High Tunnel Magnetoresistance Ratio Research Articles

Co/Pt multilayer-based magnetic tunnel junctions with a CoFeB/Ta insertion layer

We investigate properties of magnetic tunnel junctions (MTJs) having a Co/Pt multilayer as a recording layer. A CoFeB layer is inserted between MgO barrier and the recording layer in order to enhance the tunnel magnetoresistance ratio. We show that an additional layer of Ta inserted between CoFeB and Co/Pt multilayer is effective in improving the MTJ properties after annealing. A high effective magnetic anisotropy energy per unit area over 1.2 mJ/m2 is obtained after annealing at 300 °C. Using a 1.6 nm-thick CoFeB insertion layer, both high thermal stability factor of 92 and high tunnel magnetoresistance ratio of 91% are achieved in a MTJ with 17 nm in diameter.

Magnetic properties of graphene/BN/Co(111) and potential spintronics

Using the Vienna ab initio simulation package (VASP) we have explored structural, adsorption, and magnetic properties of graphene/BN/Co(111) with the help of semi empirical and non-local van der Waals functional methods such as DFT-D2, vdW-DF2, optPBE-vdW, and optB88-vdW. In the most stable optimized structure, one carbon is on top of B atom and the other carbon is on top of hollow site of BN layer. No buckling geometry is found in both graphene and BN layers with vdW-DF2 method. In contrast, all three other approaches show that a buckling feature takes place in BN layer adjacent to Co substrate and due to this buckling an adsorption energy of graphene is significantly enhanced. Very interestingly, the graphene has an energy gap only in one spin band due to the influence of Co and an insulating state in BN layer adjacent to graphene is observed. In addition, the Co layer displays half metallic states as well. This feature suggests that the graphene/BN/Co(111) can be utilized for ideal tunneling magnetoresistance (TMR) structure because a high TMR ratio is expected.

MgO-Based Double Barrier Magnetic Tunnel Junctions With Synthetic Antiferromagnetic Free Layer

CoFeB/Ru/CoFeB has been used as the middle free layer in MgO-based double barrier magnetic tunnel junctions (DBMTJs). The tunneling magnetoresistance (TMR) ratio, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sub> (bias voltage at half maximum TMR ratio) and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</sub> (output voltage, defined as V multiplied by TMR ratio) have been investigated as a function of annealing temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> ) and Ru thickness (t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ru</sub> ) in the free layer. Magnetization data reveal that the two CoFeB layers in CoFeB/Ru/CoFeB are antiferromagnetically coupled. Compared with V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sub> of only 0.31 V for single barrier MTJs (SBMTJs) annealed at 375°C, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sub> for DBMTJs is up to 0.66 V. By increasing Ta, TMR ratio first increases, reaching the highest critical value, and then decreases. The highest TMR ratio is 181% with t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ru</sub> = 0.6 nm, which is much higher than that obtained in the DBMTJs with the pure CoFeB as the free layer. The improved TMR ratio is mainly due to the relatively thorough crystallization of the CoFeB layers in the free layer. The thermal annealing has been proven to be an effective method to remove the dissimilarity of the top and bottom CoFeB/MgO interfaces. V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</sub> in the positive and negative voltage branches follows the same trend as that of TMR ratio with T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> .

High TMR Ratio in ${\rm Co}_{2}{\rm FeSi}$ and ${\rm Fe}_{2}{\rm CoSi}$ Based Magnetic Tunnel Junctions

Magnetic tunnel junctions with Fe1+xCo2-xSi (0 < x < 1) electrodes and MgO barrier were prepared on MgO substrates by magnetron co-sputtering. Maximum tunnel magnetoresistance (TMR) ratios of 262 % at 15 K and 159 % at room temperature were observed for x=0.75. Correlations of the annealing temperature dependent atomic ordering and TMR amplitude are discussed. The high TMR for an intermediate stoichiometry is ascribed to the adjustment of the Fermi energy within a minority spin pseudo gap.

A 71%-Area-Reduced Six-Input Nonvolatile Lookup-Table Circuit Using a Three-Terminal Magnetic-Tunnel-Junction-Based Single-Ended Structure

A single-ended circuit using three-terminal magnetic tunnel junction (3T-MTJ) devices is proposed for a compact nonvolatile lookup-table (NV-LUT) circuit. The use of 3T-MTJ devices makes a high tunnel magneto-resistance ratio used in the circuit, because read-current path is separated from the write-current path. By utilizing single-ended circuit structure, the NV-LUT circuit becomes quite simple without reference circuit. In fact, the effective area of the proposed 6-input NV-LUT circuit is only 29% the size of the corresponding CMOS-based implementation using two-terminal-MTJ-based nonvolatile static random access memory cells, with a simulation program with integrated circuit emphasis (SPICE) simulation under a 90 nm CMOS technology.

Nonequilibrium spin transport through a diluted magnetic semiconductor quantum dot system with noncollinear magnetization

The spin-dependent transport through a diluted magnetic semiconductor quantum dot (QD) which is coupled via magnetic tunnel junctions to two ferromagnetic leads is studied theoretically. A noncollinear system is considered, where the QD is magnetized at an arbitrary angle with respect to the leads’ magnetization. The tunneling current is calculated in the coherent regime via the Keldysh nonequilibrium Green’s function (NEGF) formalism, incorporating the electron–electron interaction in the QD. We provide the first analytical solution for the Green’s function of the noncollinear DMS quantum dot system, solved via the equation of motion method under Hartree–Fock approximation. The transport characteristics (charge and spin currents, and tunnel magnetoresistance (TMR)) are evaluated for different voltage regimes. The interplay between spin-dependent tunneling and single-charge effects results in three distinct voltage regimes in the spin and charge current characteristics. The voltage range in which the QD is singly occupied corresponds to the maximum spin current and greatest sensitivity of the spin current to the QD magnetization orientation. The QD device also shows transport features suitable for sensor applications, i.e., a large charge current coupled with a high TMR ratio.

Boron Composition Dependence of Magnetic Anisotropy and Tunnel Magnetoresistance in MgO/CoFe(B) Based Stack Structures

We investigated magnetic anisotropy and tunnel magnetoresistance (TMR) properties in MgO/(Co0.25Fe0.75)100-xBx stack structures with x=0, 15, 20, and 25 (in at.%). After annealing at 350°C, the easy axis of magnetization switches from in-plane to perpendicular direction in 1.5-nm-thick CoFeB with the B composition near x=15 . The effective magnetic anisotropy energy density (Keff) shows a maximum of 1.9×105 J/m3 in the 1.5 nm-thick CoFeB film with x=20 annealed at 350 °C. Keff is determined by the competition between contributions of interface anisotropy energy per effective CoFeB thickness (Ki/t*, where t* is the effective CoFeB layer thickness) and demagnetization energy (-MS2/2μ0) . Bulk magnetic anisotropy energy (Kb) is negligibly small with comparison to those two terms. To obtain MgO/ferromagnetic stack structure with a high Keff, materials and structures that reduce demagnetization energy while maintaining a high Ki and a thin t* have to be explored. In MTJs with the higher B compositions, high TMR ratio is obtained at higher annealing temperature. High TMR ratio of 136% is observed in a MTJ with x=25 annealed at 350 °C.

Highly Spin-Polarized Tunneling in Epitaxial Magnetic Tunnel Junctions with a Co2MnSi Electrode and a MgO Barrier with Improved Interfacial Structural Properties

Fully epitaxial magnetic tunnel junctions (MTJs) with a Heusler alloy Co2MnSi electrode and a MgO barrier were prepared on MgO-buffered MgO(001) substrates with various layer structures to elucidate the contribution of coherent tunneling to the spin-dependent tunneling characteristics of these MTJs, which potentially feature both the half-metallicity of Co2MnSi and coherent tunneling. MTJs consisting of (from the lower side) Co50Fe50 (CoFe)/MgO/Co2MnSi or CoFe-buffered Co2MnSi/MgO/CoFe showed almost identical, high tunnel magnetoresistance (TMR) ratios of 335% at 290 K (1049% at 4.2 K) and 340% at 290 K (879% at 4.2 K), respectively. In contrast, MTJs consisting of MgO-buffered Co2MnSi/MgO/CoFe showed a lower TMR ratio of 173% at 290 K (448% at 4.2 K). The higher TMR ratios obtained for CoFe/MgO/Co2MnSi MTJs and CoFe-buffered Co2MnSi/MgO/CoFe MTJs can be ascribed to the enhanced contribution of coherent tunneling that originated from decreased misfit dislocation densities at the lower and upper interfaces with a MgO barrier.

Fabrication of Fully Epitaxial CoFe/MgO/CoFe Magnetic Tunnel Junctions on Ge(001) Substrates via a MgO Interlayer

Fully epitaxial magnetic tunnel junctions (MTJs) with Co50Fe50 (CoFe) electrodes and a MgO barrier were fabricated on Ge(001) single-crystal substrates via a MgO interlayer. Microfabricated CoFe/MgO/CoFe MTJs with a 10-nm-thick MgO interlayer showed a high tunnel magnetoresistance (TMR) ratio of 218% at 293 K, which is encouraging for monolithic integration of MTJs and Ge field-effect transistors for constructing future-generation nonvolatile logic circuits featuring ultralow-power consumption. Furthermore, MTJs with even a decreased MgO interlayer thickness of 1.0 nm showed a relatively high TMR ratio of 110% at 293 K, suggesting the promise of heterostructures consisting of MTJ/MgO interlayer/Ge(001) as a key device structure for spin injection into a Ge channel from an MTJ.

Low magnetisation alloys for in-plane spin transfer torque devices

Magnetic properties of thin films of three in-plane alloy systems, namely, Co40Ni40−xCrxB20, Co40Fe40−xNixB20, and Co40Fe40−xCrxB20 are investigated with a view to reducing the critical current density (jc) for spin transfer torque switching in MgO barrier magnetic tunnel junction nanopillars. In all three systems, the saturation magnetisation, Ms, decreases with increasing Cr or Ni substitution. The Co40Fe40−xCrxB20 alloy system is found to be the best in terms of reducing Ms, while maintaining a high tunnel magnetoresistance ratio. Ferromagnetic resonance experiments revealed that the damping coefficient of CoFeCrB alloy remains unchanged with increasing Cr content, but jc is reduced by a factor of four to 4.9 × 105 A/cm2 by using a Co40Fe32.7Cr7.3B20 free layer while maintaining a magnetoresistance of around 60 %.

Low frequency noise in La0.7Sr0.3MnO3 based magnetic tunnel junctions

Magnetic tunnel junctions based on manganites can exhibit a high tunneling magnetoresistance ratio due to the almost full spin polarization at the Fermi level. However, the performances of magnetic tunnel junction devices are also strongly linked to their noise characteristics. Here, we present a low frequency noise study on fully epitaxial La0.7Sr0.3MnO3/SrTiO3/La0.7Sr0.3MnO3/La0.66Sr0.33Mn0.995Ru0.005O3 tunnel spin valves with tunneling magnetoresistance ratios larger than 100%. We evidence non-conventional low frequency noise dependence on temperature related to the magnetic fluctuations and structural phase transitions in the structure. We present also a comparison with the low frequency noise exhibited in Fe/MgO/Fe magnetic tunnel junctions.

Improved tunneling magnetoresistance in (Ga,Mn)As/AlOx/CoFeB magnetic tunnel junctions

We fabricated (Ga,Mn)As/AlOx/Co40Fe40B20 magnetic tunnel junctions with ferromagnetic semiconductor/insulator/ferromagnetic metal (S/I/F) structure. The treatments of pre-annealing and post-plasma cleaning on the (Ga,Mn)As film were introduced before the growth of the subsequent layers. A high tunneling magnetoresistance (TMR) ratio of 101% is achieved at 2 K, and the spin polarization of (Ga,Mn)As, P = 56.8%, is deduced from Jullière’s formula. The improved TMR ratio is primarily due to the improved magnetism of (Ga,Mn)As layer by low-temperature annealing and cleaned interface between (Ga,Mn)As and AlOx attained by subsequent plasma cleaning process.

Sub-Poissonian shot noise in CoFeB/MgO/CoFeB-based magnetic tunneling junctions

We measured the shot noise in the CoFeB/MgO/CoFeB-based magnetic tunneling junctions with a high tunneling magnetoresistance ratio (over 200% at 3 K). Although the Fano factor in the antiparallel configuration is close to unity, it is observed to be typically 0.91±0.01 in the parallel configuration. It indicates the sub-Poissonian process of the electron tunneling in the parallel configuration due to the relevance of the spin-dependent coherent transport in the low bias regime.

C-doped ZnO nanowires: Electronic structures, magnetic properties, and a possible spintronic device

Electronic structures, magnetic properties, and spin-dependent electron transport characteristics of C-doped ZnO nanowires have been investigated via first-principles method based on density functional theory and nonequilibrium techniques of Green's functions. Our calculations show that the doping of carbon atoms in a ZnO nanowire could induce strong magnetic moments in the wire, and the electronic structures as well as the magnetic properties of the system sensitively depend on partial hydrogenation. Based on these findings, we proposed a quasi-1d tunneling magnetic junction made of a partially hydrogenated C-doped ZnO nanowire, which shows a high tunneling magnetoresistance ratio, and could be the building block of a new class of spintronic devices.

Metallic Spintronics and Surface Science

Electrons convey not only an electric charge but also a magnetic charge. Spintronics aims to achieve electronic devices with new capabilities by using both the charge and spin of electrons in solids. This area is now developing with advances in surface science. With the reduction of junction sizes into nanometer-scale, it has become impossible to improve spintronic devices without the investigation of surface/interface structures. In this article, our studies on exploration of correlation between surface/interface structures in magnetic tunnel junctions yielding high tunnel magnetoresistance ratio and their magneto-transport properties by using a scanning tunneling microscopy are presented. Then, recent studies on development of new ordered alloys with perpendicular magnetic anisotropy by using a monoatomic alternative deposition technique are introduced topically.

A perpendicular-anisotropy CoFeB–MgO magnetic tunnel junction

Magnetic tunnel junctions (MTJs) with ferromagnetic electrodes possessing a perpendicular magnetic easy axis are of great interest as they have a potential for realizing next-generation high-density non-volatile memory and logic chips with high thermal stability and low critical current for current-induced magnetization switching. To attain perpendicular anisotropy, a number of material systems have been explored as electrodes, which include rare-earth/transition-metal alloys, L1(0)-ordered (Co, Fe)-Pt alloys and Co/(Pd, Pt) multilayers. However, none of them so far satisfy high thermal stability at reduced dimension, low-current current-induced magnetization switching and high tunnel magnetoresistance ratio all at the same time. Here, we use interfacial perpendicular anisotropy between the ferromagnetic electrodes and the tunnel barrier of the MTJ by employing the material combination of CoFeB-MgO, a system widely adopted to produce a giant tunnel magnetoresistance ratio in MTJs with in-plane anisotropy. This approach requires no material other than those used in conventional in-plane-anisotropy MTJs. The perpendicular MTJs consisting of Ta/CoFeB/MgO/CoFeB/Ta show a high tunnel magnetoresistance ratio, over 120%, high thermal stability at dimension as low as 40 nm diameter and a low switching current of 49 microA.

CoFeB Inserted Perpendicular Magnetic Tunnel Junctions with CoFe/Pd Multilayers for High Tunnel Magnetoresistance Ratio

The effect of Co20Fe60B20 insertion on tunnel magnetoresistance (TMR) properties of MgO barrier magnetic tunnel junctions (MTJs) was studied with Co90Fe10/Pd multilayer electrodes showing perpendicular anisotropy. The TMR ratio, which depended on the sputtering power density of the CoFeB layer, reached 43% at 1.77 W/cm2 in MTJs with a 1.8-nm-thick CoFeB layer inserted on both sides of the MgO barrier. With increasing CoFeB layer thickness, the optimal annealing temperature (Ta) realizing the maximum TMR ratio increased along with the TMR ratio. The MTJs with 3.0-nm-thick CoFeB deposited at 1.77 W/cm2 showed a TMR ratio of 91% at Ta = 250 °C, where the perpendicular magnetic anisotropy is maintained.

Effect of defects in Heusler alloy thin films on spin-dependent tunnelling characteristics ofCo2MnSi/MgO/Co2MnSi andCo2MnGe/MgO/Co2MnGe magnetic tunnel junctions

Fully epitaxial magnetic tunnel junctions (MTJs) with Co-based Heusler alloyCo2MnSi electrodes and a MgO tunnel barrier were fabricated with various values of Mn compositionα forCo2MnαSi inCo2MnαSi/MgO/Co2MnαSi MTJs. The tunnel magnetoresistance (TMR) ratios at both 4.2 Kand room temperature (RT) increased systematically with increasingα inCo2MnαSi electrodes from Mn-deficient compositions (α < 1) up to a certainMn-rich composition (α > 1), demonstrating high TMR ratios of 1135% at 4.2 K and 236% at RT for MTJs with Mn-richCo2MnαSi electrodes withα = 1.29. Identicallyfabricated Co2MnβGeδ/MgO/Co2MnβGeδ (δ = 0.38) MTJs showed similar dependence of the TMR ratio on Mn compositionβ, demonstrating relatively high TMR ratios of 650% at 4.2 K and 220% at RT forβ = 1.40.The Mn composition dependence of the TMR ratio at both 4.2 K and RT observed commonly for bothCo2MnSi/MgO/Co2MnSi andCo2MnGe/MgO/Co2MnGe MTJs can be attributed to suppressed minority-spin in-gap states around the Fermi level for Mn-richCo2MnSi andCo2MnGe electrodes.

GaMnAs-based magnetic tunnel junctions with an AlMnAs barrier

We investigate the spin-dependent transport of GaMnAs-based magnetic tunnel junctions (MTJs) containing a paramagnetic AlMnAs barrier with various thicknesses. The barrier height of AlMnAs with respect to the Fermi level of GaMnAs is estimated to be 110 meV. We observe tunneling magnetoresistance (TMR) ratios up to 175% (at 2.6 K), which is higher than those of the GaMnAs-based MTJs with other barrier materials in the same temperature region. These high TMR ratios can be mainly attributed to the relatively high crystal quality of AlMnAs and the suppression of the tunneling probability at the in-plane wave vector k∥∼0.

Tunneling spectroscopy and magnon excitation in Co2FeAl/MgO/Co–Fe magnetic tunnel junctions

Magnetic tunnel junctions with the Heusler compound Co2FeAl as the soft electrode are prepared. Pinned Co–Fe is used as the hard reference electrode. The junctions show a high tunnel magnetoresistance ratio of 273% at 13 K. The electronic transport characteristics are investigated by tunneling spectroscopy—dI/dV and d2I/dV2 are discussed. In the parallel magnetic state the tunneling spectra are asymmetric with respect to the bias voltage, with a pronounced bias-independent region. In the antiparallel state the dependence on bias voltage is much stronger and the curves are symmetric. The findings can be explained with a gap in the minority density of states of Co2FeAl.