Heavy Metal Layer Research Articles

Control of coherent phonon dynamics in CoFeB/heavy metal heterojunction structures for future hybrid phononic and spintronic devices

Abstract The heterojunction structure of CoFeB/heavy metal has shown significant potential for spintronics, where both electrons and magnons potentially can serve as information carriers. However, another promising information carrier, coherent phonons, has not been fully explored for hybrid phononic and spintronic devices. In this study, we used time-resolved pump-probe spectroscopy to investigate the dynamic behavior of coherent phonons. We observed variations in reflectivity spectra, corresponding to changes in phonon frequency and relaxation times, with different thicknesses of the heavy metal and CoFeB layers. The experimental results demonstrated a decrease in coherent phonon oscillation frequency as the thickness of the CoFeB and heavy metal layers increased. These findings were further supported by first-principles calculations, which showed that the frequency of the optical modes is suppressed due to interface relaxation between the magnetic and heavy metal layers.

Achieving a comparable transverse magneto-optical Kerr effect by spin–orbit field driven magnetoplasmonic

In this study, we propose and simulate a magnetoplasmonics heterostructure that utilizes spin–orbit fields to generate an internal magnetic field and create a significant magneto-optical effect. Our approach offers a new way to overcome the challenges of using permanent magnets or magnetic coils in conventional magnetoplasmonics, such as high-power consumption and non-scalability. We demonstrate that it is possible to create an appropriate amount of magnetic field using spin–orbit fields induced by the spin-Hall effect, such that the consumption power becomes reasonable and the dimensions could be miniaturized. This approach will be an important development in the field of magneto-optics, as it can lead to enhanced transverse magneto-optical Kerr effect in the present of surface plasmon polaritons. The proposed nanostructure consists of a ferromagnetic film adjacent to a heavy metal layer, both sandwiched between two noble metal films, and deposited on a dielectric prism. The strength of the Kerr signal strongly depends on the thickness of the ferromagnetic layer, with the maximum effect observed at a thickness of 5nm. This concept has potential for various nanophotonic and spintronic applications, particularly for developing high-speed active plasmonic devices for ultrafast light modulation.

Engineering of Co/MgO interface with combination of ultrathin heavy metal insertion and post-oxidation for voltage-controlled magnetic anisotropy effect

The voltage-controlled magnetic anisotropy (VCMA) effect in ferromagnet/insulator junctions provides an effective way to manipulate electron spins, which can form the basis of future magnetic memory technologies. Recent studies have revealed that the VCMA effect can be strongly tuned by a process of “interface engineering” exploiting ultrathin heavy metal layers and an electron depletion effect. To further decrease the numbers of electrons, chemical reactions, such as surface oxidation of ferromagnets, may also be an effective way to achieve this depletion. However, the knowledge of combined effect of heavy metal layers and oxidation is still lacking. Here, we demonstrate that dual interfacial engineering using an insertion of heavy metals (Pt or Re) and a post-oxidation process can have a remarkable effect on the perpendicular magnetic anisotropy and the VCMA effect. Interestingly, a strong enhancement of the perpendicular magnetic anisotropy is observed by dual interfacial engineering with Pt insertion, although it does not occur with Pt insertion or surface oxidation alone. Furthermore, even a sign reversal of the additional VCMA effect due to the ultrathin heavy metal layers is observed by utilizing dual interfacial engineering. These findings provide another degree of freedom for designing voltage-controlled spintronic devices and pave the way to interfacial spin–orbit engineering for the VCMA effect.

Contributions of interfacial spin–orbit coupling to magnetic and spintronic properties in AuPt/ferromagnet bilayers

We investigate the relationships between various magnetic and spintronic properties within AuPt/ferromagnet (FM) bilayers (FM = CoFe, CoFeB, Py, and Co). A linear correlation between the volume and surface magnetic anisotropies is identified, potentially influenced by the magnetoelastic effect. The FM thickness dependence of the magnetic damping indicates that spin-memory loss due to the interfacial spin–orbit coupling (ISOC) and spin pumping to the heavy-metal layer contribute little to the damping. Instead, a notable contribution from two magnon scattering to the damping is recognized in AuPt/(Co, CoFe, CoFeB) bilayers, possibly originating from a magnetic inhomogeneity due to the ISOC. In addition, in contrast to the magnetic damping, spin–orbit-torque efficiencies are unlikely related to the ISOC in AuPt/FM systems. This work offers valuable insights into the correlations among magnetic and spintronic parameters arising from the interfaces, ultimately aiding in the advancement of magnetic memory and information processing systems.

Interfacial Spintronic THz Emission

AbstractThe broken inversion symmetry at the ferromagnet (FM)/heavy‐metal (HM) interface leads to spin‐dependent degeneracy of the energy band, forming spin‐polarized surface states. As a result, the interface serves as an effective medium for converting spin accumulation into 2D charge current through the inverse Rashba–Edelstein effect. Exploring and assessing this spin‐to‐charge conversion (SCC) phenomenon at the FM/HM interface can offer a promising avenue to surpass the presumed limits of SCC in bulk HM layers. Spintronic heterostructures are utilized as a platform to measure the SCC experienced by photoexcited spin currents. Therefore, FM/HM heterostructures emitting terahertz electric field upon illumination by femtosecond laser pulses enable quantitative measure of the ultrafast SCC process. This results demonstrate a robust interfacial spin‐to‐charge conversion (iSCC) within a synthetic antiferromagnetic heterostructure, specifically for the NiFe/Ru/NiFe configuration, by isolating the SCC contribution originating from the interface and the bulk heavy‐metal (HM). Through the measurements of the emitted terahertz pulse, the iSCC at the NiFe/Ru interface is identified to be ≈27% of the strength as compared to SCC from the highest spin‐Hall conducting heavy‐metal, Pt. The results thus highlight the significance of interfacial engineering as a promising pathway for achieving efficient ultrafast spintronic devices.

External manipulation of the spin-orbit torque and magnetization switching in gradient CoPd single layer via hydrogen

Recent findings regarding spin-orbit torques (SOTs) and current-induced magnetization switching in ferromagnetic (FM) single layers have attracted substantial attention due to the advantage of not necessitating the use of heavy-metal layers. Nevertheless, despite prior studies on the interior structural engineering of the SOT, the external techniques for manipulating the SOT in the FM single layer remains elusive, which is indispensable for the practical application of the single layer SOT devices. Here, we demonstrate external manipulation of SOT generation in CoPd single layer through the fabrication of CoPd film with a composition gradient, utilizing the H2-absorption property of Pd. It is found that the H-induced strain within the CoPd film plays a pivotal role in generating SOT. Meanwhile, we demonstrate that the critical current density required for the current-induced magnetization switching is markedly diminished with the application of H2 due to the enhanced SOT generation and reduced perpendicular magnetic anisotropy energy. Our findings offer a straightforward method for external manipulation of single layer SOT devices, and hold the potential for applications of the spintronic devices.

The influence of exchange-bias on the resonance frequencies and damping in IrMn3/[Co/Pt] multilayers

In order to realise single on-chip three axis magnetic field sensing technology without the need for convoluted die stacking, perpendicular media must be exploited. This poses additional challenges in both static-bias and the dynamics of the stack. Presented here are ferromagnetic resonance measurements (FMR) on exchange-biased multilayers of IrMn3/[Co/Pt]n. We chart the evolution of the different anisotropies caused by both magnetic field annealing and increasing repetition number n. The damping of these stacks does not obey the usual symmetry seen in single and bilayer samples with in-plane anisotropy, but instead follows a sin2(θ) dependence. This, we propose, is entirely dominated by spin diffusion from the Co layers into the adjacent heavy metal layers. This is modelled using a simple effective circuit model, providing a guide for the optimisation of complex multilayers.

Terahertz Spin-Conductance Spectroscopy: Probing Coherent and Incoherent Ultrafast Spin Tunneling.

Thin-film stacks | consisting of a ferromagnetic-metal layer and a heavy-metal layer are spintronic model systems. Here, we present a method to measure the ultrabroadband spin conductance across a layer between and at terahertz frequencies, which are the natural frequencies of spin-transport dynamics. We apply our approach to MgO tunneling barriers with thickness d = 0-6 Å. In the time domain, the spin conductance Gs has two components. An instantaneous feature arises from processes like coherent spin tunneling. Remarkably, a longer-lived component is a hallmark of incoherent resonant spin tunneling mediated by MgO defect states, because its relaxation time grows monotonically with d to as much as 270 fs at d = 6.0 Å. Our results are in full agreement with an analytical model. They indicate that terahertz spin-conductance spectroscopy will yield new and relevant insights into ultrafast spin transport in a wide range of spintronic nanostructures.

Ferroelectric Control of Spin‐Orbitronics

AbstractSpin‐orbit coupling refers to the relativistic interaction between the spin and orbital motions of electrons. This interaction leads to numerous intriguing phenomena, including spin‐orbit torques, spin–momentum locking, topological spin textures, etc., that have recently gained prominence in the field of spin‐orbitronics. In particular, the emerging ferroelectric control is recognized and validated as an effective means to enhance energy efficiency across a broad spectrum of spin‐orbitronic devices. Here, cutting‐edge research on ferroelectric control of spin‐orbitronics (FECSO) by means of spontaneous polarization, reversible ferroelectric switching, and multiferroic coupling, are comprehensively reviewed. Two fascinating topics are mainly discussed: topological spin texture and spin‐charge interconversion. The classification of control mechanisms for different interactions in FECSO is summarized first. Then, from the perspective of material classification, the ferroelectric‐controlled spin‐orbit coupling with tunable topological spin texture in oxide systems, magnetic metal multilayers, and 2D van der Waals materials is reviewed. Subsequently, the ferroelectric‐tunable spin‐charge interconversion on heavy metal layers, oxide interfaces, and ferroelectric Rashba semiconductors is highlighted. In the end, the challenges and forthcoming prospects of FECSO are discussed. This work may provide pertinent and forward‐thinking guidance to accelerate the ongoing advancement of this field.

Comparison of Current Induced Domain Wall Motion Driven by Spin Transfer Torque and by Spin Orbit Torque in Ferrimagnetic GdFeCo Wires

Current-induced domain wall motion (CIDWM) in magnetic wires can be driven by spin transfer torque (STT) originating from transferring angular momentums of spin-polarized conducting electrons to the magnetic DW and can be driven by spin orbit torque (SOT) originating from the spin Hall effect (SHE) in a heavy metal layer and Dzyaloshinsky Moriya (DMI) generated at an interface between a heavy metal layer and a magnetic layer. In this work, we carried out a comparative study of CIDWM driven by STT and by SOT in ferrimagnetic GdFeCo wires with magnetic perpendicular anisotropy based on structures of SiN (10 nm)/GdFeCo (8 nm)/SiN (10 nm) and Pt (5 nm)/GdFeCo (8 nm)/SiN (10 nm). We found that CIDWM driven by SOT exhibited a much lower critical current density (JC), and much higher DW mobility (µDW). Our work might be useful for the realization and the development of low-power and high-speed memory devices.

Large enhanced thermal stability of perpendicular magnetic anisotropy films in magnetic tunnel junctions

For magnetic sensor and magnetic random access memory, the thermally tolerant need to be improved at a relatively high operation temperature. In this work, the correlations between the thermal stability and perpendicular magnetic anisotropy films with different heavy metal layers, structures and CoFeB compositions were investigated in details. It was found that the thin films adopting W or Mo heavy metal layer in the bottom structure, and the thin films adopting Mo heavy metal layer in the top structure exhibit a relatively good operation stability, no matter what the CoFeB composition is. Moreover, the thermally robust perpendicular magnetic anisotropy was obtained in the CoFeB with a relatively large Co atomic composition. According to the element distribution states analysis and oxidation conditions at CoFeB/MgO interface, the reasons are attributed to the structural- and composition-dependent Fe-O bonding. This research helps to design and optimize the ultra-low-power and thermally stable magnetic devices based on the suitable heavy metal layers, structures, and CoFeB compositions.

Enhanced controllable triplet proximity effect in superconducting spin–orbit coupled spin valves with modified superconductor/ferromagnet interfaces

In a superconductor/ferromagnet hybrid, a magnetically controlled singlet-to-triplet Cooper pair conversion can modulate the superconducting critical temperature. In these triplet superconducting spin valves, such control usually requires inhomogeneous magnetism. However, in the presence of spin–orbit coupling from an interfacial heavy metal layer, the singlet/triplet conversion rate and, thus, critical temperature can be controlled via the magnetization direction of a single homogeneous ferromagnet. Here, we report significantly enhanced controllable pair conversion to a triplet state in a Nb/Pt/Co/Pt superconducting spin valve in which Pt/Co/Pt is homogenously magnetized and proximity-coupled to a superconducting layer of Nb. The Co/Pt interface furthest away from Nb is modified by a sub-nanometer-thick layer of Cu or Au. We argue that the enhancement is most likely associated from an improvement of the Co/Pt interface due to the insertion of Cu and Au layers. Additionally, the higher normalized orbital moments in Au measured using x-ray magnetic circular dichroism shows that increasing spin–orbit coupling enhances the triplet proximity effect—an observation supported by our theoretical calculations. Our results provide a pathway to enhancing triplet pair creation by interface engineering for device development in superspintronics.

Asymmetric magnetization switching and programmable complete Boolean logic enabled by long-range intralayer Dzyaloshinskii-Moriya interaction

After decades of efforts, some fundamental physics for electrical switching of magnetization is still missing. Here, we report the discovery of the long-range intralayer Dzyaloshinskii-Moriya interaction (DMI) effect, which is the chiral coupling of orthogonal magnetic domains within the same magnetic layer via the mediation of an adjacent heavy metal layer. The effective magnetic field of the long-range intralayer DMI on the perpendicular magnetization is out-of-plane and varies with the interfacial DMI constant, the applied in-plane magnetic fields, and the magnetic anisotropy distribution. Striking consequences of the effect include asymmetric current/field switching of perpendicular magnetization, hysteresis loop shift of perpendicular magnetization in the absence of in-plane direct current, and sharp in-plane magnetic field switching of perpendicular magnetization. Utilizing the intralayer DMI, we demonstrate programable, complete Boolean logic operations within a single spin-orbit torque device. These results will stimulate investigation of the long-range intralayer DMI effect in a variety of spintronic devices.

Growth of thin-film magnetic nanostructures promising for spintronics applications

Multilayered metallic nanostructures are promising for the fabrication of spin valves based on the giant magnetoresistive effect and for studies of the nature of topological magnetism, aimed at the development of new nanoscale data storage and transfer devices, e.g. those based on magnetic skyrmions. It is still an important task to develop methods of synthesis and configuration of thin-film nanostructures and control of spin textures in those nanostructures under electric and spin currents generated as a result of the spin Hall effect in external electric fields. Thin-film polycrystalline ferromagnetic / heavy metal Ru(10nm)/Co(0,8)/Ru(2), Ru(10)/Co(0,8)/Ru(2)/W(4), Pt(5)/Co(0,8)/MgO(2)/Pt(2) and Pt(15)/Co(0,8)/MgO(2)/Pt(2) nanostructures have been synthesized using magnetron sputtering. Electric contacts and Hall structures with different conductive bridge thicknesses have been synthesized on the specimens using electron beam photolithography. Experimental vibration magnetometric data have been utilized to calculate magnetic parameters of the specimens, i.e., saturation magnetization, magnetic anisotropy energy and field and coercive force as functions of ferromagnetic and heavy metal layer types. The domain structure of the specimens has been studied using Kerr microscopy. The electrical resistivity has been simulated and the critical current and current density of the nanostructures have been assessed. We show that all the film specimens exhibit perpendicular magnetic anisotropy and can be used in the studies of current-induced phenomena and spin moment transfer processes in nanostructures.

The role of magnetic anisotropy in the magnetoresistance of Cr2O3/Al2O3 thin film antiferromagnets

The magnetic states of antiferromagnetic insulating thin films are a promising medium for information storage, but characterization of these states has proven to be challenging. One approach is via magnetotransport measurements in an adjacent heavy metal layer. To this end, we synthesized and characterized a series of Cr2O3 films and bilayers on Al2O3 substrates with three different orientations: m-plane, a-plane, and c-plane. X-ray diffraction results demonstrated orientation control of the Cr2O3 thin film, with m-plane films displaying a higher degree of mosaic spread than the a- and c-plane films. Reciprocal space maps showed that the films are mostly relaxed, although there was a small and different degree of strain in each orientation. The m-plane films were under 2% compressive strain, the a-plane film was under 0.5% compressive strain, and the c-plane film was completely relaxed to bulk values. To probe the magnetic state of the films, we measured the angular dependent magnetoresistance of Cr2O3/Pt bilayers for each orientation. We found a nontrivial temperature dependence of the sign of the magnetoresistance, pointing to the complex interplay between the exchange and anisotropy energies that vary with orientation. We propose that strain and mosaic spread may contribute to a difference in magnetic anisotropies among the samples and the resulting temperature dependence of the magnetoresistance. This work demonstrates the importance of considering the competition between antiferromagnetic exchange and magnetic anisotropy when storing information in the spin state of an antiferromagnetic insulator.

Terahertz-frequency oscillator driven by spin–orbit torque in NiF2/Pt bilayers

Exploration and manipulation of terahertz signal generators are crucial steps in the creation of numerous applications. Antiferromagnets can boost output signal frequency to the terahertz range. We propose a nanometer-scale generating device that produces terahertz signals by DC-exciting in a bilayer structure. The structure comprises a heavy metal layer (Pt) and a non-collinear antiferromagnetic layer (NiF2), where the magnetic moments in NiF2 with single-ion anisotropy are excited by the spin current from the Pt layer through spin–orbit torque. The inhomogeneous dynamic behaviors of the magnetic moments of NiF2 are calculated by the Landau–Lifshitz–Gilbert equation. It is found that terahertz-frequency AC can be reliably output from the bilayer structure, with the frequency that can reach to 1.82 terahertz. The oscillator shows the best performance when the polarized direction of the spin current is along the hard-axis of NiF2. The frequency and the amplitude of the AC can be adjusted by the current density, thickness and damping constant of the NiF2 layer. The threshold currents for exciting and maintaining the stable oscillation increase with the thickness and damping constant of the NiF2.

Effective electrical manipulation of a topological antiferromagnet by orbital torques

The electrical control of the non-trivial topology in Weyl antiferromagnets is of great interest for the development of next-generation spintronic devices. Recent studies suggest that the spin Hall effect can switch the topological antiferromagnetic order. However, the switching efficiency remains relatively low. Here, we demonstrate the effective manipulation of antiferromagnetic order in the Weyl semimetal Mn3Sn using orbital torques originating from either metal Mn or oxide CuOx. Although Mn3Sn can convert orbital current to spin current on its own, we find that inserting a heavy metal layer, such as Pt, of appropriate thickness can effectively reduce the critical switching current density by one order of magnitude. In addition, we show that the memristor-like switching behaviour of Mn3Sn can mimic the potentiation and depression processes of a synapse with high linearity—which may be beneficial for constructing accurate artificial neural networks. Our work paves a way for manipulating the topological antiferromagnetic order and may inspire more high-performance antiferromagnetic functional devices.

Spin–Orbit Torque and Current‐Driven Switching in Pt100‐yTby/Co/AlOx Trilayers

AbstractTo decrease the energy consumption for the electrical manipulation of magnetization, the enhancement of the spin Hall effect through alloying is widely investigated, but the use of rare earth elements is rarely mentioned. This work reports the modification of the spin Hall effect on Pt by doping rare earth Tb atoms. The spin–orbit torque (SOT) performance is significantly enhanced in Pt100‐yTby alloyed heavy metal (HM) layer. Compared with the Tb‐free sample, the damping‐like effective field per unit current density increases to 1.9 times in the samples with Tb content between 5% and 10%. The critical current density for magnetization reversal is greatly reduced by 65% in a device with Pt87Tb13 HM layer and the in‐plane assistant field as small as ±20 Oe is sufficient for the deterministic switching in the same device. By magneto‐optical Kerr effect imaging, it is confirmed that the increased in‐plane field can effectively compensate the Dzyaloshinskii–Moriya interaction (DMI), which not only helps to reduce the critical current, but also facilitates the domain wall motion and is beneficial for the switching process. All results show that the Pt‐Tb alloy is a competitive candidate for low‐power spintronic devices.

Optimizing skyrmionium movement and stability via stray magnetic fields in trilayer nanowire constructs.

Skyrmioniums, known for their unique transport and regulatory properties, are emerging as potential cornerstones for future data storage systems. However, the stability of skyrmionium movement faces considerable challenges due to the skyrmion Hall effect, which is induced by deformation. In response, our research introduces an innovative solution: we utilized micro-magnetic simulations to create a sandwiched trilayer nanowire structure augmented with a stray magnetic field. This combination effectively guides the skyrmionium within the ferromagnetic (FM) layer. Our empirical investigations reveal that the use of a stray magnetic field not only reduces the size of the skyrmionium but also amplifies its stability. This dual-effect proficiently mitigates the deformation of skyrmionium movement and boosts their thermal stability. We find these positive outcomes are most pronounced at a particular intensity of the stray magnetic field. Importantly, the required stray magnetic field can be generated using a heavy metal (HM1) layer of suitable thickness, rendering the practical application of this approach plausible in real-world experiments. Additionally, we analyze the functioning mechanism based on the Landau-Lifshitz-Gilbert (LLG) equation and energy variation. We also develop a deep spiking neural network (DSNN), which achieves a remarkable recognition accuracy of 97%. This achievement is realized through supervised learning via the spike timing dependent plasticity rule (STDP), considering the nanostructure as an artificial synapse device that corresponds to the electrical properties of the nanostructure. In conclusion, our study provides invaluable insights for the design of innovative information storage devices utilizing skyrmionium technology. By tackling the issues presented by the skyrmion Hall effect, we outline a feasible route for the practical application of this advanced technology. Our research, therefore, serves as a robust platform for continued investigations in this field.

Ultrafast THz probing of nonlocal orbital current in transverse multilayer metallic heterostructures

THz generation from femtosecond photoexcited spintronic heterostructures has become a versatile tool for investigating ultrafast spin-transport and transient charge-current in a non-contact and non-invasive manner. The equivalent effect from the orbital degree of freedom is still in the primitive stage. Here, we experimentally demonstrate orbital-to-charge current conversion in metallic heterostructures, consisting of a ferromagnetic layer adjacent to either a light or a heavy metal layer, through detection of the emitted THz pulses. Our temperature-dependent experiments help to disentangle the orbital and spin components that are manifested in the respective Hall-conductivities, contributing to THz emission. NiFe/Nb shows the strongest inverse orbital Hall effect with an experimentally extracted value of effective intrinsic Hall-conductivity, (σSOHint)eff~195Ω−1cm−1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${({\\sigma }_{{SOH}}^{{{{{\\mathrm{int}}}}}})}^{{eff}} \\sim 195{\\varOmega }^{-1}{{cm}}^{-1}$$\\end{document}, while CoFeB/Pt shows maximum contribution from the inverse spin Hall effect. In addition, we observe a nearly ten-fold enhancement in the THz emission due to pronounced orbital-transport in W-insertion heavy metal layer in CoFeB/W/Ta heterostructure as compared to CoFeB/Ta bilayer counterpart.