MgO Barrier Research Articles

Strongly suppressed 1/f noise and enhanced magnetoresistance in epitaxial Fe–V/MgO/Fe magnetic tunnel junctions

Alloying Fe electrodes with V, through reduced FeV/MgO interface mismatch in epitaxial magnetic tunnel junctions with MgO barriers, notably suppresses both nonmagnetic (parallel) and magnetic (antiparallel) state 1/f noise and enhances tunneling magnetoresistance. A comparative study of the room temperature electron transport and low frequency noise in Fe1−xVx/MgO/Fe and Fe/MgO/Fe1−xVx magnetic tunnel junctions with 0≤x≤0.25 reveals that V doping of the bottom electrode for x<0.1 reduces in nearly two orders of magnitude the normalized nonmagnetic and magnetic 1/f noise. We attribute the enhanced TMR and suppressed 1/f noise to strongly reduced misfit and dislocation density.

Reduced low frequency noise in electron beam evaporated MgO magnetic tunnel junctions

We compare low frequency noise in magnetic tunnel junctions with MgO barriers prepared by electron-beam evaporation with those prepared by radiofrequency sputtering, both showing a high tunneling magnetoresistance. The normalized noise parameter in the parallel state of junctions with evaporated barriers is at least one order of magnitude lower than that in junctions with sputtered barriers, and exhibits a weaker bias dependence. The lowest normalized noise is in the 10−11 μm2 range. A lower density of oxygen vacancies acting as charge trap states in the evaporated MgO is responsible for the lower noise.

Incidence of in situ annealing on the nanoscale topographical/electrical properties of the tunnel barrier in sputtered epitaxial Fe/MgO/Fe multilayers

Technological improvements in the magnetotransport performance of Fe/MgO/Fe stacks require nanoscale control over the topographical and electrical properties of the ultrathin MgO barrier. We have statistically investigated the incidence of in situ annealing of the lower Fe layer on the nanoscale topographical/electrical properties of Fe/MgO bilayers and the structural and magnetic properties of Fe/MgO/Fe/Co multilayers prepared by sputtering. This annealing step improves the crystal quality of both the lower Fe and the upper Fe/Co layers, leading to an enhanced saturated magnetic moment. Finally, this annealing step substantially mitigates the presence of nanohills on the lower Fe layer and improves the uniformity of the height and/or the thickness of the MgO tunnel barrier. Our results pave the way for studies of nanoscale transport on micrometre-sized devices through a better understanding of, and control over, nanoscale hotspots in the tunnel barrier.

Barrier degradation of tunneling magnetoresistance device with MgO barrier and low resistance-area product

MgO barrier degradation is studied in a tunneling magnetoresistance head with low resistance-area product. As the stress current is increased, the resistance is significantly reduced before the barrier breakdown, while the magnetoresistance ratio remains almost unvaried. At the same time, the bias dependence of the resistance becomes less affected by the bias polarity, suggesting that slight degradation occurs at the interface between MgO and the ferromagnetic electrode. Just before the breakdown, the bias dependence shows an increasing tendency, indicating the defect accumulation inside the MgO barrier. The results are helpful for understanding the mechanisms of barrier degradation, which is critical for developing future magnetic tunneling junction devices.

Tunnel magnetoresistance properties and film structures of double MgO barrier magnetic tunnel junctions

The authors fabricated double MgO barrier magnetic tunnel junctions (MTJs) with 3-nm-thick Co40Fe40B20 free layer. When annealed at 350 °C, tunnel magnetoresistance (TMR) ratio at room temperature was 130%, much lower than that (297%) of single MgO barrier MTJs processed and annealed under the same condition. The middle CoFeB free layer sandwiched between the two MgO barriers was found to be mostly amorphous. Replacement of the Co40Fe40B20 free layer by a highly oriented Co50Fe50 layer and a composite Co50Fe50/Co40Fe40B20 layer led to the enhanced TMR ratios up to 165% and 212% at annealing temperature of 350 °C, respectively.

Direct measurement of the spin polarization of Co2FeAl in combination with MgO tunnel barriers

It is a truth universally acknowledged that a Heusler compound in possession of a good order must be in want of a high spin polarization. In the present work, we investigated the spin polarization of the Heusler compound Co2FeAl by spin polarized tunneling through a MgO barrier into a superconducting Al–Si electrode. The measured spin polarization of P=55% is in good agreement with the previously obtained tunnel magnetoresistance values and compared to the data by other groups.

Interlayer exchange coupling and current induced magnetization switching in magnetic tunnel junctions with MgO wedge barrier

Current induced magnetization switching and interlayer exchange coupling (IEC) in sputtered CoFeB/MgO/CoFeB exchange-biased magnetic tunnel junctions with an extremely thin (0.96–0.62 nm) MgO wedge barrier is investigated. The IEC is found to be ferromagnetic for all samples and the associated energy increases exponentially down to a barrier thickness of 0.7 nm. Nanopillars with resistance area product ranging from 1.8 to 10 Ω μm2 and sizes of 0.13 μm2 down to 0.03 μm2 and tunneling magnetoresistance values of up to 170% were prepared. We found, that the critical current density increases with decreasing MgO barrier thickness. The experimental data and theoretical estimations show that the barrier thickness dependence of the spin transfer torque can largely be explained by a reduction in the tunnel current polarization at very small barrier thickness.

Giant oscillations in spin-dependent tunneling resistances as a function of barrier thickness in fully epitaxial magnetic tunnel junctions with a MgO barrier

Giant oscillations in spin-dependent tunneling resistances as a function of MgO barrier thickness $({t}_{\text{MgO}})$ were observed for fully epitaxial magnetic tunnel junctions with Heusler alloy ${\text{Co}}_{2}{\text{Cr}}_{0.6}{\text{Fe}}_{0.4}\text{Al}$ electrodes and a MgO barrier. The oscillations in tunneling resistances were well approximated by a superposition of an exponential function of $\text{exp}(a{t}_{\text{MgO}}+b)$ and a periodic function of $1+C\text{ }\text{cos}[(2\ensuremath{\pi}/T){t}_{\text{MgO}}+\ensuremath{\varphi}]$ with significantly large amplitudes $C\ensuremath{\sim}0.16\ifmmode\pm\else\textpm\fi{}0.01$ even at 293 K for both parallel and antiparallel magnetization orientations. The period was found to be almost independent of temperature and bias voltage $(V)$. The amplitudes $C$ showed only weak dependence on $V$ at least up to 0.2 V. These features should be a key to understand the origin of the pronounced oscillations.

Inter-diffusion study in MgO tunneling magneto-resistive (TMR) system by XPS

In this paper, we investigated the elemental inter-diffusion in MgO TMR system, namely, between MgO barrier and free layer (CoFeB, NiFe or their combination) interface and the oxygen diffusion into the capping layers (Ta, Ru, TaN) at elevated temperatures using simple sheet film stack to simplify the results interpretation. Boron, cobalt, iron, and nickel show various diffusion tendencies into the MgO barrier after annealing the sheet film stack. Oxygen has different penetration depth into single CoFeB free layer upon annealing under N 2 + Ar protective atmosphere for different capping layers. Ru and TaN capping layer provide much better O 2 diffusion barrier, compared with Ta capping layer. This could potentially change the boron segregation tendency at free layer and capping layer interface and thus affect the interface crystallization process and lattice matching between the crystallized CoFeB free layer and the MgO(0 0 1) barrier layer. All these effects will impact the overall TMR performance.

Metallic Mg insertion in rf deposited MgO barrier

Metallic Mg insertions in rf deposited MgO barrier of magnetic tunnel junctions structures were investigated in a resistance-area (RA) range from 1 to 1000 Ω μm2. For the first time, investigations on Mg insertions above the MgO barrier and simultaneously on both sides of the barrier are reported. It is shown that for RA larger than 5 Ω μm2, a bottom Mg insertion does not improve the tunnel magnetoresistance (TMR) ratio compared to a sample with no Mg insertion. Furthermore, a top Mg insertion yields a lower TMR ratio decreasing as the Mg thickness is increased. On the other side, for RA lower than 5 Ω μm2, there is no significant difference between top and bottom Mg insertions indicating that in this region, the MgO crystallization occurs mainly during the annealing process. In the RA range investigated, there is no significant difference in coupling field for different insertions. In very low RA regions between 1 and 10 Ω μm2, an increase in TMR is observed for 0.3 nm insertions simultaneously below an above the barrier.

Evidence of spin-polarized direct elastic tunneling and onset of superparamagnetism in MgO magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) with crystalline MgO barriers are currently being used in a variety of applications, namely forefront magnetic sensors and memories. In this work we probed the temperature $(T)$ dependence of the transport and magnetic properties of MgO-based MTJs with different CoFeB free layer thicknesses (${t}_{fl}=1.55$, 1.65, 1.95, and 3.0 nm). All samples have the same insulating MgO barrier with a nominal thickness of 1.35 nm. Our results show that the tunnel magnetoresistance (TMR) temperature behavior is mainly due to a strong variation of the conductance $(G)$ of the antiparallel state. Also, we provide evidence that direct elastic tunneling is the dominant mechanism determining the temperature dependence of the tunneling conductance and TMR in the studied MgO MTJs. Intrinsic to this mechanism is the thermal smearing of the electron energies near the Fermi level which then plays a key role in $G(T)$, especially in the parallel state where the overall change in $G$ is very small. Furthermore, we show a clear change in the MTJ properties as the free layer thickness is reduced. Besides the typical decrease of TMR related with the loss of spin polarization, we were able to probe the thickness dependence of the spin wave $\ensuremath{\alpha}$ parameter. MTJ with the thinner free layer show both absence of hysteresis in the room temperature TMR cycles and interesting freezing effects in the zero and field cool magnetization curves at low temperatures, revealing the discontinuous nature of thin free layers.

Giant tunneling magnetoresistance with electron beam evaporated MgO barrier and CoFeB electrodes

Electron-beam (EB) evaporated MgO grows with (001) texture on amorphous CoFeB when the deposition rate is kept below 5 pm/s. Magnetic tunnel junctions (MTJs) fabricated using this method exhibit ∼240% magnetoresistance at room temperature for a 2.5 nm thick EB-MgO barrier, which is similar to the value for a radio frequency (rf) sputtered barrier with the same junction geometry. The average barrier height of the EB-MgO is 0.48 eV, which is higher than previously reported values for rf-MgO barriers and it increases with increasing annealing temperature. Our results show that EB-MgO could be a simpler alternative to rf-MgO in MTJs without any compromise in the tunnelling magnetoresistance.

Interface structure of half-metallic Heusler alloyCo2MnSithin films facing an MgO tunnel barrier determined by x-ray magnetic circular dichroism

Using x-ray absorption spectroscopy and x-ray magnetic circular dichroism, we performed an element-specific investigation of the spin magnetic moments $({m}_{\text{spin}})$ of Mn and Co of ${\text{Co}}_{2}\text{MnSi}$ ultrathin layers grown on an Fe underlayer to stabilize the ferromagnetism and facing an MgO barrier. The experimentally observed dependence of the ${m}_{\text{spin}}$ values of Co and Mn on the ${\text{Co}}_{2}\text{MnSi}$ layer thickness in the ultrathin region is qualitatively in good agreement with the one theoretically obtained for a MnSi-terminated interface of the ${\text{Co}}_{2}\text{MnSi}$ layer facing an MgO barrier. This clarification would provide a strong basis for further advancement of applications of half-metallic Heusler alloy electrodes into spintronic devices.

Coherent tunneling and giant tunneling magnetoresistance inCo2FeAl/MgO/CoFemagnetic tunneling junctions

Spin-dependent coherent tunneling has been experimentally observed in high-quality sputtered-deposited ${\text{Co}}_{2}\text{FeAl}/\text{MgO}/\text{CoFe}$ epitaxial magnetic tunneling junctions (MTJs). Consequently, the microfabricated MTJs manifest a very large tunnel magnetoresistance (TMR) at room temperature and an unexpectedly TMR oscillation as a function of MgO barrier thickness. First-principles electronic band calculations confirm the pronounced coherent tunneling effect and are in good agreement with the experimental data. The present work demonstrates the importance of coherent tunneling for large TMR with Heusler alloys

Understanding tunneling magnetoresistance during thermal annealing in MgO-based junctions with CoFeB electrodes

The competition between the interface crystallization and diffusion processes, their influence on the onset of symmetry-filtering coherent tunneling of ${\ensuremath{\Delta}}_{1}$ band electrons in the MgO-based magnetic tunnel junctions is investigated. Systematic study of the transport and magnetoresistance during thermal annealing of these junctions shows a unique behavior of the tunneling conductance in the parallel state. The optimal annealing time for achieving giant tunneling magnetoresistance at different temperatures is determined. The evolution of magnetoresistance consists of three distinct regions, responsible by different contributions from CoFeB electrodes and the MgO barrier. The whole phenomenon can be understood through an empirical model based on the Landauer tunneling picture.

CoFeB Inserted Perpendicular Magnetic Tunnel Junctions with CoFe/Pd Multilayers for High Tunnel Magnetoresistance Ratio

The effect of Co20Fe60B20 insertion on tunnel magnetoresistance (TMR) properties of MgO barrier magnetic tunnel junctions (MTJs) was studied with Co90Fe10/Pd multilayer electrodes showing perpendicular anisotropy. The TMR ratio, which depended on the sputtering power density of the CoFeB layer, reached 43% at 1.77 W/cm2 in MTJs with a 1.8-nm-thick CoFeB layer inserted on both sides of the MgO barrier. With increasing CoFeB layer thickness, the optimal annealing temperature (Ta) realizing the maximum TMR ratio increased along with the TMR ratio. The MTJs with 3.0-nm-thick CoFeB deposited at 1.77 W/cm2 showed a TMR ratio of 91% at Ta = 250 °C, where the perpendicular magnetic anisotropy is maintained.

Evolution of barrier-resistance noise in CoFeB/MgO/CoFeB tunnel junctions during annealing

The low-frequency resistance noise in sputtered-deposited magnetic tunnel junctions with MgO barriers has been measured as a function of annealing time at different annealing temperatures. The noise has a 1/f spectrum and it is quantified by a Hooge-like parameter α given in units of μm2. Unannealed devices have the highest noise levels and their α parameters exhibit a pronounced dependence on the voltage bias across the junction. A significant increase in tunneling magnetoresistance (TMR) is observed for short annealing times (on the order of minutes) at high temperatures and it is correlated with a large reduction in noise and in its bias dependence. The maximum TMR and minimum noise levels are reached at a later time that depends on temperature, being shorter at higher annealing temperatures. Devices annealed at 380 and at 430 °C exhibit the same minimum noise levels, α≈2×10−10 μm2. The origin of the resistance noise, its annealing time evolution, and its bias dependence are discussed and they are attributed to vacancy defects in the MgO barriers.

Normal-state conductance used to probe superconducting tunnel junctions for quantumcomputing

Here we report normal-state conductance measurements of three different types ofsuperconducting tunnel junctions that are being used or proposed for quantum computingapplications: p-Al/a-AlO/p-Al, e-Re/e-AlO/p-Al, and e-V/e-MgO/p-V, where p stands forpolycrystalline, e for epitaxial, and a for amorphous. All three junctions exhibitedsignificant deviations from the parabolic behavior predicted by the WKB approximationmodels. In the p-Al/a-AlO/p-Al junction, we observed enhancement of tunnelingconductances at voltages matching harmonics of Al–O stretching modes. On the otherhand, such Al–O vibration modes were missing in the epitaxial e-Re/e-AlO/p-Al junction.This suggests that absence or existence of the Al–O stretching mode might be related tothe crystallinity of the AlO tunnel barrier and the interface between the electrode and thebarrier. In the e-V/e-MgO/p-V junction, which is one of the candidate systems for futuresuperconducting qubits, we observed suppression of the density of states at zero bias. Thisimplies that the interface is electronically disordered, presumably due to oxidationof the vanadium surface underneath the MgO barrier, even if the interface wasstructurally well ordered, suggesting that the e-V/e-MgO/p-V junction will not besuitable for qubit applications in its present form. This also demonstrates thatthe normal-state conductance measurement can be effectively used to screen outlow quality samples in the search for better superconducting tunnel junctions.

All magnesium diboride Josephson junctions with MgO and native oxide barriers

We present results on all-MgB2 tunnel junctions, where the tunnel barrier is deposited MgO or native-oxide of base electrode. For the junctions with MgO, the hysteretic I-V curve resembles a conventional underdamped Josephson junction characteristic with critical current-resistance product nearly independent of the junction area. The dependence of the critical current with temperature up to 20 K agrees with the [Ambegaokar and Baratoff, Phys. Rev. Lett. 10, 486 (1963)] expression. For the junctions with native-oxide, conductance at low bias exhibits subgap features while at high bias reveals thick barriers. As a result no supercurrent was observed in the latter, despite the presence of superconducting-gaps to over 30 K.

High-Jc MgB2 Josephson junctions with operating temperature up to 40 K

Sandwich-type MgB2/MgO/MgB2 Josephson junctions with Au or MgB2 interconnection were fabricated using hybrid physical-chemical vapor deposited MgB2 thin films and RF-magnetron-sputtered MgO barrier. The junctions show properties similar to those in high-Jc Nb junctions with Jc up to 275 kA/cm2 at 4 K, which remains nonzero up to 40 K. Critical current modulations by applied magnetic field and constant voltage steps under microwave radiation were observed. Combined with the larger energy gaps in MgB2 than in Nb, the junctions presented here allow simple MgB2 digital circuits to work over 20 K or with a clock speed above 1 THz.