Relative Magnetization Research Articles

Emergent Correlated Phases in Rhombohedral Trilayer Graphene Induced by Proximity Spin-Orbit and Exchange Coupling.

The impact of proximity-induced spin-orbit and exchange coupling on the correlated phase diagram of rhombohedral trilayer graphene (RTG) is investigated theoretically. By employing abinitio-fitted effective models of RTG encapsulated by transition metal dichalcogenides (spin-orbit proximity effect) and ferromagnetic Cr_{2}Ge_{2}Te_{6} (exchange proximity effect), we incorporate the Coulomb interactions within the random-phase approximation to explore potential correlated phases at different displacement fields and doping. We find a rich spectrum of spin-valley resolved Stoner and intervalley coherence instabilities induced by the spin-orbit proximity effects, such as the emergence of a spin-valley-coherent phase due to the presence of valley-Zeeman coupling. Similarly, proximity exchange removes the phase degeneracies by biasing the spin direction, enabling a magnetocorrelation effect-strong sensitivity of the correlated phases to the relative magnetization orientations (parallel or antiparallel) of the encapsulating ferromagnetic layers.

Molecular rectification induced by magnetization alignment in organic-ferromagnetic devices.

Spin-dependent transport in ferromagnet/organic-ferromagnet/ferromagnet junctions is investigated theoretically under different alignment of magnetization orientations. The results demonstrate a significant current rectification at low bias voltages, and the rectifying direction relies on the relative magnetization orientation in each component. The orbital analysis demonstrates two underlying mechanisms for the rectification, the slight structural asymmetry of the molecule from spin radicals and distinct spin match between conducting electrons and the magnetic molecule upon the reversal of bias. The latter is responsible for the strong low-bias rectification and relies on the magnetization alignment. The effects of parameter strength, temperature and size on the rectification are discussed. This work explores a new route to achieve high-performance molecular rectifiers operating at low bias with controlled rectifying direction.

Wide-field imaging of the magnetization process in soft magnetic-thin film using diamond quantum sensors

The magnetization process of a soft magnetic CoFeB-SiO2 thin film was imaged using diamond quantum sensors with perfectly aligned nitrogen-vacancy centers along the [111] direction formed by CVD. Around the film edge, the easy and hard axes directions exhibited different responses to the external magnetic field, consistent with ones observed by magneto-optical Kerr effect microscopy. Moreover, quantum diamond imaging could observe discontinuous magnetization along domain walls as non-uniform magnetic charges (MCs). Quantum diamond imaging would help in visualization through MCs, such as irregularity in the material and relative orientation of magnetizations in neighboring domains.

Non-linear GMI decoding in 3D printed magnetic encoded systems

The nonlinear giant magnetoimpedance (GMI) effect was explored as a highly sensitive sensing technology in 3D-printed magnetic encoded systems. Magnetic nanoparticles with low (magnetite, Fe3O4) and high (Co ferrite, Co0.7Fe2.3O4) magnetic remanence were embedded (10 wt.%) in a polymeric matrix of Polylactic Acid (PLA) and Poly-ε-caprolactone (PCL) and extruded in magnetic filaments to be 3D printed by the Fused Deposition Modelling technique (FDM). Two different geometries were constructed namely, individual magnetic strips and fixed barcoded pieces. The stray magnetic fields generated by the magnetic nanoparticles were detected through the non-linear (second harmonic) GMI voltage using a soft magnetic CoFeSiB wire as the nucleus sensor. The decoding response was analyzed as a function of the magnetization remanence of the nanoparticles, the distance between the individual magnetic strips, and the position (height) of the GMI decoding sensor. It has been shown that modification of the net magnetization direction of each individual fixed strip within the barcode geometry is possible through the application of local external magnetic fields. This possibility improves the versatility of the 3D binary encoding system by adding an additional state (0 without nanoparticles, 1 or -1 depending on the relative orientation of the net magnetization along the strips) during the codifying procedure.

Spin-wave nonreciprocity at the spin-flop transition region in synthetic antiferromagnets

We investigate the frequency nonreciprocity in CoFeB/Ru/CoFeB synthetic antiferromagnets near the spin-flop transition region, where the magnetic moments in the two ferromagnetic layers are noncollinear. Using conventional Brillouin light scattering, we perform systematic measurements of the frequency nonreciprocity as a function of an external magnetic field. At near zero magnetic field, where the antiparallel alignment of the magnetic moments in the two layers is established, we observe a significant frequency nonreciprocity of up to a few GHz, which vanishes when the relative magnetization orientation switches into the parallel configuration at a large magnetic field. For the intermediate values of the magnetic field, where the system transitions from the antiparallel to the parallel orientation, a nonmonotonous dependence of the frequency nonreciprocity is found, with a maximum frequency shift around the spin-flop critical point. This nontrivial dependence of the nonreciprocity versus field is attributed to the nonmonotonous dependence of the dynamic dipolar interaction, which is the main factor that causes asymmetry in the dispersion relation. Furthermore, we found that the sign of the frequency shift changes even without switching the polarity of the bias field. These results show that one can precisely control the nonreciprocal propagation of spin waves via field-driven magnetization reorientation.

Octupole-driven magnetoresistance in an antiferromagnetic tunnel junction

The tunnelling electric current passing through a magnetic tunnel junction (MTJ) is strongly dependent on the relative orientation of magnetizations in ferromagnetic electrodes sandwiching an insulating barrier, rendering efficient readout of spintronics devices1–5. Thus, tunnelling magnetoresistance (TMR) is considered to be proportional to spin polarization at the interface1 and, to date, has been studied primarily in ferromagnets. Here we report observation of TMR in an all-antiferromagnetic tunnel junction consisting of Mn3Sn/MgO/Mn3Sn (ref. 6). We measured a TMR ratio of around 2% at room temperature, which arises between the parallel and antiparallel configurations of the cluster magnetic octupoles in the chiral antiferromagnetic state. Moreover, we carried out measurements using a Fe/MgO/Mn3Sn MTJ and show that the sign and direction of anisotropic longitudinal spin-polarized current in the antiferromagnet7 can be controlled by octupole direction. Strikingly, the TMR ratio (about 2%) of the all-antiferromagnetic MTJ is much larger than that estimated using the observed spin polarization. Theoretically, we found that the chiral antiferromagnetic MTJ may produce a substantially large TMR ratio as a result of the time-reversal, symmetry-breaking polarization characteristic of cluster magnetic octupoles. Our work lays the foundation for the development of ultrafast and efficient spintronic devices using antiferromagnets8–10.

The negative interface capacitance and its anisotropy in magnetic tunnel junctions

The capacitance of magnetic tunnel junctions (MTJs) with the tunneling magnetocapacitance (TMC) effect has drawn lots of attention but there is still a lack of research on the capacitance anisotropy. In this work, a study of the capacitance in MgO-based MTJ is performed. The negative capacitance is observed and indicates the importance of interface capacitance in the MTJ. In further research, the interface capacitance depends on the relative orientation of magnetization in the two ferromagnetic layers (FMs), exhibiting a strong anisotropy. The Debye-Fröhlich model combined with the magnetization dependence of relaxation time is introduced to explain the behavior. Theory and experiment results of the relaxation time as a function of the relative angle of the two FM magnetizations are well fitted. It reveals that the TMC originates from the interface charge accumulation and spin-dependent tunneling. All the results suggest that the interface capacitance with apparent anisotropy plays an essential role in the TMC.

Length-induced large magnetoresistance in polyacene molecular spin valves

With the help of first-principles method, an unusual length effect in polyacene molecular spin valves is revealed. It is found that beyond a critical length, the magnitude of tunneling magnetoresistance is significantly enhanced from 10% to 2500%. Theoretical analysis demonstrates that such significant increase of magnetoresistance is related to the nonmagnetic-antiferromagnetic phase transition of polyacene molecules, which is determined by the relative magnetization orientation of the ferromagnetic electrodes. The molecular gap shows distinct length dependence with the change of molecular magnetic state, which keeps decreasing with length in the nonmagnetic state while a fine gap is obtained in the antiferromagnetic state. This work indicates a valid way to obtain considerable tunneling magnetoresistance in long molecular spin valves.

Large magnetoresistance in a Co/MoS2/graphene/MoS2/Co magnetic tunnel junction

We demonstrate a large magnetoresistance (MR) in a $\mathrm{Co}/\mathrm{Mo}{\mathrm{S}}_{2}/\mathrm{graphene}/\mathrm{Mo}{\mathrm{S}}_{2}/\mathrm{Co}$ magnetic tunnel junction by means of ab initio transport calculations. A Co electrode turns out to be an excellent spin injector for a $\mathrm{Mo}{\mathrm{S}}_{2}/\mathrm{graphene}/\mathrm{Mo}{\mathrm{S}}_{2}$ barrier. The transmission spectrum, current-voltage characteristics, spin injection efficiency, and magnetoresistance are calculated for the modeled device at various bias voltages in the parallel and antiparallel magnetic configurations. A remarkable change in the transmission spectrum and a subsequent change in total current through the junction have been observed, when the relative magnetic orientations of the electrodes are altered. The huge change in current due to the change in the relative magnetic orientation of the Co electrodes produces a high magnetoresistance up to $1270%$. The obtained values of the device parameters clearly indicate that a $\mathrm{Mo}{\mathrm{S}}_{2}/\mathrm{graphene}/\mathrm{Mo}{\mathrm{S}}_{2}$ heterostructure would be an excellent compound for highly efficient spin-valve device applications.

Electrically Reconfigurable 3D Spin‐Orbitronics

AbstractThe explosive demands of storage capacity and the von Neumann bottleneck of modern computer architectures trigger many innovations in information technology. Amongst them, nonvolatile spintronics attract considerable attentions for which can embed the computation capability into memory, enable neuromorphic, and probabilistic computing. These exciting progresses typically rely on the manipulation of the relative magnetization orientations of two magnetic layers. By extending to 3D spintronic architectures made of multiple magnetic layers (n), the exponentially increased 2n magnetic states can provide ample opportunities for implementing novel spintronic functionalities. Here, through building perpendicularly magnetized 3D spin‐orbitronic architectures – [Pt/Fe1−xTbx/Si3N4]n multilayers, it is demonstrated the electrical programing of 2n memory states via current‐induced spin–orbit torques (SOTs), and the accompanied reconfigurable multifunction in‐memory logic features in a single four‐terminal Hall device. Further, an electrical readout of these 2n states, together with the implementation of Boolean logic gates and digital circuitry such as 2–4 and 3–8 decoders, are successfully conducted. More complex logic circuits are also envisioned. The experiments thus substantiate 3D spin‐orbitronic structures as a promising platform for exponentially boosting the storage capacity and accommodating in‐memory computing that can be important for promoting the emerging 3D nanospintronics.

Dependence on sensitivity patterns of bias fields for vector measurements using optically pumped vapor magnetometers

The optical light shift effect is an all-optical technique to create magnetic bias fields to determine the vector components of the magnetic field using a scalar optically pumped magnetometer. We show that the measurement of the induced bias fields is affected by the response pattern of the scalar magnetometer. The sensitivity of the magnetometer affects the minimum total field resolution as well as the angular resolution of the vector measurement. Three configurations of multiple-laser, optically pumped cesium vapor vector magnetometers are characterized for their spatial sensitivity patterns. Each configuration has a unique response level as a function of the relative magnetic field orientation. Thus, the formulations used for angular determination are dependent on the magnetometer configuration, and the angular resolution of the vector measurement is dependent on the relative angle between the magnetic field and the optical probe beam. The spatial dependence of the light shift signals is measured, and the equations describing their responses are presented for each configuration. Equations for determining the field angles from the bias field responses are derived.

Spin valves as magnetically switchable spintronic THz emitters

In this Letter, we demonstrate that spin valve systems consisting of a magnetically soft free Fe layer and a magnetically hard Fe fixed layer pinned by an antiferromagnetic Ir23Mn77 layer, which are separated by a Pt or W spacer, are efficient switchable spintronic terahertz (THz) emitters. We show that a small external magnetic field on the order of 10 mT is sufficient to switch the magnetization orientation of the free layer and, therefore, to change the relative magnetization orientation of the two Fe layers. This goes along with a change in the THz RMS amplitude by up to more than one order of magnitude and allows fast switching of the emitter from a high to a low THz power state and vice versa.

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.

Influence of Current Redistribution in Switching Models for Perpendicular STT-MRAM

The outstanding improvement in the performance of modern integrated circuits is supported by the continuous down-scaling of semiconductor devices. However, this leads to a substantial increase in leakages, which results in growing stand-by power consumption. A viable path to mitigate these issues is the introduction of nonvolatility in integrated circuits. Spin-transfer torque (STT) magnetoresistive random access memory (MRAM) is a promising candidate. It is competitive with conventional nonvolatile flash memory as it combines high-speed, excellent endurance, and low costs. The range of potential STT-MRAM applications goes from automotive and Internet-of-Things to embedded memories on System-on-Chips and L3 caches [1]. The core of an STT-MRAM device is a magnetic tunnel junction, where the relative orientation of its magnetic layers provides a way of storing the binary information. Switching between the two possible configurations is achieved by passing an electric current through the structure. The electrons get spin-polarized by the reference layer and, when entering the free layer, act via the exchange interaction on its magnetization providing the torque. With a sufficiently large current density, the magnetization of the free layer can be flipped.The usual approach for micromagnetic simulations of STT switching is to assume a constant and uniform current density [2]. In circuits and applications, however, the voltage, rather than the current density, is fixed during the switching process. As the tunneling resistance in an MTJ depends on the relative magnetization orientation of the two layers, the current depends on time. Moreover, the magnetization of the free layer is nonuniform during switching, which results in a position (and time) dependent current density J(r,t) (Fig. 1). In order to evaluate the validity and the limits of the fixed current density approach in computing the switching time, we compare it with a model in which the total current is fixed, but redistributed according to the position-dependent resistance [3], determined by the local magnetization orientation. In addition, the fixed current and the fixed current density models are compared with a realistic approach in which the voltage during switching is kept constant. Herein, the value of the current in the fixed current/current density models is equal to the voltage in the fixed voltage model divided by the resistance in the initial parallel (P) or anti-parallel (AP) state. The free layer is perpendicularly magnetized. The switching time depends on the realization of the stochastic magnetic field which mimics the magnetization fluctuations at room temperature.It is demonstrated that, by slightly increasing the current from its initial value for AP to P switching and decreasing it for P to AP switching, the switching time distributions can be matched [4] for any value of the stray field induced by the pinned layer (Fig.2). Here, we systematically study the dependence of this current correction on system parameters. The dependence of the current correction on the tunneling magnetoresistance ratio (TMR) at room temperature is reported in Fig. 3. It is observed that the value of the correction increases with higher TMR. We then performed simulations at zero temperature, and in this case, the correction required to reproduce the fixed voltage results is lower than the one at room temperature (Fig. 4). However, the switching is also slower. This proves that the correction to the current is not universal and depends on the system parameters. To elaborate on the physical origin of this dependence we performed macrospin simulations. The initial magnetization orientation is slightly tilted from its perfect perpendicular orientation in order to reduce the incubation time of switching. By changing the initial tilting of the magnetization, we demonstrate that a faster switching requires a higher correction to the current value, which explains the difference between the room and zero temperature simulations (Fig. 5). Simulation results for the current correction as a function of the surface area of the structure at room temperature are shown in Fig. 6. The switching is faster for higher values of the area, and the current correction required is consequently higher, in agreement with the macrospin results. We conclude that it is possible to reproduce the switching times obtained within the realistic fixed voltage approach with the approximate fixed current density approach for a wide range of system parameters, provided that the dependence of the current correction on such parameters is taken into consideration.REFERENCES[1] S.Sakhare, M.Perumkunnil, T.H.Bao, S.Rao, W.Kim et al., Proc.IEDM, 420 (2018).[2] A.Makarov, T.Windbacher, V.Sverdlov, S.Selberherr, Semic.Sci.Tec. 31, 113006 (2016).[3] D.Aurelio, L.Torres, G.Finocchio, J.Magn.Magn.Mat. 321, 3913 (2009).[4] S.Fiorentini, R.Orio, W.Goes, J.Ender, V.Sverdlov, Proc.SISPAD, 57 (2019). Figure 1

Spin-dependent shot noise in MgO-based magnetic tunnel junctions under noncollinear magnetization alignment

We report experimental measurements of shot noise in MgO-based magnetic tunnel junctions (MTJs) upon angular magnetization rotation in the free layer. The normalized shot noise (the Fano factor) is found to be sub-Poissonian and exhibits a sinusoidal-like variation over the relative magnetic orientation between the free and the pinned layer inside the MTJs. The observed sub-Poissonian statistics provides direct evidence of electron sequential tunneling mediated by localized states inside the tunnel barrier. The variation of the Fano factor can be interpreted in terms of a semi-classical model that we propose. Based on this model, the variation of the Fano factor can be used to reveal microscopic details of the tunneling barrier.

Tunneling magnetoresistance in ferromagnet/organic-ferromagnet/metal junctions*Project supported by the National Natural Science Foundation of China (Grant Nos. 11974215, 11704230, 11674197, and 11874242), the Natural Science Foundation of Shandong Province, China (Grant No. ZR2019MA043), and the Taishan Scholar Project of Shandong Province, China.

Spin-dependent transport in ferromagnet/organic-ferromagnet/metal junctions is investigated theoretically. The results reveal a large tunneling magnetoresistance up to 3230% by controlling the relative magnetization orientation between the ferromagnet and the central organic ferromagnet. The mechanism is explained by distinct efficient spin-resolved tunneling states in the ferromagnet between the parallel and antiparallel spin configurations. The key role of the organic ferromagnet in generating the large magnetoresistance is explored, where the spin selection effect is found to enlarge the difference of the tunneling states between the parallel and antiparallel configurations by comparing with the conventional organic spin valves. The effects of intrinsic interactions in the organic ferromagnet including electron–lattice interaction and spin coupling with radicals on the magnetoresistance are discussed. This work demonstrates a promising potential of organic ferromagnets in the design of high-performance organic spin valves.

Mapping out the magneto-Seebeck coefficient

Spintronics In the Seebeck effect, a temperature difference across a device generates voltage. If the thermal gradient is imposed across a magnetic tunnel junction—with two magnetized layers separated by an insulating tunnel barrier—the magnitude of the generated voltage depends on the relative orientation of the magnetization in the two layers. Transport measurements of this so-called magneto-Seebeck tunneling typically reveal only the signal averaged over the device. Friesen et al. created an atomic-scale version of this experiment by using a scanning tunneling microscope with a spin-polarized tip that they scanned across the surface of a magnetic sample. By heating the tip, they were able to map out the spatial dependence of the spin-resolved Seebeck coefficient. Science , this issue p. [1065][1] [1]: /lookup/doi/10.1126/science.aat7234

Magnon Valve Effect between Two Magnetic Insulators.

The key physics of the spin valve involves spin-polarized conduction electrons propagating between two magnetic layers such that the device conductance is controlled by the relative magnetization orientation of two magnetic layers. Here, we report the effect of a magnon valve which is made of two ferromagnetic insulators (YIG) separated by a nonmagnetic spacer layer (Au). When a thermal gradient is applied perpendicular to the layers, the inverse spin Hall voltage output detected by a Pt bar placed on top of the magnon valve depends on the relative orientation of the magnetization of two YIG layers, indicating the magnon current induced by the spin Seebeck effect at one layer affects the magnon current in the other layer separated by Au. We interpret the magnon valve effect by the angular momentum conversion and propagation between magnons in two YIG layers and conduction electrons in the Au layer. The temperature dependence of the magnon valve ratio shows approximately a power law, supporting the above magnon-electron spin conversion mechanism. This work opens a new class of valve structures beyond the conventional spin valves.

Demands for spin-based nonvolatility in emerging digital logic and memory devices for low power computing

Miniaturization of semiconductor devices is the main driving force to achieve an outstanding performance of modern integrated circuits. As the industry is focusing on the development of the 3nm technology node, it is apparent that transistor scaling shows signs of saturation. At the same time, the critically high power consumption becomes incompatible with the global demands of sustaining and accelerating the vital industrial growth, prompting an introduction of new solutions for energy efficient computations. Probably the only radically new option to reduce power consumption in novel integrated circuits is to introduce nonvolatility. The data retention without power sources eliminates the leakages and refresh cycles. As the necessity to waste time on initializing the data in temporarily unused parts of the circuit is not needed, nonvolatility also supports an instanton computing paradigm. The electron spin adds additional functionality to digital switches based on field effect transistors. SpinFETs and SpinMOSFETs are promising devices, with the nonvolatility introduced through relative magnetization orientation between the ferromagnetic source and drain. A successful demonstration of such devices requires resolving several fundamental problems including spin injection from metal ferromagnets to a semiconductor, spin propagation and relaxation, as well as spin manipulation by the gate voltage. However, increasing the spin injection efficiency to boost the magnetoresistance ratio as well as an efficient spin control represent the challenges to be resolved before these devices appear on the market. Magnetic tunnel junctions with large magnetoresistance ratio are perfectly suited as key elements of nonvolatile CMOS-compatible magnetoresistive embedded memory. Purely electrically manipulated spin-transfer torque and spin-orbit torque magnetoresistive memories are superior compared to flash and will potentially compete with DRAM and SRAM. All major foundries announced a near-future production of such memories. Two-terminal magnetic tunnel junctions possess a simple structure, long retention time, high endurance, fast operation speed, and they yield a high integration density. Combining nonvolatile elements with CMOS devices allows for efficient power gating. Shifting data processing capabilities into the nonvolatile segment paves the way for a new low power and high-performance computing paradigm based on an in-memory computing architecture, where the same nonvolatile elements are used to store and to process the information.

Angle and rotational direction dependent horizontal loop shift in epitaxial Co/CoO bilayers on MgO(100)

Co/CoO belongs to the typical exchange bias systems which have been investigated for decades. Surprisingly, Co/CoO thin film systems epitaxially grown on MgO(100) substrates show a strong influence of the relative orientation of the average uncompensated antiferromagnetic magnetization with respect to the cooling field direction, giving rise to unexpected asymmetric angular dependencies of the horizontal loop shift as well as the sign of the transverse magnetization peaks in magneto-optical Kerr effect (MOKE) experiments. In this paper, we provide a broad overview of the influence of cooling field orientation and sample orientation on magnetization reversal processes in this system.