Spin Tunnel Junctions Research Articles

Microwave artificial neurons based on magnetic tunnel junction nano-oscillators for image recognition and denoising

We report a microwave artificial neuron based on magnetic tunnel junction spin torque nano-oscillator (STNO). Based on STNO neurons, the three-layer full connected artificial neural network (ANN) are constructed to recognize handwritten digit with a produced accuracy of up to 90.95%. To address noise inherent in real circuits leads to poor recognition accuracy, we harness the frequency mutation characteristics of the STNO output near the critical threshold current for denoising handwritten digits corrupted by noise. Before and after denoising using the aforementioned ANN, the recognition accuracy is improved from 55.98% to 88.76%.

Influence of the Shape of a Spin-Tunnel Element on the Dependence of Its Magnetoresistance

A theoretical and experimental study of the dependence of the magnetoresistance for two spin-tunnel junctions (STJs) of ellipsoidal shape has been made. The one-sided homogeneous magnetization reversal mode of an ellipsoidal STJ with different aspect ratios has been experimentally selected. Despite the reverseinhomogeneous remagnetization, this selection has allowed for the calculation of the magnetic parameters of these elements by developing the Stoner-Wohlfarth theory.

Domain wall-magnetic tunnel junction spin–orbit torque devices and circuits for in-memory computing

There are pressing problems with traditional computing, especially for accomplishing data-intensive and real-time tasks, that motivate the development of in-memory computing devices to both store information and perform computation. Magnetic tunnel junction memory elements can be used for computation by manipulating a domain wall, a transition region between magnetic domains, but the experimental study of such devices has been limited by high current densities and low tunnel magnetoresistance. Here, we study prototypes of three-terminal domain wall-magnetic tunnel junction in-memory computing devices that can address data processing bottlenecks and resolve these challenges by using perpendicular magnetic anisotropy, spin–orbit torque switching, and an optimized lithography process to produce average device tunnel magnetoresistance TMR = 171% and average resistance-area product RA = 29 Ω μm2, close to the RA of the unpatterned film. Device initialization variation in switching voltage is shown to be curtailed to 7%–10% by controlling the domain wall initial position, which we show corresponds to 90%–96% accuracy in a domain wall-magnetic tunnel junction full adder simulation. Repeatability of writing and resetting the device is shown. A circuit shows an inverter operation between two devices, showing that a voltage window is large enough, compared to the variation noise, to repeatably operate a domain wall-magnetic tunnel junction circuit. These results make strides in using magnetic tunnel junctions and domain walls for in-memory and neuromorphic computing applications.

Synthetic Antiferromagnetic Structures in Technology of Spintronic Devices

In this paper, we consider synthetic antiferromagnetic (SAF) structures and their influence in a fixed layer of a spin-tunnel junction on the magnitude of the magnetoresistive effect and temperature stability. The SAF structure consists of two ferromagnetic (FM) layers, between which a nonmagnetic film is located. The ion exchange interaction of two FM layers has an oscillating character and depends on the nonmagnetic layer thickness and the surface roughness of the FM layers. The mechanism of SAF magnetization reversal with fixation by an antiferromagnetic layer and in its absence, as well as the effect of the sequence of deposition of layers in the SAF structure on its properties, are considered. The main applications of SAF structures as a part of spintronic devices is described.

A nonlocal spin Hall magnetoresistance in a platinum layer deposited on a magnon junction

Magnetoresistance effects are used in a variety of devices including hard disk drives and magnetic random access memories. In particular, giant magnetoresistance and tunnelling magnetoresistance can be used to create spin valves and tunnel junctions in which the resistance depends on the relative magnetization orientations of two ferromagnetic conducting layers. Here, we report a magnetoresistance effect that occurs in a platinum layer deposited on a magnon junction consisting of two insulating magnetic yttrium iron garnet (YIG) layers separated by an antiferromagnetic nickel oxide spacer layer. The resistance of the platinum layer is found to depend on the magnetization of the YIG layer in direct contact with it (an effect known as spin Hall magnetoresistance), but also the magnetization of the adjacent YIG layer in the junction. The resistance of the platinum layer is higher when the two YIG layers are aligned antiparallel than when parallel. We assign this behaviour to a magnonic nonlocal spin Hall magnetoresistance in which spin-carrying magnon propagation across the junction affects spin accumulation at the metal interface and hence modulates the spin Hall magnetoresistance. The effect could be used to develop spintronic and magnonic devices that have spin transport properties controlled by an all-insulating magnon junction and are thus free from Joule heating. A magnetoresistance effect that occurs in a platinum layer deposited on a magnon junction consisting of two insulating magnetic yttrium iron garnet layers separated by an antiferromagnetic nickel oxide spacer layer could be used to create spintronic and magnonic devices that are free from Joule heating.

Voltage-Controlled Magnonic Spin Tunneling Junction

We theoretically investigate the effective exchange interaction, $J_\mathrm{eff}$, mediated by conductive electrons within a nonmagnetic metal spacer, in the presence of a bias voltage, sandwiched by two ferromagnetic insulators. On the basis of the tight-binding model, we show the voltage and spacer thickness dependences of $J_\mathrm{eff}$, and its contorollability is demonstrated. We also propose a new magnonic device with the functions of both field effect transistor and non-volatile memory.

Tunable Spin Seebeck Diode with Magnonic Spin Tunneling Junction

We theoretically investigate the spin--wave spin current induced by the spin Seebeck effect in magnonic spin tunneling junctions (MSTJs) for arbitrary magnetization directions. We show that the MSTJ functions as a \textit{tunable} spin Seebeck diode in which the tunneling spin current can be turned on and off with high efficiency by controlling the magnetization direction.

A high-speed single sideband generator using a magnetic tunnel junction spin torque nano-oscillator

An important property of spin-torque nano-oscillators (STNOs) is their ability to produce a frequency modulated (FM) signal, which is very critical for communication applications. We here demonstrate a novel single sideband (SSB) modulation phenomenon using a magnetic tunnel junction (MTJ)-based STNO, which saves transmission bandwidth and in principle should minimize attenuation for wireless communication. Experimentally, lower single sidebands (LSSBs) have been successfully demonstrated over a wide range of modulation frequency, fm = 150 MHz-1 GHz. The observed LSSBs are determined by the intrinsic properties of the device, which can be modeled well by a nonlinear frequency and amplitude modulation formulation and reproduced in macrospin simulations. Moreover, our macrospin simulation results show that the range of modulation current and modulation frequency for generating SSBs can be controlled by the field-like torque and biasing conditions.

Design of magnetic tunnel junction‐based tunable spin torque oscillator at nanoscale regime

This study proposes a spintronic based compact tunable nano-sized RF oscillator. The proposed design provides parametric performance improvement as compared with the designs already reported in literature. This design also offers higher operating frequency up to range of several GHz, which can be tuned by a DC bias current. The proposed magnetic tunnel junction-spin torque oscillators (MTJ-STO) model overcomes the limitation of low output power which is prime issue in spin torque oscillators (STOs). This is achieved by employing multi-stage amplifier with impedance matching stages. In addition to that, mutual phase locking mechanism of STOs is introduced to act as a remedy for broadening of the spectrum linewidth, which is a critical issue in traditional oscillators. The hybrid model presented in this study contains spin torque based MTJ, which is compatible with CMOS technology. The modelling of proposed circuit has been done in Verilog-A and simulation results have been verified using HSPICE.

Defect induced enhancement of exchange bias by swift heavy ion irradiation in zinc ferrite–FeNiMoB alloy based bilayer films

Exchange biased systems consisting of ferromagnetic (FM)–antiferromagnetic (AFM) interfaces are increasingly being investigated because of their application potential in spin valves and tunnel junctions. In bilayer systems, ion irradiation is capable of modifying the interface and thereby offers unique opportunities to tailor exchange field. In the present study, irradiation with 100MeV Ag8+ ions is utilized to alter the exchange bias field in zinc ferrite–FeNiMoB bilayer system. The thin films which were deposited by RF sputtering technique and annealed at 600°C were irradiated at various fluences. Structural and magnetic studies were carried out by using Glancing X Ray Diffractometer (GXRD) and Superconducting Quantum Interference Device Vibrating Sample Magnetometer (SQUID VSM) respectively. It was observed that the as deposited films exhibited exchange bias and on ion irradiation, bias field could be enhanced at certain fluences. The enhancement in bias field is attributed to defects created in the antiferromagnet as a result of ion irradiation. The experimental result is fitted in accordance with the diluted antiferromagnet model. Coercivity was also found to vary with ion fluence. Ion fluence was thus effectively used to enhance bias field as well as coercivity in the bilayer consisting of zinc ferrite–FeNiMoB.

Comprehensive and Macrospin-Based Magnetic Tunnel Junction Spin Torque Oscillator Model-Part I: Analytical Model of the MTJ STO

Magnetic tunnel junction (MTJ) spin torque oscillators (STO) have shown the potential to be used in a wide range of microwave and sensing applications. To evaluate potential uses of MTJ STO technology in various applications, an analytical model that can capture MTJ STO's characteristics, while enabling system- and circuit-level designs, is of great importance. An analytical model based on macrospin approximation is necessary for these designs since it allows implementation in hardware description languages. This paper presents a new macrospin-based, comprehensive and compact MTJ STO model, which can be used for various MTJ STOs to estimate the performance of MTJ STOs together with their application-specific integrated circuits. To adequately present the complete model, this paper is divided into two parts. In Part I, the analytical model is introduced and verified by comparing it against measured data of three different MTJ STOs, varying the angle and magnitude of the magnetic field, as well as the DC biasing current. The proposed analytical model is suitable for being implemented in Verilog-A and used for efficient simulations at device-, circuit- and system-levels. In Part II, the full Verilog-A implementation of the analytical model with accurate phase noise generation is presented and verified by simulations.

Spin-dependent thermoelectric effects in transport through a nanoscopic junction involving a spin impurity

Conventional and spin-related thermoelectric effects in transport through a magnetic tunnel junction with a large-spin impurity, such as a magnetic molecule or atom, embedded into the corresponding barrier are studied theoretically in the linear-response regime. The impurity is described by the giant spin Hamiltonian, with both uniaxial and transverse magnetic anisotropy taken into account. Owing to the presence of transverse component of magnetic anisotropy, spin of a tunneling electron can be reversed during scattering on the impurity, even in the low temperature regime. This reversal appears due to exchange interaction of tunneling electrons with the magnetic impurity. We calculate Seebeck and spin Seebeck coefficients, and analyze their dependence on various parameters of the spin impurity and tunnel junction. In addition, conventional and spin figures of merit, as well as the electronic contribution to heat conductance are considered. We also show that pure spin current can be driven by a spin bias applied to the junction with spin impurity, even if no electron transfer between the electrodes can take place. The underlying mechanism employs single-electrode tunneling processes (electrode-spin exchange interaction) and the impurity as an intermediate reservoir of angular momentum.

Current driven magnetization dynamics of a self-polarised synthetic ferrimagnet

Spin torque driven excitations in spin valves and tunnel junctions are often investigated for a two magnetic layer system for which a polarizer (fixed magnetization) and a free layer can be distinguished. In the search for improved microwave properties and to understand the role of different coupling mechanisms between the magnetic layers, here, the excitation spectrum of an exchange coupled two layer synthetic ferrimagnet (SyF) system is investigated numerically with spin momentum transfer acting on both layers simultaneously. This self-polarised two layer system does not contain an external polarizer, and excitation of coupled modes arises due to the mutual spin transfer torque and the Ruderman-Kittel-Kasuya-Yosida interlayer exchange coupling. The current-field state diagrams of static and dynamic states are reported as a function of the interlayer exchange coupling strength. The numerically determined critical boundaries are well reproduced by an analytical stability analysis. The dynamic steady states reveal an optic-like mode at low magnetic fields, which becomes progressively acoustic-like for increased magnetic fields and currents. The frequency of these modes can be tuned by the film thickness and the strength of the interlayer exchange interaction. The results presented here will provide an important guide for designing spin torque oscillators that exploit the dynamic coupling between layers and, furthermore, they will provide a basis to test analytical models of spin torque driven coupled excitations.

Neutron scattering study of magnetic phase separation in nanocrystalline La5/8Ca3/8MnO3

We demonstrate that magnetic phase separation and competing spin order in the colossal magnetoresistive (CMR) manganites can be directly explored via tuning strain in bulk samples of nanocrystalline La${}_{1\ensuremath{-}x}$Ca${}_{x}$MnO${}_{3}$. Our results show that strain can be reversibly frozen into the lattice in order to stabilize coexisting antiferromagnetic domains within the nominally ferromagnetic metallic state of La${}_{5/8}$Ca${}_{3/8}$MnO${}_{3}$. The measurement of tunable phase separation via magnetic neutron powder diffraction presents a direct route of exploring the correlated spin properties of phase separated charge/magnetic order in highly strained CMR materials and opens a potential avenue for realizing intergrain spin tunnel junction networks with enhanced CMR behavior in a chemically homogeneous material.

Temporal Coherence of MgO Based Magnetic Tunnel Junction Spin Torque Oscillators

Single-shot, time-resolved measurements are presented to investigate the temporal coherence of the microwave emission for MgO based magnetic tunnel junction spin torque oscillators. The time-domain data reveal that the steady state regime obtained from frequency-domain analysis can be subdivided into two regimes as a function of spin polarized current amplitude. According to these two regimes, two mechanisms that limit the temporal coherence are identified. At low current, extinctions of the steady state oscillations lead to a very short coherence time on the order of a few nanoseconds, while at higher current, the extinctions vanish and the coherence time saturates around 40 ns. As an important result it is shown that the latter is limited by frequency fluctuations. Quenching these frequency fluctuations suggests an intrinsic linewidth that is by a factor of 20 below the one of the free running oscillator.

Large magnetoresistance in a manganite spin tunnel junction usingLaMnO3as the insulating barrier

A spin-tunnel-junction based on manganites, with La1 xSrxMnO3 (LSMO) as ferromagnetic metallic electrodes and the undoped parent compound LaMnO3 (LMO) as insulating barrier, is here theoretically discussed using double exchange model Hamiltonians and numerical techniques. For an even number of LMO layers, the ground state is shown to have anti-parallel LSMO magnetic moments. This highly resistive, but fragile, state is easily destabilized by small magnetic fields, which orient the LSMO moments in the direction of the field. The magnetoresistance associated with this transition is very large, according to Monte Carlo and Density Matrix Renormalization Group studies. The influence of temperature, the case of an odd number of LMO layers, and the differences between LMO and SrTiO3 as barriers are also addressed. General trends are discussed. The study of strongly correlated electronic systems (SCES) continues attracting the attention of the Condensed Matter community. These materials present complex phase diagrams that illustrates the competition which exists among phases with very different physical properties, such as d-wave superconductivity, antiferroand ferro-magnetic order, charge- and orbital-order, multiferroic behavior, and several others. Moreover, this complexity and phase competition lead to self-organized nano-scale inhomogeneities which are believed to generate giant responses, as in the famous colossal magnetoresistance (CMR) effect of the Mn-oxides known as manganites. 1

Interface structure and transport of complex oxide junctions

The interface structure and magnetism of hybrid magnetic tunnel junction-spin filter devices have been investigated and correlated with their transport properties. Magnetic tunnel junctions made of a spinel NiMn2O4 tunnel barrier sandwiched by theoretically predicted half-metallic electrodes, perovskite La0.7Sr0.3MnO3 and spinel Fe3O4, exhibit very high crystalline quality as observed by transmission electron microscopy. Structurally abrupt interfaces allow for the distinct magnetic switching of the electrodes as well as large junction magnetoresistance. The change in the magnetic anisotropy observed at the spinel-spinel interface is indicative of a thin interdiffused magnetically soft interfacial layer. The strong exchange coupling at this interface allows for low background magnetoresistance, and a spin-filter effect with when the barrier is ferrimagnetic.

La 0.6Sr 0.4MnO 3 ramp-edge junctions using (LaAlO 3) 0.3–(SrAl 0.5Ta 0.5O 3) 0.7 barrier

We have fabricated La 0.6Sr 0.4MnO 3 (LSMO)/(LaAlO 3) 0.3–(SrAl 0.5Ta 0.5O 3) 0.7 (LSAT)/LSMO spin tunnel junctions with a small area by using a ramp-edge geometry. The ramp angle was estimated to be about 25° from measurements of an atomic force microscope. The thickness of a counter LSMO electrode, a bottom LSMO electrode and an insulating LSAT barrier were 100, 200, and 4 nm, respectively. The 1-μm-wide junction showed a tunnel magneto-resistance (TMR) behavior with the TMR ratio of 550% at 100 K. Depending on the directions of the external magnetic field, different TMR characteristics were observed. It may reflect domain structures of the counter and the bottom electrodes, since the ramp-edge junctions are fabricated at the film edges of the base electrode.

Improved tunneling magnetoresistance in interface engineered (La,Sr)MnO3 junctions

Spin tunnel junctions were fabricated and characterized with various insulating barriers and interface structures employing (La,Sr)MnO3 (LSMO) as the ferromagnetic electrodes. Junctions with LaAlO3 barriers exhibited systematically larger tunnel magnetoresistance (TMR) (TMR ratio ∼230% at 10K) than those with SrTiO3 barriers (∼50%). When two unit cells of nondoped LaMnO3 are inserted between LSMO and SrTiO3 at both interfaces in the SrTiO3-barrier junction, the TMR was also significantly enhanced to ∼170%. These results, including the temperature dependence of TMR, qualitatively agree with the characteristics of the local magnetization at the interface that was evaluated by magnetization-induced second harmonic generation (MSHG) for the corresponding interface structures. However, slight deviations appear as a systematic suppression of TMR for all the junctions at high temperatures compared with MSHG results. Also, TMR results appear to be more degraded than MSHG results for the SrTiO3-barrier junctions. The barrier thickness dependence of the TMR response revealed that a spin-independent and inelastic conduction channel is a possible origin for the apparent suppression of TMR compared with MSHG.

X-Ray Diffraction Study of Ordered Antiferromagnets for Tunnel Junctions

Antiferromagnets (AF) such as MnPt and MnNi can be used in spin valves and tunnel junctions due to their high exchange coupling, high blocking temperature, and fair corrosion resistance. They are used as pinning layer in a AF/FM/barrier/FM structure, where FM is a ferromagnet such as CoFe and the barrier is an ultra-thin insulating layer. However, as deposited MnPt and MnNi films with thickness around 20 nm are in the fcc phase, and show no exchange bias in AF/FM bilayers. A transition from fcc to fct is required, and takes place upon annealing. We present an X-ray diffraction study of the phase transition in glass/Ta 7nm/Ru 3 nm/MnPt 20 nm/CoFe 5 nm/Ta 3 nm AF/FM bilayers. We observe the MnPt phase transition from fcc to fct around 250°C. We correlate the phase transition with the change of the bilayers magnetic properties.