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Magnetic skyrmions are nanoscale topologically nontrivial spin texture that offer great promise as information carriers for the next-generation spintronic computing schemes. However, the current skyrmion-based logic gates require precise control of skyrmion collisions through clocked synchronizers, which leads to the complexity of logic implementation. To address this challenge, we propose a clockless skyrmion logic gate that can be implemented into large-scale computing networks without skyrmions synchronization. The clockless operations are achieved in a cross-shaped skyrmion track based on the skyrmion gating method with the aid of the voltage-controlled magnetic anisotropy gate. A complete set of Boolean operations and a cascaded full adder can be realized through the different combinations of inputs and interconnections of these logic gates. This computing paradigm paves the way for the design of a highly efficient and robust computing architecture using fully skyrmion-based logic devices.

InP-based high electron mobility transistors (HEMTs) are potential candidates for sub-millimeter wave and terahertz satellite communications due to their ultrahigh frequency performance. Therefore, the study of their irradiation reliability is extremely urgent. In this paper, a 2 MeV proton irradiation experiment has been carried out in InP-based HEMTs, and damage mechanisms have been systematically studied, including dc and rf characteristics. The experimental results show that InP-based HEMTs have wondrously excellent radiation tolerance. The degradation of electrical characteristics occurs only when the irradiation fluence is higher than 1 × 1013 H+/cm2. The drain saturation current and the maximum transconductance have, respectively, decreased by 7.1% and 5.4% at a fluence of 1 × 1014 H+/cm2. Different from the other III–V HEMTs, the irradiated InP-based HEMTs exhibited an abnormality in the “peak collapse” of transconductance. Rf characteristics' parameters demonstrate slighter degradation compared to dc transconductance. Transmission line model (TLM) measurement and Schottky barrier calculation have shown that there is no noticeable degradation of an Ohmic contact and a Schottky contact; therefore, the main possible reason for device degradation comes from the interior of a semiconductor structure. Furthermore, device simulation indicates that defects introduced by irradiation on the upper and lower heterojunction interface of the channel and the interface of the gate recess should be responsible for degradation. Our experiments show that InP-based HEMTs have excellent radiation resistance, and they have good prospects for applications in radiation environments.

The operation speed and the energy-efficiency of magnetic random access memory (MRAM) is largely controlled by Gilbert damping of magnetic layers. The ultrafast laser pulse may offer an opportunity to tune the interfacial spin pumping, which can then control the Gilbert damping. Here, we have investigated the ultrafast laser induced magnetization precession, especially the magnetic damping, of a series of Ta/CoFeB/MgO thin films using the pump–probe time-resolved magneto-optical Kerr effect (TR-MOKE) measurements. The pump fluence dependence of the magnetic damping has been found to vary with the thickness tCoFeB of the nanoscale CoFeB layer. Remarkably, the intrinsic damping constant α0 has been found to decrease with the increase in the pump fluence when the thickness of the CoFeB layer is less than 1.2 nm. This fluence-dependent behavior of α0 is attributed to the fluence-dependent contribution of the Ta/CoFeB interface induced spin pumping effect. The ultrafast laser pulses effectively enhance the interfacial spin pumping effect via tuning the spin diffusion length of the adjacent Ta layer, from 2.4 to 7.1 nm. Our findings provide insights into the ultrafast laser pulse driven magnetic dynamics and interfacial spin manipulation in Ta/CoFeB/MgO structures.

Skyrmions have attracted significant interest due to their topological spin structures and fascinating physical features. The skyrmion phase arises in materials with Dzyaloshinskii-Moriya (DM) interaction at interfaces or in volume of non-centrosymmetric materials. However, although skyrmions were generated experimentally, one critical intrinsic relationship between fabrication, microstructures, magnetization and the existence of skyrmions remains to be established. Here, two series of CoFeB ultrathin films with controlled atomic scale structures are employed to reveal this relationship. By inverting the growth order, the amount of defects can be artificially tuned, and skyrmions are shown to be preferentially formed at defect sites. The stable region and the density of the skyrmions can be efficiently controlled in the return magnetization loops by utilizing first-order reversal curves to reach various metastable states. These findings establish the general and intrinsic relationship from sample preparation to skyrmion generation, offering an universal method to control skyrmion density., 25 pages, 5 figures

We present a field-free spin–orbit torque magnetic random access memory (SOT-MRAM) element using an in-plane ferromagnet (FL1)/coupling layer/perpendicular ferromagnet (FL2) as a composite free layer. By using micromagnetic simulations, we investigate the magnetic switching of the composite free layer in different conditions. Leveraging on interlayer exchange coupling, a field-free and efficient spin–orbit torque-induced reversal of perpendicular magnetization is realized, which can reduce the switching current density of SOT-MRAM. When the current density is increased to a certain value, the oscillations of magnetization are observed. Furthermore, by adjusting the magnetic anisotropy of FL1 and FL2, multilevel magnetization states can be achieved by varying the amplitude of the writing current. This work paves the way toward practical spin–orbit torque-based memory, oscillating, and logic devices.

Ion etching is an essential step in the processing of spintronic devices. In this work, we investigated the role of argon ion (Ar+) bombardment in the spin pumping and inverse spin Hall effect in the Y3Fe5O12/Pt (YIG/Pt) heterostructure. The inverse spin Hall voltage is found to reduce by an order of two when the argon ion bombardment is employed on the YIG surface before the deposition of the Pt layer. This giant inhibition of spin injection efficiency is undesirable. In this work, we propose an experimental technique for its recovery via a chemical route. The interface property and chemical state were identified by transmission electron microscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy. We found that the argon ion bombardment on the YIG surface leads to an increase in the ratio of Fe2+ ions in the YIG/Pt interface region. Moreover, the interface magnetic moment reduces in the presence of Fe2+ ions, which resulted in the decrease in spin injection efficiency. A strong oxidizing solution (a mixture of concentrated H2SO4 and 30% H2O2 of 1:1 volume ratio) was used to recover the valence of iron and subsequently the interface magnetic moment. Our results are helpful for the understanding of the importance of interface properties and the optimization of spintronic device processing technology.

The voltage impulse-induced large, nonvolatile, and tunable magnetization switching in a Ni80Co20/Pb(Mg, Nb)O3-PbTiO3 (PMN-PT) structure was investigated at room temperature. Ni80Co20 was deposited onto a specified PMN-PT substrate with defect dipoles. By exploiting defect dipoles, a distinct and stable strain memory state was achieved at zero electric field. It induces and sustains two distinct magnetization states when removing an electric field via the magnetoelectric coupling effect. Via the detailed x-ray diffraction and piezoresponse force microscopy analyses, the polarization switching pathway and the lattice strain in response to the in situ electric field were investigated to understand the microscopic mechanisms behind the nonvolatile magnetic memory. Furthermore, the impulse electric field can be selected in the range between the coercive field and the saturation field of the PMN-PT, leading to a wide range controlling technique. This work provides a promising way to produce a large and nonvolatile magnetic memory in magnetoelectric heterostructure and is significant for ultra-low-power information storage devices.

Here, the spin-torque ferromagnetic resonance signal and the spin Hall magnetoresistance induced by the spin Hall effect of W/Cu/CoFeB heterostructures with different Cu layer thicknesses ( t Cu) have been systemically studied. The effective spin mixing conductance g eff ↑ ↓, the interfacial spin transparency T, and the real spin–orbit torque efficiency ( J s J c ) real show a significant increase compared to the W/CoFeB heterostructure. ( J s J c ) real reaches its maximum of ∼0.54, increased up to ∼50% at the optimized t Cu ∼ 0.52 nm according to our theoretical prediction. More importantly, the intrinsic spin Hall angle of W, θ SH int ∼ 0.79 ± 0.20, has also been obtained after the correction of the inverse spin Hall effect and T. This suggests that the Cu insertion improves the interface quality and, therefore, assists the spin transport in the heterostructures, which potentially improves the performance of next-generation spintronic devices.

A spin–orbit torque-based logic device that combines the Dzyaloshinskii–Moriya interaction and asymmetric shape anisotropy is proposed in this paper. Through magnetization switching of the magnetic tunnel junction (MTJ), four distinct input cases are determined to perform all of the 16 basic Boolean functions by a single magnetic tunnel junction (MTJ). With various combinations of the tilted angle of the MTJ and the value of current density, a single MTJ can serve as a reconfigurable logic gate.

Gilbert damping is of crucial importance for spintronic devices due to its practical effect on the response time and energy consumption. Lower damping would enable more energy-efficient excitations and, thus, less current is needed. The Gilbert damping constant of the Co2FeAl film, a half-metal material important for spintronics with its 100% spin polarization at the Fermi level, has shown an abnormal increase at the thin film limit due to the inter-diffusion of Al atoms. Here, we report that the Gilbert damping of ultra-thin Co2 FeAl 1 + x films of nanometer thicknesses can be effectively tuned by delicately controlling the stoichiometric ratio during the growth. Gilbert damping has been found to be the lowest of 0.065 in Co2 FeAl 1 + 0.1, which is deduced by ∼ 50 % compared to that in Co2FeAl. We have further found that the damping constant of the ultra-thin Co2 FeAl 1 + 0.1 film is restored to the value of 0.062 of the nominal stoichiometric Co2FeAl by compensating the Al composition deficiency, as supported by 10.37% of composition change from scanning transmission electron microscopy with energy-dispersive-spectroscopy. This work offers a unique path to manipulate the Gilbert damping constant in ultra-thin Co2FeAl films by Al concentration control.

An energy efficient technique has shown to produce a three-state magnetic memory cell in a [011]-poled Ni80Co20/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) heterostructure. Via the magnetic field deposition, a 45° magnetic easy axis of the NiCo film was induced in the film plane. By using a strong converse magnetoelectric coupling between the NiCo film and the PMN-PT, the magnetic moments of NiCo can be modulated to [001] and [1-10] directions of PMN-PT by selecting an appropriate electric field (E-field). Consequently, large, medium, and small anisotropic magnetoresistance (AMR) values are obtained by fixing a measuring current along the [001] direction. The required E-field significantly reduces due to the initial direction of NiCo along the 45° direction. The tunability of the AMR ratio is as large as ∼87%. These results indicate that an energy efficient approach to generate magnetic storage by using only a small E-field rather than a magnetic field with a high energy consumption was realized. This work shows great potential for the development of ultra-low power and high-density magnetoresistive memory devices.

In this work, undoped semiconductors, germanium (Ge) and germanium tin (GeSn), were grown on ferrimagnetic insulator yttrium iron garnet (YIG) thin films using ultra-high vacuum molecular beam epitaxy. The crystallinity of the structure was determined from x-ray diffraction and high-resolution transmission electron microscopy combined with energy dispersive x-ray spectroscopy. Both spin pumping and inverse spin Hall effects (ISHEs) of YIG/Ge and YIG/GeSn heterojunctions have been investigated with the help of broadband ferromagnetic resonance (FMR). We observe that the spin mixing conductances of YIG/Ge (60 nm) and YIG/GeSn (60 nm) are 5.4 × 1018 m−2 and 7.2 × 1018 m−2, respectively, responsible for giant spin current injection. Furthermore, it is found that spin pumping injects giant spin current from ferrimagnetic YIG into the Ge semiconductor. The infrared laser modulated ISHE was examined using heavy metal platinum as a spin current collector. Also, it has been noted that the variation in the power of laser irradiation significantly changed the ISHE voltage of YIG/Ge/Pt spin junctions, saturated magnetization, FMR linewidth, and Gilbert damping parameter of YIG, which could be attributed to the laser-induced thermal effect. The outcomes from this study are promising for the development of Ge-based spintronic and magnonic devices.

The atomic-scale magnetism of Co2FeAl Heusler alloys has long been an outstanding question, and with the thickness down to the nanometer scale, this becomes even more sophisticated. Here, we report a direct measurement of the Co2FeAl epitaxial thin films on the GaAs(001) substrate with the in-situ magneto-optic Kerr effect and the synchrotron-based X-ray magnetic circular dichroism techniques. Strong uniaxial magnetic anisotropy has been observed from all thicknesses of the Co2FeAl thin films between 3 unit cells (uc) and 20 uc. A critical thickness of 3 uc has been identified, below which an anti-parallel spin component of the Co atoms occurs. This anti-parallel spin component can be responsible for the significantly reduced magnetic moment and the low spin-polarization near the Fermi level of the Co2FeAl.

In this article, we investigate the effect of electric-field on the perpendicular magnetic anisotropy (PMA) and strain properties in nanoscaled CoFeB/MgO magnetic tunnel junction using tunnel magnetoresistance and piezoresponse force microscopy (PFM) measurements, respectively. We show that while the PMA change under electric-field is consistent with the previous reports, the PFM data show that the applied electric-field induces strain in a nanoscaled MgO. We demonstrate that the development of compressive and tensile strains corresponding to different polarities of applied electric-field. We discuss the interplay between the electric-field controlled PMA and strain properties. Our results may accelerate the development of magnetoelectrically controlled spintronic devices for low-power and high-density magnetic data storage applications.

We report an investigation of electric-field (EF) control of spin re-orientation as functions of the thicknesses of CoFeB free layer (FL) and MgO layer in synthetic-antiferromagnetic pinned magnetic tunnel junctions with perpendicular magnetic anisotropy. It is found that the EF modulates the coercivity (Hc) of the FL almost linearly for all FL thicknesses, while the EF efficiency, i.e., the slope of the linearity, increases as the FL thickness increases. This linear variation in Hc is also observed for larger MgO thicknesses (≥1.5 nm), while the EF efficiency increases only slightly from 370 to 410 Oe nm/V when MgO thickness increases from 1.5 to 1.76 nm. We have further observed the absence of quasi-DC unipolar switching. We discuss its origin and highlight the underlying challenges to implement the EF controlled switching in a practical magnetic memory.

We demonstrate an electric-field-tunable magnetic-field-sensor based on CoFeB/MgO magnetic tunnel junction with interfacial perpendicular magnetic anisotropy (PMA). From the dynamic lock-in measurements, we show that an applied electric-field induces a peak in sensor voltage (VSENSOR) around the free layer magnetization switching regime in response to external a.c. magnetic field. Detailed measurements of VSENSOR as functions of free layer thickness, a.c. magnetic field amplitude and frequency reveal that the sensitivity of the sensor can be up to 80.8 V cm−1 Oe−1 under −0.5 V, which can be controlled by the strength and polarity of the applied electric-field via electric-field controlled PMA. We discuss the origin of our observations based on the oscillations in the tunnel magnetoresistance, and this may trigger the development of magnetoelectrically controlled magnetic-field-sensor based on magnetic tunnel junctions.

We have investigated the electric field effects in low resistance perpendicular magnetic tunnel junction (MTJ) devices and found that the electric field can effectively reduce the coercivity (Hc) of free layer (FL) by 30% for a bias voltage Vb = −0.2 V. In addition, the bias field (Hb) on free layer is almost linearly dependent on Vb yet independent on the device size. The demonstrated Vb dependences of Hc and Hb in low resistance MTJ devices present the potential to extend the scalability of the electric field assisted spin transfer torque magnetic random access memory and improve its access speed.

A strong ⟨111⟩ preferred orientation is induced along the axis of the TbDyFe/epoxy bonded giant magnetostrictive rods by curing the epoxy under a moderate magnetic field. TbDyFe particles with a size distribution of 200–300 μm align in an epoxy matrix, showing an approximate chain structure. A high magnetostrictive performance resulting from the grain-⟨111⟩-oriented anisotropy is achieved with the longitudinal magnetostriction λ∥ up to 1360×10−6 under the prestress of 17 MPa.

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

Spin momentum transfer in a nanomagnetic device with perpendicular magnetic anisotropy for both free and fixed magnetic layers is studied. The perpendicular anisotropy is induced by using CoFe∕Pt multilayer. The magnetoresistive loop shows that the perpendicular switching fields for the free and fixed layers are 170 and 380Oe, respectively, with ΔR∕R=0.47%. Resistance-current scanning clearly shows a full out-of-plane switching of the free layer magnetization under a sweeping current, which fully excludes the effect of switching by the magnetic field generated by the current. The critical current density is around 1.0×108A∕cm2, which could be tuned by changing the CoFe∕Pt multilayer structures.

We report a novel approach to reduce the critical current density for spin transfer in nanoscale magnetic tunnel junction (MTJ) structures. By integrating a spin valve and a MTJ structure with antiparallel pinned layers, the dc critical switching current density is reduced by one order of magnitude (2×106A∕cm2) at room temperature. The magnetoresistive (MR) properties of the MTJ + spin-valve device are dominated by the MTJ layers. The MR ratio is 15.8% with resistance area product of 4.5Ωμm2. This demonstration opens a window for high-density magnetic random access memory design.