Ni SAF-based STT-MRAM stacks.

BEOL compatible high tunnel magneto resistance perpendicular magnetic tunnel junctions using a sacrificial Mg layer as CoFeB free layer cap

Perpendicularly magnetized MgO-based tunnel junctions are envisaged for future generation spin-torque transfer magnetoresistive random access memory devices. Achieving a high tunnel magneto resistance and preserving it together with the perpendicular magnetic anisotropy during BEOL CMOS processing are key challenges to overcome. The industry standard technique to deposit the CoFeB/MgO/CoFeB tunnel junctions is physical vapor deposition. In this letter, we report on the use of an ultrathin Mg layer as free layer cap to protect the CoFeB free layer from sputtering induced damage during the Ta electrode deposition. When Ta is deposited directly on CoFeB, a fraction of the surface of the CoFeB is sputtered even when Ta is deposited with very low deposition rates. When depositing a thin Mg layer prior to Ta deposition, the sputtering of CoFeB is prevented. The ultra-thin Mg layer is sputtered completely after Ta deposition. Therefore, the Mg acts as a sacrificial layer that protects the CoFeB from sputter-induced damage during the Ta deposition. The Ta-capped CoFeB free layer using the sacrificial Mg interlayer has significantly better electrical and magnetic properties than the equivalent stack without protective layer. We demonstrate a tunnel magneto resistance increase up to 30% in bottom pinned magnetic tunnel junctions and tunnel magneto resistance values of 160% at resistance area product of 5 Ω.μm2. Moreover, the free layer maintains perpendicular magnetic anisotropy after 400 °C annealing.

Development of perpendicularly magnetized Ta|CoFeB|MgO-based tunnel junctions at IBM (invited)

The discovery of perpendicular magnetic anisotropy (PMA) in Ta|CoFeB|MgO and the subsequent development of perpendicularly magnetized tunnel junctions at IBM is reviewed. The fast-turn-around method used for screening materials for interface PMA by measuring the moment/area and anisotropy field of in-plane materials as a function of CoFeB thickness is presented, including the data as a function of seed-layer material which led to the discovery of PMA in Ta|CoFeB|MgO. Magnetic and electrical data are reported for the first PMA magnetic tunnel junction we made using this material. By inserting a thin Fe layer at the Ta|CoFeB interface, a substantial increase in the PMA energy density was obtained. Pure Fe layers (which required the use of a TaMg seed) greatly improved the thermal stability, allowing annealing up to 400 °C.

Optimization of tunneling magnetoresistance of MgO based tunnel junctions by tuning the stage impedance for radio frequency sputtering of the barrier

In this article, we describe a method to maximize the tunneling magnetoresistance of magnetic tunnel junctions with MgO tunneling barrier by optimizing the stage impedance for the RF MgO deposition. The impedance can be varied continuously using a matchbox that is in place to apply a RF bias to the stage. It is measured by means of a network analyzer connected to a dummy wafer on the stage. For positive stage reactance, both resistance area product and tunneling magnetoresistance are observed to drop, related to the stage impedance resonating with the plasma sheath capacitance. At high negative stage reactance, resputtering is minimized and tunneling magnetoresistance maximized.

Ab initiostudies of the tunneling magneto-Seebeck effect: Influence of magnetic material

We found a strong influence of the composition of the magnetic material on the temperature dependence of the tunneling magneto-Seebeck effect in MgO-based tunnel junctions. We use ab initio alloy theory to consider different Fe${}_{x}$Co${}_{1\ensuremath{-}x}$ alloys for the ferromagnetic layer. Even a small change of the composition leads to strong changes in the magnitude or even in the sign of the tunneling magneto-Seebeck effect. This can explain differences between recent experimental results. In addition, changing the barrier thickness from six to ten monolayers of MgO leads to a nontrivial change of the temperature dependence. Our results emphasize that the tunneling magneto-Seebeck effect depends very crucially on and is very sensitive to material parameters and show that further experimental and theoretical investigations are necessary.

A study of the effect of iron island morphology and interface oxidation on the magnetic hysteresis of Fe-MgO (001) thin film composites

Fe-MgO tunnel junctions have received much attention for their use in hard drive read heads and other spintronic applications. The system is particularly interesting because of its magnetoresistive behavior and the abundance and low cost of its constituent elements. However, many questions remain about how the structure and chemistry of the Fe-MgO interface mediates magnetic behavior. In this study, we report on transmission electron microscopy, electron energy loss spectroscopy, and magnetic characterization of Fe-MgO composite films with various morphologies. We explore relationships between film morphology, intermixing, and the resulting effects on magnetic structure. We find the presence of oxidation at the Fe-MgO interface, with a detrimental impact on the saturation magnetization of the composite. We also observe changes in coercivity and magnetocrystalline anisotropy with film morphology and thickness. These results will inform the design of MgO-based tunnel junctions and improve our understanding of how processing conditions, resulting in morphological and chemical changes such as oxidation, affect magnetization.

Influence of interface termination on the magneto-Seebeck effect in MgO based tunnel junctions

On an ab initio level, we investigate the recently observed magneto-Seebeck effect in MgO based tunnel junctions. In particular, we considered ordered CoFe alloys as lead material. Next to the MgO barrier, there are different possible terminations of the CoFe alloy due to the assumed ordered alloy. These results show a strong influence of the termination on the temperature dependence of the magneto-Seebeck effect. In addition, we use a simple model to account for randomly ordered alloys. We propose, that by a controlled treatment of the CoFe/MgO interface the magneto-Seebeck effect can be tuned experimentally.

Spin-polarized tunneling in MgO-based tunnel junctions with superconducting electrodes

We prepared magnetic tunnel junctions with one ferromagnetic and one superconducting Al–Si electrode. Pure cobalt electrodes were compared with a Co–Fe–B alloy and the Heusler compound Co2FeAl. The polarization of the tunneling electrons was determined using the Maki–Fulde model and is discussed along with the spin–orbit scattering and the total pair-breaking parameters. The junctions were post-annealed at different temperatures to investigate the symmetry filtering mechanism responsible for the giant tunneling magnetoresistance ratios in Co–Fe–B/MgO/Co–Fe–B junctions.

Spin caloritronics in magnetic tunnel junctions:Ab initiostudies

This Letter presents ab initio calculations of the magneto-thermoelectric power (MTEP) and of the spin-Seebeck coefficient in MgO based tunnel junctions with Fe and Co leads. In addition, the normal thermopower is calculated and gives for pure Fe and Co an quantitative agreement with experiments. Consequently, the calculated values in tunnel junctions are a good estimation of upper limits. In particular, spin-Seebeck coefficients of more than 100 \mu V/K are possible. The MTEP ratio exceed several 1000% and depends strongly on temperature. In the case of Fe leads the MTEP ratio diverges even to infinity at certain temperatures. The spin-Seebeck coefficient as a function of temperature shows a non-trivial dependence. For Fe/MgO/Fe even the sign of the coefficient changes with temperature.

Random barrier double-well model for resistive switching in tunnel barriers

The resistive switching phenomenon in MgO-based tunnel junctions is attributed to the effect of charged defects inside the barrier. The presence of electron traps in the MgO barrier, which can be filled and emptied, locally modifies the conductance of the barrier and leads to the resistive switching effects. A double-well model for trapped electrons in MgO is introduced to theoretically describe this phenomenon. Including the statistical distribution of potential barrier heights for these traps leads to a power-law dependence of the resistance as a function of time, under a constant bias voltage. This model also predicts a power-law relation of the hysteresis as a function of the voltage sweep frequency. Experimental transport results strongly support this model and in particular confirm the expected power laws dependencies of resistance. They moreover indicate that the exponent of these power laws varies with temperature as theoretically predicted.

Frequency transition of spin-torque oscillator under the magnetic-field pulse in nanosecond range

We report a time domain study of the frequency transition of spin-torque oscillator (STO) under the magnetic-field pulse in nanosecond range. We fabricated the pillar-structured STO devices consisting of MgO-based tunnel junctions with CoFeB free layers. Single-shot waveforms of the STO were obtained using a real-time oscilloscope (40 GS/sec). First, we measured current dependence of the waveform to investigate the time-domain stability of the oscillation. With the increase in the dc current applied to the STO, the oscillation state changed continuously in the following order: thermal fluctuation, intermittent unsteady oscillation, steady oscillation, and chaotic oscillation. Next, we measured the response of the STO to the magnetic-field pulse with a rise time of 0.5 ns, a duration time of 10 ns, and an amplitude of 60 Oe. In this measurement, the oscillation state was kept in the above-mentioned steady state with the frequency ∼3.5 GHz and the spectral linewidth ∼50 MHz. In the presence of the magnetic-field pulse, the oscillation frequency shifted around 150 MHz and the frequency transition occurred in less than 2 ns.

Reproducible trajectory on subnanosecond spin-torque magnetization switching under a zero-bias field for MgO-based ferromagnetic tunnel junctions

One of the features of spin-transfer torque (STT)-based magnetic random access memories (spin-RAMs) is a fast write cycle; however, switching properties in the subnanosecond regime for MgO-based magnetic tunnel junctions (MTJs) are still unclear. In this work, we demonstrated subnanosecond magnetization switching by STT for MgO-based MTJs. We also discuss the thermal effect on subnanosecond STT switching, as well as the subnanosecond pulse width that is dependent on the remarkable plateau that switching probability has under a zero-bias field.

Origin of the spectral linewidth in nonlinear spin-transfer oscillators based on MgO tunnel junctions

We demonstrate the strong impact of the oscillator nonlinearity on the line broadening by studying spin-transfer-induced microwave emission in MgO-based tunnel junctions as a function of both the injected dc current and the temperature. In addition, we give clear evidences that the intrinsic noise is not dominated by thermal fluctuations but rather by the chaotization of the magnetic system induced by the spin transfer torque. A consequence is that the spectral linewidth is almost not reduced in decreasing the temperature.

Electroforming, magnetic and resistive switching in MgO-based tunnel junctions

Magnetic tunnel junctions (MTJs) are under investigation since they offer great potential for applications in magnetic memories. An interesting effect in TJs concerns non-volatile resistive switching of non-magnetic origin. We report magnetic (magnetoresistance) and structural (resistive; R) switching in MgO-based MTJs (barrier thicknesses t = 0.75, 1.35 nm). As-grown MTJs display R-switching only in the thinnest series, while thicker barrier samples need an electroforming step for R-switching to occur. Forming changes the electrical resistance temperature dependence, from tunnel- to metallic-like, revealing the formation of conductive bridges across the barrier which, leading to local high electrical fields and temperatures, are essential for resistive switching.

Electrical time-domain observation of magnetization switching induced by spin transfer in magnetic nanostructures (invited)

We have measured the distribution of switching times in spin-transfer switching induced by fast current pulses in two pillar-shaped systems: (i) spin valves and (ii) MgO-based magnetic tunnel junctions. (i) Spin valves can sustain high currents, such that the application of pulsed currents of amplitude a few times that of the static switching threshold is possible. This makes subnanosecond switching within reach. In that limit, the pulse durations leading to switching follow a multiply stepped distribution at 300K and a regular distribution at 40K. At 300K, this reflects the precessional nature of the switching, which proceeds through a small number of precession cycles. The switching time distribution can be modeled from the thermal variance of the initial magnetization orientations. At 40K, nonuniform magnetization switching occurs. (ii) In MgO-based tunnel junctions, we could follow individual time-resolved switching events with a 13GHz bandwidth. The switching proceeds through a nanosecond-scale random incubation delay during which the resistance is quiet, followed by a sudden (400ps duration) transition terminated by a pronounced ringing that is damped within 1.5ns. While the incubation delay is probabilistic, the following time dependence of the resistance is reproducible.