Layer In Junctions Research Articles

Spin-dependent resonant tunneling and magnetoresistance in Ni/graphene/h-BN/graphene/Ni van der Waals heterostructures

Abstract Future development of spintronic devices urgently requires an advanced control method of spin current. Combining graphene with other various characteristic two-dimensional crystals in different ways can produce a new class of functional materials, which offers a unique platform for spintronics. A specific van der Waals heterostructure consisting of graphene and hexagonal boron nitride (h-BN) as the barrier layer in magnetic tunnel junctions (MTJs) is proposed in this paper. This heterostructure device combines the strong spin filtering effect of graphene/ferromagnet interface and the resonant tunneling effect of graphene/h-BN/graphene van der Waals heterostructure. We demonstrate an effective approach to control spin current based on the bias tunability of the spin resonance tunneling transport. Theoretical research unambiguously reveals that spin resonance appears when the electronic spectra of spin electron in two graphene layers are aligned, which results in a large magnetoresistance and negative differential resistance in the device. The efficient bias controllability of spin current, combined with the excellent magnetoresistance behavior, can be the key building blocks in future spin-based magnetic detection and high-frequency logic devices.

An effect of capping-layer material on interfacial anisotropy and thermal stability factor of MgO/CoFeB/Ta/CoFeB/MgO/capping-layer structure

We investigate the magnetic properties of a MgO/CoFeB/Ta/CoFeB/MgO/capping-layer (Ru or Ta) structure and properties of a magnetic tunnel junction with the structure as a free layer. By using Ru instead of Ta as the capping layer, interfacial anisotropy Ki increases by a factor of ∼2 and a smaller damping constant is obtained. The increase in Ki results in an enhancement of the thermal stability factor of the free layer with the Ru capping layer compared with that with the Ta capping layer in magnetic tunnel junctions.

Defect-Assisted Tunneling Electroresistance in Ferroelectric Tunnel Junctions.

Recent experimental results have demonstrated ferroelectricity in thin films of SrTiO_{3} induced by antisite Ti_{Sr} defects. This opens a possibility to use SrTiO_{3} as a barrier layer in ferroelectric tunnel junctions (FTJs)-emerging electronic devices promising for applications in nanoelectronics. Here using density functional theory combined with quantum-transport calculations applied to a prototypical Pt/SrTiO_{3}/Pt FTJ, we demonstrate that the localized in-gap energy states produced by the antisite Ti_{Sr} defects are responsible for the enhanced electron tunneling conductance which can be controlled by ferroelectric polarization. Our tight-binding modeling, which takes into account multiple defects, shows that the predicted defect-assisted tunneling electroresistance effect is greatly amplified when the defect energy levels are brought to the Fermi energy by one of the polarization states. Our results have implications for FTJs based on conventional ferroelectric barriers with defects and can be employed for the design of new types of FTJs with enhanced performance.

Magnetic and superconducting phase diagram of Nb/Gd/Nb trilayers

We report on a study of the structural, magnetic and superconducting properties of Nb(25nm)/Gd($d_f$)/Nb(25nm) hybrid structures of a superconductor/ ferromagnet (S/F) type. The structural characterization of the samples, including careful determination of the layer thickness, was performed using neutron and X-ray scattering with the aid of depth sensitive mass-spectrometry. The magnetization of the samples was determined by SQUID magnetometry and polarized neutron reflectometry and the presence of magnetic ordering for all samples down to the thinnest Gd(0.8nm) layer was shown. The analysis of the neutron spin asymmetry allowed us to prove the absence of magnetically dead layers in junctions with Gd interlayer thickness larger than one monolayer. The measured dependence of the superconducting transition temperature $T_c(d_f)$ has a damped oscillatory behavior with well defined positions of the minimum at $d_f$=3nm and the following maximum at $d_f$=4nm; the behavior, which is in qualitative agreement with the prior work (J.S. Jiang et al, PRB 54, 6119). The analysis of the $T_c(d_f)$ dependence based on Usadel equations showed that the observed minimum at $d_f$=3nm can be described by the so called "$0$" to "$\pi$" phase transition of highly transparent S/F interfaces with the superconducting correlation length $\xi_f \approx 4$nm in Gd. This penetration length is several times higher than for strong ferromagnets like Fe, Co or Ni, simplifying thus preparation of S/F structures with $d_f \sim \xi_f$ which are of topical interest in superconducting spintronics.

Free and forced Barkhausen noises in magnetic thin film based cross-junctions

Barkhausen noise, driven by thermal fluctuations in stationary magnetic field, and Barkhausen jumps, driven by sweeping magnetic field, are demonstrated to be effects of different orders of magnitude. The critical magnetic field for domain walls depinning, followed by avalanched and irreversible magnetization jumps, is determined. Magnetoresistive response of NiFe/M/NiFe (M = Au, Ta, Ag) trilayers to stationary and sweeping magnetic field is studied by means of anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) measurements. Thermal fluctuations result in local and reversible changes of magnetization of the layers in thin film magnetic junctions, while the sweeping magnetic field results in reversible and irreversible avalanched domain motion, dependently on the ratio between the values of sweeping magnetic field and domain wall depinning field. The correlation between AMR and PHE responses to Barkhausen jumps is studied. The value of this correlation is found to be dependent on the α angle between the directions of magnetic field and current path.

Using rf voltage induced ferromagnetic resonance to study the spin-wave density of states and the Gilbert damping in perpendicularly magnetized disks

We study how the shape of the spinwave resonance lines in rf-voltage induced FMR can be used to extract the spinwave density of states and the damping within the precessing layer in nanoscale tunnel junctions that possess perpendicular anisotropy. We work with a field applied along the easy axis to preserve the uniaxial symmetry of the system. We describe the set-up to study the susceptibility contributions of the spin waves in the field-frequency space. We then identify the maximum device size above which the spinwaves can no longer be studied in isolation as the linewidths of their responses make them overlap. The rf-voltage induced signal is the sum of two voltages that have comparable magnitudes: a first voltage that originates from the transverse susceptibility and rectification by magnetoresistance and a second voltage that arises from the non-linear longitudinal susceptibility and the resultant time-averaged change of the micromagnetic configuration. The transverse and longitudinal susceptibility signals have different dc bias dependences such that they can be separated by measuring how the device rectifies the rf voltage at different dc bias voltages. The transverse and longitudinal susceptibility signals have different lineshapes; their joint studies can yield the Gilbert damping of the free layer of the device with a degree of confidence that compares well with standard FMR. Our method is illustrated on FeCoB-based free layers in which the individual spin-waves can be sufficiently resolved only for disk diameters below 200 nm. The resonance line shapes on devices with 90 nm diameters are consistent with a Gilbert damping of 0.011. This damping of 0.011 exceeds the value of 0.008 measured on the unpatterned films, which indicates that device-level measurements are needed for a correct evaluation of dissipation.

Inhomogeneous free layer in perpendicular magnetic tunnel junctions and its impact on the effective anisotropies and spin transfer torque switching efficiency

Magnetoresistive and magnetoresonance measurements carried out on patterned perpendicular magnetic tunnel junction pillars and full-sheet films reveal magnetic inhomogeneities of FeCoB free layer grown on MgO and coated with Ta. At low FeCoB thicknesses, the layer behaves as an ensemble of weakly coupled grains resulting in a decrease of the free-layer thermal stability. In contrast, for thicker layers, the grains become more strongly coupled but strong magnetic inhomogeneities remain, yielding the emergence and further increase of a second-order magnetic anisotropy term ($\ensuremath{\sim}{K}_{2\mathrm{eff}}\mathrm{co}{\mathrm{s}}^{4}\ensuremath{\theta}$), eventually resulting in an easy-cone anisotropy. We show that the static and dynamic magnetic properties of such a free layer can be successfully described by a granular model with three thickness-dependent parameters: mean perpendicular anisotropy of the grains, grain-to-grain anisotropy distribution, and intergrain exchangelike coupling strength. Easy-cone anisotropy may help reduce the stochasticity of the spin transfer torque switching. However, it arises at intermediate values of the intergrain exchange coupling where the spin transfer torque (STT) switching efficiency is degraded, as shown by multimacrospin modeling. This is due to the excitation of exchange modes contributing weakly to the STT switching process while dissipating part of the STT energy.

Ni thickness influence on magnetic properties (Co/Ni/Co/Pt) multilayers with perpendicular magnetic anisotropy

We present a study of perpendicular magnetic anisotropy in [Co/Ni(t)/Co/Pt]×8 multilayers for use as free layers in magnetic tunnel junctions (MTJ) and spin valves. The thickness t of the Ni sub-layer was varied and the resulting magnetic properties were investigated. As determined from magnetic force microscopy and magnetometry measurements, all multilayers exhibited a perpendicular magnetic anisotropy with an increase of saturation magnetization with thickness t. From the temperature dependence of the magnetization, well described by a Bloch law, we find that the spin-wave stiffness constant of the [Co/Ni(t)/Co/Pt]×8 multilayers is larger compared to (Co/Ni) multilayers. These multilayers could be the basis for spintronic devices where the reduction of total Pt content could help to reduce the damping constant while keeping the magnetic anisotropy energy relatively high. These are conflicting requirements needed for high performance magnetic memory devices.

Enhanced ferromagnetic resonance linewidth of the free layer in perpendicular magnetic tunnel junctions.

We report the frequency dependence of the ferromagnetic resonance linewidth of the free layer in magnetic tunnel junctions with all perpendicular-to-the-plane magnetized layers. While the magnetic-field-swept linewidth nominally shows a linear growth with frequency in agreement with Gilbert damping, an additional frequency-dependent linewidth broadening occurs that shows a strong asymmetry between the absorption spectra for increasing- and decreasing external magnetic field. Inhomogeneous magnetic fields produced during reversal of the reference and pinned layer complex is demonstrated to be at the origin of the symmetry breaking and the linewidth enhancement. Consequentially, this linewidth enhancement provides indirect information on the magnetic coercivity of the reference and pinned layers. These results have important implications for the characterization of perpendicular magnetized magnetic random access memory bit cells.

Observation of giant magnetoresistance in CoFeN/AlOx/CoFeN magnetic tunneling junctions employing a nitrogen-doped amorphous CoFeN free layer electrode

We examine the crystallographic and magnetic features of nitrogen-doped amorphous CoFe (CoFeN) electrodes for application as ferromagnetic free layers in magnetic tunnel junctions, in which precise control of the nitrogen content is crucial for achieving the desirable magnetic features. Incorporating nitrogen into the CoFe layer during growth provides numerous benefits including a remarkably reduced coercivity of 5 Oe, a phase transition from polycrystalline to amorphous, a low magnetization of 294 emu/cm3, and an enhanced thermal stability up to 400 °C. A high magnetic resistance ratio of about 220% was also obtained for annealed in-plane CoFeN/AlOx/CoFeN magnetic tunneling junctions containing a 1.2-nm-thick amorphous AlOx tunnel barrier. We anticipate that our experimental findings will aid in the development of a variety of future spintronic devices.

Progress of electrical control magnetization reversal and domain wall motion

Electrical control of spins in magnetic materials and devices is one of the most important research topics in spintronics. We briefly describe the recent progress of electrical manipulations of magnetization reversal and domain wall motion.This review consists of three parts:basic concepts,magnetization manipulation by electrical current and voltage methods,and the future prospects of the field.The basic concepts,including the generation of the spin current,the interaction between the spin current and localized magnetization,and the magnetic dynamic Landau-Lifshitz-Gilbert-Slonczewski equation are introduced first.In the second part,we reviewed the progress of the magnetization controlled by electrical current and voltage. Firstly we review the electrical current control of the magnetization and domain wall motion.Three widely used structures, single-layer magnets,ferromagnet/heavy metal and ferromagnet/nonmagnetic metal/ferromagnet,are reviewed when current is used to induce magnetization reversal or drive domain wall motion.In a single-layer magnetic material structure,domain wall can be effectively driven by electrical current through spin transfer torque.The factors influencing the domain wall trapping and motion are also discussed.The electrical current control of the skyrmions has big potential applications due to much lower current density.Using the Dresselhaus and Rashba spin orbital coupling,the electrical current can also directly reverse the magnetization of single magnetic or antiferromagnetic layer.Then,we review the electrical current switching the magnetization of the ferromagnetic layer in ferromagnetic/heavy metal structures,where both spin Hall effect and Rashba effect can contribute to the current switching magnetization in such device structures. To identify the relative contributions of these two mechanisms,several quantitative studies are carried,concluding that spin Hall effect plays a major role,which is summarized in this review.Finally,we review the current switching magnetization of free layers in spin valve and magnetic tunnel junctions (MTJs) by spin transfer torque.We also discuss the approaches to the decrease of the critical current density in MTJs,which is desired for future applications.Alternatively,the electric field can also be used to manipulate the magnetization,where three methods are reviewed. Applying an electric field to the ferromagnetic/piezoelectric heterostructures,which changes the crystal structure of magnetic film through piezoelectric effects,realizes the change of the magnetic anisotropy of the ferromagnetic layer.In ferromagnetic/ferroelectric heterostructures,electric field changes the spin distribution and orbital hybridization at the surface of magnetic film through the magnet-electric coupling effects,and then controls the magnetization of the ferromagnetic layer.In ferromagnetic metal (semiconductor)/dielectric/metal structure,electric field controls the electron accumulation or depletion at the surface of the ferromagnetic metal or semiconductor,the change of the electron density in the magnetic layer in turn affects the magnetic exchange interaction and magnetic anisotropy.Finally,we present the prospects for the development of electrical control magnetization reversal and domain wall motion for future applications.

Exchange stiffness in ultrathin perpendicularly magnetized CoFeB layers determined using the spectroscopy of electrically excited spin waves

We measure the frequencies of spin waves in nm-thick perpendicularly magnetized FeCoB systems, and model the frequencies to deduce the exchange stiffness of this material in the ultrathin limit. For this, we embody the layers in magnetic tunnel junctions patterned into circular nanopillars of diameters ranging from 100 to 300 nm, and we use magneto-resistance to determine which rf-current frequencies are efficient in populating the spin wave modes. Micromagnetic calculations indicate that the ultrathin nature of the layer and the large wave vectors used ensure that the spin wave frequencies are predominantly determined by the exchange stiffness, such that the number of modes in a given frequency window can be used to estimate the exchange stiffness. For 1 nm layers, the experimental data are consistent with an exchange stiffness A=20±2 pJ/m, which is slightly lower than its bulk counterpart. The thickness dependence of the exchange stiffness has strong implications for the numerous situations that involve ultrathin films hosting strong magnetization gradients, and the micromagnetic description thereof.

Ferromagnetic resonance measurements of (Co/Ni/Co/Pt) multilayers with perpendicular magnetic anisotropy

Multilayers of [Co/Ni(t)/Co/Pt]×8 with varying Ni thickness were investigated for possible use as a free layer in magnetic tunnel junctions and spintronics devices. The thickness t of the Ni sub-layer was varied from 0.3 nm to 0.9 nm and the resulting magnetic properties were compared with (Co/Ni) and (Co/Pt) multilayers. As determined from magnetic force microscopy, magnetometry and ferromagnetic resonance measurements, all multilayers exhibited perpendicular magnetic anisotropy. Compared with (Co/Pt) multilayers, the sample with t of 0.9 nm showed almost the same anisotropy field of μ0Hk = 1.15 T but the damping constant was 40% lower. These characteristics make these multilayers attractive for spin torque based magnetoresistive devices with perpendicular anisotropy.

Comparative investigation of InGaP/InGaAs/Ge triple-junction solar cells using different Te-doped InGaP layers in tunnel junctions

Heavily tellurium (Te)-doped InGaP layers in tunnel junctions (TJs) grown by using metalorganic chemical vapor deposition (MOCVD) were investigated to improve the device performance of InGaP/InGaAs/Ge triple-junction solar cells. Three different doping techniques were employed to grow the Te-doped InGaP layers in the TJ; Te doping, Te and Si co-doping and Te pre-doping. Compared to other samples, the external quantum efficiency (EQE) profiles in the InGaP top cell were found to be higher for the sample with Te pre-doping. Under a concentrated light condition, higher fill factor (FF) and conversion efficiency were also observed for the sample with Te pre-doping. These indicate that the crystalline qualities of the upper TJ, composed of a p-GaAs/n-InGaP TJ, and the InGaP top cell were improved by using the Te pre-doping method.

Perpendicular magnetic tunnel junctions with a synthetic storage or reference layer: A new route towards Pt- and Pd-free junctions.

We report here the development of Pt and Pd-free perpendicular magnetic tunnel junctions (p-MTJ) for STT-MRAM applications. We start by studying a p-MTJ consisting of a bottom synthetic Co/Pt reference layer and a synthetic FeCoB/Ru/FeCoB storage layer covered with an MgO layer. We first investigate the evolution of RKKY coupling with Ru spacer thickness in such a storage layer. The coupling becomes antiferromagnetic above 0.5 nm and its strength decreases monotonously with increasing Ru thickness. This contrasts with the behavior of Co-based systems for which a maximum in interlayer coupling is generally observed around 0.8 nm. A thin Ta insertion below the Ru spacer considerably decreases the coupling energy, without basically changing its variation with Ru thickness. After optimization of the non-magnetic and magnetic layer thicknesses, it appears that such a FeCoB/Ru/FeCoB synthetic storage layer sandwiched between MgO barriers can be made stable enough to actually be used as hard reference layer in single or double magnetic tunnel junctions, the storage layer being now a single soft FeCoB layer. Finally, we realize Pt- or Pd-free robust perpendicular magnetic tunnel junctions, still keeping the advantage of a synthetic reference layer in terms of reduction of stray fields at small pillar sizes.

Titanium nitride as a seed layer for Heusler compounds

Titanium nitride (TiN) shows low resistivity at room temperature (27 μΩ cm), high thermal stability and thus has the potential to serve as seed layer in magnetic tunnel junctions. High quality TiN thin films with regard to the crystallographic and electrical properties were grown and characterized by x-ray diffraction and 4-terminal transport measurements. Element specific x-ray absorption spectroscopy revealed pure TiN inside the thin films. To investigate the influence of a TiN seed layer on a ferro(i)magnetic bottom electrode in magnetic tunnel junctions, an out-of-plane magnetized Mn2.45Ga as well as in- and out-of-plane magnetized Co2FeAl thin films were deposited on a TiN buffer, respectively. The magnetic properties were investigated using a superconducting quantum interference device and anomalous Hall effect for Mn2.45Ga. Magneto optical Kerr effect measurements were carried out to investigate the magnetic properties of Co2FeAl. TiN buffered Mn2.45Ga thin films showed higher coercivity and squareness ratio compared to unbuffered samples. The Heusler compound Co2FeAl showed already good crystallinity when grown at room temperature on a TiN seed-layer.

Manipulation of magnetization switching and tunnel magnetoresistance via temperature and voltage control.

Magnetization switching between parallel and antiparallel alignments of two magnetic layers in magnetic tunnel junctions (MTJs) is conventionally controlled either by an external magnetic field or by an electric current. Here, we report that the manipulation of magnetization switching and tunnel magnetoresistance (TMR) in perpendicularly magnetized CoFeB/MgO/CoFeB MTJs can be achieved by both temperature and voltage. At a certain range of temperature, coercivity crossover between top and bottom magnetic layers is observed in which the TMR ratio of the MTJs is almost unmeasurable. Furthermore, the temperature range can be tuned reversibly by an electric voltage. Magnetization switching driven by the voltage reveals an unconventional phenomenon such that the voltage driven coercivity changes with temperature are quite different for top and bottom CoFeB layers. A model based on thermally-assisted domain nucleation and propagation is developed to explain the frequency and temperature dependence of coercivity. The present results of controlling the magnetization switching by temperature and voltage may provide an alternative route for novel applications of MTJs based spintronic devices.

Interface evolution analysis of graded thermoelectric materials joined by low temperature sintering of nano-silver paste

Lead free solder Sn96.5Ag3.0Cu0.5 (SAC305) and nano-silver paste were used to connect graded thermoelectric (TE) arms, respectively. A series of harsh aging tests were carried out on joints of both p-type and n-type graded TE arms. The results showed that nano-silver paste printed by low-temperature joining technique was much more suitable for the stable connection of TE materials working in medium temperature range. 316 stainless steel and nickel were proved effectively as diffusion barrier layers in p-type junctions. While for n-type TE junctions, when aging at 300 °C, (Fe, Cr, Te) ternary alloy could be detected at Bi2Te2.7Se0.3/316 stainless steel interface. The formation of ternary compound realized fine metallurgical interconnection which was benefit for the improvement of joint strength. The average joint strength for p-type and n-type junctions in graded TE arms were above 20 MPa and 30 MPa, respectively.

Effects of quantum-well states on the tunneling magnetoresistance in magnetic tunnel junctions

When a nonmagnetic spacer is inserted between the insulator and the ferromagnetic layer in magnetic tunnel junctions (MTJs), spin-polarized quantum-well (QW) states may be formed in the nonmagnetic layer. We theoretically investigate the effect of the QW states on the tunneling magnetoresistance (TMR) in MTJs with an inserted nonmagnetic spacer using a free-electron model. When the spin-dependent confinement caused by the ferromagnetic layer is very strong, the TMR may converge to a finite value as a function of the nonmagnetic spacer thickness, and another oscillation period related to the QW state may appear.

Optimization of perpendicular anisotropy of Ta-inserted double CoFeB/MgO interface MTJ’s for STT-MRAM

Ta inserts in double magnetic tunnel junctions have been shown to induce perpendicular magnetic anisotropy. We fabricated the central layers of a CoFeB/MgO based double magnetic tunnel junction with a Ta insertion layer between the free layers of the magnetic tunnel junctions. The thickness of the Ta insert and CoFeB layer were varied from 0.5 to 1.1nm and 0.9 to 1.7nm respectively, to find which minimum thickness of Ta will induce perpendicular anisotropy in the MTJ. FMR studies were performed to measure the interfacial anisotropy Ku,i. We demonstrated that the most effective stack minimized the CoFeB thickness while maximizing the Ta thickness. This was balanced with the need to keep both CoFeB layers from decoupling or becoming magnetically dead.