High TMR Research Articles

Prediction of likelihood of conception in dairy cows using milk mid-infrared spectra collected before the first insemination and machine learning algorithms

Accurate and ex-ante prediction of cows' likelihood of conception (LC) based on milk composition information could improve reproduction management on dairy farms. Milk composition is already routinely measured by mid-infrared (MIR) spectra, which are known to change with advancing stages of pregnancy. For lactating cows, MIR spectra may also be used for predicting the LC. Our objectives were to classify the LC at first insemination using milk MIR spectra data collected from calving to first insemination and to identify the spectral regions that contribute the most to the prediction of LC at first insemination. After quality control, 4,866 MIR spectra, milk production, and reproduction records from 3,451 Holstein cows were used. The classification accuracy and area under the curve (AUC) of 6 models comprising different predictors and 3 machine learning methods were estimated and compared. The results showed that partial least square discriminant analysis (PLS-DA) and random forest had higher prediction accuracies than logistic regression. The classification accuracy of good and poor LC cows and AUC in herd-by-herd validation of the best model were 76.35 ± 10.60% and 0.77 ± 0.11, respectively. All wavenumbers with values of variable importance in the projection higher than 1.00 in PLS-DA belonged to 3 spectral regions, namely from 1,003 to 1,189, 1,794 to 2,260, and 2,300 to 2,660 cm-1. In conclusion, the model can predict LC in dairy cows from a high productive TMR system before insemination with a relatively good accuracy, allowing farmers to intervene in advance or adjust the insemination schedule for cows with a poor predicted LC.

Voltage-controlled magnetic anisotropy effect through a LiF/MgO hybrid tunneling barrier

Improving the perpendicular magnetic anisotropy (PMA) and voltage-controlled magnetic anisotropy (VCMA) properties are fundamentally important for the development of voltage-controlled magnetoresistive random access memories (VC-MRAM). Recently, we reported on a large increase in PMA at an Fe/MgO interface brought about by inserting an ultrathin LiF layer at the interface. In this paper, we investigate the PMA, VCMA, and TMR properties in MTJs with an Ir-doped ultrathin ferromagnetic layer and a LiF/MgO hybrid tunneling barrier. We observed a clear increase in the interfacial PMA by a factor of 2.5 when an ultrathin 0.25 nm LiF layer was inserted. A large VCMA coefficient, exceeding −300 fJ/Vm, was also achieved while maintaining the high TMR ratio and high interfacial PMA. These results demonstrate the high potential of interface engineering using ultrathin LiF layers for spintronic devices.

Dilute magnetic semiconductor electrode based all semiconductor magnetic tunnel junction for high-temperature applications

In this paper, the spin-transport properties of Gallium Nitride/Indium Phosphide/Gallium Nitride (GaN/InP/GaN) all semiconductor magnetic tunnel junction (MTJ) has been investigated. To induce ferromagnetic properties in the electrodes made of GaN, the electrodes are doped with Manganese (Mn). The central region is made of InP, which is a large bandgap material, thereby improving the zero-bias magnetoresistance. The electrodes used in the proposed device show ferromagnetic behavior beyond room temperature without the conductivity mismatch problem. Finally, a comparison based on the proposed device's performance parameters with the devices reported in the open literature is carried out in this work. The comparison clearly shows the pre-eminence of the proposed device over the previously reported devices as it has high TMR (1.97 × 104%) and works over a wide range of temperature.

Ab-initio study of LD-HfO2, Al2O3, La2O3 and h-BN for application as dielectrics in MTJ memory device

Low-dimensional (LD) forms of HfO2, Al2O3, La2O3 and h-BN have been used as dielectric layer in the proposed MTJ memory device. Subsequently, DFT and NEGF model based atomistic computation were performed for the device using Quantum ESPRESSO. Self-consistent Field (SCF), Density of State (DOS) and Bandstructure calculations were carried out. The SCF converged uniquely for all LD materials used. Lowest energy was obtained for multilayer LD-La2O3 at −1760.7200095Ry and HfO2 at −940.365Ry, thus predicting better accuracy in solving Schrödinger equation and wave functions. The DOS plots exhibited the valence band sharp peaks at −4.5eV, −0.5eV, −0.5eV and −4.25eV for multilayer LD-HfO2, Al2O3, La2O3 and h-BN respectively. The energy band spacing manifested from DOS plots were found to be better for LD- HfO2, La2O3 and h-BN. On inspecting the bandstructure plots, higher bandgap was observed for multilayer LD-HfO2 (at 3.8eV), hence depicting it to have a better insulating property. Upon device computation, higher TMR was obtained for HfO2 based MTJ memory device (i.e. 2.5 at 0 V), showcasing better readability of bits. While consistent boundary voltages were obtained from the DTMR calculations for h-BN and HfO2 based devices, concluding them to have better stability of the memory states. The overall results obtained for different performance parameters of the MTJ memory device using LD-HfO2 and h-BN were very impressive. Thus, predicting these materials as highly promising to be used as dielectrics in a MTJ memory device.

Impact of single and double oxygen vacancies on electronic transport in Fe/MgO/Fe magnetic tunnel junctions

The combination of a low tunneling barrier height and a large tunneling magnetoresistance (TMR) ratio in MgO-class magnetic tunnel junctions (MTJs) has enabled next-generation information storage and bio-inspired computing solutions thanks to the spin transfer torque effect. Recent literature has proposed that this synergistic combination arises from the electronic properties of oxygen vacancies. To explicitly understand their impact on spin-polarized transport, we have computed the electronic and transport properties of single (F centers) and paired (M centers) oxygen vacancies using density functional theory and the projector augmented wave method. These point defects can generate energy level positions of 0.4 eV with respect to the Fermi level for FeCo electrodes irrespective of the defect’s spatial position within the MgO barrier and of the orientation of the M center. These defects promote a strong decrease in the conductance of the spin up channel in the MTJ’s parallel magnetic state that mainly accounts for an order-of-magnitude drop in TMR from ≈10000% in the ideal case toward values more in line with experiment. When placed in the middle layer of the MgO barrier, the F center introduces additional P ↑ transmission away from the Γ point. This scattering lowers TMR to 145%. In contrast, the M center merely broadens this transmission around Γ, thereby boosting TMR to 315%. Rotating a M center so as to partly point along the transmission direction sharpens transmission around Γ, further increasing TMR to 1423%. When these defects are placed at the MTJ interface, the transmission and ensuing TMR, which reaches ≈4000%, suggest that such junctions behave as an ideal MTJ only with a much lower TMR. Our results, thus, theoretically reconcile the concurrent observations of high TMR and low barrier heights in line with experimental preparation techniques such as post-deposition oxidation of metallic Mg, which can generate oxygen vacancies at the lower MTJ interface, and annealing which can promote M centers over F centers. Our theory is also in line with an origin of perpendicular magnetic anisotropy in terms of oxygen vacancies at MTJ interfaces. The effective size of these vacancies sets a limit for both the barrier thickness, in line with experiment, as well as for the MTJ’s lateral dimension. Our work provides a much-needed theoretical basis to move beyond the mostly unsuspected, fortuitous defect engineering of spintronic performance that has, thus, far propelled MgO-based spintronics and its applications.

Spin-orbit Torques and Magnetization Switching in Perpendicularly Magnetized Epitaxial Pd/Co 2 FeAl/MgO Structures

We demonstrate efficient current induced spin orbit torque switching in perpendicularly magnetized epitaxial MgO(001)//Pd/Co2FeAl/MgO heterostructures grown by magnetron sputtering. The advantage of such heterostructures for spin orbit torque MRAM devices is that they allow a unique combination of critical ingredients: i) low resistivity required for reduced electric energy consumption during writing, ii) high TMR ratio required for efficient reading and iii) strong perpendicular magnetic anisotropy for increased thermal stability.

Polycrystalline Co2Fe0.4Mn0.6Si Heusler alloy thin films with high B2 ordering and small magnetic anisotropy for magnetic tunnel junction based sensors

To measure the bio magnetic field from the heart and brain at room temperature, the magnetic tunnel junctions (MTJs) based sensor is required to achieve a high sensitivity which is defined as the TMR ratio/2Hk (where Hk is anisotropic magnetic field of the free layer). To realize both high TMR ratio and small Hk, we employed (001)-oriented polycrystalline Co-based Heusler alloy material (Co2Fe0.4Mn0.6Si) for the free layer which is known for its high spin polarization and small Hk. In this study, we optimized the crystal and magnetic properties of Co2Fe0.4Mn0.6Si free layer and succeeded in reducing Hk in polycrystalline CFMS free layer with high B2 ordering. We also demonstrated the linear output of TMR in MTJs fabricated under the optimized condition.

The Effect of Functionalization on Spin-Polarized Transport of Gallium Nitride–Based Magnetic Tunnel Junctions

Using first-principles spin-polarized density functional theory computations, the effect of functionalization (fluorination and hydrogenation) on spin-polarized transport of gallium nitride (GaN) nanosheet–based magnetic tunnel junction (MTJ) with CrO2 as electrodes is investigated. The results show fluorinated GaN–based structure exhibits better spin filtration and high TMR (maximum ~ 99%), as compared with hydrogenated GaN (maximum TMR ~ 93%)– and pristine GaN (maximum TMR ~ 83%)–based structures. In addition, fluorinated GaN nanosheet exhibits a ferromagnetic behavior with a magnetic movement of 1.0 μb per fluorine atom. The magnetic movements for pristine GaN and hydrogenated GaN sheets are reported to be 0 μb and 0.9 μb per hydrogen atom, respectively. Higher TMR, better spin filtration, and ferromagnetic behavior for fluorinated GaN–based structure open up its possibility as spin filter (injector) in MTJs and other spin-based devices.

Wide input range, fully-differential indirect current feedback instrumentation amplifier for self-x sensory systems / Symmetrischer Instrumentierungsverstärker mit indirekter Stromgegenkopplung und hoher Eingangsignalspanne für integrierte Sensorsysteme mit Self-x-Eigenschaften

Abstract This paper research presents the design of wide input range indirect current feedback-instrumentation amplifier (CFIA). In order to extend the input range without sacrificing the amplifier performance, the negative feedback is applied to the source coupled differential pairs inputs. The feedback network and the biasing current can be programmed to work at different values to meet different signal conditions or to self-correct the drift in the amplifier properties. The simulated input range Vin; P-P=1.6 V with total harmonic distortion of 0.93 % at 5 MHz frequency. Thus the proposed CFIA is very suitable to read the high speed and high common mode range TMR differential voltage sensor signal. The circuit is implemented using the CMOS 0.35 μm technology from Austriamicrosystems (AMS) and by using Cadence Virtuoso design tools.

Spin transport properties of magnetic tunnel junction based on zinc blende CrS

Abstract In this work, the spin transport characteristics of (001) zinc blende CrS/ZnSe/CrS magnetic tunnel junctions are studied with numerical simulations. The computations are performed with the density functional theory and nonequilibrium Green's function. Considering the electronegativity of different atoms, the MTJs with different interface atomic bonds are simulated. High TMR values (up to 2.4 × 1014%) are achieved. The results are illustrated with the transmission spectrum, device density of states, and the band structure. Moreover, the tunnel magnetic resistance ratio decreases with the increasing thickness of the ZnSe barrier. This work provides some physical insight and a preliminary performance assessment for the ZB CrS/ZnSe/CrS MTJ, which is promising for probable spintronic applications.

Sensing schemes for STT-MRAMs structured with high TMR in low RA MTJs

In this work, we investigated the sensing challenges of spin-transfer torque MRAMs structured with perpendicular magnetic tunnel junctions with a high tunneling magnetoresistance ratio in a low resistance-area product. To overcome the problems of reading this type of memory, we have proposed a voltage sensing amplifier topology and compared its performance to that of the current sensing amplifier in terms of power, speed, and bit error rate performance. We have verified that the proposed sensing scheme offers a substantial improvement in bit-error-rate performance. To enumerate the read operations of the proposed sensing scheme with the proposed cross-coupled capacitive feedback technique on the clamped circuity have successfully been performed a 2.5X reduction in average low power and a 13X increase in average reading speed compared with the previous works due to its device structure and the proposed circuit technique.

Effects of functionalization of carbon nanotubes on its spin transport properties

Spin-transport properties of in-plane fluorinated zigzag (8, 0) single-walled carbon nanotubes contacted by two CrO2 Half-Metallic-Ferromagnetic (HMF) electrodes are investigated using density-functional theory. Comparison of the results with pristine carbon nanotube show that fluorinated carbon nanotube (CNT) offers high TMR ∼100% and the transport phenomenon is tunneling, since there are no transmission states near Fermi level. However, for pristine CNT structure the transport phenomenon cannot be tunneling since there are significant number of transmission states near Fermi level, although high Magneto Resistance (MR) ∼97% is observed in it. Both TMR and Spin Injection Efficiency η (Spin-Filtration) are higher in fluorinated (8,0) carbon nanotube (CNT) structure, which is due to fluorinated carbon nanotube (CNT) acting as a perfect barrier in vertical (out-of-plane) arrangement resulting in negligible spin-down current (I↓) in both Parallel Configuration (PC) and Antiparallel Configuration (APC).

Tuning the tunneling magnetoresistance by using fluorinated graphene in graphene based magnetic junctions

Spin polarized properties of fluorinated graphene as tunnel barrier with CrO2 as two HMF electrodes are studied using first principle methods based on density functional theory. Fluorinated graphene with different fluorine coverages is explored as tunnel barriers in magnetic tunnel junctions. Density functional computation for different fluorine coverages imply that with increase in fluorine coverages, there is increase in band gap (Eg) of graphene, Eg ∼ 3.466 e V was observed when graphene sheet is fluorine adsorbed on both-side with 100% coverage (CF). The results of CF graphene are compared with C4F (fluorination on one-side of graphene sheet with 25% coverage) and out-of-plane graphene based magnetic tunnel junctions. On comparison of the results it is observed that CF graphene based structure offers high TMR ∼100%, and the transport of carrier is through tunneling as there are no transmission states near Fermi level. This suggests that graphene sheet with both-side fluorination with 100% coverages acts as a perfect insulator and hence a better barrier to the carriers which is due to negligible spin down current (I↓) in both Parallel Configuration (PC) and Antiparallel Configuration (APC).

First principle study on TMR effect in A-MgO-A (A = Fe, Co and Ni) magnetic tunnel junction

• The role of electrode materials in TMR devices have investigated using DFT with Non-Equilibrium Green's Function (NEGF). • TMR effect is existing in Co-MgO-Co, Fe-MgO-Fe and Ni-MgO-Ni systems. • Iron electrodes have very high TMR efficiency than nickel and cobalt electrode. Abstract Magnetic tunnel junctions are used in storage devices. Electron transport in magnetic tunnel junctions highly dependents upon spin states of electron in ferromagnetic material. Iron, cobalt and nickel are room temperature ferromagnetic materials, magnetic tunnel junctions have been constructed using these materials as electrode and MgO as insulating layer (Co-MgO-Co, Fe-MgO-Fe and Ni-MgO-Ni). Total density of states, projected density of states and transmission coefficient have been calculated using non-equilibrium Green’s function (NEGF) with density functional theory (DFT) to analyze the spin transport properties of these devices. To study the TMR efficiency of these devices, TMR percentage at zero bias conductance was calculated based on optimistic and pessimistic. Our result shows that, iron ferromagnetic electrodes have very high TMR efficiency than nickel and cobalt electrode.

Thermal FMR Spectral Characterization of Very Low RA In-Plane MgO Magnetic Tunnel Junctions

The ability to fabricate MgO magnetic tunnel junction nanopillars with low RA and high TMR opens the door to new devices such as magnetic memories or nanooscillators. Here, we address the dynamic behavior of a CoFeB/MgO/CoFeB MTJ with a synthetic-antiferromagnetic pinned layer as a potential candidate for spin-transfer torque (STT) driven applications. The unpatterned stack presents an RA of $\sim 1.4~\Omega \mu \text{m}^{2}$ and both free layer and barrier are patterned down to a $90 \times 180$ nm2 elliptical shape. The observed high-frequency spectra displayed deviations from the uniform precession modes due to additional coupling fields. A dependence of the STT magnitude on the analyzed mode was obtained, differing from previous reports, most likely due to the very low RA employed.

Spin transport in carbon nanotubes bundles: An ab-initio study

First principles investigations are performed on understanding the spin-polarized transport in carbon nanotubes and carbon nanotube bundles consisting of (8,0) and (17,0) SWCNTs kept in vertical (out-of-plane) arrangement and contacted by two CrO2 Half-Metallic-Ferromagnetic (HMF) electrodes. On comparison of the results for all the structures, it is observed that carbon nanotube bundle consisting of (17,0) CNT offers high TMR ∼100% and the transport phenomenon is tunneling, since there are no transmission states near Fermi level. However, in individual (8,0) and (17,0) CNT the transport is not because of tunneling, since there are significant number of transmission states near Fermi level. High Magneto Resistance (MR) 96% and 99% is observed in individual (8,0) and (17,0) CNTs respectively. Both TMR and Spin Injection Efficiency η (Spin-Filtration) are higher in (17,0) carbon nanotube bundle structure, which is due to carbon nanotube bundle acting as a perfect barrier in vertical (out-of-plane) arrangement resulting in negligible spin-down current (I↓) in both Parallel Configuration (PC) and Antiparallel Configuration (APC).

Challenging Issues for Terabit-Level Perpendicular STT-MRAM

Recently, many researchers have intensively studied p-STT MRAM as an alternative terra-bit-integration nonvolatile memory since current Si-based dynamic-random-access-memory has a physical scaling limit of less than 20 nm. p-STT MRAM has demonstrated a fast write time of ~10 ns, non-volatile memory operation, and extremely low power consumption. However, for satisfying a commercial specification as a terra-bit-integration nonvolatile memory, perpendicular magnetic tunneling junction (p-MTJ) of p-STT MRAM essentially needs to satisfy critical device performances for p-MTJ spin-valves such as a high TMR ratio of 150 %, a high Jex of > 0.7 erg/cm2, a good Ku of 1x107 erg/cm3, a good ¢ of ~ 74,and a low critical writing current (JC0) of 13.4 MA/cm2, which should satisfy the thermal stability such as an ex-situ annealing above 400 °C. In general, a CoFeB based p-MTJ spin-valve in p-STT-MRAM has been fabricated with the stacked structure of bottom electrode/ amorphous Ta seed layer/amorphous CoFeB free layer/MgO tunneling barrier/amorphous CoFeB pinned layer/capping layer/buffer layer/ synthetic anti-ferromagnetic layer (SyAF) /top electrode. In particular, nano-scale thick amorphous CoFeB layer has been applied for free and pinned layer. However, the mechanism by which the perpendicular magnetic anisotropy (PMA) characteristic for amorphous CoFeB based p-MTJ using amorphous Ta seed layer could be achieved has not been clarified. In our study, we will review its mechanism and the thermal stability limit for amorphous CoFeB based p-MTJ using amorphous Ta seed layer. In addition, we will present the thermal stability enhancement for amorphous CoFeB based p-MTJ using bcc crystalline seed layer and its mechanism. For amorphous CoFeB based p-MTJ using amorphous Ta seed layer, the B atoms were segregated at the interface between the MgO tunneling barrier and the CoFeB free and pined layer after the ex-situ annealing of 275 °C, producing interface-PMA (i-PMA), as shown in Fig. 1(a). However, the PMA characteristics for amorphous CoFeB based p-MTJ using amorphous Ta seed layer was rapidly degraded as the ex-situ annealing temperature increased above 400 °C, which the mechanism was not proved yet. Surprisingly, the crystalline structure of both CoFeB free layer and pinned layer sustained at the amorphous structure although the p-MTJ was ex-tu annealed at 275 °C, as shown in Fig. 2. In order to enhance the thermal stability of amorphous CoFeB based p-MTJ, a bcc crystalline seed layer is essentially necessary rather than amorphous Ta seed layer. During ex-situ annealing at 400 °C, the bcc crystalline seed layer could texture the amorphous CoFeB free and pinned layer into the bcc crystalline CoFe free layer, MgO tunneling, and CoFeB pinned layer, as shown in Fig. 3. As a result, amorphous CoFeB based p-MTJ could achieve the TMR of ~135 %, as shown in Fig. 4. In this presentation, I will challenging material issues for p-STT MRAM in the view of the perfect crystalline linearity for free layer, MgO tunneling barrier, and pinned layer, and SyAF layer.

ChemInform Abstract: MgO(001) Barrier Based Magnetic Tunnel Junctions and Their Device Applications

AbstractReview: 210 refs.

In-situ grazing incidence X-ray diffraction measurements of relaxation in Fe/MgO/Fe epitaxial magnetic tunnel junctions during annealing

The relaxation of Fe/MgO/Fe tunnel junctions grown epitaxially on (001) MgO substrates has been measured by in-situ grazing incidence in-plane X-ray diffraction during the thermal annealing cycle. We find that the Fe layers are fully relaxed and that there are no irreversible changes during annealing. The MgO tunnel barrier is initially strained towards the Fe but on annealing, relaxes and expands towards the bulk MgO value. The strain dispersion is reduced in the MgO by about 40% above 480K post-annealing. There is no significant change in the “twist” mosaic. Our results indicate that the final annealing stage of device fabrication, crucial to attainment of high TMR, induces substantial strain relaxation at the MgO barrier/lower Fe electrode interface.

MgO(001) barrier based magnetic tunnel junctions and their device applications

Spintronics has received a great attention and significant interest within the past decades, and provided considerable and remarked applications in industry and electronic information etc. In spintronics, the MgO based magnetic tunnel junction (MTJ) is an important research advancement because of its physical properties and excellent performance, such as the high TMR ratio in MgO based MTJs. We present an overview of more than a decade development in MgO based MTJs. The review contains three main sections. (1) Research of several types of MgO based MTJs, including single-crystal MgO barrier based-MTJs, double barrier MTJs, MgO based MTJs with interlayer, novel electrode material MTJs based on MgO, novel barrier based MTJs, novel barrier MTJs based on MgO, and perpendicular MTJs. (2) Some typical physical effects in MgO based MTJs, which include six observed physical effects in MgO based MTJs, namely spin transfer torque (STT) effect, Coulomb blockade magnetoresistance (CBMR) effect, oscillatory magnetoresistance, quantum-well resonance tunneling effect, electric field assisted magnetization switching effect, and spincaloric effect. (3) In the last section, a brief introduction of some important device applications of MgO based MTJs, such as GMR & TMR read heads and magneto-sensitive sensors, both field and current switching MRAM, spin nano oscillators, and spin logic devices, have been provided.