Hybrid Interface States and Spin Polarization at Ferromagnetic Metal–Organic Heterojunctions: Interface Engineering for Efficient Spin Injection in Organic Spintronics

Ferromagnetic metal–organic semiconductor (FM‐OSC) hybrid interfaces have been shown to play an important role for spin injection in organic spintronics. Here, 11,11,12,12‐tetracyanonaptho‐2,6‐quinodimethane (TNAP) is introduced as an interfacial layer in Co‐OSCs heterojunctions with an aim to tune the spin injection. The Co/TNAP interface is investigated by use of X‐ray and ultraviolet photoelectron spectroscopy (XPS/UPS), near edge X‐ray absorption fine structure (NEXAFS) and X‐ray magnetic circular dichroism (XMCD). Hybrid interface states (HIS) are observed at Co/TNAP interfaces, resulting from chemical interactions between Co and TNAP. The energy level alignment at the Co/TNAP/OSCs interface is also obtained, and a reduction of the hole injection barrier is demonstrated. XMCD results confirm sizeable spin polarization at the Co/TNAP hybrid interface.     

E‐Field Control of Exchange Bias and Deterministic Magnetization Switching in AFM/FM/FE Multiferroic Heterostructures

The coexistence of electrical polarization and magnetization in multiferroic materials provides great opportunities for novel information storage systems. In particular, magnetoelectric (ME) effect can be realized in multi­ferroic composites consisting of both ferromagnetic and ferroelectric phases through a strain mediated interaction, which offers the possibility of electric field (E‐field) manipulation of magnetic properties or vice versa, and enables novel multiferroic devices such as magnetoelectric random access memories (MERAMs). These MERAMs combine the advantages of FeRAMs (ferroelectric random access memories) and MRAMs (magnetic random access memories), which are non‐volatile magnetic bits switchable by electric field (E‐field). However, it has been challenging to realize 180° deterministic switching of magnetization by E‐field, on which most magnetic memories are based. Here we show E‐field modulating exchange bias and for the first time realization of near 180° dynamic magnetization switching at room temperature in novel AFM (antiferromagnetic)/FM (ferromagnetic)/FE (ferroelectric) multiferroic heterostructures of FeMn/Ni₈₀Fe₂₀/FeGaB/PZN‐PT (lead zinc niobate–lead titanate). Through competition between the E‐field induced uniaxial anisotropy and unidirectional anisotropy, large E‐field‐induced exchange bias field‐shift up to $ {{{\Delta H_{ex}}}\over{{H_{ex}}}} = 218\%$ and near 180° deterministic magnetization switching were demonstrated in the exchange‐coupled multiferroic system of FeMn/Ni₈₀Fe₂₀/FeGaB/PZN‐PT. This E‐field tunable exchange bias and near 180° deterministic magnetization switching at room temperature in AFM/FM/FE multiferroic heterostructures paves a new way for MERAMs and other memory technologies.     

Tuning a Binary Ferromagnet into a Multistate Synapse with Spin–Orbit‐Torque‐Induced Plasticity

Ferromagnets with binary states are limited for applications as artificial synapses for neuromorphic computing. Here, it is shown how synaptic plasticity of a perpendicular ferromagnetic layer (FM1) can be obtained when it is interlayer exchange‐coupled by another in‐plane ferromagnetic layer (FM2), where a magnetic field‐free current‐driven multistate magnetization switching of FM1 in the Pt/FM1/Ta/FM2 structure is induced by spin–orbit torque. Current pulses are used to set the perpendicular magnetization state, which acts as the synapse weight, and spintronic implementation of the excitatory/inhibitory postsynaptic potentials and spike timing‐dependent plasticity are demonstrated. This functionality is made possible by the action of the in‐plane interlayer exchange coupling field which leads to broadened, multistate magnetic reversal characteristics. Numerical simulations, combined with investigations of a reference sample with a single perpendicular magnetized Pt/FM1/Ta structure, reveal that the broadening is due to the in‐plane field component tuning the efficiency of the spin–orbit torque to drive domain walls across a landscape of varying pinning potentials. The conventionally binary FM1 inside the Pt/FM1/Ta/FM2 structure with an inherent in‐plane coupling field is therefore tuned into a multistate perpendicular ferromagnet and represents a synaptic emulator for neuromorphic computing, demonstrating a significant pathway toward a combination of spintronics and synaptic electronics.     

Ferroelectric and Ferromagnetic Behavior of Pb(Zr0.52Ti0.48)O3–Co0.9Zn0.1Fe2O4 Multilayered Thin Films Prepared via Solution Processing

Multilayered multiferroic nanocomposite films of Pb(Zr_0.52Ti_0.48)O₃ (PZT) and Co_0.9Zn_0.1Fe₂O₄ (CZFO) are prepared on general Pt/Ti/SiO₂/Si substrates via a simple solution‐processing method. Structural characterization by X‐ray diffraction and electron microscopy techniques reveals good surface and cross‐sectional morphologies of these multilayered thin films. In particular, at room temperature strong ferroelectric and ferromagnetic responses are simultaneously observed in the multilayered thin films, depending on the deposited sequences and volume fractions of ferroelectric PZT phase and magnetic CZFO phase.     

Ultrathin Epitaxial Ferromagnetic γ‐Fe2O3 Layer as High Efficiency Spin Filtering Materials for Spintronics Device Based on Semiconductors

In spintronics, identifying an effective technique for generating spin‐polarized current has fundamental importance. The spin‐filtering effect across a ferromagnetic insulating layer originates from unequal tunneling barrier heights for spin‐up and spin‐down electrons, which has shown great promise for use in different ferromagnetic materials. However, the low spin‐filtering efficiency in some materials can be ascribed partially to the difficulty in fabricating high‐quality thin film with high Curie temperature and/or partially to the improper model used to extract the spin‐filtering efficiency. In this work, a new technique is successfully developed to fabricate high quality, ferrimagnetic insulating γ‐Fe₂O₃ films as spin filter. To extract the spin‐filtering effect of γ‐Fe₂O₃ films more accurately, a new model is proposed based on Fowler–Nordheim tunneling and Zeeman effect to obtain the spin polarization of the tunneling currents. Spin polarization of the tunneled current can be as high as ?94.3% at 2 K in γ‐Fe₂O₃ layer with 6.5 nm thick, and the spin polarization decays monotonically with temperature. Although the spin‐filter effect is not very high at room temperature, this work demonstrates that spinel ferrites are very promising materials for spin injection into semiconductors at low temperature, which is important for development of novel spintronics devices.     

Emergence of Multispinterface and Antiferromagnetic Molecular Exchange Bias via Molecular Stacking on a Ferromagnetic Film

Heterointerfaces may exhibit unexpected physical properties distinct from intrinsic properties of component materials. In particular, metal–organic interfaces can drive unique interfacial spin moments, which are often called molecular spinterface. Here, van der Waals stacking of molecular layers may lead to variations in the intra/interlayer exchange coupling resulting in multiple ground states, which is highly desired for multifunctional magnetic devices. In this report, the emergence of molecular multispinterface of paramagnetic cobalt‐octaethyl‐porphyrin (CoOEP) layers in a Fe/CoOEP heterostructure is demonstrated through the interfacial layer and a successive antiferromagnetic molecular spin chain. The disentangled interfacial ferromagnetic spins lead to multiple magnetic ground states and behave as additional spin‐dependent scattering centers, as evidenced through the magnetotransport study. In addition, the antiferromagnetic molecule spin chain derives tunable exchange bias, which signifies the dominance of the antiferromagnetic interfacial interaction. Theoretical calculations demonstrate spin configurations of the molecular chain and the antiferromagnetic interfacial coupling through oxygen intermediaries. The development of the molecular multispinterface and controllable exchange bias therein will provide a promising route for the active control of multivalued data processing at the nanoscale.     

Dirac Cone Spin Polarization of Graphene by Magnetic Insulator Proximity Effect Probed with Outermost Surface Spin Spectroscopy

The effects of the proximity contact with magnetic insulator on the spin‐dependent electronic structure of graphene are explored for the heterostructure of single‐layer graphene (SLG) and yttrium iron garnet Y₃Fe₅O₁₂ (YIG) by means of outermost surface spin spectroscopy using a spin‐polarized metastable He atom beam. In the SLG/YIG heterostructure, the Dirac cone electrons of graphene are found to be negatively spin polarized in parallel to the minority spins of YIG with a large polarization degree, without giving rise to significant changes in the π band structure. Theoretical calculations reveal the electrostatic interfacial interactions providing a strong physical adhesion and the indirect exchange interaction causing the spin polarization of SLG at the interface with YIG. The Hall device of the SLG/YIG heterostructure exhibits a nonlinear Hall resistance attributable to the anomalous Hall effect, implying the extrinsic spin–orbit interactions as another manifestation of the proximity effect.     

Interface‐Assisted Sign Inversion of Magnetoresistance in Spin Valves Based on Novel Lanthanide Quinoline Molecules

Molecules are proposed to be an efficient medium to host spin‐polarized carriers, due to their weak spin relaxation mechanisms. While relatively long spin lifetimes are measured in molecular devices, the most promising route toward device functionalization is to use the chemical versatility of molecules to achieve a deterministic control and manipulation of the electron spin. Here, by combining magnetotransport experiments with element‐specific X‐ray absorption spectroscopy, this study shows the ability of molecules to modify spin‐dependent properties at the interface level via metal–molecule hybridization pathways. In particular, it is described how the formation of hybrid states determines the spin polarization at the relevant spin valve interfaces, allowing the control of macroscopic device parameters such as the sign and magnitude of the magnetoresistance. These results consolidate the application of the spinterface concept in a fully functional device platform.     

Energy level alignment and interactive spin polarization at organic/ferromagnetic metal interfaces for organic spintronics

Energy level alignment and spin polarization at tetracyanoquinodimethane/Fe and acridine orange base/Fe interfaces are investigated by means of photoelectron spectroscopy and X-ray magnetic circular dichroism (XMCD), respectively, to explore their potential application in organic spintronics. Interface dipoles are observed at both hybrid interfaces, and the work function of Fe is increased by 0.7 eV for the tetracyanoquinodimethane (TCNQ) case, while it is decreased by 1.2 eV for the acridine orange base (AOB) case. According to XMCD results, TCNQ molecule has little influence on the spin polarization of Fe surface. In contrast, AOB molecule reduces the interfacial spin polarization of Fe significantly. Induced spin polarization of the two organic molecules at the interfaces is not observed. The results reveal the necessity of investigating the magnetic property changes of both the OSC and the FM during the process of energy level alignment engineering.     

Anti‐Ferromagnet Controlled Tunneling Magnetoresistance

The requirement for high‐density memory integration advances the development of newly structured spintronic devices, which have reduced stray fields and are insensitive to magnetic field perturbations. This could be visualized in magnetic tunnel junctions incorporating anti‐ferromagnetic instead of ferromagnetic electrodes. Here, room‐temperature anti‐ferromangnet (AFM)‐controlled tunneling anisotropic magnetoresistance in a novel perpendicular junction is reported, where the IrMn AFM stays immediately at both sides of AlO_x tunnel barrier as the functional layers. Bi‐stable resistance states governed by the relative arrangement of uncompensated anti‐ferromagnetic IrMn moments are obtained here, rather than the traditional spin‐valve signal observed in ferromagnet‐based tunnel junctions. The experimental observation of room‐temperature tunneling magnetoresistance controlled directly by AFM is practically significant and may pave the way for new‐generation memories based on AFM spintronics.     

Probing Local and Global Ferroelectric Phase Stability and Polarization Switching in Ordered Macroporous PZT

We describe the characterization, ferroelectric phase stability and polarization switching in strain‐free assemblies of PbZr_0.3Ti_0.7O₃ (PZT) nanostructures. The 3‐dimensionally ordered macroporous structures present uniquely large areas and volumes of PZT where the microstructure is spatially modulated and the composition is homogeneous. Variable temperature powder X‐ray diffraction (XRD) studies show that the global structure is crystalline and tetragonal at room temperature and undergoes a reversible tetragonal to cubic phase transition on heating/cooling. The measured phase‐transition temperature is 50–60 °C lower than bulk PZT of the same composition. The local ferroelectric properties were assessed using piezoresponse force spectroscopy that reveal an enhanced piezoresponse from the nanostructured films and demonstrate that the switching polarization can be spatially mapped across these structures. An enhanced piezoresponse is observed in the nanostructured films which we attribute to the formation of strain free films, thus for the first time we are able to assess the effects of crystallite‐size independently of internal stress. Corresponding polarization distributions have been calculated for the bulk and nanostructured materials using a direct variational method and Landau‐Ginzburg‐Devonshire (LGD) theory. By correlating local and global characterization techniques we have for the first time unambiguously demonstrated the formation of tetragonal and ferroelectric PZT in large volume nanostructured architectures. With the wide range of materials available that can be formed into such controlled architectures we conclude that this study opens a pathway for the effective studies of nanoscale ferroelectrics in uniquely large volumes.     

Ferroelectric Materials: Probing Local and Global Ferroelectric Phase Stability and Polarization Switching in Ordered Macroporous PZT (Adv. Funct. Mater. 5/2011)

We describe the characterization, ferroelectric phase stability and polarization switching in strain‐free assemblies of PbZr_0.3Ti_0.7O₃ (PZT) nanostructures. The 3‐dimensionally ordered macroporous structures present uniquely large areas and volumes of PZT where the microstructure is spatially modulated and the composition is homogeneous. Variable temperature powder X‐ray diffraction (XRD) studies show that the global structure is crystalline and tetragonal at room temperature and undergoes a reversible tetragonal to cubic phase transition on heating/cooling. The measured phase‐transition temperature is 50–60 °C lower than bulk PZT of the same composition. The local ferroelectric properties were assessed using piezoresponse force spectroscopy that reveal an enhanced piezoresponse from the nanostructured films and demonstrate that the switching polarization can be spatially mapped across these structures. An enhanced piezoresponse is observed in the nanostructured films which we attribute to the formation of strain free films, thus for the first time we are able to assess the effects of crystallite‐size independently of internal stress. Corresponding polarization distributions have been calculated for the bulk and nanostructured materials using a direct variational method and Landau‐Ginzburg‐Devonshire (LGD) theory. By correlating local and global characterization techniques we have for the first time unambiguously demonstrated the formation of tetragonal and ferroelectric PZT in large volume nanostructured architectures. With the wide range of materials available that can be formed into such controlled architectures we conclude that this study opens a pathway for the effective studies of nanoscale ferroelectrics in uniquely large volumes.     

电极对PZT铁电薄膜性能的影响

用溶胶－凝胶法制备ＰＺＴ铁电薄膜。以Ｐｔ／Ｔｉ／ＳｉＯ２／Ｓｉ为底电极，Ａｕ为上电极，形成金属－铁电薄膜－金属结构的铁电电容器。研究电极对ＰＺＴ铁电薄膜结构和电性能的影响，实验发现，金属Ｔｉ的厚度会影响ＰＺＴ铁电薄膜的结构。界面层的存在使介电系数、自发极化、矫顽电压、漏电流都与薄膜的厚度有关。     

A Ferroelectric Ferromagnetic Composite Material with Significant Permeability and Permittivity

Composite materials containing both ferroelectric and ferromagnetic phases have been synthesized from nanometer‐sized powders of BaTiO₃ (ferroelectric phase) and NiCuZn ferrite (ferromagnetic phase) by a standard ceramic method. The coexistence of magnetic and electric hysteresis in the composite material has been observed at room temperature. Upon the application of magnetic and electric fields, the magnetization and electric polarization of the composite material can easily be tuned based on the changing BaTiO₃ content of the materials studied. These composite materials exhibit both excellent dielectric and soft‐magnetic properties with a variation of the frequency. Our results strongly suggest that this composite material may be the best candidate for the development of truly integrated passive filters. Due to the combination of both inductance and capacitance in one material, the adoption of an integrated passive filter could greatly reduce the size of printed circuit boards and could efficiently suppress electromagnetic interference, thereby enabling significant miniaturization of electronic elements and devices.     

Charge current polarization and magnetoresistance in ferromagnetic/organic semiconductor/ferromagnetic devices

Spin-polarized injection and transport in ferromagnetic/organic semiconductor/ferromagnetic devices are studied theoretically. Based on the spin diffusion theory and Ohm’s law, we obtain the charge current polarization and the magnetoresistance, which takes into account the special carriers in organic semiconductors. From the calculation, it is found that the charge current polarization decreases exponentially from the ferromagnetic layer into the organic layer and polarons are effective spin carriers in organic semiconductors for polarized charge current. To get an apparent magnetoresistance in an organic device, it is better to adopt a spin-dependent interface, and the thickness of the organic interlayer is much smaller than the spin diffusion length. Spin polarons are effective carriers for gaining remarkable magnetoresistance in ferromagnetic/organic semiconductor/ferromagnetic devices.     

C60/NiFe combination as a promising platform for molecular spintronics

Spintronics based on ferromagnetic metals and organic semiconductors has attracted great interest in recent years. Molecular-based spintronic devices, such as magnetic tunnel junctions, have been demonstrated with performances competing with those of conventional inorganic devices. Still, there is huge margin for improvement, as many details about the injection of spin-polarized electrons into the molecular layer remain not completely understood. In order to achieve better understanding and control of the physical mechanisms, it is necessary to explore various combinations of ferromagnetic metals and organic semiconductors.In this letter, we study the properties of the combination between the ferromagnetic metal NiFe (commonly known as Permalloy or Py) and the molecular system C₆₀. We produced C₆₀/Py bilayers and characterized them structurally, electrically and magnetically. The C₆₀ films grow smoothly on both Py and SiO₂ substrates, and we estimate that a 5-nm-thick C₆₀ film covers completely the surface underneath without leaving pinholes and can be therefore used in a vertical device, as confirmed by electrical characterization. Furthermore, the C₆₀ film is robust against the deposition of the top metal electrode, being the intermixing layer of only 1-2 nm at the C₆₀/Py interface. Finally, we show that the magnetic properties of Py are not affected by the deposition sequence, and that a 5-nm-thick Py layer on top of a C₆₀ layer keeps its magnetic properties intact.These results show that the combination between Py and C₆₀ provides a robust template platform for the development of molecular spintronics, and can be used later on for more sophisticated investigations, such as the role of the interfaces in the spin injection.     

Magneto-optical behaviors at a 2-D ferromagnetic/organic semiconductor interface for singlet fission

This article reports the magneto-optical effects on the singlet fission of the p-type organic semiconductor, tetracene, from a ferromagnetic/semiconductor interface between thin films of cobalt and tetracene. We experimentally show that this interface has two effects on the thin films of tetracene: spin interactions and electrical polarization. The experimental tools used to study the interface include magnetic field effect photoluminescence (MFE_PL), photoluminescence and absorption. Spin interaction effects are shown by MFE_PL data, where we observe a large increase in the maximum MFE_PL when cobalt is introduced, as well as changes in the hyperfine interactions at low magnetic fields. Electrical polarization is analyzed with photoluminescence and absorption measurements, showing small changes in the energy difference between the HOMO and LUMO levels of tetracene, as well as an increase in the electron-phonon coupling in tetracene. Also, electrical polarization is shown to increase electrical interactions between tetracene molecules. Therefore, we conclude that using spin interactions and electrical polarization from the ferromagnetic/organic semiconductor interface can tune the properties of tetracene, ultimately enhancing singlet fission. This work gives new insight to understand the singlet fission process using a ferromagnetic interface. These changes can be further utilized in photovoltaic applications based on this singlet fission material and be applied to other similar types of singlet fission organic semiconductors.     

Tunable Low‐Field Magnetoresistance in (La0.7Sr0.3MnO3)0.5:(ZnO)0.5 Self‐Assembled Vertically Aligned Nanocomposite Thin Films

Tunable and enhanced low‐field magnetoresistance (LFMR) is observed in epitaxial (La_0.7Sr_0.3MnO₃)_0.5:(ZnO)_0.5 (LSMO:ZnO) self‐assembled vertically aligned nanocomposite (VAN) thin films, which have been grown on SrTiO₃ (001) substrates by pulsed laser deposition (PLD). The enhanced LFMR properties of the VAN films reach values as high as 17.5% at 40 K and 30% at 154 K. They can be attributed to the spin‐polarized tunneling across the artificial vertical grain boundaries (GBs) introduced by the secondary ZnO nanocolumns and the enhancement of spin fluctuation depression at the spin‐disordered phase boundary regions. More interestingly, the vertical residual strain and the LFMR peak position of the VAN films can be systematically tuned by changing the deposition frequency. The tunability of the physical properties is associated with the vertical phase boundaries that change as a function of the deposition frequency. The results suggest that the tunable artificial vertical GB and spin‐disordered phase boundary in the unique VAN system with vertical ferromagnetic‐insulating‐ferromagnetic (FM‐I‐FM) structure provides a viable route to manipulate the low‐field magnetotransport properties in VAN films with favorable epitaxial quality.     

Effect of spin excited states on electron transport through an organic ferromagnetic device

Spin excited states in an organic ferromagnet are proposed and investigated on the basis of an extended SSH + Heisenberg (SSH = Su-Schrieffer-Heeger) model. It is found that a spin excited state will form a local distortion of the spin density wave (SDW) of π-electrons while the lattice configuration of main chain has no obvious change. Then the spin-polarized transport properties through an organic ferromagnetic device are investigated with the Landauer-Büttiker formula and Green’s function method. It is obtained that the current will be spin polarized due to the existence of SDW in the ferromagnetic molecule. Both the total current and the spin-polarized current will be modulated when the SDW is excited. The total current through the device is suppressed by the spin excitation of side radicals, through which a conductance switch function may be realized. Compared with ground state, the spin polarization has no obvious change in a low spin excited state and the device still has spin-filter function. Finally, spin excitations induced by temperature is studied and we find that an organic ferromagnetic device can hold a high spin polarization when temperature is not too high.