Spin-dependent Tunnel Junctions Research Articles

Growth of FeSe on general substrates by metal-organic chemical vapor deposition and the application in magnet tunnel junction devices

Single-phase tetragonal FeSe films were grown on c-plane sapphire, SiO 2, GaAs (100) and Si (100) substrates by low-pressure metal-organic chemical vapor deposition method. X-ray diffraction analysis shows that all the FeSe thin films on different substrates are of (00l) orientation. Spin-dependent magnet tunnel junction with Fe/ZnSe/FeSe structure were fabricated, and the tunneling magnetic resistance ratio decreased with increasing the thickness of ZnSe layer in the range of 10–20 nm.

Measurement of magnetic parameters of nanometer-thick conducting magnetic films using anisotropic magnetoresistive effect

The angular dependences of anisotropic magnetoresistance (AMR) are measured in conducting ferromagnetic films of nanometer thickness and layered structures containing such films and having the shape of narrow ribbons. These structures are used for preparing spin-dependent magnetic tunnel junctions possessing a giant magnetoresistance. The possibility of determining the main magnetic parameters, which are important for preparing magnetic junctions, by AMR angular measurements is demonstrated experimentally. The magnetic anisotropy axis, the saturating magnetic field, and the coercivity are determined in a 25-nm-thick permalloy (Py) film, in the structures FeMn film (15 nm)-Py film (10 nm) deposited by RF magnetron sputtering on a oxidized silicon substrate, as well as in the structure FeMn (15 nm)-Py (10 nm)-SiC (1.5 nm)-Py (10 nm) deposited on a sitall substrate. It is shown that, under the same conditions of Py films deposition, the magnetic anisotropy axis in the FeMn-Py structure is turned through 90° relative to the anisotropy axis of Py in structures without FeMn layers. The value of the exchange bias fields of the magnetization reversal measured in the structure FeMn (15 nm)-Py (10 nm)-SiC (1.5 nm)-Py (10 nm) by the AMR method is in good agreement with the result of measurement by the inductive method.

Formation of tunnel barrier using a pseudo-atomic layer deposition method and its application to spin-dependent tunneling junction

The tunneling barrier is crucial to the overall performance in magnetic tunnel junctions. We have suggested a new formation method for the tunnel barrier, which has utilized pseudo-atomic layer deposition with sputtering. As is well known, all metallic thin films oxidize more or less under atmospheric conditions. Using this phenomenon, an ultra-thin metallic layer was prepared and exposed to the oxygen ambient repeatedly to reach a desired thickness for the tunnel barrier. From transmission electron microscopy, the tunnel barrier has been confirmed to have a clear and smooth interface between magnetic layers and the tunnel barrier. From atomic force microscopy, it has also been confirmed to have a low surface roughness. The fabricated magnetic tunnel junction has been shown to exhibit tunnel resistivities from 60 to 92 kΩ μm2 and a maximum tunneling magnetoresistance ratio of 40%.

Magnetostriction effect of amorphous CoFeB thin films and application in spin-dependent tunnel junctions

CoFeB thin films and magnetic tunnel junctions using them are studied for magnetostriction effect. The single-layer films were sputter deposited with excellent soft magnetic properties including a high saturation magnetization of 1.5T, a near-zero hard axis coercivity, a low easy axis coercivity of 2.0Oe, and an induced magnetic anisotropy field of 32Oe. The saturation magnetostriction constant is measured to be 31ppm. Magnetic tunnel junctions (MTJs) were fabricated and tested for potential strain gauge applications. The gauge factor for the magnetostrictive MTJs, a measure of strain sensitivity, is many times of the best piezoresistive devices.

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.

Field orientation dependence of magnetoresistance in spin-dependent tunnel junctions

The dependence of magnetotransport on field orientation is an important issue in spintronics-related devices where the applied field is not necessarily in the ideal field-in-plane (FIP) geometry. In this study, we perform tunneling magnetoresistance (TMR) measurements on Co-Al/sub 2/O/sub 3/-CoFe-NiFe spin-dependent tunnel (SDT) junctions prepared at different conditions with varying field orientation ranging from FIP to field-perpendicular-to-plane (FPP). The TMR ratio decreases drastically, whereas the switching field of Co increases when the field direction is set close to FPP. Furthermore, in a situation near FPP, a peculiar TMR looping behavior is observed for one set of samples. Interface effect is thought to be related.

Use of the Anisotropic Magnetoresistance Effect for Direct Measurement of the Coercivity and Exchange Bias Fields of Magnetization Reversal in Conducting Magnetic Films of Nanometer Thickness

The anisotropic magnetoresistance effect has been used for direct measurement of the coercivity and exchange bias fields of magnetization reversal in conducting ferromagnetic films of nanometer thickness and in sandwich structures containing such films, which are used in spin dependent tunneling junctions featuring colossal magnetoresistance. The measurements have been performed for 25-nm-thick permalloy (Py) films obtained by RF magnetron sputtering on oxidized silicon substrates and for FeMn(15 nm)/Py(10 nm)/SiC(1.5 nm)/Py(10 nm) structures on glass ceramic substrates. The results of measurements performed using the proposed method are in satisfactory agreement with the data obtained by the induction method.

70% TMR at Room Temperature for SDT Sandwich Junctions With CoFeB as Free and Reference Layers

Spin dependent tunneling (SDT) wafers were deposited using dc magnetron sputtering. SDT junctions were patterned and connected with one layer of metal lines using photolithography techniques. These junctions have a typical stack structure of Si(100)-Si/sub 3/N/sub 4/-Ru-CoFeB-Al/sub 2/O/sub 3/-CoFeB-Ru-FeCo-CrMnPt with the antiferromagnet CrMnPt layers for pinning at the top. High-resolution transmission electron microscopy (HRTEM) reveals that the CoFeB has an amorphous structure and a smooth interface with the Al/sub 2/O/sub 3/ tunnel barrier. Although it is difficult to pin the amorphous CoFeB directly from the top, the use of a synthetic antiferromagnet (SAF) pinned layer structure allows sufficient rigidity of the reference CoFeB layer. The tunnel junctions were annealed at 250/spl deg/C for 1 h and tested for magneto-transport properties with tunnel magnetoresistive (TMR) values as high as 70.4% at room temperature, which is the highest value ever reported for such a sandwich structure. This TMR value translates to a spin polarization of 51% for CoFeB, which is likely to be higher at lower temperatures. These junctions also have a low coercivity (Hc) and a low parallel coupling field (Hcoupl). The combination of a high TMR, a low Hc, and a low Hcoupl is ideal for magnetic field sensor applications.

Thickness dependence of magnetic coupling strength and thermal stability in a spin-dependent tunnel junction

The change of magnetic coupling strength between two ferromagnetic layers, separated by an insulating barrier, was investigated as a function of the barrier thickness (T B ) and thermal annealing temperatures. The magnetic junctions consist of Ta/CoFe/AlO x /NiFe/Ta layers with three different nominal thickness of T B = 1.3, 1.6, and 2.0 nm. Isothermal magnetization at room temperature revealed that, while the junction with a lower T B showed a higher magnetic coupling strength, thermal annealing at T = 225 °C increased (and diminished) the coupling strength of the junctions with T B = 1.3 and 1.6 nm (and 2.0 nm), respectively. This observation was utilized to understand consistently the magneto-resistance behavior and specific junction resistance of the junctions as a function of thermal annealing temperature. This study demonstrated that the physical properties of a magnetic tunnel junction, such as magneto-resistance ratio, specific junction resistance and their thermal stability, were substantially influenced by the insulating barrier structure as well as the interface quality between the layers.

He-RBS, He-ERDA and heavy ion-ERDA analysis of Si/Ta 70 Å/CoFe 35 Å/HfAlO x/CoFe 35 Å/Ta 30 Å systems

Low resistance spin-dependent tunnel junctions are investigated for read-head applications due to their large tunnelling magnetoresistance effect and low junction resistance-area product. Typical full junction structures can be Ta 70 Å/NiFe 70 Å/MnIr 80 Å/CoFe 35 Å/Hf x Al y O z 10 Å/CoFe 35 Å/NiFe 40 Å/Ta 30 Å, annealed to temperatures up to 250 °C. We analyzed simpler structures with RBS experiments at a grazing angle of incidence in order to study the junction only. Heavy ion- and He-ERDA experiments were also done. The composition of the HfAlO barrier is determined, showing that a slightly substoichiometric oxide is formed. On annealing at 240 °C, the interfaces of the barrier become sharper. TEM results confirm that the barrier is continuous after annealing. This is related to an increase in the tunnel magnetoresistance signal from 4% to 13.5% after annealing.

Spin-dependent tunneling junctions with superparamagnetic sensing layers

Spin-dependent tunneling (SDT) junctions with superparamagnetic (SPM) sensing layers have been made using sputtering deposition and photolithography fabrication techniques. The SPM-SDT junctions have zero hysteresis with reasonable sensitivity, and the output signal is bipolar relative to the external magnetic field without using any biasing-an inherent advantage in using a unidirectional pinned layer and an in-plane-isotropic SPM free layer in the SDT structure. This novel SPM-SDT junction is the first of its kind ever reported and offers great potential for commercial applications. The SPM-SDT devices are especially suitable for low-power and low-field applications such as solid-state compassing, magnetic medium detection, nondestructive evaluation, biomedical sensing, and mobile electronics device applications.

Simultaneous magnetic force microscopy and magnetoresistance characterization of a magnetic tunnel junction with in situ applied field

For the first time, both the magnetoelectronic properties and the magnetic domain structure of a magnetic tunnel junction (MTJ) have been characterized simultaneously. In situ tunneling magnetoresistance measurements for a spin-dependent tunnel junction were directly compared to magnetic domain behavior imaged using magnetic force microscopy (MFM) under the same applied fields, allowing for detailed investigation of electronic behavior relative to magnetic behavior. From the results, it was found that the resistance in the hysteretic field range (less than the coercivity of Hc=800 A/m or 10 Oe) corresponded with the complex magnetic domain behavior observed in the MFM images and qualitatively matched micromagnetically modeled junctions.

Spin dependent tunneling junctions with reduced Neel coupling

A new structure of spin dependent tunneling (SDT) junctions has been demonstrated to have a much reduced Neel coupling field between the free and pinned ferromagnetic layers comparing with conventional SDT structures. The new structure consists of a modified synthetic-antiferromagnetic composite layer as the pinned layer with two Ru spacer layers and three ferromagnetic layers. The Neel coupling field is much reduced for both top- and bottom-pinned SDT structures using this new composite pinned layer. Furthermore, the net magnetic moment is kept at zero for the composite pinned layer to minimize the fringe field after patterning. The coupling reduction can be understood by considering the additive contribution from the first two interfaces with Ru in the composite pinned layer, which cancels that from the pinned layer interface with the barrier. By properly spacing these three most important interfaces, reducing the coupling to basically zero is realized. The coupling reduction allows the elimination of an on-chip bias coil used to correct the coupling, therefore simplifying the electronics and reducing the power to operate the SDT sensors. The new SDT structure has potential impacts on many SDT and spin valve devices such as magnetoresistive sensors, galvanic isolators, magnetic logic, and MRAM devices.

Continuous thin barriers for low-resistance spin-dependent tunnel junctions

The occurrence of pinholes in thin barrier low-resistance junctions degrades the TMR signal and increases the coupling field between pinned and free layers. The tunnel junction coupling field (Hf), junction resistance and TMR signal dependence on the barrier thickness was studied for various types of barriers (HfOx,HfAlOx,ZrAlOx,AlOx). Micromagnetic simulation was employed to simulate the coupling field versus pinhole density. From the coupling field results, HfOx makes the thinnest continuous barriers, followed by doped HfAlOx and ZrAlOx, and then AlOx. HfAlOx and ZrAlOx offer the best compromise between low resistance (1–5 Ω μm2) and reasonable TMR (12%–14%). Pure HfOx can be made with R×A products of 0.4 Ω μm2 but the TMR does not exceed 5.5%.

Magnon scattering and tunneling through localized states in magnetic tunnel junctions

Spin-dependent tunnel junctions with Co75Fe25 electrodes and a Fe50Mn50 pinning layer were fabricated with tunneling magnetoresistance of about 20% at room temperature. The Al2O3 barriers were formed by ultra-violet light assisted oxidation. The resistance × area (R×A) product was about 375 kΩ μm2 in the parallel alignment of the magnetizations. The effect of barrier impurities has been investigated via tunneling conductance as a function of temperature and bias voltage for as-deposited and annealed samples. The spin wave parameter α was determined to be 3.06×10−5 and 2.03×10−5 K−3/2 before and after annealing, respectively. The improved barrier properties after annealing are explained by inelastic hopping via several localized states and reduced magnon scattering. We propose a qualitative model of the barrier homogenization during annealing which supports former Rutherford backscattering and x-ray photoelectron spectroscopy studies.

Enhancement of switching stability of tunneling magnetoresistance systems with artificial ferrimagnets

In the study of spin dependent magnetic tunneling junctions, the switching stability of the magnetically hard layer is a crucial issue for long-term use in magnetic random access memory. After N switching cycles of the soft layer, the hard layer would be demagnetized due to the stray field from the domain wall created during switching of the soft layer. Therefore, reducing the stray field from the soft layer is the way to increase switching stability. In this study, we propose a structure which replaces the usual soft layer (typically permalloy, Fe, or Co) with an artificial ferrimagnet to reduce the stray field. The artificial ferrimagnet consists of a trilayer with an interlayer that antiferromagnetically couples two ferromagnetic layers of unequal thickness. The total stray field from the artificial ferrimagnet structure can be approximated as the sum of the stray fields from the two ferromagnetic layers. Since the sign of the stray field of the two layers is opposite, due to antiferromagnetic coupling, the total stray field is reduced due to cancellation. Since the magnitude of the stray field depends on the magnetic properties and the thickness of each layer and the distance from the magnetic layer, we can tailor the two magnetic layers of the artificial ferrimagnet structure to minimize the total stray field.

Low-resistance spin-dependent tunnel junctions with HfAlO/sub x/ barriers for high-density recording-head application

Spin-dependent tunnel junctions with the structure (Ta 70 /spl Aring//NiFe 70 /spl Aring//MnIr 80 /spl Aring//CoFe 35 /spl Aring//HfAlO/sub x//CoFe 35 /spl Aring//NiFe 40 /spl Aring//TiW(N) 150 /spl Aring/) were fabricated on top of 600-/spl Aring/-thick ion-beam-smoothed low-resistance Al electrodes. HfAlO/sub x/ barriers were formed by natural oxidation (5 min at 1 torr in pure O/sub 2/) of 5-/spl Aring/-thick (2-/spl Aring/ Hf+3-/spl Aring/ Al) films or 6-/spl Aring/-thick (2-/spl Aring/ Hf+4-/spl Aring/ Al) films. Resistance/spl times/area (R/spl times/A) products of 0.65 /spl Omega//spl times//spl mu//sup 2/ and 2.1 /spl Omega//spl times//spl mu/m/sup 2/ were achieved with 9.5% and 13.5% tunnel magnetoresistance signal (TMR), respectively. Current inhomogeneity effects on the measured (R/spl times/A) products and TMR values were calculated in particular for junctions with resistance below 1 /spl Omega//spl times//spl mu/m/sup 2/. Transmission electron microscopy indicates that HfAlO/sub x/ forms a continuous amorphous barrier that follows conformally the topography of the bottom electrode. X-ray photoelectron spectroscopy analysis indicates that 2.5% metallic Hf is left inside the barrier closer to the bottom electrode. These low-resistance tunnel junctions are attractive for read-head applications at recording densities above 100 Gbit/in/sup 2/.

Determination of the thickness of Al2O3 barriers in magnetic tunnel junctions

The barrier thickness in magnetic spin-dependent tunnel junctions with Al2O3 barriers has been measured using grazing incidence x-ray reflectivity and by fitting the tunneling current to the Simmons model. We have studied the effect of glow discharge oxidation time on the barrier structure, revealing a substantial increase in Al2O3 thickness with oxidation. The greater thickness of barrier measured using grazing incidence x-ray reflectivity compared with that obtained by fitting current density–voltage to the Simmons electron tunneling model suggests that electron tunneling is localized to specific regions across the barrier, where the thickness is reduced by fluctuations due to nonconformal roughness.

Effect of natural oxidation conditions on low resistance spin tunnel junctions

Spin dependent tunnel junctions with AlOx barrier were fabricated by in situ natural oxidation of a 7 Å (or 8 Å) thick Al film. Junctions were deposited on top of preannealed, ion-beam smoothed, 600 Å thick Al bottom electrodes. By changing the oxidation time from 10 min to 10 h with the oxygen pressure of 10 and 100 Torr, the top-pinned junctions have resistances varying from 31 to 406 Ω μm2 and the tunneling magnetoresistance (TMR) varies from 8.5% to 19.6% for the as deposited state. For the bottom-pinned junctions, the oxygen pressure was varied from 0.5 to 100 Torr, and oxidation time ranged from 5 min to 2 h. Bottom-pinned junction resistances as low as 10–12 Ω μm2 were obtained with TMR ranging from 14% to 17% for the junctions oxidized at the lower pressure (0.5 Torr). Rutherford backscattering analysis (RBS) indicates an O/Al ratio of 1.29±0.34 denoting incomplete oxidation of the Al for junctions oxidized at the lower pressures and short times. Junctions oxidized at higher pressures (⩾10 Torr) can reach 25%–30% TMR, with resistances ranging from 30 to 70 Ω μm2. RBS shows near stoichiometric Al2O3 oxide composition (O/Al=1.51±0.43) in these barriers.

Low resistance spin-dependent tunnel junctions with ZrAlOx barriers

Spin-dependent tunnel junctions with ZrAlOx barriers were fabricated with low resistance×area product 4 Ω×μm2, and tunnel magnetoresistance of 15.2%. Barrier fabrication was done by natural oxidation (5 min, at oxidation pressures ranging from 0.5 to 10 Torr). The junctions were deposited on top of 600 Å thick, ion beam smoothed, low resistance, Al electrodes. X-ray photoelectron spectroscopy analysis indicates the presence of AlOx, ZrO2, some remnant metallic Zr, but no metallic Al in the as-deposited barriers. High resolution transmission electron microscopy indicates that ZrAlOx forms an amorphous barrier that is smoother than pure crystalline ZrOx or pure amorphous AlOx barriers. These low resistance tunnel junctions are attractive for read head applications above 100 Gbit/in2 where competitive signal to noise ratios imply resistance×area product below a few Ω×μm2, and tunneling magnetoresonance signals near or above 20%.