High-density Magnetic Random Access Research Articles

Voltage controllable Pt/Co/AlOx superparamagnetic films

Electrochemical reaction induced by gate voltage between metallic cobalt and cobalt oxides was found to remarkably reshape average volume of superparamagnetic particles in Pt/Co/AlOx stacks at room temperature and thus modulate superparamagnetic properties, such as saturation fields and saturated magnetization, of the superparamagnetic films. The interfacial reaction was indicated in anomalous Hall transport measurement. This voltage-controllable superparamagnetism observed in Pt/Co/AlOx films could be utilized in a new architecture of nonvolatile and high density magnetic random access memories or neuromorphic computing.

Impact of boron diffusion at MgO grain boundaries on magneto-transport properties of MgO/CoFeB/W magnetic tunnel junctions

Controlling the B diffusion in CoFeB-MgO based magnetic tunnel junctions (MTJs) is important to realize the large tunneling magnetoresistance (TMR) ratio, which is required to realize high density magnetic random access memories (MRAMs). Here we investigate the detailed microstructure of the MgO-CoFeB MTJs having W underlayer and overlayer in comparison with well-studied MTJs with Ta underlayer and overlayer. Detailed microstructure analysis based on transmission electron microscopy and electron energy loss spectroscopy revealed that B diffused into Ta under-/overlayers and crystallization of CoFeB to CoFe efficiently occurred upon annealing at 300 °C and 400 °C in MTJs with Ta underlayers. In contrast, B diffused into the [001] tilt grain boundaries of MgO barrier in the case of MTJs with W under-/overlayers. These structural differences well explain the difference in post annealing temperature dependence of TMR ratio and resistance area product (RA) of MTJs with Ta and W under-/overlayers.

Nanoscale Spin-Transfer Torque MRAM Etching Using Various Gases

As the volatile dynamic random access memory (DRAM) device faces a physical limitation, new memory devices are actively investigated to solve this problem. Among them, spin transfer torque magnetic random access memory (STT-MRAM) has received a lot of attention because of its very high speed of device, endurance, low power consumption of device, and high density memory cell in addition to the non-volatility of information. Especially, the MTJ memory elements which consist of CoxFeyBz/MgO/CoxFeyBz multilayer structure are the critical building blocks for high density magnetic random access memory with high tunneling magnetoresistance(TMR). However, the dry etching of MTJ stack with sub 70 nm is a challenging step in the fabrication of STT-MRAM. The etching of nanoscale MTJ induces oxidation in magnetic layer, pattern degradation and damage or sidewall redeposition at patterned MTJ sidewall especially. In this study, for successful etching of MTJ and MTJ related materials, RIB (reactive ion beam) etching using NF3, CH3OH, and CO/NH3 gas mixture and ICP etching using Ar, He, CO/NH3 and so on were conducted. Etch characteristics of magnetic tunnel junction (MTJ) stacks masked with W were investigated precisely using field emission scanning electron microscopy (FE-SEM) and TEM (Transmission electron microscopy). Simulation of MTJ material and hardmask material was conducted with SRIM simulation to observe tendency with etch process of blank unit thin film samples. In addition, the magnetic hysteresis curves were analyzed by a vibrating sample magnetometer (VSM). The results showed that, among the various gas combinations, the use of He gas improved etch selectivity of MTJ materials over W and also improved the etch profile of MTJ features. Also the He etch gas could be beneficial in reducing damage. The detailed etch mechanism and results on the etching of MTJ materials will be presented.

A finite element based real-space phase field model for domain evolution of ferromagnetic materials

This paper proposes a real-space phase field model solved by finite element method for giant magnetostrictive materials. The model is based on the thermodynamic theory of ferromagnetic materials and employs the time-dependent Ginzburg-Laudau (TDGL) equation to predict the domain evolution process. We have derived the corresponding finite element formulation which takes the mechanical displacement, the magnetic potential, and the magnetization vector as the field variables, according to variational principle. A multi-field coupling finite element has been developed through the ABAQUS user element subroutine module to simulate the coupling between mechanical and magnetic characteristics for microstructures with arbitrary geometries and boundary conditions. The simulation results of a general magnetic nanoring coincide well with the reports in literature, which confirms the validity of the phase field method developed. The magnetization processes have also been investigated for nanorings with different geometries. The results suggest that the vortex chirality and multi-stable magnetization states can be controlled by changing both the symmetry of loading situation and the geometrical configuration. The phase field model solved by finite element method is expected to be a useful tool for the design of high density magnetic random access memories.

Co/Ni]-CoFeB hybrid free layer stack materials for high density magnetic random access memory applications

Alternative free layer materials with high perpendicular anisotropy are researched to provide spin-transfer-torque magnetic random access memory stacks' sufficient thermal stability at critical dimensions of 20 nm and below. We demonstrate a high tunnel magetoresistance (TMR) MgO-based magnetic tunnel junction stack with a hybrid free layer design made of a [Co/Ni] multilayer and CoFeB. The seed material on which the [Co/Ni] multilayer is deposited determines its switching characteristics. When deposited on a Pt seed layer, soft magnetic switching behavior with high squareness is obtained. When deposited on a NiCr seed, the perpendicular anisotropy remains high, but the squareness is low and coercivity exceeds 1000 Oe. Interdiffusion of the seed material with the [Co/Ni] multilayers is found to be responsible for the different switching characteristics. In optimized stacks, a TMR of 165% and low resistance-area (RA) product of 7.0 Ω μm2 are attained for free layers with an effective perpendicular magnetic anisotropy energy of 1.25 erg/cm2, which suggests that the hybrid free layer materials may be a viable candidate for high density magnetic random access memory applications.

Cu-Containing [Pt/Co] Multilayers With Low Saturation Magnetization Suitable for the Pinned Structure

We prepared [Pt/(Co, Cu)] multilayers with a very thin (i.e., 0.2 nm thick) Pt layer and a thicker (i.e., 0.4, 0.5, or 0.6 nm thick) (Co, Cu) layer and investigated the effects of the Cu content in the (Co, Cu) layer and the postannealing temperature on the magnetic properties of the multilayers. The samples in the as-deposited state and those annealed up to 500 °C for 1 h all exhibited perpendicular magnetic anisotropy (PMA). The PMA strength and saturation magnetization both decreased with increasing content of Cu in the samples. The saturation magnetization of the Cu containing multilayers annealed at 500 °C substantially decreased, probably owing to the change in the microstructure from a layered structure in the as-deposited samples to a bulk phase in the annealed ones, as supported by high resolution transmission electron microscopy images. The effective PMA energy density of the samples annealed at high temperatures, on the other hand, remained nearly unchanged and even increased slightly indicating a high postannealing stability of Cu containing multilayers. The Cu-containing multilayers may find potential application to high density magnetic random access memory as the pinned structure of magnetic tunnel junctions because the multilayers showed low saturation magnetizations in the range 164-753 emu/cm\(^{3}\) and high postannealing stability.

The dipolar interaction in CoFeB/MgO/CoFeB perpendicular magnetic tunnel junction

Ultrathin CoFeB/MgO/CoFeB system with perpendicular magnetic anisotropy is a promising candidate for the high density magnetic random access memory. However, a dipolar interaction between the CoFeB layers may introduce a minor loop shift (Hs) and causes uncertainty during the operation. In this report, we systematically studied the dipolar effect in these structures and found that the coupling may be either ferromagnetic or antiferromagnetic (15 Oe > Hs > −15 Oe) depending upon the CoFeB thickness (0.9–1.4 nm). A modified Fabry-Perot model, which accounts the Bloch wave interference, may explain the present observations of the dipolar effect in the perpendicular junctions of CoFeB/MgO/CoFeB.

Influence of thermal agitation on the electric field induced precessional magnetization reversal with perpendicular easy axis

We investigated the influence of thermal agitation on the electric field induced precessional magnetization switching probability with perpendicular easy axis by solving the Fokker-Planck equation numerically with finite difference method. The calculated results show that the thermal agitation during the reversal process crucially influences the switching probability. The switching probability can be achieved is only determined by the thermal stability factor Δ of the free layer, it is independent on the device dimension, which is important for the high density device application. Ultra-low error rate down to the order of 10−9 can be achieved for the device of thermal stability factor Δ of 40. Low damping factor α material should be used for the free layer for high reliability device applications. These results exhibit potential of electric field induced precessional magnetization switching with perpendicular easy axis for ultra-low power, high speed and high density magnetic random access memory (MRAM) applications.

Effects of defects on the switching properties of the nanostructured cells of a single layer and a synthetic ferrimagnet

The switching properties of nanostructured magnetic cells containing defects are investigated by using micromagnetic simulation. A particular emphasis is placed on the comparison of the results for the cells of a single magnetic layer (SL) and a synthetic ferrimagnet (SyF). A different switching behavior is observed in the relatively large cell sizes; the SL cells switch incoherently, but the SyF cells switch coherently, even though the thickness asymmetry of the SyF is quite large. The formation of vortices around the defects is mainly responsible for the incoherent switching in the SL cells, while the formation of a flux-closure structure is behind the reason for the coherent rotation in the SyF cells. At a small cell, suitable for high density magnetic random access memory (MRAM), however, the switching occurs coherently in both the SL and SyF cells. These results indicate that defects, which can be introduced to the cells during nanofabrication, do not significantly affect the switching properties and also the thermal stability of the high density MRAM cells.

Magnetic dead layer in amorphous CoFeB layers with various top and bottom structures

The magnetic dead layer (MDL) in amorphous CoFeB layers is investigated for four different unit structures. These structures are relevant to the synthetic ferrimagnetic (SyF) free layer structure in magnetic tunnel junctions used for high density magnetic random access memory (MRAM). The MDL results for these unit structures are then converted to those for the constituent interfaces of the SyF free layer structure. These MDL results are critically tested by fabricating the synthetic ferrimagnetic free layer structures with various thickness asymmetries. The observed switching properties of these tested structures are in good agreement with those expected from the effective thicknesses after the MDL correction, confirming the accuracy of the present results for the MDLs at the constituent interfaces.

Calculation of the magnetic energy barrier in nanostructured cells of synthetic ferrimagnets

A method is proposed to calculate the magnetic energy barrier of nanostructured cells of synthetic ferrimagnets. An important feature of the method is the use of an equation for the total energy that contains the magnetostatic fields at the saddle point as parameters of the energy equation. With no suitable methods of accessing the magnetic configuration at the unstable saddle point, it is difficult to obtain the saddle point magnetostatic fields. This difficulty is overcome with the use of equations that link the magnetostatic fields at the saddle point and critical fields, which are readily obtained by micromagnetic simulation. The present method is essentially based on the micromagnetic simulation, and, therefore, it should provide accurate results for the magnetic energy barrier. A contour diagram showing the thermal stability parameter is constructed as a function of the cell geometry and the thickness asymmetry, and the result should be of great value in designing magnetic cells for high density magnetic random access memory.

Switching behavior of a Stoner–Wohlfarth particle subjected to spin-torque effect

Recently, the current-induced spin-transfer torque has been proposed as a convenient writing process in high density magnetic random access memory. A spin-polarized current can switch the magnetization of a ferromagnetic layer more efficiently than a current induced magnetic field. Our paper discusses the switching properties of a Stoner–Wohlfarth magnetic particle for the case when spin torques and external field pulses are simultaneously present. The theoretical investigation of precessional motion is described by using Landau–Lifschitz–Gilbert equation with a spin-transfer torque term included. The main goal is to determine the parameters of field pulse for that the fast and stable switching can be achieved.

Patterned nanoring magnetic tunnel junctions

Patterned nanoring (NR) magnetic tunnel junctions (MTJs) with outer diameters between 100 and 400nm and narrow ring widths between 25 and 30nm were fabricated. The NR structure consists of CoFeB electrodes and Al-oxide barrier. The tunneling magnetoresistance (TMR) ratio in the range of 20%–50% for the NR-MTJs with the resistance-area product lower than 50Ωμm2 were observed at room temperature. These NR-MTJs allow current-induced magnetization switching with a low switching current density of around 9×106A∕cm2. Due to the small stray field and high TMR ratio, NR-MTJs offer superior prospects for very high density magnetic random access memory, recording medium, and other spintronic devices.

Thermal stability of a nanostructured trilayer synthetic antiferromagnet

Thermal stability of a nanostructured trilayer synthetic antiferromagnet is investigated in the Worledge model. No or minimal shape anisotropy effect is predicted from the model due to the simplifying assumption that the self-demagnetizing field is equal to the dipole field. A slight relaxation of the assumption causes a large impact on the thermal stability. In the case that the dipole field is 90% of the demagnetizing field, the energy barrier to spin flop is doubled to 80kT in a typical ellipsoid cell (212×106nm2) for high density magnetic random access memory. The thermal stability of half-selected cells in the writing scheme based on the spin flop is also improved by relaxing the assumption over the applied field range relevant to magnetic random access memory applications.

Heteroepitaxial growth of sixfold symmetric osmium on Si (111) and Si (100)

Growing sixfold symmetric osmium (Os) epitaxial films with suitable buffer layers was developed. Using a Cu buffer layer, the lattice mismatch between Os (0002) and Si (100) was significantly reduced from >30% to ∼7% to grow Os films with twin relationships and weak sixfold symmetries. On the other hand, the Cu∕Au buffer layer was selected to form a fcc (111) surface mesh on H–Si (111)-1×1, and therefore sixfold symmetric Os films were grown due to the lower lattice mismatch. Such growth properties may be applied in the high density magnetic random access memory manufacturing processes to connect the magnetic tunnel junction growth and Cu metal line directly.

Composite free layer for high density magnetic random access memory with lower spin transfer current

A magnetic tunnel junction (MTJ) structure with a composite free layer consisting of a nanocurrent-channel (NCC) layer sandwiched by two CoFe layers was proposed and investigated. The NCC layer increased the local spin current density inside the free layer and thus enhanced the writing capability for MTJ devices. In comparison with the conventional MTJ design with a single free layer, the intrinsic critical switching current density was reduced from 2.4×107to8.5×106A∕cm2 by using the composite free layer. On the other hand, the thermal stability factor of the composite free layer, KuV∕kBT, is around 149, which is almost the same as the value (159) for the MTJ device with a single free layer. The MTJ structure with the composite free layer is a candidate to solve the scaling problem for high density magnetic random access memory.

Ballistic magnetoresistance in a magnetic nanometer sized contact: An effective gate for spintronics

We present experimental results of unprecedented large magnetoresistance obtained in stable electrodeposited Ni–Ni nanocontacts 10–30 nm in diameter. The contacts exhibit magnetoresistance of up to 700% at room temperature and low applied fields and, therefore, act as very effective spin filters. These large values of the magnetoresistance are attributed to spin ballistic transport through a magnetic “dead layer” at the contact of width of about 1 nm or smaller. Nanometer sized, high sensitive magnetoresistive sensors could become key elements for magnetic storage in the terabit/in.2 range and in high density magnetic random access memories.