Injection Of Spin-polarized Carriers Research Articles

Probing Spin-Dependent Ballistic Charge Transport at Single-Nanometer Length Scales.

The coherent transport of charge and spin is a key requirement of future devices for quantum computing and communication. Scattering at defects or impurities may significantly reduce the coherence of quantum-mechanical states, thereby affecting the device functionality. While numerous methods exist to experimentally assess charge transport, the real-space detection of a material's ballistic spin transport properties with nanometer resolution remains a challenge. Here we report on a novel approach that utilizes a combination of spin-polarized scanning tunneling microscopy (SP-STM) and the recently introduced molecular nanoprobe (MONA) technique. It relies on the local injection of spin-polarized charge carriers from a magnetic STM tip and their detection by a single surface-deposited phthalocyanine molecule via reversible electron-induced tautomerization events. Based on the particular electronic structure of the Rashba alloy BiAg2, which is governed by a spin-momentum-locked surface state, we prove that the current direction inverses upon tip magnetization reversal.

Thickness dependence of magnetic and magnetotransport properties of textured Fe3O4 ferrimagnetic films on SiO2/n-Si(001)

The efficient injection of spin-polarized charge carriers from a ferro- or ferrimagnet into a semiconductor is an urgent problem. Half-metal magnetite (Fe3O4) is an attractive candidate for the role of a spin injector in spintronic devices. Here, the influence of the thickness of magnetite films on their electrical, magnetic, and magnetotransport properties is presented. Polycrystalline magnetite films with (311)-texture and thickness ranging from 15 to 250 nm are grown on the oxidized silicon surface by reactive iron deposition in the molecular oxygen atmosphere. The saturation magnetization is found to increase nonmonotonically with the film thickness and reaches its maximum value at 170 nm, which is close to that of a single crystal (∼480 emu/cm3). It is shown that for films with a thickness of more than 150 nm, there is a rapid decrease in resistivity by almost three times. The critical film thickness at which the coercive force decreases to the lowest value of ∼250 Oe is 100 nm. The magnetoresistance value reaches − 2.5 % already at a thickness of 50 nm and remains almost unchanged with a subsequent increase in the thickness of the textured film. Since, at a small thickness, textured films have higher magnitudes of conductivity and magnetization than conventional polycrystalline films, the textured-F3O4/SiO2/n-Si(001) junction with amorphous SiO2 barrier is promising for Si-based spintronic devices.

Enhanced Spin Accumulation in Semiconductor at Room Temperature Using Ni0.65Zn0.35Fe2O4(NZFO) as Spin Injector in NZFO/MgO/p-Si Device

In this article, the fabrication of a Ni0.65Zn0.35Fe2O4/MgO/p-Si heterostructure device has been optimized using the pulsed laser deposition (PLD) technique, and a detailed investigation of its structural, electrical, and magnetic features has been performed experimentally. The electronic and magneto-transport characteristics have been explored in the temperature range of 100–300 K. The current-voltage (I-V) characteristics of the heterojunction have been recorded, which displayed an excellent rectifying magnetic tunnel diode-like behavior throughout that temperature regime. The application of an external magnetic field parallel to the plane of the NZFO film causes the current (I) across the junction to decrease, clearly indicating positive junction magnetoresistance (JMR) of the heterostructure. The root of displaying positive magnetoresistance in our heterojunction has been well justified using the standard spin injection model. The electrical injection of spin-polarized carriers and its accumulation and detection in a p-Si channel have been demonstrated using the NZFO/MgO tunnel contact using a three-terminal (3-T) Hanle device. The parameters such as spin lifetime (99 ps), spin diffusion length (276 nm), and spin polarization (0.44) have been estimated from the Hanle curve detected in our heterostructure at room temperature, making the Ni0.65Zn0.35Fe2O4/MgO/p-Si device a very favorable promising junction structure in the field of spintronics for several device appliances in the future.

Intensity equations for birefringent spin lasers

Semiconductor spin lasers are distinguished from their conventional counterparts by the presence of spin-polarized carriers. The transfer of angular momentum of the spin-polarized carriers to photons provides important opportunities for the operation of lasers. With the injection of spin-polarized carriers, which lead to the circularly polarized light, the polarization of the emitted light can be changed an order of magnitude faster than its intensity. This ultrafast operation of spin lasers relies on a large birefringence, usually viewed as detrimental in spin and conventional lasers. We introduce a transparent description of spin lasers using intensity equations, which elucidate the influence of birefringence on the intensity and polarization modulation of lasers. While intensity modulation is independent of birefringence, for polarization modulation an increase in birefringence directly increases the resonant frequency. Our results for dynamical operation of lasers provide a guide for their spin-dependent response and spintronic applications beyond magnetoresistance.

Robust and Efficient Electrical Injection of Spin-Polarized Carriers from a Zero-Spin-Gap Half-Metal V2NiAl into n-Si at Room Temperature

In recent times, an engrossing and newly discovered class of materials with unique electronic band properties, namely, zero-spin-gap half-metal, has gained acute attraction worldwide because of its...

Spin-polarized carrier injection through hybrid ferromagnetic electrode for enhanced optical efficiency of organic light-emitting diodes

Abstract Organic light-emitting diodes (OLEDs) with an enhanced optical efficiency were fabricated using a hybrid ferromagnetic (FM) electrode that injected spin-polarized carriers (spin-polarized current). The optical efficiency of the fluorescent OLEDs was increased through the injected spin-polarized carriers, which increased the formation ratio of singlet to triplet exciton. The OLED with the hybrid FM electrode exhibited enhancements of 66.6% in the external quantum efficiency at 50 mA/cm2 and 31.8% in the power efficiency at 500 cd/m2, compared with a reference device at room temperature. The hybrid FM electrode was designed using the finite-difference time-domain method to obtain a high optical efficiency while preventing spectrum distortion.

Electrical Control of Spin-Injection Using Mixed Dimensional van der Waals Heterostructures

We demonstrate that van der Waals heterostructures based on CrI3, MoS2 and a 3D semiconductor source may allow for all electrical control of spin polarized carrier injection into MoS2. We demonstrate this possibility using a simple resonant tunneling device structure and model it using the transfer matrix method. Our results show that electrically controlled spin polarized carrier injection is theoretically possible at low device bias.

Unexpected Outstanding Room Temperature Spin Transport Verified in Organic-Inorganic Hybrid Perovskite Film.

Fundamental understanding on the spin transport properties of semiconducting organic-inorganic hybrid perovskites (OIHP) is of great importance for advancing their applications for spin-optoelectronic devices. Herein, the study of spin-pumping induced inverse spin Hall effect in Ni80Fe20(Py)/CH3NH3PbCl3-xIx/Pt trilayers with different OIHP spacer thicknesses concludes the spin diffusion length in CH3NH3PbCl3-xIx as large as 61 ± 7 nm at room temperature. In addition, spin-valves with a structure of Ni80Fe20(Py)/CH3NH3PbCl3-xIx/AlOx/Co was fabricated as well. Using a ∼160 nm-thick CH3NH3PbCl3-xIx spacer, the present spin valve exhibits a positive magnetoresistance (MR) of 0.57% at 10 K. Thus, the present spin related results demonstrate that electrical spin-polarized carrier injection, transport, and detection, which are essential in spintronic devices, can be successfully established in OIHP films. The outstanding spin transport in the present CH3NH3PbCl3-xIx should be owing to its highly out-of-plane oriented crystalline texture and Rashba spin splitting at domain boundaries between crystallographic orientations. These results demonstrate OIHP as very attractive materials for spintronics.

Spin polarized carrier injection driven magneto-optical Kerr effect in Cr-doped ZnO nanorods

Hydrothermal growth of Cr doped ZnO nanorods (NRs) thin films was grown on glass substrates. The strong ferromagnetic (FM) and antiferromagnetic (AFM) ordering was observed in 3% and 7% of Cr doped samples. The optical excitation of the Cr doped ZnO NRs is responsible for injection of the spin-polarized carriers in the 3% Cr doped ZnO NRs through longitudinal magneto-optical Kerr effect (MOKE). We determined a negative anisotropic magnetoresistance (∼23%) originated from spin orbit coupling due to sp-d exchange interaction. we calculated the process of photon induced inverse spin Hall angle (θISHE∼3.94×10−2) close to the MOKE saturated rotation angle (θk∼0.046) for 3% Cr: ZnO. These results can open a new path of optical spin detectors for next-generation spintronic device technology.

Effect of Charge Transfer upon Li- and Na-Ion Insertion in Fine-Grained Graphitic Material as Probed by NMR.

We investigated the insertion-extraction behaviors of Li and Na ions in graphitic materials using solid-state NMR. A unique advantage of high-degree 13C-isotope enrichment of graphitic material allowed sensitive and metastable graphite intercalation compounds to be measured in a short time. Ex situ 13C magic-angle spinning NMR spectra of 13C fine-grained graphite are presented as a function of state-of-charge. The observations are discussed with respect to graphite intercalation phenomena, which include the effects of charge transfer and the demagnetizing field. Dramatic narrowing of the 13C NMR signal in metal-intercalated graphite evidences quasi-complete charge transfer occurring between lithium and graphite host material and resulting in reducing the macroscopic field effects. Upon Na insertion, incomplete charge transfer is observed and explained by inaccessibility of graphitic interlayer space for Na ions in our study. In addition, critical issues of reversibility of Li- and Na-ion electrochemical cells and solid electrolyte interphase formation are considered on the atomic scale. The knowledge gained in the present work can be applied to advanced high-power-density electrode materials for safe and fast-charging metal-ion batteries or for novel spintronic concepts with controlled spin-polarized charge carrier injection and transport combined with the possibility to manipulate magnetic anisotropy.

Spin-optoelectronic devices based on hybrid organic-inorganic trihalide perovskites

Recently the hybrid organic-inorganic trihalide perovskites have shown remarkable performance as active layers in photovoltaic and other optoelectronic devices. However, their spin characteristic properties have not been fully studied, although due to the relatively large spin-orbit coupling these materials may show great promise for spintronic applications. Here we demonstrate spin-polarized carrier injection into methylammonium lead bromide films from metallic ferromagnetic electrodes in two spintronic-based devices: a ‘spin light emitting diode’ that results in circularly polarized electroluminescence emission; and a ‘vertical spin valve’ that shows giant magnetoresistance. In addition, we also apply a magnetic field perpendicular to the injected spins orientation for measuring the ‘Hanle effect’, from which we obtain a relatively long spin lifetime for the electrically injected carriers. Our measurements initiate the field of hybrid perovskites spin-related optoelectronic applications.

Spin-dependent giant junction magnetoresistance in simple Fe/p-Si(001) Schottky heretrojunction at low temperature

We report on the giant junction magnetoresistance observed at low temperature in Fe/p-Si Schottky heterojunction. The device shows good rectifying characteristics at room temperature and a dual—Schottky as well as magnetic diode—characteristics at low temperature, below 50 K. Formation of a magnetic field-dependent potential barrier due to electrical injection of spin-polarized carriers from the ferromagnetic electrode into the semiconductor is speculated to result in such large junction resistance. The magnetoresistance value is of the order of $$10^{4}\%$$ at 10 K and saturates at $$\sim 0.5\,\hbox {kOe}$$ , showing dual functionality—working as a magnetic diode as well as a magnetoresistive element.

Tunable electronic structure and magnetic coupling in strained two-dimensional semiconductor MnPSe3

The electronic structures and magnetic properties of strained monolayer MnPSe3 are investigated systematically via first-principles calculations. It is found that the magnetic ground state of monolayer MnPSe3 can be significantly affected by biaxial strain engineering, while the semiconducting characteristics are well-preserved. Owing to the sensitivity of the magnetic coupling towards structural deformation, a biaxial tensile strain of approximately 13% can lead to an antiferromagnetic (AFM)- ferromagnetic (FM) transition. The strain-dependent magnetic stability is mainly attributed to the competition of the direct AFM interaction and indirect FM superexchange interaction between the two nearest-neighbor Mn atoms. In addition, we find that FM MnPSe3 is an intrinsic half semiconductor with large spin exchange splitting in the conduction bands, which is crucial for the spin-polarized carrier injection and detection. The sensitive interdependence among the external stimuli, electronic structure, and magnetic coupling makes monolayer MnPSe3 a promising candidate for spintronics.

Studies of spin related processes in fullerene C60 devices

We have investigated spin related processes in fullerene C60 devices using a several experimental techniques, which include magnetic field effect of photocurrent and electroluminescence in C60-based diodes; spin polarized carrier injection in C60-based spin-valves; and pure spin current generation in NiFe/C60/Pt trilayer devices.

Organic Spin-Valves and Beyond: Spin Injection and Transport in Organic Semiconductors and the Effect of Interfacial Engineering.

Since the first observation of the spin-valve effect through organic semiconductors, efforts to realize novel spintronic technologies based on organic semiconductors have been rapidly growing. However, a complete understanding of spin-polarized carrier injection and transport in organic semiconductors is still lacking and under debate. For example, there is still no clear understanding of major spin-flip mechanisms in organic semiconductors and the role of hybrid metal-organic interfaces in spin injection. Recent findings suggest that organic single crystals can provide spin-transport media with much less structural disorder relative to organic thin films, thus reducing momentum scattering. Additionally, modification of the band energetics, morphology, and even spin magnetic moment at the metal-organic interface by interface engineering can greatly impact the efficiency of spin-polarized carrier injection. Here, progress on efficient spin-polarized carrier injection into organic semiconductors from ferromagnetic metals by using various interface engineering techniques is presented, such as inserting a metallic interlayer, a molecular self-assembled monolayer (SAM), and a ballistic carrier emitter. In addition, efforts to realize long spin transport in single-crystalline organic semiconductors are discussed. The focus here is on understanding and maximizing spin-polarized carrier injection and transport in organic semiconductors and insight is provided for the realization of emerging organic spintronics technologies.

Dynamic spin filtering at the Co/Alq3 interface mediated by weakly coupled second layer molecules.

Spin filtering at organic-metal interfaces is often determined by the details of the interaction between the organic molecules and the inorganic magnets used as electrodes. Here we demonstrate a spin-filtering mechanism based on the dynamical spin relaxation of the long-living interface states formed by the magnet and weakly physisorbed molecules. We investigate the case of Alq3 on Co and, by combining two-photon photoemission experiments with electronic structure theory, show that the observed long-time spin-dependent electron dynamics is driven by molecules in the second organic layer. The interface states formed by physisorbed molecules are not spin-split, but acquire a spin-dependent lifetime, that is the result of dynamical spin-relaxation driven by the interaction with the Co substrate. Such spin-filtering mechanism has an important role in the injection of spin-polarized carriers across the interface and their successive hopping diffusion into successive molecular layers of molecular spintronics devices.

Impact of Tunnel-Barrier Strength on Magnetoresistance in Carbon Nanotubes

With the relentless demand for smaller computing components, low-dimensional objects such as quantum wires and graphene nanoribbons are of special interest for future spintronics applications. A key issue is the injection of spin-polarized charge carriers into these systems via tunneling. In this study, the role of barrier strength in carrier injection is investigated both experimentally and theoretically, using carbon nanotubes (CNTs) as a model system. The results show how to understand, and control, the performance of CNT-based spin valves.

Atomic-Scale Engineering of Abrupt Interface for Direct Spin Contact of Ferromagnetic Semiconductor with Silicon.

Control and manipulation of the spin of conduction electrons in industrial semiconductors such as silicon are suggested as an operating principle for a new generation of spintronic devices. Coherent injection of spin-polarized carriers into Si is a key to this novel technology. It is contingent on our ability to engineer flawless interfaces of Si with a spin injector to prevent spin-flip scattering. The unique properties of the ferromagnetic semiconductor EuO make it a prospective spin injector into silicon. Recent advances in the epitaxial integration of EuO with Si bring the manufacturing of a direct spin contact within reach. Here we employ transmission electron microscopy to study the interface EuO/Si with atomic-scale resolution. We report techniques for interface control on a submonolayer scale through surface reconstruction. Thus we prevent formation of alien phases and imperfections detrimental to spin injection. This development opens a new avenue for semiconductor spintronics.

(Invited) Importance of Interface Engineering in Organic Spintronics

Organic semiconductors provide a route to revolutionary changes in many areas of device technology with their unique electronic properties and functionalities, which can be easily tailored by chemical synthesis and molecular engineering.[1] In particular, an emerging research area referred to as organic spintronics has initiated a range of device concepts by exploiting spin-related properties in organic semiconductors.[2] Currently, there are two major interests in organic spintronics technology.[3] One of them is to realize a system enabling the injection/transport/detection of spin polarized charge carriers in organic semiconductors.[3] The other one is to understand and explore the magnetic field effects (MFEs) of organic semiconductors for the development and improvement of organic-based devices such as organic light emitting diodes and magnetic sensors.[3] It has been known that in organic-based devices, the interfaces between active materials play a critical role in their performance and functionality because the interfaces greatly affect the injection/transport/extraction of charge carriers in organic semiconductors.[4] Recent findings suggest that the interface engineering is also very crucial in organic spintronics technology such as enabling efficient spin-polarized charge carrier injection and tuning of organic MFEs.[5-7] In this presentation, we will discuss our efforts towards the realization of efficient spin injection and transport in organic semiconductors as well as the modification of organic MFEs by interface engineering. We experimentally show the importance of band alignment between a metal electrode and an organic semiconductor at their interface for efficient spin injection/detection.[6] We will also show how one can utilize a molecular self-assembled monolayer by inserting it between a metal electrode and an organic semiconductor in order to change the characteristics of spin injection into an organic semiconductor from a ferromagnetic metal as well as in order to tune the organic MFEs.[7] 1. Editorial, Nat. Mater. 12, 591 (2013). 2. C. Boehme and J. M. Lupton, Nat. Nano. 8, 612 (2013). 3. W. Wagemans, et al., SPIN, 1, 93 (2011). 4. H. Ma, et al., Adv. Funct. Mater. 20, 1371 (2010). 5. S. Sanvito, Nat. Phys. 6, 562 (2010). 6. H.-J. Jang, et al., Appl. Phys. Lett. 101, 102412 (2012). 7. H.-J. Jang, et al., ACS Nano 8, 7192 (2014).

Direct epitaxial integration of the ferromagnetic semiconductor EuO with silicon for spintronic applications.

Following a remarkable success of metallic spintronics, tremendous efforts have been invested into the less developed semiconductor spintronics, in particular, with the aim to produce three-terminal spintronic devices, e.g., spin transistors. One of the most important prerequisites for such a technology is an effective injection of spin-polarized carriers into a nonmagnetic semiconductor, preferably one of those currently used for industrial applications such as Si-a workhorse of modern electronics. Ferromagnetic semiconductor EuO is long believed to be the best candidate for integration with Si. Although EuO proved to offer optimal conditions for effective spin injection into silicon and in spite of considerable efforts, the direct epitaxial stabilization of stoichiometric EuO thin films on Si without any buffer layer has not been demonstrated to date. Here we report a new technique for control of EuO/Si interface on submonolayer level. Using this technique we solve a long-standing problem of direct epitaxial growth on silicon of thin EuO films which exhibit structural and magnetic properties of EuO bulk material. This result opens up new possibilities in developing all-semiconductor spintronic devices.