Spin injection in metals and semiconductors

The spin lifetime and diffusion length of electrons are transport parameters that define the scale of coherence in spintronic devices and circuits. As these parameters are many orders of magnitude larger in semiconductors than in metals, semiconductors could be the most suitable for spintronics. So far, spin transport has only been measured in direct-bandgap semiconductors or in combination with magnetic semiconductors, excluding a wide range of non-magnetic semiconductors with indirect bandgaps. Most notable in this group is silicon, Si, which (in addition to its market entrenchment in electronics) has long been predicted a superior semiconductor for spintronics with enhanced lifetime and transport length due to low spin-orbit scattering and lattice inversion symmetry. Despite this promise, a demonstration of coherent spin transport in Si has remained elusive, because most experiments focused on magnetoresistive devices; these methods fail because of a fundamental impedance mismatch between ferromagnetic metal and semiconductor, and measurements are obscured by other magnetoelectronic effects. Here we demonstrate conduction-band spin transport across 10 mum undoped Si in a device that operates by spin-dependent ballistic hot-electron filtering through ferromagnetic thin films for both spin injection and spin detection. As it is not based on magnetoresistance, the hot-electron spin injection and spin detection avoids impedance mismatch issues and prevents interference from parasitic effects. The clean collector current shows independent magnetic and electrical control of spin precession, and thus confirms spin coherent drift in the conduction band of silicon.

Spin injection from a ferromagnetic metal into a semiconductor driven by a constant phonon flow is considered. It is shown that diffusive (as opposed to electrical) spin injection gives rise to a highly spin-polarized current through the semiconductor. In other words, phonon-wind-driven injection under certain conditions eliminates the conductivity mismatch responsible for reduced spin polarization of the electric current injected from a ferromagnetic metal into a semiconductor.

Future spintronics devices will be built from elemental blocks allowing the electrical injection, propagation, manipulation and detection of spin-based information. Owing to their remarkable multi-functional and strongly correlated character, oxide materials already provide such building blocks for charge-based devices such as ferroelectric field-effect transistors (FETs), as well as for spin-based two-terminal devices such as magnetic tunnel junctions, with giant responses in both cases. Until now, the lack of suitable channel materials and the uncertainty of spin-injection conditions in these compounds had however prevented the exploration of similar giant responses in oxide-based lateral spin transport structures. In this paper, we discuss the potential of oxide-based spin FETs and report magnetotransport data that suggest electrical spin injection into the LaAlO(3)-SrTiO(3) interface system. In a local, three-terminal measurement scheme, we analyse the voltage variation associated with the precession of the injected spin accumulation driven by perpendicular or longitudinal magnetic fields (Hanle and 'inverted' Hanle effects). The spin accumulation signal appears to be much larger than expected, probably owing to amplification effects by resonant tunnelling through localized states in the LaAlO(3). We give perspectives on how to achieve direct spin injection with increased detection efficiency, as well on the implementation of efficient top gating schemes for spin manipulation.

Interplay of electronic structure, magnetism, strain, and defects in carbide MXenes

Injection of spin currents into solids is crucial for exploring spin physics and spintronics. There has been significant progress in recent years in spin injection into high-resistivity materials, for example, semiconductors and organic materials, which uses tunnel barriers to circumvent the impedance mismatch problem; the impedance mismatch between ferromagnetic metals and high-resistivity materials drastically limits the spin-injection efficiency. However, because of this problem, there is no route for spin injection into these materials through low-resistivity interfaces, that is, Ohmic contacts, even though this promises an easy and versatile pathway for spin injection without the need for growing high-quality tunnel barriers. Here we show experimental evidence that spin pumping enables spin injection free from this condition; room-temperature spin injection into GaAs from Ni(81)Fe(19) through an Ohmic contact is demonstrated through dynamical spin exchange. Furthermore, we demonstrate that this exchange can be controlled electrically by applying a bias voltage across a Ni(81)Fe(19)/GaAs interface, enabling electric tuning of the spin-pumping efficiency.

The current induced spin-orbit-torque (SOT) [1-4] is the key physical parameter describing the generation of magnetization dynamics in bilayers composed by a heavy metal (HM) layer and a ferromagnetic (FM) layer. Such bilayers, especially when associated with a magnetic tunnel junction at the FM layer (then forming the so called 3-terminal device), are considered as promising elements to be applied in novel magnetic memories, microwave generators and even bio-inspired types of computing [1]. Therefore, the robust and accurate determination of the SOT in HM/FM bilayers is of high relevance for optimizing future applications.In HM/FM bilayers the SOT is mainly caused by the spin Hall effect (SHE) of the HM layer, by which a spin current is injected into the FM layer. 3-terminal devices with application as magnetic memories adopt mainly FM films with perpendicular magnetic anisotropy, while for sensor applications also the configuration with in-plane magnetic anisotropy is of interest. The determination and quantification of the SOT can be performed in both cases by different techniques. While in a 3-terminal device the magnetization dynamics of observed via the tunnel magnetoresistance effect, in HM/FM bilayers other methods can be employed, such as the SOT-ferromagnetic resonance (FMR), the harmonic Hall voltage measurement and the spin Hall magnetoresistance (SMR). The SMR is a straightforward method when the FM layer is an insulator, while with metallic FM, in which the testing electric current in not confined in the HM layer only, it is accompanied by additional effects, such as the anisotropic magnetoresistance or the anomalous Nernst effect [5]. Therefore, it is important to analyse the measurement results by accurate models taking into account all aspects. The dominant theoretical approach for SHE-SOT is the drift-diffusion model considering spin accumulation at the interface between the FM and HM by introducing a spin mixing conductance. However, the agreement with experiments may be poor if spin dissipation at the interface (spin memory loss, spin dependent scattering, etc.) is disregarded [2].In this paper we analyze transport measurements on FM/HM bilayers as a function of the HM thickness by employing a non-equilibrium thermodynamic approach to SHE-SOT [6] with the aim to reach a semi-quantitative agreement. The study was performed on HM/FM bilayers with HM=Pt (wedge 4-19nm), Ta (wedge 4-19nm), and FM=Fe60Co20B20 (2nm) prepared at Singulus Technologies AG. All samples were annealed for 2h at 310°C in a magnetic field of 1T in order to develop an in-plane magnetic anisotropy (y axis). For the transport measurements the films were patterned into µm-size Hall bars/crosses (along x axis) by optical lithography and ion beam etching with Ti/Au contact pads. Magneto-optic Kerr effect (MOKE) and SMR measurements were performed following the protocol of [5], while SOT-FMR and harmonic Hall voltage measurements and analysis were based on references [3,4].Fig.1 shows the comparison between MOKE loops and the SMR signal. The loop along the y axis is more rectangular and indicates the magnetic easy anisotropy axis. The SMR signal is much more pronounced with the field along the x axis, while it shows a small peak close to the coercivity when the field is along the y axis. With MOKE the anisotropy was determined and a dependence on HM thickness was observed.Fig.2 presents the summary of the SOT-FMR and harmonic Hall voltage measurements for HM/FM bilayers with different thickness of HM. FMR analysis enabled determination of effective magnetization and magnetization damping (α = 0.015 for Pt and 0.0075 for Ta), while anomalous Hall effect and harmonic Hall measurements resulted in SOT efficiencies dependence vs. HM thickness. While damping-like SOT efficiency for Ta reaches ξDL-Ta = -0.35 and ξDL-Pt = 0.15 for Pt, with high resistivity difference (ρTa > 200 µΩcm and 24 < ρPt < 52 µΩcm) taken into account, the spin Hall conductivity for Pt of σSH-Pt = 2.9 (105 S/m) exceeds the one for Ta σSH-Ta = 1.7 (105 S/m) and is among the highest values reported [8].The HM thickness dependence of the SMR is analyzed by considering both the finite conductance at the interface [7] (an approach valid for small thicknesses) and thermodynamic approach [6] (valid for larger thicknesses). The results are compared with the harmonic Hall voltage measurements the SOT and the SOT efficiency and discussed in relation to the key parameters as the spin Hall angle and the spin diffusion length.Acknowledgements: WS acknowledges National Science Centre Grant No. UMO-2015/17/D/ST3/00500, Poland. Microfabrication was performed at the Academic Centre for Materials and Nanotechnology of AGH. **

The spin injection and accumulation in metallic lateral spin valves with transparent interfaces are studied using dc injection current. Unlike ac-based techniques, this allows the investigation of the effects of the direction and magnitude of the injected current. We find that the spin accumulation is reversed by changing the direction of the injected current, whereas its magnitude does not change. The injection mechanism for both current directions is thus perfectly symmetric, leading to the same spin injection efficiency for both spin types. This result is accounted for by a spin-dependent diffusion model. Joule heating increases considerably the local temperature in the spin valves when high-current densities are injected ($\ensuremath{\sim}80--105\text{ }\text{K}$ for $1--2\ifmmode\times\else\texttimes\fi{}{10}^{7}\text{ }\text{A}\text{ }{\text{cm}}^{\ensuremath{-}2}$), strongly affecting the spin accumulation.

Materials with 100% conduction electron spin polarization are possible candidates for electrical spin injection into non-magnetic semiconductors, as they do not suffer from the conduction mismatch problem encountered by ferromagnetic metals with incomplete spin polarization. However, I show that truly half-metallic contacts are extremely challenging to fabricate, and will in many cases suffer from other problems such as the presence of a Schottky barrier that prevents the injection of any carriers at all. The suggested solution is to insert a tunnel barrier to accommodate both the Schottky barrier and any deviations from a true half-metallic nature. The largest spin injection we have obtained using NiMnSb is (6% for polycristalline films on AlO x tunnel barriers, and 2.2% or less for epitaxial single crystalline films. This is much below what is obtained using traditional contacts such as (Co)Fe, and suggests that the experimental NiMnSb/GaAs interfaces have a low spin polarization. On the other hand, using a Zener tunnel junction to convert polarized holes into polarized electrons, we have obtained 80% spin injection from the ferromagnetic semiconductor (Ga,Mn)As into non-magnetic GaAs, showing that this material possesses a very high spin polarization.

There have been several recent experiments involving spin injection and accumulation in mesoscopic metal samples, using lateral spin valve structures. The results have been interesting from the perspectives of both fundamental physics and applications. The resistance change associated with spin accumulation, ▵R, has been predicted to scale inversely with sample volume and this inverse scaling has been observed over ten decades. A value of ▵R = 1 Ohm has been reported, at room temperature, in a thin film Al wire structure with transverse dimensions of roughly 100 nm. High values of ▵R have been observed in samples characterized by electrode interface resistances that vary by 6 decades. Results are discussed within the framework of Johnson-Silsbee theory, and factors that limit inverse scaling are identified. Lateral spin valves with dimensions of tens of nm may be competitive for device applications. Structures with ▵R of order 1 Ohm are superior to CPP spin valves of comparable dimensions. Prospects for improved performance, the plausibility of a lateral spin valve with output levels of 10 Ohms and output impedance of 50 Ohms, and relevance to hard drive read heads and integrated nonvolatile random access memory applications are discussed.

Because of their extremely weak spin-orbit coupling and hence very long spin relaxation time, molecular semiconductors have considered to be very promising materials in spintronics. Since the first observation of spin transport in molecular semiconductors in 2002, molecular spintronics, as an emerging subject for studying spin-correlated phenomenon of electrons in molecular matrix, has attracted considerable research interests in the previous decade. Molecular spin valve is the most prototypical device in molecular spintronic, which should be the best test bed to study spintronic processes in molecules, including spin injection, spinterfacial effect, spin transport, as well as novel spintronic functions. Thus, to achieve highly reproducible molecular spin valves with high performances is of significant importance to the field of molecular spintronics, in which the efficient spin injection-transport process is the necessary precondition. However, according to current studies, there are several unexpected challenges for enhancing spin injection in spin valves, such as top electrode diffusion into molecular layer during device fabrication and the bad energy level alignment between ferromagnetic electrodes and molecules. To get away from these problems and improve device performance, considerable contributions have already been made to enhance spin injection from perspective of creating novel fabrication methods, optimizing device structure as well as various interfacial engineering methods. Besides, the spinterfacial effects at the interface between ferromagnetic electrodes and molecules are also considered to be crucial for spin injection. In latest studies, hybridized interface state (HIS) was proposed to characterize interaction between ferromagnetic electrode and the first molecular layer, and its spin-density of state at Fermi level of ferromagnetic electrode was changed. Therefore, spin polarization was influenced; as well its electronic structure at HIS was rebuilt to affect spin injection. In this review, research advances regarding spin transport mechanisms are also summarized, including tunneling transport and hopping transport separately. According to few recent reports, it is found that the hopping spin transport mode may avail longer spin transport distance especially at room temperature. Moreover, functional spintronic devices, integrated the spin transport and molecular functions (such as optical, electrical or spinterfacial) in one chip, are briefly introduced, which attracted considerable research interest very recently due to the great potential for applications in future. To summarize, the plentiful contributions regarding spintronic device, including performance optimization as well as mechanism study, have greatly facilitated the development of molecular spintronics in the past decade. However, molecular spintronics is an interdisciplinary filed of chemistry, physics and materials, thus exploring novel molecules that are suitable for spintronic applications must be one of the most important issues in future development of molecular spintronics.

Electrical creation of spin accumulation in [formula omitted]-type germanium

Dynamical spin injection based on ferromagnetic resonance (FMR) has been intensively investigated owing to its simple structure with variable applications [1]. When the FMR is excited in ferromagnetic-metal (FM)/nonmagnetic-metal (NM) bilayer thin films, the spin current flows from the FM layer into the NM layer. Although the mechanism of the dynamical spin injection is mainly explained by spin pumping, FMR-heating could be a possible reason for the dynamical spin injection [2].To investigate the dominant contribution for the dynamical spin injection, we measured the temperature dependence of the inverse spin Hall signal in a CoFeB/Pt/CoFeB trilayer structure. Fig.1 shows the schematic illustration of the measurement system. Here, the bottom Cu layer acts as the coplanar wave guide for genrating a high frequency magnetic field, resulting in the efficient excitation of the FMR of the trilayer structure. In addition, since the Cu layer with sufficiently low electrical resistivity is directly connected to the trilayer, we are able to purely observe inverse spin Hall signal in the Pt due to the dynamical spin injection with eliminate the spurious signals such as the rectified anisotropic magnetoresistance and anomalous Nernst effects.[3] The observed inverse spin Hall signals at room temperature (297K) and low temperature(77K) are shown in Fig.2. It should be emphasized that the sign of the inverse spin Hall signal at 77 K is opposite from that at 297 K. This can be explained by the change of the thermal gradient in CoFeB/Pt/CoFeB/Cu/Sub, indicating that the FMR heating is a dominant mechanism for the dynamical spin injection. **

The authors demonstrate efficient hole spin injection from a ferromagnetic metal (Ni) contact in a forward biased light emitting Schottky diode (LESD) fabricated on a GaAs based heterostructure with a quantum well (QW). The spin polarization of the injected holes was detected by measuring the circular polarization of the electroluminescence (EL) from the near surface InGaAs∕GaAs QW. An intermediate gold layer has been used in order to improve the spin injection efficiency. Over 40% degree of circular polarization of the EL has been observed at T=2K for the LESD structure with Au–Ni–Au Schottky contact.

The spin injection into 2D electron gas (2DEG) in AlN/GaN heterostructures is studied by magneto‐transport measurements. An ultrathin AlN layer at the hetero‐interface acts as a barrier to form high‐quality 2DEG in the triangular quantum well and a tunneling barrier for the spin injection to overcome the conductance mismatch issue. In this study, Hanle signals and inversed Hanle signals are observed, proving that the spin injection is achieved in the 2DEG in the AlN/GaN heterostructure rather than in the interfacial states. The spin‐relaxation time in 2DEG at 8 K is found to be as long as 860 ps, which almost keeps constant with bias and decreases with increasing temperature. The spin‐relaxation process is illustrated as Rashba spin‐orbit coupling dominated D'yakonov Perel’ mechanisms above 8 K. These results show the promising potential of 2DEG in AlN/GaN heterostructures for spin field‐effect transistor applications.

Organic–inorganic hybrid perovskites (OIHPs) have been explosively investigated mainly due to their potential applications in optoelectronics. Despite the electronic charge transport, phenomena regarding the spin‐polarized electronic transport in OIHPs‐based spintronic devices and the role of ferromagnet/OIHP spinterfaces remain unclear. In this work, the spin injection, accumulation, transport, and detection at room temperature for a vertical perovskite spin valve (PeSV) consisting of Ni/CH3NH3PbI3−xClx/Ni is reported. An in‐plane anisotropic magnetoresistance (AMR) and a PeSV related magnetoresistance (MR) show remarkable magnetic switching behaviors due to the formation of Ni/CH3NH3PbI3−xClx spinterfaces, and the ferromagnetic coupling between two spin quantization axes of the spinterfaces. With assists of capacitance–frequency (C − − f) measurements under magnetic fields, the spin accumulation that occurs at the Ni/CH3NH3PbI3−xClx interface can be detected at the spin parallel (↑↑) and antiparallel (↑↓) configurations. Owing to a strong orbital interaction at the Ni/CH3NH3PbI3−xClx hybrid interface, the spin‐sensitive electron paramagnetic spectroscopy (EPR) reveals significant change of the magnetic moment (µ). It is believed that the solution processed CH3NH3PbI3−xClx and the formation of the Ni/CH3NH3PbI3−xClx spinterface may hold an exceptionally important role for future hybrid optospintronic applications.