Spin-to-charge conversion (SCC) involving topological surface states (TSS) is one of the most promising routes for highly efficient spintronic devices for terahertz (THz) emission. Here, the THz generation generally occurs mainly via SCC consisting in efficient dynamical spin injection into spin-locked TSS. In this work, we demonstrate sizable THz emission from a nanometric thick topological insultator (TI)/ferromagnetic junction - SnBi$_2$Te$_4$/Co - specifically designed to avoid bulk band crossing with the TSS at the Fermi level, unlike its parent material Bi$_2$Te$_3$. THz emission time domain spectroscopy (TDS) is used to indicate the TSS contribution to the SCC by investigating the TI thickness and angular dependence of the THz emission. This work illustrates THz emission TDS as a powerful tool alongside angular resolved photoemission spectroscopy (ARPES) methods to investigate the interfacial spintronic properties of TI/ferromagnet bilayers.
Topological insulators are quantum materials involving time-reversal protected surface states making them appealing candidates for the design of the next generation of highly efficient spintronic devices. The very recent observation of large transient spin-charge conversion and subsequent powerful THz emission from Co/Bi1−xSbx bilayers clearly demonstrates such potentiality and feasibility for the near future. Among the exotic properties appearing in and at the surface of such quantum materials, spin-momentum locking and Rashba-Edelstein effects remain as key ingredients to effectively convert the spin degree of freedom into a charge or a voltage signal. In this work, we extend our analyses to the quantification of orbital momentum-locking and related orbital charge conversion effects in Bi.85Sb.15 via orbital Rashba-Edelstein effects. In that sense, we will provide some clear theoretical and numerical insights implemented by multiorbital and slab tight-binding methods to clarify our recent experimental results obtained by THz spectroscopy in the time domain. Published by the American Physical Society 2024
We present magnetotransport measurements as well as micromagnetic simulations of magnetization reversal in magnetic nanowires having strong transverse magnetocrystalline anisotropy. The interplay between exchange, demagnetizing, and magnetocrystalline energies gives rise to a micromagnetic configuration involving vortices with alternative chirality along the wire. Despite the complexity of the field angle-dependent magnetization reversal process, a very good agreement is obtained between experiments and simulations. This provides evidence that the reported magnetization process appears generally in nanowires with strong transverse magnetocrystalline anisotropy. Moreover an analytical expression is established for the angular dependency of the core switching field. The simulations indicate the occurrence of Bloch points during the core reversal.
The development of spin light emitting diode (spin‐LED) with spin injectors with perpendicular magnetic anisotropy (PMA) is a prerequisite for the conversion of carrier spin polarization to the circular polarization (PC) of photon without magnetic field for practical applications. In our previous study, a maximum PC at zero field (20% at 25K, 8% at 300K) was reported with an ultrathin perpendicular MgO/CoFeB spin injector with Ta capping layer [1]. To achieve a good PMA, post‐annealing is indispensable with a narrow optimal window around 250°C. Recent work shows that the PMA can be increased by 20%, by replacing Ta layer with a Mo layer [2]. Moreover, Mo capping increases the temperature stability of the spin injector, allowing post annealing up to 425°C. The specific crystal structure, local chemistry and local bonding of all staking layer that make up the perpendicular spin injector remain unclear, hindering the establishment of the relationships between the material properties and the nanoscale structure. The interfacial anisotropy depends critically on the crystal structure. The composition, chemical states, and local defects in CoFeB and in MgO are known to be critical for transport and magnetic properties. Experimentally the structural and chemical issues in CoFeB‐MgO have been addressed utilising HRTEM, EDS, XPS, secondary‐mass spectroscopy. A consistent picture has not emerged yet due to the difficulty of probing local atomic details. In particular there is many contradictory results on the fate of B following annealing, including diffusion into Ta [3], segregation at the CoFe interface [4], B diffusion into MgO forming a magnesium boride phase [5]. Here, we combine HRTEM, aberration corrected STEM (HAADF and BF) and spatially resolved EELS to follow the structure and the local chemistry of MgO\ CoFeB capped with Ta or Mo, before and after annealing up to 400°C. The spin injectors consist in 2.5 nm of MgO / 1.2 nm CoFeB / 5 nm Ta or Mo deposited on a GaAs‐based LED (Fig 1). For FeCoB layer caped with Ta at temperature higher than 250°C, annealing favours the diffusion of B into Ta when CoFe crystallises then Ta diffuses trough the CoFe layer into the MgO barrier (Fig 2). For CoFeB layer caped with Mo, boron stays in the CoFe layer close to the CoFe‐Mo interface (Fig 3). Mo diffusion into the CoFe layer also occurs, but for higher temperature than for Ta diffusion. In extension we'll show that for both structures, annealing has a strong influence on the Fe/Co ratio at the MgO‐CoFe interface and at the CoFe‐capping layer (Fig 2 and 3). The influence of the structure and of local chemical composition on the properties of the spin injector will be discussed.
Perpendicularly magnetized spin injector with high Curie temperature is a prerequisite for developing spin optoelectronic devices on 2D materials working at room temperature (RT) with zero applied magnetic field. Here, we report the growth of Ta/CoFeB/MgO structures with a large perpendicular magnetic anisotropy (PMA) on full coverage monolayer (ML) MoS2. A large perpendicular interface anisotropy energy of 0.975mJ/m2 has been obtained at the CoFeB/MgO interface, comparable to that observed in magnetic tunnel junction systems. It is found that the insertion of MgO between the ferromagnetic metal (FM) and the 2D material can effectively prevent the diffusion of the FM atoms into the 2D material. Moreover, the MoS2 ML favors a MgO(001) texture and plays a critical role to establish the large PMA. First principle calculations on a similar Fe/MgO/MoS2 structure reveal that the MgO thickness can modify the MoS2 band structure, from an indirect bandgap with 7ML-MgO to a direct bandgap with 3ML-MgO. Proximity effect induced by Fe results in a splitting of 10meV in the valence band at the Γ point for the 3ML-MgO structure while it is negligible for the 7ML-MgO structure. These results pave the way to develop RT spin optoelectronic devices on 2D transition-metal dichalcogenide materials.
Using an advanced tight-binding approach, we estimate the anisotropy of the tunnel transmission associated with the rotation of the 5/2 spin of a single Mn atom forming an acceptor state in GaAs and located near an AlGaAs tunnel barrier. Significant anisotropies in both in-plane and out-of-plane geometries are found, resulting from the combination of the large spin-orbit coupling associated with the p-d exchange interaction, cubic anisotropy of heavy-hole dispersion and the low C2v symmetry of the chemical bonds.
We have measured the inverse spin Hall effect (ISHE) in \textit{n}-Ge at room temperature. The spin current in germanium was generated by spin pumping from a CoFeB/MgO magnetic tunnel junction in order to prevent the impedance mismatch issue. A clear electromotive force was measured in Ge at the ferromagnetic resonance of CoFeB. The same study was then carried out on several test samples, in particular we have investigated the influence of the MgO tunnel barrier and sample annealing on the ISHE signal. First, the reference CoFeB/MgO bilayer grown on SiO$_{2}$ exhibits a clear electromotive force due to anisotropic magnetoresistance and anomalous Hall effect which is dominated by an asymmetric contribution with respect to the resonance field. We also found that the MgO tunnel barrier is essential to observe ISHE in Ge and that sample annealing systematically lead to an increase of the signal. We propose a theoretical model based on the presence of localized states at the interface between the MgO tunnel barrier and Ge to account for these observations. Finally, all of our results are fully consistent with the observation of ISHE in heavily doped $n$-Ge and we could estimate the spin Hall angle at room temperature to be $\approx$0.001.
Optically excited THz spintronic emitters have become promising THz sources, permitting a gapless broadband spectrum. We experimentally investigate THz spintronic emission based on the spin Hall effect in ferromagnetic/metal heterostructures with different femtosecond pulse excitations. We demonstrate the combination of high efficiencies (>10 -5 ) and ultrabroad emission with a 23 fs high power Ytterbium laser.
The emission of circularly polarized light from a single quantum dot relies on the injection of carriers with well-defined spin polarization. Here we demonstrate single dot electroluminescence (EL) with a circular polarization degree up to 35% at zero applied magnetic field. The injection of spin-polarized electrons is achieved by combining ultrathin CoFeB electrodes on top of a spin-LED device with p-type InGaAs quantum dots in the active region. We measure an Overhauser shift of several microelectronvolts at zero magnetic field for the positively charged exciton (trion X+) EL emission, which changes sign as we reverse the injected electron spin orientation. This is a signature of dynamic polarization of the nuclear spins in the quantum dot induced by the hyperfine interaction with the electrically injected electron spin. This study paves the way for electrical control of nuclear spin polarization in a single quantum dot without any external magnetic field.
