Ferromagnetic Interface Research Articles

Aspects of long range spin–triplet correlations in superconductor/ferromagnet hetero-structures

The notion of competing ferromagnetic (F) and superconducting (S) orders in F/S hybrid structures was transformed by the first realization of ferromagnetic Josephson π-junctions and the almost simultaneous prediction of a possibility of spin–triplet correlations in such structures, almost two decades back. Such hybrid structures in various configurations are now studied as rich sources of emergent states and new effects. Unlike the spin singlet Cooper pairs, the spin triplet Cooper pairs are much less affected by the exchange field of a ferromagnet and, therefore, immediately finds interest in the field of spintronics. Theoretically, it has been shown that the basic protocol for spin–singlet to spin–triplet supercurrent conversion is the presence of magnetic non-collinearity at the superconductor–ferromagnet interface. Therefore, almost all experiments in this direction have utilized transport measurements on F/S systems with artificial magnetic non-collinearity formed by combination of several ferromagnetic layers next to the superconducting layer. Here we highlight two aspects of studying these heterostructures. Firstly we show that natural magnetic inhomogeneities, found in domain walls of ferromagnets, can also be used to achieve singlet–triplet conversion, instead of artificial magnetic inhomoheneities. This possibility was explored via transport measurements in nano-scale planar Nb–Ni–Nb junctions and nano-SQUIDs, where a domain wall was pinned at the Josephson junction barrier. By this method we were able to show Josephson coupling across about 70 nm of strong ferromagnetic planar barrier. Secondly we show that spin–triplet correlations at the F/S interface are robust enough to be probed by the diamagnetic screening currents at the interface. This was probed by studying the change in sperconducting transition temperature of Nb/Co/Py/Nb multilayers in presence of small in-plane magnetic field. The Co/Py combination, which is a soft-hard type magnetic exchange spring, worked as magnetic inhomogeneity for triplet generation at the interface of the superconducting Nb. These observations may promote new experiments in the field of superconducting-spintronics.

Molecular rectification assisted by spin-polarized hybrid interfacial states

Based on first-principles method, we construct molecular rectifiers by designing asymmetric hybrid interfacial states. The devices contain a ferromagnetic interface with sp3-d hybrid interfacial states and a nonmagnetic interface without hybrid interfacial states. The transport calculation demonstrates that obvious charge-current rectification and spin-current rectification are realized. The analysis of spin-resolved transmission reveals that the asymmetric shift of hybrid interfacial states plays an important role in determining the rectification ratio as well as the spin polarization orientation of the spin current, and hence the form of spin-current rectification. The effect of molecular length is further discussed. A length-induced increase of rectification ratio is observed for the charge-current rectification, while the largest spin-current rectification ratio appears at a specific length. This work demonstrates the potential of hybrid interfacial states in designing excellent molecular spinterface devices.

Emergent phenomena at oxide interfaces studied with standing-wave photoelectron spectroscopy

Emergent phenomena at complex-oxide interfaces have become a vibrant field of study in the past two decades due to the rich physics and a wide range of possibilities for creating new states of matter and novel functionalities for potential devices. The electronic-structural characterization of such phenomena presents a unique challenge due to the lack of direct yet nondestructive techniques for probing buried layers and interfaces with the required Ångstrom-level resolution, as well as element and orbital specificity. In this Review, we survey several recent studies wherein soft x-ray standing-wave photoelectron spectroscopy—a relatively newly developed technique—is used to investigate buried oxide interfaces exhibiting emergent phenomena such as metal-insulator transition, interfacial ferromagnetism, and two-dimensional electron gas. The advantages, challenges, and future applications of this methodology are also discussed.

A Spin‐Orbit Torque Ratchet at Ferromagnet/Antiferromagnet Interface via Exchange Spring

AbstractThe antiferromagnet (AFM) and ferromagnet (FM) interface is a unique branch of magnetics of broad scientific interest. AFMs play an important role in spin‐orbit torque devices based on their ability to generate spin‐polarized current and exchange bias when combined with FMs. In this study, an interesting spin‐orbit torque (SOT) ratchet involving the exchange spring effect in an IrMn/CoFeB bilayer device with perpendicular anisotropy and exchange bias is developed. The combined use of electrical and spectroscopic analysis reveals that the exchange spring in IrMn/CoFeB bilayer yields unidirectional anisotropy, resulting in a collinear/orthogonal AFM/FM spin configuration at the interface upon switching CoFeB magnetization upward/downward. The ratcheting characteristics resulting from unidirectional anisotropy manifest in SOT switching. In this process, magnetization against the exchange spring features digital‐like switching with a sharp transition, whereas the reverse function is characteristic of analog switching with a gradual transition tail. The dual digital‐analog characteristics of the IrMn/CoFeB bilayer may be of benefit in neuromorphic and memory applications.

Research of spinterface in organic spintronic devices

Spintronics are attractive to the utilization in next-generation quantum-computing and memory. Compared with inorganic spintronics, organic spintronics not only controls the spin degree-of-freedom but also possesses advantages such as chemical tailorability, flexibility, and low-cost fabrication process. Besides, the organic spin valve with a sandwich configuration that is composed of two ferromagnetic electrodes and an organic space layer is one of the classical devices in organic spintronics. Greatly enhanced or inversed magnetoresistance (MR) sign appearing in organic spin valve is induced by the unique interfacial effect an organic semiconductor/ferromagnetic interface. The significant enhancement or inversion of MR is later proved to be caused by the spin-dependent hybridization between molecular and ferromagnetic interface, <i>i.e.</i>, the spinterface. The hybridization is ascribed to spin-dependent broadening and shifting of molecular orbitals. The spinterface takes place at one molecular layer when attaching to the surface of ferromagnetic metal. It indicates that the MR response can be modulated artificially in a specific device by converting the nature of spinterface. Despite lots of researches aiming at exploring the mechanism of spinterface, several questions need urgently to be resolved. For instance, the spin polarization, which is difficult to identify and observe with the surface sensitive technique and the inversion or enhancement of MR signal, which is also hard to explain accurately. The solid evidence of spinterface existing in real spintronic device also needs to be further testified. Besides, the precise manipulation of the MR sign by changing the nature of spinterface is quite difficult. According to the above background, this review summarizes the advance in spinterface and prospects future controllable utilization of spinterface. In Section 2, we introduce the basic principle of spintronic device and spinterface. The formation of unique spinterface in organic spin valve is clarified by using the difference in energy level alignment between inorganic and organic materials. Enhancement and inversion of MR sign are related to the broadening and shifting of the molecular level. In Section 3, several examples about identification of spinterface are listed, containing characterization by surface sensitive techniques and identification in real working devices. In Section 4 some methods about the manipulation of spinterface are exhibited, including modulation of ferroelectric organic barrier, interface engineering, regulation of electronic phase separation in ferromagnetic electrodes, etc. Finally, in this review some unresolved questions in spintronics are given, such as multi-functional and room-temperature organic spin valve and improvement of the spin injection efficiency. Spinterface is of great importance for both scientific research and future industrial interest in organic spintronics. The present study paves the way for the further development of novel excellent organic spin valves.

Controllable magnetic anisotropy and spin orientation of a prototypical easy-plane antiferromagnet on a ferromagnetic support

The control of the spin orientation in antiferromagnets is of key importance in modern spintronics. Here, the authors demonstrate controllable antiferromagnetic spin switching in a synthetically fabricated epitaxial ferromagnet/antiferromagnet bilayer. Thanks to the precise nanoengineering of ferromagnet anisotropy and interface exchange coupling, two orthogonal spin states in the antiferromagnet can be either thermally stabilized in a field-free manner or written by a small external magnetic field.

Electron-magnon coupling and quasiparticle lifetimes on the surface of a topological insulator

The fermionic self-energy on the surface of a topological insulator proximity coupled to ferro- and antiferromagnetic insulators is studied. An enhanced electron-magnon coupling is achieved by allowing the electrons on the surface of the topological insulator to have a different exchange coupling to the two sublattices of the antiferromagnet. Such a system is therefore seen as superior to a ferromagnetic interface for the realization of magnon-mediated superconductivity. The increased electron-magnon-coupling simultaneously increases the self-energy effects. In this paper we show how the inverse quasiparticle lifetime and energy renormalization on the surface of the topological insulator can be kept low close to the Fermi level by using a magnetic insulator with a sufficient easy-axis anisotropy. We find that the antiferromagnetic case is most interesting from both a theoretical and an experimental standpoint due to the increased electron-magnon coupling, combined with a reduced need for easy-axis anisotropy compared to the ferromagnetic case. We also consider a set of material and instrumental parameters where these self-energies should be measurable in angle-resolved photoemission spectroscopy experiments, paving the way for a measurement of the interfacial exchange coupling strength.

Ta cap-induced stabilization of interfacial ferromagnetism and enhanced magnetoelectricity in ultrathin FeRh films

We report ab initio electronic structure calculations to study the effect of Ta cap on the magnetic properties and the magnetoelectric response of Ta/FeRh/MgO nanojunctions. The calculations reveal that the Ta cap reverses the magnetic phase stability in ultrathin FeRh films leading to the stabilization of the ferromagnetic phase and interfacial reconstructed ferromagnetism. We demonstrate that the Ta cap induces a large charge transfer which in turn reduces dramatically the magnetic moments of the interfacial Fe atoms regardless the magnetic configuration and enhances the in-plane (out-of-plane) magnetization orientation of the antiferromagnetic (AFM) (ferromagnetic (FM)) phase compared to the uncapped bilayer. The Ta cap modifies substantially and enhances the magnetoelectric response under an external electric field, where the voltage controlled magnetic anisotropy (VCMA) changes from ∨-shape in the AFM to linear behavior in the FM phase with large VCMA efficiency. These findings demonstrate the manipulation of magnetic ordering of FeRh films with heavy metal capping and electric field which can promote the application of FeRh alloy in MeRAM.

Nanoscale Domain Wall Engineered Spin-Triplet Josephson Junctions and SQUID.

Spin-singlet Cooper pairs convert to spin-triplet Cooper pairs on passing through a magnetically noncollinear structure at a superconductor(S)/ferromagnet(F) interface. In this context, the generation of triplet supercurrents through intrinsic ferromagnetic domain walls, which are naturally occurring noncollinear magnetic features, was proposed theoretically in the past decade. However, an experimental demonstration has been lacking in the literature, particularly because of the difficulty in accessing a single domain wall, which is typically buried between two domains in a ferromagnetic material. By patterning a ferromagnetic nanoconstriction, we have been able to realize a nanoscale S/F/S planar junction, where a single domain wall (pinned at the nanoconstriction) acts as a Josephson barrier. In this geometry, we are able to show the predicted long-range triplet supercurrent across a ferromagnetic barrier exceeding 70 nm. Using this technique, we have demonstrated a ferromagnetic planar nano-SQUID device consisting of two Nb/Ni/Nb spin-triplet Josephson junctions.

Unveiling the spin-to-charge current conversion signal in the topological insulator Bi2Se3 by means of spin pumping experiments

We report an investigation of the spin-to-charge current conversion in sputter-deposited films of the topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ onto single crystalline layers of yttrium iron garnet (YIG-${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$) and polycrystalline films of permalloy ($\mathrm{Py}\text{\ensuremath{-}}{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$). Pure spin current was injected into the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ layer by means of the spin pumping process produced by the spin precession in microwave driven ferromagnetic resonance of the ferromagnetic film. The spin-to-charge current conversion occurring at the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$/ferromagnet interface is attributed to the inverse Rashba-Edelstein effect (IREE). From the data, we verified that the voltage generated by the spin-to-charge current conversion process in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ has the same polarity as the inverse spin Hall effect in Ta, which is opposite to the one in Pt. Also, from the dependence of the voltage on the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ thickness we were able the to calculate the IREE length values and found that $1.2\phantom{\rule{0.16em}{0ex}}\mathrm{pm}\ensuremath{\le}|{\ensuremath{\lambda}}_{\mathrm{IREE}}|\ensuremath{\le}2.2\phantom{\rule{0.16em}{0ex}}\mathrm{pm}$. This result allows us to conclude that indeed the surface states of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ have a dominant role in the spin-to-charge current conversion process, and the mechanism based on the spin diffusion process plays a secondary role.

Polarity and charge redistribution induced emergent interfacial ferromagnetism in non-magnetic LaNiO3/SrMnO3 superlattices

Artificial heterostructures composed of strongly correlated oxides can host many up-and-coming phenomena. The interfacial ferromagnetism can be observed at the interface of paramagnetic LaNiO3 (LNO) and G-type antiferromagnetic SrMnO3 (SMO). Through the macroscopic magnetic and transport measurements and the microscopic x-ray magnetic circular dichroism signals, the obvious interfacial ferromagnetic behavior exhibited in the thicker superlattices. Using x-ray absorption spectroscopy and x-ray photoelectron spectroscopy measurements, the polar mismatch-induced charge redistribution in Ni and Mn ions accurately demonstrates the dependence on the varying thickness of LNO/SMO superlattices. These results suggest that the ferromagnetic double-exchange interaction of Mn3+–Mn4+ increases and the ferromagnetic super-exchange interaction of Ni2+–Mn4+ decreases with the increasing thickness of LNO/SMO superlattices. Consequently, the obvious ferromagnetic behavior in the thicker superlattices is induced by the double-exchange interaction. For the thinner LNO/SMO superlattice, the interfacial super-exchange ferromagnetic state is offset by the double-exchange ferromagnetism. This is the crucial reason behind the fact that ferromagnetism is hard to exist in the prior LNO/SMO superlattices. Our findings highlight the intrinsic mechanism responsible for the interfacial ferromagnetism and the significance of understanding polar mismatch at the interface of heterostructures.

Spin filtering by proximity effects at hybridized interfaces in spin-valves with 2D graphene barriers

We report on spin transport in state-of-the-art epitaxial monolayer graphene based 2D-magnetic tunnel junctions (2D-MTJs). In our measurements, supported by ab-initio calculations, the strength of interaction between ferromagnetic electrodes and graphene monolayers is shown to fundamentally control the resulting spin signal. In particular, by switching the graphene/ferromagnet interaction, spin transport reveals magneto-resistance signal MR > 80% in junctions with low resistance × area products. Descriptions based only on a simple K-point filtering picture (i.e. MR increase with the number of layers) are not sufficient to predict the behavior of our devices. We emphasize that hybridization effects need to be taken into account to fully grasp the spin properties (such as spin dependent density of states) when 2D materials are used as ultimately thin interfaces. While this is only a first demonstration, we thus introduce the fruitful potential of spin manipulation by proximity effect at the hybridized 2D material / ferromagnet interface for 2D-MTJs.

Protonation control of spin transport properties in magnetic single-molecule junctions

The protonation-controlled conductance switching of 4,4′-vinylenedipyridine (44VDP) molecular junctions was experimentally reported by Brooke et al. (Nano Lett 18(2): 1317–1322, 2018), where a change induced by protonation in the bonding at the molecule–metal interface was proposed as the key ingredient. Here, we perform a first-principles study on the spin-dependent transport properties of 44VDP molecular junctions modulated by protonation. The switching mechanism in the experiment is clarified; namely, the weak coupling strength at the molecule–metal interface is triggered by protonation of pyridyl groups in the 44VDP molecule. In particular, the protonation process modifies the organic–ferromagnetic spinterface, which reduces the number of hybrid interface states and causes an inversion of tunneling magnetoresistance from positive to negative values. Furthermore, a protonation-induced excellent spin-filtering effect is realized. This work sheds light on the mechanism of protonation at the organic–ferromagnetic interface and provides a promising way to realize multifunctional devices in organic spintronics.

Large interfacial spin-orbit torques in layered antiferromagnetic insulator NiPS3 /ferromagnet bilayers

Finding efficient ways of manipulating magnetic bits is one of the core goals in spintronic research. Electrically-generated spin-orbit torques (SOTs) are good candidates for this and the search for materials capable of generating highly-efficient SOTs has gained a lot of traction in the recent years. While antiferromagnet/ferromagnet bilayer structures have been employed extensively for passive applications, e.g. by using exchange bias fields, their active properties are not yet widely employed. Here we show the presence of large interfacial SOTs in bilayer of a ferromagnet and the two-dimensional layered antiferromagnetic insulator NiPS$_3$. We observe a large in-plane damping-like interfacial torque, showing a torque conductivity of $\sigma_\mathrm{DL} \approx 1 \times 10^{5} \mathrm{(\frac{\hbar}{2e}) /(\Omega m)}$ even at room temperature, comparable to the best devices reported in the literature for standard heavy-metal-based and topological insulators-based devices. Additionally, our devices also show an out-of-plane field-like torque arising from the NiPS$_3$/ferromagnet interface, further indicating the presence of an interfacial spin-orbit coupling in our structures. Temperature-dependent measurements reveal an increase of the SOTs with a decreasing temperature below the N\'eel temperature of NiPS$_3$ ($T_N \approx 170 \mathrm{K}$), pointing to a possible effect of the magnetic ordering on our measured SOTs. Our findings show the potential of antiferromagnetic insulators and two-dimensional materials for future spintronic applications.

Correlating magnetic structure and magnetotransport in semimetal thin films of Eu1-x Sm x TiO3.

We report on the evolution of the average and depth-dependent magnetic order in thin-film samples of biaxially stressed and electron-doped EuTiO3 for samples across a doping range < 0.1 to 7.8 × 1020 cm-3. Under an applied in-plane magnetic field, the G-type antiferromagnetic ground state undergoes a continuous spin-flop phase transition into in-plane, field-polarized ferromagnetism. The critical field for ferromagnetism slightly decreases with an increasing number of free carriers, yet the field evolution of the spin-flop transition is qualitatively similar across the doping range. Unexpectedly, we observe interfacial ferromagnetism with saturated Eu2+ moments at the substrate interface at low fields preceding ferromagnetic saturation throughout the bulk of the degenerate semiconductor film. We discuss the implications of these findings for the unusual magnetotransport properties of this compound.

Deterministic Spin-Orbit Torque Switching by a Light-Metal Insertion.

Spin-orbit torque (SOT) switching of magnetization is a promising emerging technology for nonvolatile spintronic memory and logic applications. However, deterministic switching of perpendicular magnetization with SOTs requires an additional symmetry breaking, which is typically provided by an external magnetic field, making it impractical for applications. In this work, we disclose that by the insertion of a slightly asymmetric light-metal layer at the heavy metal-ferromagnet interface of SOT heterostructures, current-induced out-of-plane effective magnetic fields are introduced that enable deterministic switching without an external magnetic field. We obtain uniform perpendicular magnetic anisotropy and switching current density despite the asymmetry of the light-metal layer, and we show the scalability of our approach by studying device sizes that differ by 2 orders of magnitude. Our work provides a practical route for utilization of SOTs for magnetization switching on the wafer scale and paves the way for the practical application of SOT-based technology.

Anomalous Josephson Hall effect charge and transverse spin currents in superconductor/ferromagnetic-insulator/superconductor junctions

Interfacial spin-orbit coupling in Josephson junctions offers an intriguing way to combine anomalous Hall and Josephson physics in a single device. We study theoretically how the superposition of both effects impacts superconductor/ferromagnetic insulator/superconductor junctions' transport properties. Transverse momentum-dependent skew tunneling of Cooper pairs through the spin-active ferromagnetic insulator interface creates sizable transverse Hall supercurrents, to which we refer as anomalous Josephson Hall effect currents. We generalize the Furusaki-Tsukada formula, which got initially established to quantify usual (tunneling) Josephson current flows, to evaluate the transverse current components and demonstrate that their amplitudes are widely adjustable by means of the spin-orbit coupling strengths or the superconducting phase difference across the junction. As a clear spectroscopic fingerprint of Josephson junctions, well-localized subgap bound states form around the interface. By analyzing the spectral properties of these states, we unravel an unambiguous correlation between spin-orbit coupling-induced asymmetries in their energies and the transverse current response, founding the currents' microscopic origin. Moreover, skew tunneling simultaneously acts like a transverse spin filter for spin-triplet Cooper pairs and complements the discussed charge current phenomena by their spin current counterparts. The junctions' universal spin-charge current cross ratios provide valuable possibilities to experimentally detect and characterize interfacial spin-orbit coupling.

Perpendicular magnetic anisotropy of Pd/Co55Mn25Si20/NiO/Pd sputtering films

This paper carries out research on perpendicular magnetic anisotropy (PMA) of the Pd/Co55Mn25Si20(CMS)/NiO trilayer stack, which is a structure innovatively combining heavy metal (HM)/ferromagnetic (FM) interface (Pd/CMS), FM/oxide interface (CMS/NiO), and FM/antiferromagnetic (AFM) interface (CMS/NiO). The maximum effective PMA energy, obtained from the Pd (6 nm)/Co55Mn25Si20(5.5 nm)/NiO(1.0 nm) trilayer film which is sputter-deposited on glass at a substrate temperature of 350 °C, is 0.695 Merg/cm3. By appropriately increasing the Co content, a wider FM layer thickness range (3–8 nm) that achieved PMA was obtained. The oxide in the traditional FM/oxide structure was replaced by the antiferromagnetic NiO to obtain a wider oxide layer thickness of 0.4–7.6 nm, or even wider.

Magnon-mediated superconductivity on the surface of a topological insulator

We study superconductivity on the surface of a topological insulator, mediated by magnetic fluctuations in an adjacent ferromagnetic or antiferromagnetic insulator. Superconductivity can arise from effective interactions between helical fermions induced by interfacial fermion-magnon interactions. For both ferromagnetic and antiferromagnetic insulators, these fermion-fermion interactions have the correct structure to facilitate pairing between particles located on the same side of the Fermi surface, also known as Amperean pairing. In antiferromagnets, the strength of the induced interactions can be enhanced by coupling the topological insulator asymmetrically to the two sublattices of the antiferromagnet. This effect is further amplified by next nearest neighbor frustration in the antiferromagnetic insulator. The enhancement makes the induced interactions significantly stronger in the antiferromagnetic case, as compared to the ferromagnetic case. These results indicate that an uncompensated antiferromagnetic interface might be a better candidate than a ferromagnetic interface for proximity-induced magnon-mediated superconductivity on the surface of a topological insulator.

Tuning Interfacial Spins in Antiferromagnetic–Ferromagnetic–Heavy-Metal Heterostructures via Spin-Orbit Torque

Antiferromagnets are outstanding candidates for the next generation of spintronic applications, with great potential for downscaling and decreasing power consumption. Recently, the manipulation of bulk properties of antiferromagnets has been realized by several different approaches. However, the interfacial spin order of antiferromagnets is an important integral part of spintronic devices, thus the successful control of interfacial antiferromagnetic spins is urgently desired. Here, we report the high controllability of interfacial spins in antiferromagnetic / ferromagnetic / heavy metal heterostructure devices using spin-orbit torque (SOT) assisted by perpendicular or longitudinal magnetic fields. Switching of the interfacial spins from one to another direction through multiple intermediate states is demonstrated. The field-free SOT-induced switching of antiferromagnetic interfacial spins is also observed, which we attribute to the effective built-in out-of-plane field due to unequal upward and downward interfacial spin populations. Our work provides a precise way to modulate the interfacial spins at an antiferromagnet / ferromagnet interface via SOT, which will greatly promote innovative designs for next generation spintronic devices.