Spin Transfer Research Articles

L10 FePd-based perpendicular magnetic tunnel junctions with 65% tunnel magnetoresistance and ultralow switching current density

L10 FePd is increasingly recognized as a potential candidate for magnetic tunnel junctions (MTJs), yet there remains room for enhancing device performance. In this work, we fabricated fully-integrated L10 FePd-based perpendicular MTJ devices and achieved a significant increase in tunnel magnetoresistance, reaching ∼65%, compared to the previous record of 25%. Notably, we observed bi-directional switching with a low switching current density of about 1.4 × 105 A/cm2, which outperforms the typical spin-transfer torque (STT) MTJ by about one order of magnitude. We propose two possible mechanisms to elucidate the switching process and associated device performance: (1) The voltage-controlled exchange coupling-driven switching of the bottom CoFeB layer; (2) The STT-driven switching of the exchange-coupled L10 FePd–CoFeB composite. While additional research is necessary, these findings may further advance the integration of L10 FePd into spintronic devices, potentially enabling low-energy memory and logic technologies.

Data-cell-variation-tolerant triple sampling non-destructive self-reference sensing scheme of STT-MRAM

Inevitable process variations (PVT) brought by both the magnetic tunneling junction (MTJ) and MOSFET based on the complementary metal-oxide semiconductor (CMOS) technology become a major obstacle for the mass production of spin transfer torque magnetic random access memory (STT-MRAM). The detriment of the process variations leads to a serious degradation in the fundamental yield with the shrinkage of the technology nodes. However, the conventional data-cell-variation-tolerant (DCVT) sense scheme cannot get the target read yield due to the limited sense margin (SM). To resolve this problem, a DCVT triple sampling non-destructive self-reference sensing scheme (TSNS) is proposed in the paper, which doubles the SM, with lower power consumption and better SM compared with the conventional DCVT sense scheme. Monte Carlo simulation with industry-compatible 65-nm model parameters results show that the proposed sensing scheme shows over 2.5 times higher SM and less power consumption compared to the previous self-reference circuit. The proposed sensing scheme can get the target read yield with lower power consumption.

Ultralow Electric Current-Assisted Magnetization Switching due to Thermally Engineered Magnetic Anisotropy.

Control of magnetic anisotropy in thin films with perpendicular magnetic anisotropy is of paramount importance for the development of spintronics with ultralow-energy consumption and high density. Traditional magnetoelectric heterostructures utilized the synergistic effect of piezoelectricity and magnetostriction to realize the electric field control of magnetic anisotropy, resulting in additional fabrication and modulation processes and a complicated device architecture. Here, we have systematically investigated the electric current tuning of the magnetic properties of the metallic NiCo2O4 film with intrinsic perpendicular magnetic anisotropy. Ferrimagnetic-to-paramagnetic phase transition has been induced through Joule heating, resulting in a rapid decrease of both magnetic coercivity and moment. An ultralow current density of 2.5 × 104 A/cm2, which is 2 to 3 orders magnitude lower than that of conventional spin transfer torque devices, has been verified to be effective for the control of the magnetic anisotropy of NiCo2O4. Successful triggering of magnetic switching has been realized through the application of a current pulse. These findings provide new perspectives toward the electric control of magnetic anisotropy and design of spintronics with an ultralow driving current density.

Magnetization reversal via domain wall motion in vertical high-aspect-ratio nanopillar with two magnetic junctions

A vertical ferromagnetic (FM) nanopillar can be used as magnetic memory owing to characteristics such as its high storage capacity and high thermal stability. The perpendicular shape anisotropy (PSA) of the pillar enables its magnetization direction to be stabilized. A pillar with a high aspect ratio exhibits both strong PSA and magnetization with high thermal stability. Reversing the magnetization direction of such a pillar using the current flowing through it is a significant challenge in spintronics. However, spin injection from another FM layer alone cannot reverse the magnetization of pillars of which the length exceeds 100 nm. This motivated us to propose a magnetic junction (MJ) consisting of a high-aspect-ratio FM nanopillar with two thin FM layers. Using micromagnetic simulation, we demonstrate the magnetization reversal of a 150 nm-long pillar with a diameter of 15 nm. The simulation revealed that the magnetization of the pillar reverses because of the spin transfer torque induced by the spin injection from the two thin FM layers and the spin-polarized current (SPC) flowing in the pillar in the longitudinal direction. During the magnetization reversal process, a domain wall (DW) first forms at one end of the pillar due to the spin injection. Then, driven by the SPC, the DW moves to the other end of the pillar, and the magnetization is reversed. The magnetization direction of the pillar, controlled by changing the direction of the current flowing through the pillar, can be evaluated from the respective magnetoresistance values of the two MJs. Alternatively, by pinning the DW in the pillar, a three-value magnetic memory can be developed. In addition, multi-bit and analog memories can be developed by controlling the pinning position of the DW. The high-aspect-ratio pillar-writing scheme is foreseen to pave the way for the practical development of next-generation spintronic devices.

A Radiation-Hardened Triple Modular Redundancy Design Based on Spin-Transfer Torque Magnetic Tunnel Junction Devices

Integrated circuits suffer severe deterioration due to single-event upsets (SEUs) in irradiated environments. Spin-transfer torque magnetic random-access memory (STT-MRAM) appears to be a promising candidate for next-generation memory as it shows promising properties, such as non-volatility, speed, and unlimited endurance. One of the important merits of STT-MRAM is its radiation hardness, thanks to its core component, a magnetic tunnel junction (MTJ), being capable of good function in an irradiated environment. This property makes MRAM attractive for space and nuclear technology applications. In this paper, a novel radiation-hardened triple modular redundancy (TMR) design for anti-radiation reinforcement is proposed based on the utilization of STT-MTJ devices. Simulation results demonstrate the radiation-hardened performance of the design. This shows improvements in the design’s robustness against ionizing radiation.

Implementing bidirectional logic with backhopping in magnetic tunnel junctions

A bidirectional logic gate has been designed based on the backhopping phenomenon observed in magnetic tunnel junctions (MTJ) at high bias. The magnetization dynamics of each magnetic layer of the MTJ—having materials and geometry of a standard spin-transfer torque magnetic random access memory device—is calculated using the coupled Landau–Lifshitz–Gilbert equation-based theoretical framework. A circuit design interconnecting the MTJs has been proposed to simulate a two-input NAND gate. The results in both forward and reverse directions agree well with those found from the Boltzmann distribution, thereby demonstrating the equiprobability of all valid states.

Correlations of dihadron polarization in central, peripheral, and ultraperipheral heavy-ion collisions

While jet quenching in relativistic heavy-ion collisions has been extensively studied over decades, the polarization of quenched hadrons has rarely been discussed. It has recently been proposed that the correlations of dihadron polarization in e+e− and pp collisions provide a novel probe of the longitudinal spin transfer from hard partons to hadrons without requiring the colliding beams to be polarized. To support realistic experimental measurement of dihadron polarization with sufficient luminosity, we extend the aforementioned study to relativistic heavy-ion collisions by convoluting the vacuum fragmentation of partons with their energy loss inside the quark-gluon plasma. We find that while the correlation functions of Λ−Λ (or Λ−Λ¯) polarization in peripheral collisions is consistent with those in pp collisions, clear enhancement can be seen in central collisions. These correlation functions appear sensitive to different assumptions in the de Florian-Stratmann-Vogelsang (DSV) of parton fragmentation functions and, therefore, could place additional constraints on the spin-dependent fragmentation functions of quarks and gluons. The correlation of dihadron polarization has also been explored in ultraperipheral heavy-ion collisions, which provides a cleaner probe of fragmentation functions of quarks produced by energetic photon-photon and photon-Pomeron interactions. Published by the American Physical Society 2024

First-principle study of spin transport property in L10-FePd(001)/graphene heterojunction

In our previous work, we synthesized a metal/2D material heterointerface consisting of L10-ordered iron–palladium (FePd) and graphene (Gr) called FePd(001)/Gr. This system has been explored by both experimental measurements and theoretical calculations. In this study, we focus on a heterojunction composed of FePd and multilayer graphene referred to as FePd(001)/m-Gr/FePd(001), where m represents the number of graphene layers. We perform first-principles calculations to predict their spin-dependent transport properties. The quantitative calculations of spin-resolved conductance and magnetoresistance (MR) ratio (150%–200%) suggest that the proposed structure can function as a magnetic tunnel junction in spintronics applications. We also find that an increase in m not only reduces conductance but also changes transport properties from the tunneling behavior to the graphite π-band-like behavior. Additionally, we investigate the spin-transfer torque-induced magnetization switching behavior of our junction structures using micromagnetic simulations. Furthermore, we examine the impact of lateral displacements (sliding) at the interface and find that the spin transport properties remain robust despite these changes; this is the advantage of two-dimensional material hetero-interfaces over traditional insulating barrier layers such as MgO.

Anisotropy-assisted magnon condensation in ferromagnetic thin films

We theoretically demonstrate that adding an easy-axis magnetic anisotropy facilitates magnon condensation in thin yttrium iron garnet (YIG) films. Dipolar interactions in a quasi-equilibrium state stabilize room-temperature magnon condensation in YIG. Even though the out-of-plane easy-axis anisotropy generally competes with the dipolar interactions, we show that adding such magnetic anisotropy may even assist the generation of the magnon condensate electrically via the spin-transfer torque mechanism. We use analytical calculations and micromagnetic simulations to illustrate this effect. Our results may explain the recent experiment on Bi-doped YIG and open a pathway toward applying current-driven magnon condensation in quantum spintronics. Published by the American Physical Society 2024

Realizing polarization-dependent unidirectional magnon channel in antiferromagnetic domain wall

Achieving unidirectional spin wave (or magnon) transport in domain wall (DW) represents the key step for designing functional magnonic devices. Here, we theoretically investigate the propagation behavior of spin waves (SWs) in antiferromagnetic DW when the Dzyaloshinskii–Moriya interaction (DMI) and/or spin transfer torque (STT) are considered. On the one hand, we find that the DMI lifts the degeneracy of magnon bands, from which one can obtain pure right- or left-handed polarized SWs. On the other hand, the nonreciprocal attenuation of magnons induced by STT is identified. Interestingly, we realize the polarization-dependent unidirectional propagation of SWs when the nonadiabatic coefficient β exceeds a critical value. Moreover, the micromagnetic simulations verify the theoretical predictions with good agreement. Our work provides a simple method for achieving unidirectional magnons with desired polarity in antiferromagnetic DW, which is indispensable for future magnonic computing and communication.

Optimizing Motion‐Induced Spin Transfer

In this article, the spin transfer between two ferromagnetic insulators is studied. There is a narrow gap between the ferromagnetic insulators so that they are weakly interacting with each other. One of the ferromagnetic insulators is moving at a constant speed while the other is at rest; hence, the system is out of equilibrium. In the presence of the shearing motion, the interaction amplitude is periodically modulated at the Doppler frequency. A unitary transformation allows us to regard the periodic modulation of the interaction amplitude as an effective potential, which drives the spin transfer. The amount of the spin current is controlled by the spectral overlap and the carrier population difference between the two ferromagnetic media. If the spectra of the two ferromagnets are moderately broadened, the overlap in the spectral domain increases, enlarging the spin current. However, too much broadening spoils the spectral overlap and, hence, the spin current. This implies that there is an optimal condition for maximizing the spin transfer.

Design and Assessment of Hybrid MTJ/CMOS Circuits for In-Memory-Computation

Hybrid magnetic tunnel junction/complementary metal oxide semiconductor (MTJ/CMOS) circuits based on in-memory-computation (IMC) architecture is considered as the next-generation candidate for the digital integrated circuits. However, the energy consumption during the MTJ write process is a matter of concern in these hybrid circuits. In this regard, we have developed a novel write circuit for the contemporary three-terminal perpendicular-MTJs that works on the voltage-gated spin orbit torque (VG+SOT) switching mechanism to store the information in hybrid circuits for IMC architecture. Investigation of the novel write circuit reveals a remarkable reduction in the total energy consumption (and energy delay product) of 92.59% (95.81) and 92.28% (42.03%) than the conventional spin transfer torque (STT) and spin-Hall effect assisted STT (SHE+STT) write circuits, respectively. Further, we have developed all the hybrid logic gates followed by nonvolatile full adders (NV-FAs) using VG+SOT, STT, and SHE+STT MTJs. Simulation results show that with the VG+SOT NOR-OR, NAND-AND, XNOR-XOR, and NV-FA circuits, the reduction in the total power dissipation is 5.35% (4.27%), 5.62% (3.2%), 3.51% (2.02%), and 4.46% (2.93%) compared to STT (SHE+STT) MTJs respectively.

Longitudinal and transverse spin transfer to Λ and Λ¯ hyperons in polarized p+p collisions at s=200 GeV

The longitudinal and transverse spin transfers to Λ (Λ¯) hyperons in polarized proton-proton collisions are expected to be sensitive to the helicity and transversity distributions, respectively, of (anti)strange quarks in the proton, and to the corresponding polarized fragmentation functions. We report improved measurements of the longitudinal spin transfer coefficient, DLL, and the transverse spin transfer coefficient, DTT, to Λ and Λ¯ in polarized proton-proton collisions at s=200 GeV by the STAR experiment at RHIC. The dataset includes longitudinally polarized proton-proton collisions with an integrated luminosity of 52 pb−1, and transversely polarized proton-proton collisions with a similar integrated luminosity. Both datasets have about twice the statistics of previous results and cover a kinematic range of |ηΛ(Λ¯)|<1.2 and transverse momentum pT,Λ(Λ¯) up to 8 GeV/c. We also report the first measurements of the hyperon spin transfer coefficients DLL and DTT as a function of the fractional jet momentum z carried by the hyperon, which can provide more direct constraints on the polarized fragmentation functions. Published by the American Physical Society 2024

Stark control of solid-state quantum memory with spin-wave storage

Quantum memories for quantum communication need to be able to store photons for an extended time and then to release them on demand. This can be achieved in atomic frequency comb ensemble-based quantum memories by control pulses that transfer the excitation to and from long-lived spin states. However, such pulses can give rise to coherent and incoherent noise due to their interaction with the memory ensemble. In this paper, we experimentally demonstrate the ability to switch off the coherent noise from such control pulses during the echo emission in a spin-wave quantum memory, using the linear Stark effect in rare-earth-ion doped crystals. By applying an electric-field pulse, the echo emission was coherently switched off prior to the first spin transfer pulse, and the stored data pulse was restored only when both an optical recall pulse and a rephasing electrical pulse were applied, giving a high degree of control of both desired and undesired emissions. We estimate the effectiveness of this technique by turning off the free-induction decay of a narrow ensemble of ions. This technique can thus improve the noise performance of spin-wave storage at the single-photon level by quenching coherent optical radiation created by strong control pulses. The method demonstrated here represents a proof of principle that the spin-wave storage scheme can be combined with Stark control. The combined scheme serves as an addition to the toolbox of techniques that can be used to realize a full version of a quantum repeater. Published by the American Physical Society 2024

Single-nanometer CoFeB/MgO magnetic tunnel junctions with high-retention and high-speed capabilities

Making magnetic tunnel junctions (MTJs) smaller while meeting performance requirements is critical for future electronics with spin-transfer torque magnetoresistive random access memory (STT-MRAM). However, it is challenging in the conventional MTJs using a thin CoFeB free layer capped with an MgO layer because of increasing difficulties in satisfying the required data retention and switching speed at smaller scales. Here we report single-nanometer MTJs using a free layer consisting of CoFeB/MgO multilayers, where the number of CoFeB/MgO interfaces and/or the CoFeB thicknesses are engineered to tailor device performance to applications requiring high-data retention or high-speed capability. We fabricate ultra-small MTJs down to 2.0 nm and show high data retention (over 10 years) and high-speed switching at 10 ns or below in sub-5-nm MTJs. The stack design proposed here proves that ultra-small CoFeB/MgO MTJs hold the potential for high-performance and high-density STT-MRAM.

Spin-dependent transport and spin transfer torque in a borophene-based spin valve.

This article presents a theoretical analysis of spin-dependent transport and spin-transfer torque in a borophene-based ferromagnetic/normal/ferromagnetic junction. This study focuses on borophene nanoribbons (BNRs) as a basis for spin valve numerical calculations for the investigation of conduction in both configurations where the magnetization vectors of the leads are parallel or antiparallel to each other (P and AP configurations, respectively), magnetoresistance (MR), and spin transfer torque (STT). The Landauer formalism and non-equilibrium Green's function (NEGF) approaches are used to derive the spin-dependent tunneling currents in the Magnetic Tunnel Junction (MTJ). The results of the calculations for a zigzag BNR show that the conductance is always larger than e2/h for the P configuration of lead magnetizations. For the AP configuration, the conductance becomes zero in specific energy ranges. A perfect MR plateau is found for the junction in the absence of disorder, which proves to be an excellent spin valve candidate. The variations of STT with Fermi energy and the relative angle between the magnetizations of two electrodes are studied for different strengths of ferromagnetic magnetization. The STT per unit bias voltage, as a function of Fermi energy, decreases significantly near the Dirac point energy. A sinusoidal oscillatory pattern can be evidently observed in the STT at unit bias voltage V versus the angle between the magnetizations of two electrodes, which amplifies as M increases. Borophene has unique properties, including low density and high hardness, heat resistance, and electrical conductance, which make it a promising candidate for spintronics. This article provides a comprehensive analysis of the spin-dependent properties of borophene and its potential applications in spintronics.

Investigation of the exact spin channels in laser-induced spin dynamics in two mononuclear Cu(II) complexes.

In the quest to harness the potential of nanospintronic applications, we analyze and investigate the spin channels for the ultrafast spin dynamics in mononuclear Cu2+(tdp)Cl2 (Cutdp) and Cu2+(tdp)Cl2·MeCN (Cutdp·MeCN) using a high-level ab initio many-body theory. In that spirit, we select two slightly different polymerizations arising from one parent complex. We establish the difference in magnetic behavior between the two complexes which arises solely from the geometrical differences. We calculate the static magnetic properties, such as the magnetic anisotropy of the complexes, which is analyzed by means of the magnetic moment of the ground state. The asymmetry of the core Cu-Cl-Cu-Cl axial plane unit is also reflected in the ground state absorption spectra of the two complexes. Comparisons with the experimental data are in good agreement with the exception of one peak in the theoretical calculations for each of the complexes, confirming the reliability of theoretical methods employed. A major finding in this work is the distinction between classical and coherent superpositions of Λ processes. We employ the selective blocking and retention (SBR) technique to find the unique path or paths for spin dynamic scenarios like spin flip and spin transfer. Additionally, we also present two different scenarios in which intermediate states are involved in spin dynamic processes, (i) classical superposition of Λ processes (i.e., there are many unique paths for transition, even with just one intermediate state the transition completes successfully), and (ii) collective coherent superposition of Λ processes (i.e., there is only one path for the transition, which requires more than one intermediate state to be in a specific coherent superposition). As a consequence, we gain insight into the type of correlations (static or dynamic) involved in a particular spin dynamic scenario.

Effect of spin transfer torque on antiferromagnetic film: computer simulations

Effect of spin transfer torque on antiferromagnetic film: computer simulations

Left-handed polarized spin waves induced by spin polarized electric currents in ferromagnetic domain walls

Polarization refers to the orientation of the wave oscillation which is a fundamental property of wave. It has been used widely to encode information in photonics and phononics. In magnonics, spin wave also has been used for transmitting and processing information. However, exploiting the spin wave polarization to design devices has not been achieved yet in ferromagnets as only the right-handed polarized spin waves can be accommodated in ferromagnets. Our eariler study suggests that the left-handed polarized spin waves can be introduced into ferromagnets by appling a spin-polarized electric current, thus making it possible to design spin wave devices with polarization encoding. But the critical current needed to induce left-handed polarized spin wave in a uniformly magnetized ferromagnet is too high to be realized experimentally. Magnetic domain wall can serve as spin wave guide, and the cutoff frequency of spin wave in a domain wall approaches zero. In this work, the dispersion relationship and propagation characteristics of spin wave in a Bloch domain wall are studied based on the Landau-Lifshitz equation in the presence of a spin-polarized electrical current. It is found that the stable left-handed spin wave can be generated in the domain wall with only a small current density. Micromagnetic simulations confirm the theoretical analysis results. In addition, due to the different excitation efficiencies and spin transfer torque induced propagating nonreciprocity of left- and right-handed polarized spin wave, it is possible to excite selectively the left- and right-handed polarized spin wave, as well as nearly linearly polarized spin waves. This study provides a practical and feasible solution for designing spin wave devices based on the polarization coding technique.

Using single and double laser pulses on the molecular Ni4@C48H36 system to design integrated nanospintronic units.

The accomplishment of long-distance spin transfer scenarios between several magnetic centers is a big challenge for building and supporting spin-logic units for developing future all-optical magnetic unit operations. Using high-level quantum chemistry theory CCSD and EOM-CCSD, we systematically study the ultrafast laser-induced spin-dynamics process on a carbon-based material, to which four magnetic centers are attached. We show that the CCSD method with the 6-31G basis set calculation is sensitive to the C-Ni bond length. The spin density distribution, which is computed using EOM-CCSD with LanL2DZ+ECP calculations, Mulliken population analysis, including spin-orbit-coupling (SOC) and a magnetic field, fulfills the requirements for achieving spin dynamics processes. Different local spin-flip and spin-transfer processes are accomplished within the subpicosecond regime. The impact of the propagation direction of the laser pulse by switching their polar and the azimuthal angles in spherical coordinates on the spin dynamics processes is analyzed. Double laser pulses with time delay δt ≥ 200 × FWHM yield in a realistic magnetic field gradient selectively a lateral resolution, which corresponds to distances smaller than the CMOS scale (2 nm in 2024) while our system size is comparable to the CMOS scale. Here Λ and V processes with two quasi-degenerate intermediate levels are used. We propose a model of an integrated spin-logic processor created from an array of individual spin-logic blocks, which are realized by four magnetic centers Ni. The findings of this study demonstrate the enormous potential of using laser-induced spin dynamics as the fundamental mechanism for future molecular magnetic technology.