Resistance Area Research Articles

Effects of barrier sputtering parameters on Co 80Fe 10B 10/MgO/Co 80Fe 10B 10 magnetic tunnel junctions

The influence of the Ar sputter pressure and sputter power on the growth of MgO tunnel barriers is investigated. It is found that radio frequency (RF) sputtering of MgO onto amorphous SiO 2 from a target-facing-target (TFT) gun results in smooth films with a poor (0 0 1) texture when the Ar sputter is less than 1.3×10 −3 mbar, whereas the use of a high-sputter pressure of 13×10 −3 mbar gives rougher but better oriented MgO films. The transfer of these deposition parameters to the growth of Co 80Fe 10B 10/MgO/Co 80Fe 10B 10 magnetic tunnel junctions (MTJs) reveals a clear dependence of the tunneling magnetoresistance (TMR) on barrier roughness and texture perfection. A maximum TMR value of 72% is measured after annealing MTJs with MgO barriers grown under high Ar sputter pressures. In addition, the resistance-area (RA) product of the MTJs increases by two orders of magnitude if the sputter pressure is increased from 1.3×10 −3 to 13×10 −3 mbar. Contrary to these effects, the properties of the MTJs are nearly independent of magnetron sputter power.

Ultralow resistance-area product of 0.4Ω(μm)2 and high magnetoresistance above 50% in CoFeB∕MgO∕CoFeB magnetic tunnel junctions

An ultralow resistance-area (RA) product of 0.4Ω(μm)2 was achieved in CoFeB∕MgO∕CoFeB magnetic tunnel junctions with a high magnetoresistance ratio of 57% at room temperature. Various growth conditions for polycrystalline MgO(001) tunneling barrier were optimized to improve the crystalline orientation of the MgO(001) layer, which resulted in a significant enhancement of magnetoresistance in an ultralow RA region below 1Ω(μm)2. Removal of residual H2O molecules from a growth chamber was especially effective in improving the crystalline orientation. The present achievements will enable the development of highly sensitive read heads for ultrahigh-density hard disk drives.

Intrinsic Reliability of AlOx-Based Magnetic Tunnel Junctions

We present a detailed investigation into the intrinsic tunnel barrier reliability in AlOx-based magnetic tunnel junctions (MTJs) as a function of aluminum thickness and oxidation time. The intrinsic reliability is measured as the ramped breakdown voltage <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(V _ bd)$</tex> at room temperature for both positive and negative polarity. We find that <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ V_ bd$</tex> generally increases with the resistance-area (RA) product of the MTJ. While this dependence is quite strong at low RA, it gradually weakens for higher RA. At fixed RA, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$V_ bd$</tex> also depends on the original Al film thickness with better properties for thicker Al. Finally, we observe a polarity dependence of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$V_ bd$</tex> which changes sign as the MTJ goes from thin Al to thick Al. We attribute the polarity dependence to the different quality of the top and bottom interfaces and conclude that the interface emitting the tunneling electrons primarily governs the barrier reliability.

Tunable spin-tunnel contacts to silicon using low-work-function ferromagnets

Magnetic tunnel junctions have become ubiquitous components appearing in magnetic random-access memory, read heads of magnetic disk drives and semiconductor-based spin devices. Inserting a tunnel barrier has been key to achieving spin injection from ferromagnetic (FM) metals into GaAs, but spin injection into Si has remained elusive. We show that Schottky barrier formation leads to a huge conductivity mismatch of the FM tunnel contact and Si, which cannot be solved by the well-known method of adjusting the tunnel barrier thickness. We present a radically different approach for spin-tunnelling resistance control using low-work-function ferromagnets, inserted at the FM/tunnel barrier interface. We demonstrate that in this way the resistance-area (RA) product of FM/Al2O3/Si contacts can be tuned over eight orders of magnitude, while simultaneously maintaining a reasonable tunnel spin polarization. This raises prospects for Si-based spintronics and presents a new category of ferromagnetic materials for spin-tunnel contacts in low-RA-product applications.

Highly Charged Ion Modified Magnetic Tunnel Junctions

ABSTRACTWe have used highly charged ions (HCIs) such as Xe44+ to modify ultrathin aluminum oxide barriers in magnetic tunnel junctions (MTJs) in order to controllably adjust their electrical properties independently of oxide thickness. We have reduced the resistance area (RA) product of our MTJ devices by up to three orders of magnitude down to our present measurement uncertainty limit of 30 Ω·μm2 by varying the HCI dose. Preliminary experiments indicate that HCI modified Co/Al2O3/Co MTJs have a reduced magnetoresistance (MR) of ≈ 1% at room temperature as compared to ≈ 10% for undosed devices. The goal of this effort is to fabricate a magnetic field sensor in current-perpendicular-to-plane (CPP) geometry with an RA optimized for hard drive read heads. This is an improvement over presently demonstrated CPP architectures based on giant magnetoresistance or tunnel junctions, whose RAs are either to low or too high.

Spin transfer switching current reduction in magnetic tunnel junction based dual spin filter structures

Spin-transfer-driven magnetization switching was studied in single magnetic tunneling junctions (MTJ: Ta∕PtMn∕CoFe∕Ru∕CoFeB∕Al2O3∕CoFeB∕Ta) and dual spin filters (DSF: Ta∕PtMn∕CoFe∕Ru∕CoFeB∕Al2O3∕CoFeB∕spacer∕CoFe∕PtMn∕Ta) having resistance-area (RA) product in the range of 10–30Ωμm2 and tunnel magnetoresistance (TMR) of 15%–30%. The intrinsic critical current density (Jc0) was estimated by extrapolating experimentally obtained critical current density (Jc) versus pulse width (τ) data to a pulse width of 1ns. Jc, extrapolated to τ of 1ns (∼Jc0), was 7×106 and 2.2×106A∕cm2, respectively, for the MTJ and improved DSF samples having identical free layers. Thus, a significant enhancement of the spin transfer switching efficiency is seen for DSF structures compared to the single MTJ case.

Influence of thickness and band structure of insulating barriers on resistance and tunneling magnetoresistance properties of magnetic tunnel junctions with Al-oxide and Ti-alloyed Al-oxide barriers

Abstract We investigated the influence of insulating barrier thickness and the Ti composition dependence of the band structure of Al-oxide on the resistance and tunneling magnetoresistance (TMR) behavior of the magnetic tunnel junction (MTJ). Low resistance × area (RA) value (1.1 MΩ μm2) was achieved by decreasing the Al-oxide thickness down to 1.0 nm. However, this led to the partial oxidation of the bottom ferromagnetic (FM) electrode of the junction and non-continuous thin barriers by the occurrence of pinholes, with low TMR ratio of 8.3%. For an alternative for low RA value, we developed a new Ti-alloyed Al-oxide (TiAlOx) that had lower band gap than Al-oxide as an insulating barrier of MTJ. As the Ti concentration increased up to 5.33 at.% Ti in Al, the RA value of the MTJs was reduced from 9.5 to 0.69 MΩ μm2, owing to the band-gap reduction of TiAlOx caused by the formation of extra bands, mainly composed of Ti-3d orbitals, within the band gap. It was analyzed that TiAlOx has localized d states in the band gap below the conduction band. In addition, the TMR ratio increased with the Ti concentration and reached a maximum of 49% at 5.33 at.% Ti owing to the microstructural evolution of Ti–Al alloy film in the pre-oxidation state.

Spin transfer effect in magnetic tunnel junction with a nano-current-channel Layer in free layer

The current-induced magnetization switching (spin transfer effect) in a low resistance-area (RA) product magnetic tunnel junction (MTJ) device with critical current density of 1.4/spl times/10/sup 7/ A/cm/sup 2/ was demonstrated. The RA product of the MTJ is 4.2 /spl Omega//spl mu/m/sup 2/ and the magnetoresistance (MR) ratio induced by current is up to 16%. An MTJ structure with a novel nano-current-channel (NCC) layer inserted into the free layer for the current-induced magnetization switching by lower current density was proposed and prototyped. By using the current confined effect, the local current density in the integrated free layer was sufficiently high to switch the magnetization locally. Such local magnetization reversal helped to reverse the magnetic moments around together with the polarized current and spread out the switching of the entire free layer through the superparamagnetic nano-channels. The critical current density was reduced to 4.2/spl times/10/sup 6/ A/cm/sup 2/, which is only one quarter of that for a pure MTJ structure.

Magnetic tunnel junctions using reactively sputtered Al/sub 2/O/sub 3/ barriers

Magnetic tunnel junction (MTJ) films were deposited using reactively sputtered AlOx barriers. In contrast to earlier attempts we show that sizable tunnel magnetoresistance (TMR) can be achieved at moderate resistance area (RA) products using this method. For instance, 45% at 580 /spl Omega//spl mu/m/sup 2/ is obtained using Co/sub 70/Fe/sub 30/ electrodes. The barrier formation can be understood by monitoring the oxidation state of the target in the presence of oxygen flow. Target oxidation plays a very important role in determining the final thickness and stoichiometry of the barrier, and is easily monitored by target voltage measurements. In addition to complete MTJ films, isolated barriers were also deposited. X-ray fluorescence (XRF) was used to determine their thickness and oxygen content.

Large magnetoresistance ratio of 10% by Fe50Co50 layers for current-confined-path current-perpendicular-to-plane giant magnetoresistance spin-valve films

We have realized a large magnetoresistance (MR) ratio of 10.2% by current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) spin-valve films having current-confined-path (CCP) structure formed by AlCu-NOL (nano-oxide-layer). CPP-GMR with conventional Co90Fe10 pinned and free layers showed an MR ratio and a ΔRA (the change of resistance area product) were 4% and 20mΩμm2, respectively, at a small RA (resistance area product) of 500mΩμm2. By replacing the Co90Fe10 layers by Fe50Co50 layers both for pinned and free layers, we have successfully realized a MR ratio and a ΔRA of 7.5% and 37.5mΩμm2, respectively, at a small RA of 500mΩμm2. Moreover, a large MR ratio of 10.2% and a large ΔRA of 418mΩμm2 were realized at a relatively large RA of 4100mΩμm2. This large MR ratio by using Fe50Co50 layers was due to a larger spin-dependent interface scattering factor γ of 0.72 for the interface between Fe50Co50 and Cu, which was improved from a γ of 0.62 for the interface between Co90Fe10 and Cu.

Low resistance spin-dependent magnetic tunnel junction with high breakdown voltage for current-induced-magnetization-switching devices

Spin-dependent magnetic tunnel junctions (MTJs) with pure AlOx barriers were fabricated by one-step and two-step natural oxidation processes, respectively (500mTorr 20min; 500mTorr 5min and 1Torr 10min). Preoxidized Al barrier thickness varies from 5to7Å. In this work, a multilayer structure with a low resistance of 0.8Ω∕sq and rms of 1.54Å was developed as the bottom electrode. MTJs with the following structure Ta(30Å)∕NiFe (40Å)∕MnIr (80Å)∕CoFe (30Å)∕Al+oxidation∕CoFe (30Å)∕NiFe (40Å)∕Ta (200Å) were magnetically annealed at 230°C for 30min to set the exchange bias field in the MnIr∕CoFe bilayer. Resistance×area (RA) products varying from 0.5to13Ωμm2 were achieved with tunneling magnetoresistance ratios varying from 8% to 18%. Breakdown voltages higher than 450mV were obtained for a sample with RA 0.5Ω×μm2, which allows a current of 9×107A∕cm2 to flow through the MTJ without damaging the barrier. Current-induced magnetization switching based on spin transfer or spin torque effect with a current density of 1.4×107A∕cm2 for a developed MTJ cell was achieved.

Fabrication of ferromagnetic single-electron tunneling devices by utilizing metallic nanowire as hard mask stencil

The stacked magnetic tunnel junctions (MTJs) are microfabricated into ferromagnetic single-electron tunneling devices (F-SETs) by using electron-beam lithography. The F-SETs have a couple of small MTJs (30×500nm2–0.1×100μm2), which are connected via a metallic nanowire. The large tunnel magnetoresistance ratio as much as 40% (at RT) and small junction area dependence of the RA (resistance×area) are obtained. The electrostatic energy of F-SETs estimated from the minimum junction area corresponds to the temperature of 1K, which is high enough to observe Coulomb blockade phenomena in a dilution refrigerator.

Electrical characteristics and interface structure of magnetic tunnel junctions with hafnium oxyfluoride barrier

We have studied the effects of fluorine inclusion on the electrical transport characteristics and interface structure of the hafnium oxide barrier in a magnetic tunnel junction. The tunneling magnetoresistance (TMR) and resistance-area (RA) as a function of oxidation time show that the TMR ratio of the hafnium oxyfluoride barrier is higher (8.3%) than that of the hafnium oxide barrier (5.7%) at their optimum conditions, and the oxyfluoride barrier junctions maintain a high TMR ratio even when the RA product increases by three orders of magnitude. X-ray photoelectron spectroscopy analysis shows that the fluorine atoms in the oxyfluoride barrier play an important role in the formation of a barrier with uniform composition. We believe that the initial fluoride layer is causing the subsequent oxygen diffusion to slow down, resulting in the formation of a defect-free hafnium oxide layer. These results are consistent with what we have found for aluminum oxyfluoride barriers.

Effect of nitrogen plasma treatment at the Al2O3/Fe interface in magnetic tunnel junction

We investigated the effects of nitrogen plasma treatment on top surface of Fe pinned layer for short times (tex=0, 10, 30, and 60 s) in magnetic tunnel junctions and annealing of the junctions. The nitrogen-treated junctions show much reduced magnetoresistance (MR) ratio and significantly lower resistance-area (RA) products compared with the untreated junction, i.e., MR≈3%, RA≈30 kΩ μm2 for tex=10 s and MR≈10%, RA≈60 kΩ μm2 for tex=0 s. The untreated junction showed enhanced MR ratio up to about 17% and higher RA (≈70 kΩ μm2) upon thermal annealing at Ta=230 °C, as expected. For the nitrogen-treated junctions, while the MR ratio also increases up to about 16% upon annealing at Ta=230 °C, which is almost the same value as the one of the optimal reference junction, the RA values of the annealed junctions still keep as low as their initial values. We believe that the redistribution of nitrogen during the annealing process is responsible for the change of properties of nitrogen-treated junction. The bias dependence of MR and the estimation of effective barrier height and thickness are studied and found to be consistent with the observed changes in nitrogen-treated junctions.

New plasma source with low electron temperature for fabrication of an insulating barrier in ferromagnetic tunnel junctions

A new plasma source, characterized as low electron temperature of 1 eV and high density of 10/sup 12/ cm/sup -3/, is introduced to the Al oxidation process in the magnetic tunnel junction (MTJ) fabrication. The MTJ fabricated with this new plasma source shows a high magnetoresistance ratio of about 50%. As a peculiar feature, the monotonous decrease of resistance area (RA) product is observed with increasing the postannealing temperature of MTJ. The decrease of the RA product is due to the decrease of the effective barrier width, which is a favorable feature to realize a low-resistance MTJ.

PtMn-based spin-dependent tunneling materials with thin alumina barrier fabricated by two-step natural oxidation

Spin-dependent tunneling (SDT) materials with bottom-pinned structure (substrate/Ta/NiFeCr/PtMn/CoFe/t Al2O3/CoFe/NiFe/Ta) are fabricated by magnetron sputtering in ultrahigh vacuum. In this study, a two-step natural oxidation was used, in which the second Al layer was deposited and naturally oxidized after the natural oxidation of the first Al layer. The top and bottom leads were also patterned into bow-tie shaped structures. The two-step oxidation process results in a perfectly decoupled pinned and free layer in a film with a total as-deposited aluminum thickness of 7 Å, whereas, the one-step oxidation process gives rise to strongly coupled magnetic layers in a film with this thickness of aluminum. By using this two-step natural oxidation technique, an optimum tunneling magnetic resistance (TMR) ratio of 29.3% and resistance×area (RA) product of 34 Ω μm2 were achieved in junctions with 8 Å barrier (5+3 Å). In conclusion, a two-step oxidation process was used to fabricate spin-dependent tunneling devices with low RA product and high TMR ratio for head applications.

Resistance–area product of diodes in a long-wavelength infrared HgCdTe mosaic array

A long wavelength infrared (LWIR) 2D (mosaic) diode array has been studied by numerically solving the diffusion equation in terms of thermally generated carriers in a n +-on-p HgCdTe diode in an array environment. The results are presented in terms of the resistance–area ( RA) product, in the diffusion-limited case. The results are compared with analytical expressions in the limiting case of the infinite diode. For a finite diode, with a definite junction depth, and a diode size that is smaller than the pitch, the RA, obtained from quasi-3D calculations, is smaller than that expected for the infinite diode case, the deviation being greater for small diodes. Commonly in the literature, the theoretical values of the infinite (1D) diode – which are overestimates – are stated as experimental targets. In the present calculations, the volume of the diode is considered to consist of two parts: one that contributes to the lateral diffusion current that is collected by the four lateral faces of the diode junction, and another that is the `normal' diffusion current, collected by the planar part of the junction from the volume `under' the diode. For the infinite diode case, only the latter component exists. The effect of the perimeter-to-area ratio on the RA in an array environment has been studied. The effective diffusion length associated with the finite diode geometry in an array differs from the standard diffusion length.

Low resistance magnetic tunnel junctions and their interface structures

Effects of interface structure and oxidation state were studied in stacked magnetic tunnel junction (MTJ) structures with top and bottom antiferromagnetic layers to obtain optimum resistance and high tunneling magnetoresistance (TMR) ratios for read heads. The roughness of the NiFe surface and the Al coverage were significantly improved by introduction of O2 surfactant gas on the Ta-seed-layer surface, which increased TMR ratios of the MTJ with low resistance area (RA) products of less than 10 Ω μm2. Furthermore, it was found that avoidance of Ni oxidation and Co oxidation at the tunnel barrier interface is essential to obtaining high TMR ratios, and that a good Al coverage and Fe–oxide formation may enhance TMR ratios when Fe-rich magnetic materials are used. For the top-type and bottom-type structures, a TMR ratio of 12%–17% with RA products of 6–7 Ω μm2 was obtained, which provides sufficient performance for read heads.

Dependence of zero-biasresistance-area product and quantum efficiency onperimeter-to-area ratio in a variable-area diode array

The dependence of the zero-bias resistance-area(RA) product and the quantum efficiency (η) ofvariable-area diode arrays is numerically calculated bysolving the diffusion equation in a cylindrical,three-dimensional geometry in the thick base approximation.The calculation is done for long-wavelength IR HgCdTen+-on-p diffusion-limited photodiodes at 77 K. Theinverse resistance-area product 1/(RA) and thesquare root of the quantum efficiency, η1/2, areplotted against the perimeter-to-area (P/A) ratio. The1/RA results are fitted to a quadratic dependence onthe P/A ratio. The dependence of the 1/RA on theminority carrier diffusion length, the junction depth and thesurface recombination velocity (SRV) is evaluated. An empiricalexpression is proposed that largely accounts for thedependence of the coefficients of the quadratic on theseparameters and is more general than those used in previousstudies. The results are also in reasonable agreement with theresults of Briggs, expressed in terms of the parameterf3D, that are valid for zero junction depth andzero SRV. The slope of the quantum efficiency versus P/Aplot, which is approximately a straight line, is related to aneffective length Lopt, that also depends on thediffusion length, junction depth, SRV and the absorptioncoefficient α. The parameter Lopt varies asα1/2.

Low resistance spin-dependent tunneling junctions with naturally oxidized tunneling barrier

We investigated the effect of aluminum thickness on RA (resistance area product) and MR ratio in spin-dependent tunnel junctions. Very low RA values below 40 /spl Omega/ /spl mu/m/sub 2/ and a maximum MR ratio of 6% were obtained in Ni/sub 81/Fe/sub 19//Al/sub 2/O/sub 3//Ni/sub 81/Fe/sub 19/ tunneling junctions fabricated using natural oxidation of aluminum. The MR ratio significantly increased with use of Co/sub 90/Fe/sub 10/ electrodes. Using natural oxidation of 6 /spl Aring/ aluminum, RA and MR ratio of these junctions are /spl sim/50 /spl Omega/ /spl mu/m/sup 2/ and /spl sim/28%, respectively. The MR ratio and the junction resistance strongly depend on the aluminum thickness. The optimum thickness of aluminum is 6-7 /spl Aring/ fur natural oxidation.