Low-resistance Magnetic Tunnel Junctions Research Articles

Voltage-Induced Ferromagnetic Resonance in Magnetic Tunnel Junctions

We demonstrate excitation of ferromagnetic resonance in CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) by the combined action of voltage-controlled magnetic anisotropy (VCMA) and spin transfer torque (ST). Our measurements reveal that GHz-frequency VCMA torque and ST in low-resistance MTJs have similar magnitudes, and thus that both torques are equally important for understanding high-frequency voltage-driven magnetization dynamics in MTJs. As an example, we show that VCMA can increase the sensitivity of an MTJ-based microwave signal detector to the sensitivity level of semiconductor Schottky diodes.

Parametric oscillator based on nonlinear vortex dynamics in low-resistance magnetic tunnel junctions

Radiofrequency vortex spin-transfer oscillators based on magnetic tunnel junctions with very low resistance area product were investigated. A high power of excitations has been obtained characterized by a power spectral density containing a very sharp peak at the fundamental frequency and a series of harmonics. The observed behaviour is ascribed to the combined effect of spin transfer torque and Oersted-Amp\`ere field generated by the large applied dc-current. We furthermore show that the synchronization of a vortex oscillation by applying a ac bias current is mostly efficient when the external frequency is twice the oscillator fundamental frequency. This result is interpreted in terms of a parametric oscillator.

Nanofabrication of 30 nm Devices Incorporating Low Resistance Magnetic Tunnel Junctions

In this paper the electron-beam lithography conditions and the nanofabrication process are described for current-perpendicular-to-plane (CPP) pillar devices with 30 nm critical dimensions. This work combines a RAITH-150 tool with a negative e-beam resist (AR-7520) so that dense nanopillar arrays are patterned fast into large area samples. The resist dilution and coating conditions are optimized, aiming at its thickness reduction down to 80 nm. The exposure parameters are tuned for different geometries and dimensions, so that features down to 30 nm are exposed with good accuracy (+/- 1.9 nm) and reproducibility. The complete integration of these nanoelements into CPP devices involved electron beam lithography, ion milling for pattern transfer and chemical-mechanical polishing (CMP). Results on devices incorporating very low resistance-area (R x A) MTJ films deposited by Ion beam assisted deposition are shown, for MTJ stacks with R x A down to 0.8 omega x microm2. Device characterization includes electrical measurement of the pillar resistance and the transfer curves under dc magnetic fields (TMR up to 40%).

Low resistance magnetic tunnel junctions with MgO wedge barrier

We investigated the dependence of tunnel magnetoresistance (TMR), the resistance area (RA) product, crystallographic structure, the interlayer exchange coupling (IEC) and the coercive (Hc) fields in Co40Fe40B20/MgO/Co40Fe40B20 exchange biased spin valves on the Ar partial pressure, in the range from 1 mTorr to 15 mTorr, during the MgO sputtering. For all the metallic layers a linear dynamic deposition (LDD) with a constant wafer velocity was used, whereas for the MgO deposition a LDD wedge technology with gradient velocity. The crystallographic texture of MgO(001), IEC and Hc fields, TMR and RA significantly depend on the Ar partial pressure. We found that high Ar partial pressure (above 5.6 mTorr) resulted in weaker (001) texture, lower interplanar distances of the MgO barrier and higher IEC field. Low Ar partial pressure gives smoother MgO surfaces and provides good barrier quality which results in higher TMR and RA values

The effect of pinhole formation/growth on the tunnel magnetoresistance of MgO-based magnetic tunnel junctions

In this study, we focus on how the formation and enlargement of metallic pinholes in MgO barriers (induced by large electrical currents) affect the tunnel magnetoresistance (TMR) of low and high resistance (R) magnetic tunnel junctions. The junctions were deposited by physical vapor deposition with barrier thicknesses of either 0.75 or 1.35 nm. For the parallel state, temperature-dependent R(T) measurements readily revealed a metallic conductance in the low-R sample, indicating that pinholes are already present in its thin barrier; a slight R(T) decrease with increasing temperature is observed for the high-R junction. After applying large current pulses to the low-R sample, we observe that the initially small R-decrease (∼6%) is accompanied by a significant TMR increase (∼20% at 20 K). Higher applied electrical currents continue to decrease R, leading to a gradual but steady TMR decrease. In contrast, the high-R sample exhibits a sharp and immediate decrease in TMR as soon as the first pinhole is formed. The origin of these effects will be discussed in terms of a thermally induced improvement of the barrier/electrode interfaces and the possibility of a magnetoresistance mechanism occurring through the metallic pinholes.

Effect of resistance feedback on spin torque-induced switching of nanomagnets

In large magnetoresistance devices spin torque-induced changes in resistance can produce GHz current and voltage oscillations which can affect magnetization reversal. In addition, capacitive shunting in large resistance devices can further reduce the current, adversely affecting spin torque switching. Here, we simultaneously solve the Landau–Lifshitz–Gilbert equation with spin torque and the transmission line telegrapher's equations to study the effects of resistance feedback and capacitance on magnetization reversal of both spin valves and magnetic tunnel junctions. While for spin valves parallel (P) to anti-parallel (AP) switching is adversely affected by the resistance feedback due to saturation of the spin torque, in low resistance magnetic tunnel junctions P–AP switching is enhanced. We study the effect of resistance feedback on the switching time of magnetic tunnel junctions, and show that magnetization switching is only affected by capacitive shunting in the pF range.

Interfacial Oxidation Enhanced Perpendicular Magnetic Anisotropy in Low Resistance Magnetic Tunnel Junctions Composed of Co/Pt Multilayer Electrodes

Low-resistance magnetic tunnel junctions utilizing perpendicular magnetic anisotropy of Co/Pt multilayer electrodes have been investigated. In these junctions, AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> tunnel barrier has been prepared by repeated natural oxidation processes. Each natural oxidation process has been optimized by varying the thickness of pre-oxidized Al layer prior to oxidation. The perpendicular magnetic anisotropy of Co/Pt multilayer has been found to be dependent on the degree of oxidation at the interface of tunnel barrier and Co adjacent layer. It has been observed that magnetization of Co/Pt multilayer is tilted when Co layer adjacent to tunnel barrier is relatively thick, which is attributed to the lack of interfacial anisotropy contribution near the interface of Co adjacent layer and AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> barrier. Interfacial oxidation treated on the monolayer thick Co adjacent layer has enhanced perpendicular magnetic anisotropy of the entire Co/Pt multilayer electrodes, enabling sharp transition between well-defined parallel and anti-parallel states at micro-fabricated perpendicular magnetic tunnel junctions of ~100 Omega ldr mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> resistance-area product.

Micromagnetic study of hotspot and thermal effects on spin-transfer switching in magnetic tunnel junctions

The hotspot and current self-heating effects on the spin-transfer-induced magnetization switching are investigated for low resistance magnetic tunnel junctions. Two kinds of spin torque theories, one for ohmic-like conduction from randomly distributed hotspots and the other for tunnel conduction from an insulator barrier, are combined together in this study by using a parallel resistor model. We find that the spin torque amplitude is locally enhanced in the hotspot region due to the large current density, which leads to a strong reduction of the current switching threshold (Jc), in a way that the enhanced spin torque induces the local magnetization near the hotspot switching first, and then drives the switching spreading through the whole free layer. The current self-heating effect is also studied; the free layer temperature increases only a few degrees at an applied current close to Jc∼1×107A∕cm2, which results in a slight decrease in Jc for the low resistance junctions with hotspots.

Spin-transfer-induced magnetization switching in magnetic tunnel junctions

Micromagnetic simulations for spin-transfer-induced magnetization dynamics in nanoscale low-resistance magnetic tunnel junctions (MTJs) are performed by using the modified Landau-Lifshitz equation in which a spin-transfer model for MTJs is included. Calculations show that critical switching current density (Jc) is about (4–8)×106A∕cm2, consistent with the experimental data, lower than the values found in metallic spin-valve nanopillars (typically in the order of ∼107A∕cm2). A slight asymmetry of the critical current is observed in the magnetization switching loop, due to the asymmetry of the spin-torque factor. The switching time is related to the applied current, and our calculation results show that a longer switching time, 10–15ns, is needed when the applied current density J is close to Jc, and it decreases down to 1ns for J&amp;gt;2×107A∕cm2.

Low-resistance magnetic tunnel junctions with anMgO−Al2O3composite tunnel barrier: Asymmetric transport characteristics and free electron modeling of a self-limited oxidation bilayer

Low-resistance magnetic tunnel junctions with an $\mathrm{Mg}\mathrm{O}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ composite tunnel barrier have been grown. From the theoretical point of view, current-voltage and magnetoresistance-voltage characteristics are predicted to be asymmetric. These asymmetries are studied as a function of barrier thicknesses for given experimental MgO and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ barrier heights. From an experimental point of view, the bottom alumina barrier acts as a diffusion barrier allowing the complete oxidation of the thin deposited Mg layer. As a result, composite ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}∕\mathrm{Mg}\mathrm{O}$ tunnel barriers show a lower area resistance and a magnetoresistance signal at nonzero applied voltage that is predicted to be equivalent as single ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ tunnel barriers with the same total thickness. Current-voltage and magnetoresistance-voltage characteristics are shown to be asymmetric at high voltages.

Magnetization switching by spin-polarized current in low-resistance magnetic tunnel junction with MgO [001] barrier

Current-induced magnetization switching (CIMS) was demonstrated in low resistance magnetic tunnel junctions (MTJs) with thin MgO [001] barrier. The resistance change by CIMS was more than 100%, which is much larger than the previous report in Al-O based MTJs. The switching current density was about 2/spl times/10/sup 7/ A/cm/sup 2/, which was comparable with that reported values in metallic multilayers.

Low-resistance magnetic tunnel junctions prepared by partial remote plasma oxidation of 0.9nm Al barriers

Current perpendicular to the plane read-head elements suitable for high-density magnetic storage require low resistance while maintaining a reasonable magnetoresistive (MR) signal (R×A&amp;lt;1Ωμm2 and MR&amp;gt;20% for areal densities &amp;gt;200Gb∕in2). This letter shows that competitive low R×A junctions can be produced using underoxidized barriers starting from 0.9nm thick Al layers. For as-deposited junctions, tunneling magnetoresistance (TMR) ∼20% for R×A∼2–15Ωμm2 is obtained, while in the R×A∼60–150Ωμm2 range, TMR values between 40% to 45% are achieved. A limited number of junctions exhibits considerably lower R×A values with respect to the average, while keeping a similar MR (down to 0.44Ωμm2 with TMR of 20% and down to 2.2Ωμm2 with TMR of 52%). Experimental data suggest that current confinement to small regions (barrier defects/hot spots) may explain these results.

Observation of spin-transfer switching in deep submicron-sized and low-resistance magnetic tunnel junctions

The spin-transfer effect has been studied in magnetic tunnel junctions (PtMn/CoFe/Ru/CoFe/Al2O3/CoFe/NiFe) with dimensions down to 0.1×0.2 μm2 and resistance–area product RA in the range of 0.5–10 Ω μm2 (ΔR/R=1%–20%). Current-induced magnetization switching is observed with a critical current density of about 8×106 A/cm2. The attribution of the switching to the spin-transfer effect is supported by a current-induced ΔR/R value identical to the one obtained from the R versus H measurements. Furthermore, the critical switching current density has clear dependence on the applied magnetic field, consistent with what has been observed previously in the case of spin-transfer-induced switching in metallic multilayer systems.

Hot-spot mediated current-induced resistance change in magnetic tunnel junctions

Experimental results on the current-induced resistance change in low-resistance (10-60 /spl Omega//spl middot//spl mu/m/sup 2/) magnetic tunnel junctions with AlO/sub x/ barriers are presented. The observed current-induced resistance change remains unaltered even when the ferromagnetic electrodes are saturated by an external field of 8.5 kOe and, therefore, is not related to the relative magnetization configuration of both electrodes. The critical switching current increases with decreasing barrier thickness (increasing hot-spot density), as well as with increasing junction area. Repeated switching (4000 pulses) leads to junction resistance decrease by 3%-4% and a decrease of the relative current-induced resistance change of about 20%, indicating an increase of hot spots or pinhole density upon current stressing. Critical current increases as current pulsewidth decreases.

Ion-beam deposited low resistance magnetic tunnel junctions prepared by a two-step oxidation process

Spin tunnel junctions with ultrathin (&amp;lt;1 nm) AlOx tunnel barriers were fabricated by ion-beam deposition. The tunnel barrier was formed by natural oxidation of an ultrathin Al layer (4–8.5 Å thick) by either a single-step or a two-step oxidation process. The magnetic and transport properties of these tunnel junctions indicate the barrier to be pinhole free for tAl⩾6.5 Å. The minimum Al thickness at which pinholes form depends on ion acceleration voltage used for Al deposition. Single-step oxidation of an 8 Å Al layer leads to a magnetic tunnel junction with a 25% tunnel magnetoresistance (TMR) and R×A of 14 Ω μm2. The two-step oxidation procedure was used for thinner Al layers. TMR ranges from 18%–23% at 8–10 Ω μm2 resistance values, for a total Al thickness of 6.5 Å. Junction resistance has been further decreased down to 4.35 Ω μm2 with a TMR of 10.6% by two-step oxidizing a 6 Å Al layer.

Current-induced switching in low resistance magnetic tunnel junctions

Current-induced magnetization switching (CIMS) in low resistance tunnel junctions is reported at critical current densities of 1.9×106 A/cm2 for tunnel junction areas ranging from 2 to 3 μm2, and junction resistances from 6 to 20 Ω μm2. Typical tunnel magnetic resistance values for these junctions range from 15% to 21% (measured in an external magnetic field) and 10% to 14% resistance changes are obtained by CIMS. Micromagnetic simulation indicates that vortex fields and spin transfer effects cannot fully account for the observed current-induced switching. Although able to explain the observed transition from a parallel or antiparallel state to a vortex state, it fails to explain the switch back to the original state, at a comparable but symmetrical critical current density.

Current-induced magnetization switching in magnetic tunnel junctions

Current-induced magnetization switching (CIMS) in low-resistance magnetic tunnel junctions was shown at average critical current densities (Jc=1.33×106 A/cm2). When large vertical currents pass through the junctions, spin-transfer torque, and vortex fields can rotate the magnetization of the free layer from the initial parallel state to a vortex state, resulting in 10.8% CIMS resistance change at zero-bias current, which is about half of the resistance change (22%) induced when switching is created by an external field. A micromagnetic simulation including the spin-transfer torque and the vortex field correctly predicts the critical negative-current-inducing switching from the parallel state into the vortex state, but fails to explain the reverse switching from the vortex state into the parallel state at an approximately symmetric positive critical current. Lead fields were analyzed and found to be not the cause of the observed switching. The very small dependence of the switching currents on an external magnetic field suggests the existence of hot-spots where local current densities may be much larger.

Growth and properties of Cu3N films and Cu3N/γ′-Fe4N bilayers

Copper nitride films were grown by molecular-beam epitaxy of copper in the presence of nitrogen from a radio-frequency atomic source on (001) γ′-Fe4N/(001)MgO or directly on MgO substrates. The structural properties of the Cu3N films were found to be very dependent on the substrate and on the deposition temperature. At optimal growth conditions, the Cu3N films grow epitaxial on both substrates. The Cu3N films grown on MgO were characterized optically to be insulators with an energy gap of 1.65 eV. On γ′-Fe4N, Cu3N films with a thickness of only 6 nm, were grown as closed layers, epitaxial and rather smooth (root-mean-square roughness of 0.7 nm). This material has ideal properties to be used as a barrier in low resistance magnetic tunnel junctions.

Shot noise in low-resistance magnetic tunnel junctions

Shot noise measurements made on low-resistance magnetic tunnel junctions show results inconsistent with the resistance of the samples examined. The results yield lower than expected shot noise which is consistent with parallel path conduction (pinholes). A simple electrical model shows this should be true for noise measurements as well as for the tunneling magnetoresistance (TMR) as the resistance-area product (R×A) is reduced. The model suggests that a correlation between TMR and shot noise should exist assuming the presence of pinholes and that shot noise offers a useful experimental monitoring technique. The difficulties of making these measurements related to high frequency 1/f noise are discussed. Noise results are presented for high R×A and low R×A low-resistance samples which, in this particular case, both show the influence of pinhole shunting.

Magnetic tunnel junctions with AlN and AlNxOy barriers

Nonoxide tunnel barriers such as AlN are of interest for magnetic tunnel junctions to avoid the oxidation of the magnetic electrodes. We have investigated the fabrication and properties of thin AlN-based barriers for use in low resistance magnetic tunnel junctions. Electronic, magnetic and structural data of tunnel valves of the form Ta (100 Å)/PtMn (300 Å)/CoFe20 (20 Å–25 Å)/barrier/CoFe20 (10–20 Å)/NiFe16 (35–40 Å)/Ta (100 Å) are presented, where the barrier consists of AlN, AlNxOy or AlN/AlOx with total thicknesses between 8 and 15 Å. The tunnel junctions were sputter deposited and then lithographically patterned down to 2×2 μm2 devices. AlN was deposited by reactive sputtering from an Al target with 20%–35% N2 in the Ar sputter gas at room temperature, resulting in stoichiometric growth of AlNx (x=0.50±0.05), as determined by RBS. TEM analysis shows that the as-deposited AlN barrier is crystalline. For AlN barriers and AlN followed by natural O2 oxidation, we obtain tunnel magnetoresistance &amp;gt;10% with specific junction resistance Rj down to 60 Ωμm2.