Consistent Relationship Between the Tunnel Magnetoresistance of CoFeB/MgO/CoFeB Junctions and X-Ray Diffraction Properties

We have investigated the effect of Ar pressure during MgO sputtering on the tunnel magnetoresistance (TMR) and resistance area (RA) product of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs). The TMR of MTJs with a thin MgO tunnel barrier deposited at different Ar pressures (1.3, 4, 10, and 25 mTorr) shows a consistent relationship with x-ray diffraction (XRD) properties of thick MgO films deposited with the same conditions. The deposition of the MgO-barrier at 1.3 mTorr results in a low TMR ratio and a high RA product due to the disordered MgO barrier and the oxidation of the bottom electrode of the MTJ, while the deposition at 25 mTorr results in a rough MgO barrier, and thereby gives rise to a large shift of switching field of the free layer due to the orange-peel coupling.      

Thermally stable MnIr based spin valve type tunnel junction with nano-oxide layer over 400 °C

We have investigated the annealing effect of magnetic tunnel junctions (MTJs) with or without nano-oxide layer (NOL). For MTJ without NOL, TMR ratio increased up to 300 °C and the highest value was 21.6%. On the other hand, TMR ratio of MTJ with NOL increased up to 400 °C and the highest value was 22.7%. As shown in the auger electron spectroscopy (AES) and transmission electron microscopy (TEM) results, this improved thermal stability is due to NOL in the pinned layer. Mn diffusion into Al–O barrier is blocked and interface of Al–O is smothered by NOL. These may be the main reasons of high thermal stability of MTJ with NOL.     

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.     

Interface effects in spin-dependent tunneling

In the past few years the phenomenon of spin-dependent tunneling (SDT) in magnetic tunnel junctions (MTJs) has aroused enormous interest and has developed into a vigorous field of research. The large tunneling magnetoresistance (TMR) observed in MTJs garnered much attention due to possible application in random access memories and magnetic field sensors. This led to a number of fundamental questions regarding the phenomenon of SDT. One such question is the role of interfaces in MTJs and their effect on the spin polarization of the tunneling current and TMR. In this paper we consider different models which suggest that the spin polarization is primarily determined by the electronic and atomic structure of the ferromagnet/insulator interfaces rather than by their bulk properties. First, we consider a simple tight-binding model which demonstrates that the existence of interface states and their contribution to the tunneling current depend on the degree of hybridization between the orbitals on metal and insulator atoms. The decisive role of the interfaces is further supported by studies of spin-dependent tunneling within realistic first-principles models of Co/vacuum/Al, Co/Al₂O₃/Co, Fe/MgO/Fe, and Co/SrTiO₃/Co MTJs. We find that variations in the atomic potentials and bonding strength near the interfaces have a profound effect resulting in the formation of interface resonant states, which dramatically affect the spin polarization and TMR. The strong sensitivity of the tunneling spin polarization and TMR to the interface atomic and electronic structure dramatically expands the possibilities for engineering optimal MTJ properties for device applications.     

Crossover from Kondo-assisted suppression to co-tunneling enhancement of tunneling magnetoresistance via ferromagnetic nanodots in MgO tunnel barriers

The dependence of the tunneling magnetoresistance (TMR) of planar magnetic tunnel junctions on the size of magnetic nanodots incorporated within MgO tunnel barriers is explored. At low temperatures, in the Coulomb blockade (CB) regime, for smaller nanodots the conductance of the junction is increased at low bias consistent with Kondo-assisted tunneling and the TMR is suppressed. For slightly larger nanodots but within the CB regime, the TMR is enhanced at low bias, consistent with co-tunneling. Magnetic tunnel junctions (MTJ) exhibit giant magnetoresistance in small magnetic fields that arises from the flow of spin-polarized current through an ultrathin tunnel barrier separating two magnetic electrodes. The current through an MTJ device depends on the magnetic orientation of the electrodes and is typically higher when the electrode moments are parallel than when they are antiparallel. It has recently been demonstrated that the spin polarization of the tunneling current can be greatly enhanced by using crystalline tunnel barriers formed from MgO as compared with conventional amorphous barriers formed from alumina, due to spin filtering across the MgO layer. The magneto-transport properties of magnetic granular alloys and magnetic tunnel junction devices with magnetic nanodots embedded in amorphous dielectric matrices, and tunnel barriers, respectively, have been studied by several groups, but no systematic studies of the dependence on these properties on the nanodot size have been made.     

A Novel Design and Fabrication of Magnetic Random Access Memory Based on Nano-ring-type Magnetic Tunnel Junctions

Nano-ring-type magnetic tunnel junctions （NR-MTJs） with the layer structure of Ta（5）/Ir22Mn78（10）/ Co75Fe25（2）/Ru（0.75）/CoooFe20B20（3）/Al（0.6）-oxide/Co60Fe20B20（2.5）/Ta（3）/Ru（5） （thickness unit： nm） were nano-fabricated on the Si（100）/SiO2 substrate using magnetron sputtering deposition combined with the optical lithography, electron beam lithography （EBL） and Ar ion-beam etching techniques. The smaller NR-MTJs with the inner- and outer-diameter of around 50 and 100 nm and also their corresponding NR-MTJ arrays were nano-patterned. The tunnelling magnetoresistance （TMR ＆ R） versus driving current （I） loops for a spin-polarized current switching were measured, and the TMR ratio of around 35% at room temperature were observed. The critical values of switching current for the free Co60Fe20B20 layer relative to the reference Co6oFe2oB2o layer between parallel and anti-parallel magnetization states were between 0.50 and 0.75 mA in such NR-MTJs. It is suggested that the applicable MRAM fabrication with the density and capacity higher than 256 Mbit/inch2 even 6 Gbite/inch2 are possible using both I NR-MTJ＋1 transistor structure and current switching mechanism based on based on our fabricated 4×4 MRAM demo devices.     

Tuning exchange spring effects in FePt/Fe(Co) magnetic bilayers

Structural and magnetic properties of exchange spring magnets consisting of hard magnetic (FePt) and soft magnetic (Fe and Co) bilayers, prepared by ion beam sputtering method are studied via X-ray diffraction (XRD), magneto-optic Kerr effect (MOKE) and vibrating sample magnetometry (VSM). Thin tracer layers of ⁵⁷Fe were introduced in the soft layer in order to observe the Fe spin structure and interfacial diffusion by Conversion Electron Mössbauer Spectroscopy (CEMS). The observed in-plane exchange spring behavior extends also to the magnetic hard layer, whose switching field can be tuned in an unexpected manner via the top soft magnetic layer. To explain the observed phenomenon it is suggested that the increased switching field, found in the system with a Co/Fe bilayer acting as a single soft magnetic layer, is compatible with a peculiar behavior of the stiffness coefficient of the heterogeneous soft magnetic layer. According to this observation, possibilities to maximize the exchange spring effects via suitably chosen non-homogeneous soft magnetic layers are open.     

Interfacial interaction in Fe/MgO/Fe magnetic tunneling junctions

We present a study on interfacial structures and tunneling magnetoresistance (TMR) in Fe/MgO/Fe junctions using a MgO(111) film with {100} facets. It is shown using X-ray photoelectron spectroscopy that a FeO layer occurs at MgO/Fe rather than Fe/MgO interface, which could be used to tune the TMR effect. At the Fe/MgO interface, such a change in electronic structure is attributed to the band bending associated with a change in thickness of Fe films. The present study provides a new understanding on the Fe/MgO/Fe interfacial behavior and metal/oxide barriers involving electron transport.     

Investigation on etch characteristics of nanometer-sized magnetic tunnel junction stacks using a HBr/Ar plasma

The etch characteristics of CoFeB magnetic films and magnetic-tunnel-junction (MTJ) stacks masked with Ti films were investigated using an inductively coupled plasma reactive ion etching in a HBr/Ar gas mix. The etch rate, etch selectivity, and etch profile of the CoFeB films were obtained as a function of the HBr concentration. As the HBr gas was added to Ar, the etch rate of the CoFeB films, and the etch selectivity to the Ti hard mask, gradually decreased, but the etch profile of the CoFeB films was improved. The effects of the HBr concentration and etch parameters on the etch profile of the MTJ stacks with a nanometer-sized 70 x 100 nm2 pattern were explored. At 10% HBr concentration, low ICP RF power, and low DC-bias voltage, better etch profiles of the MTJ stacks were obtained without redeposition. It was confirmed that the protective layer containing hydrogen, and the surface bombardment of the Ar ions, played a key role in obtaining a steep sidewall angle in the etch profile. Fine-pattern transfer of the MTJ stacks with a high degree of anisotropy was achieved using a HBr/Ar gas chemistry.     

Characterization of Domain Switching Behavior of MTJ Cells Using Magnetic Force Microscopy (MFM) and R–H Loop Analysis

Correlation between electrical and magnetic properties of magnetic tunnel junctions (MTJ) for magnetic random access memory (MRAM) was studied. The MTJ (Ta/NiFeCr/ PtMn/CoFe/Ru/CoFe/Al₂O₃/CoFe/NiFe/Ta) was analyzed by utilizing R-H loops and MFM images. We verified that a kink in an R-H loop comes from a vortex domain of free layer. In addition, we also observed a close relationship between a domain switching behavior and an irregular R-H curve. These results would be useful for the characterization of the MTJ cell, thereby optimizing the process to realize an ultrahigh density MRAM.     

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%).     

Spin transfer torque and tunneling magnetoresistance dependences on finite bias voltages and insulator barrier energy

We investigate the dependence of perpendicular and parallel spin transfer torque (STT) and tunneling magnetoresistance (TMR) on the insulator barrier energy of the magnetic tunnel junction (MTJ). We employed the single orbit tight binding model combined with the Keldysh non-equilibrium Green's function method in order to calculate the perpendicular and parallel STT and the TMR in the MTJ with finite bias voltages. The dependences of the STT and TMR on the insulator barrier energy are calculated for semi-infinite half metallic ferromagnetic electrodes. We find a perfect linear relation between the parallel STT and the tunneling current for a wide range of insulator barrier energy. Furthermore, the TMR also depends on the insulator barrier energy, contradicting Julliere's simple model.     

MgO-Based Tunnel Junction Material for High-Speed Toggle Magnetic Random Access Memory

We report the first demonstration of a magnetoresistive random access memory (MRAM) circuit incorporating MgO-based magnetic tunnel junction (MTJ) material for higher performance. We compare our results to those of AlOx-based devices, and we discuss the MTJ process optimization and material changes that made the demonstration possible. We present data on key MTJ material attributes for different oxidation processes and free-layer alloys, including resistance distributions, bias dependence, free-layer magnetic properties, interlayer coupling, breakdown voltage, and thermal endurance. A tunneling magnetoresistance (TMR) greater than 230% was achieved with CoFeB free layers and greater than 85% with NiFe free layers. Although the TMR with NiFe is at the low end of our MgO comparison, even this MTJ material enables faster access times, since its TMR is almost double that of a similar structure with an AlO$_ x$barrier. Bit-to-bit resistance distributions are somewhat wider for MgO barriers, with sigma about 1.5% compared to about 0.9% for AlO$_ x$. The read access time of our 4 Mb toggle MRAM circuit was reduced from 21 ns with AlO$_ x$to a circuit-limited 17 ns with MgO.     

Spin-transfer switching current distribution and reduction in magnetic tunneling junction-based structures

Spin transfer switching current distribution within a cell and switching current reduction were studied at room temperature for magnetic tunnel junction-based structures with resistance area product (RA) ranged from 10 to 30 /spl Omega/-/spl mu/m/sup 2/ and TMR of 15%-30%. These were patterned into current perpendicular to plane configured nanopillars having elliptical cross sections of area /spl sim/0.02 /spl mu/m/sup 2/. The width of the critical current distribution (sigma/average of distribution), measured using 30 ms current pulse, was found to be 3% for cells with thermal factor (KuV/k/sub B/T) of 65. An analytical expression for probability density function p(I/I/sub c0/) was derived considering a thermally activated spin transfer model, which supports the experimental observation that the thermal factor is the most significant parameter in determining the within-cell critical current distribution. Spin-transfer switching current reduction was investigated through enhancing effective spin polarization factor /spl eta//sub eff/ in magnetic tunnel junction-based dual spin filter (DSF) structures. The intrinsic switching current density (J/sub c0/) was estimated by extrapolating experimental data of critical current density (J/sub c/) versus pulse width (/spl tau/), to a pulse width of 1 ns. A reduction in intrinsic switching current density for a dual spin filter (DSF: Ta/PtMn/CoFe/Ru/CoFeB/Al2O3/CoFeB/spacer/CoFe/PtMn/Ta) was observed compared to single magnetic tunnel junctions (MTJ: Ta/PtMn/CoFe/Ru/CoFeB/Al2O3/CoFeB/Ta). J/sub c/ at /spl tau/ of 1 ns (/spl sim/J/sub c0/) for the MTJ and DSF samples were 7/spl times/10/sup 6/ and 2.2/spl times/10/sup 6/ A/cm/sup 2/, respectively, for identical free layers. Thus, a significant enhancement of the spin transfer switching efficiency is seen for DSF structure compared to the single MTJ case.     

A two-axis magnetometer using a single magnetic tunnel junction

Based on a qualitative study of the Stoner-Wohlfarth model, we point out that driving a magnetic tunnel junction (MTJ) with an alternative two-dimensional magnetic field allows to measure simultaneously two components of an external magnetic field. Only one single MTJ without a pinning layer is needed to measure both components of a magnetic field parallel to the junction plane. The response of the magnetometer does not depend on the resistance of the junction or the amplitude of its variations. A prototype has been manufactured and encouraging experimental results are presented. Sensitivities higher than 500 V/T and a noise level of 2 /spl mu/T//spl radic/Hz are reported.     

Possible Spin Pumping Effects on Spin Torque Induced Magnetization Switching in Magnetic Tunneling Junctions

Dependence of spin torque induced magnetization switching upon interfacial insulating layers properties of magnetic tunneling junctions (MTJ) are studied. For the same magnetic properties and patterning geometric dimensions, changes in MTJ interfacial insulating layers properties reveal interesting magnetization switching behaviors. These behaviors cannot be explained by conventional Landau-Lifshitz-Gilbert equation with a spin torque term and an intrinsic ferromagnetic relaxation damping. However the magnetization switching dynamics can be understood through assumption of spin pumping effects in magnetic tunneling junctions. This is not only important for fundamental understanding of spin and electronic transport in MTJ but also important for practical trade-offs between critical switching current and MTJ resistance for spin torque random access memory.     

Tunneling current-induced butterfly-shaped domains and magnetization switching in DBMTJs

Double barrier magnetic tunnel junctions (DBMTJs) with the layer architecture of Ta (5 nm)/Cu (20 nm)/Ni/sub 79/Fe/sub 21/(10 nm)/Ir/sub 22/Mn/sub 78/ (12 nm)/Co/sub 75/Fe/sub 25/ (4 nm)/Al (0.9 nm)-oxide/Ni/sub 79/Fe/sub 21/(3 nm)/Al (0.9 nm)-oxide/Co/sub 75/Fe/sub 25/(4 nm)/Ir/sub 22/Mn/sub 78/ (12 nm)/Py(10 nm)/Cu(30 nm)/Ta(5 nm) were mircofabricated. At room temperature, TMR ratio of 18.7% and 28.4%, resistance-area products(RS) of around 10.3 and 12.7 k/spl Omega//spl mu/m/sup 2/ and coercivity H/sub C/ of 17.5 and 2.0 Oe, were obtained for the DBMTJ at the as-deposited state and the after annealing state respectively. The micromagnetics simulations show that the dynamic butterfly-shaped domains and magnetization switching may occur in the free layer when a tunneling current of the order of 1 to 20 mA passes though the DBMTJ. It decreases the magnetization in the free layer, which may be the one of the reasons of the low TMR ratio observed in the DBMTJ.     

A Three-Terminal Approach to Developing Spin-Torque Written Magnetic Random Access Memory Cells

Using a self-aligned fabrication process together with multiple-step aligned electron beam lithography, we have developed a nanopillar structure where a third contact can be made to any point within a thin-film multilayer stack. This substantially enhances the versatility of the device by providing the means to apply independent electrical biases to two separate parts of the structure. Here, we demonstrate a joint magnetic spin-valve (SV)/tunnel junction structure sharing a common free layer nanomagnet contacted by this third electrode. A spatially nonuniform spin-polarized current flowing into the free layer via the low-resistance SV path can reverse the magnetic orientation of the free layer as a consequence of the spin-torque (ST) effect, by nucleating a reversal domain at the spin injection site that propagates across the free layer. The free layer magnetic state can then be read out separately via the higher resistance magnetic tunnel junction (MTJ). This three-terminal structure provides a strategy for developing high-performance ST magnetic random access memory (ST-MRAM) cells, which avoids the need to apply a large voltage across a MTJ during the writing step, thereby enhancing device reliability, while retaining the benefits of a high-impedance MTJ for read-out.      

Magneto-optical switching in nonlinear all-garnet magnetophotonic crystals

Magnetic field control over the optical switching is demonstrated for a nonlinear all-garnet magnetophotonic crystal with a microcavity (MC) layer possessing both third-order Kerr nonlinearity and magnetooptical activity. A significant enhancement of the effective refractive index of the microcavity layer is observed, that results in the light-induced spectral shift of the MC mode of 2 nm. It is shown that application of the external magnetic field leads to an increase of switching contrast by a factor of two. Possible mechanisms of photorefractive effect involved in the optical switching in all-garnet magnetophotonic crystals are discussed.