MOdulating Spin currents with Electrically driven Spin accumulation (MOSES)
Descrizione
The manipulation of spin currents for information and communication technologies is the goal of spintronics, just as the control of charge currents is the key component of electronics. However, controlling spin currents with external stimuli has proven elusive. Unlike electrical charge, spin is not a conserved physical quantity in solids, and it interacts weakly with externally applied fields. Moreover, spin injection in solids is an inefficient process. In classical spintronic devices, spin-polarized currents are obtained by exploiting the exchange interaction between conduction electron spins and magnetic materials. An innovative approach (spin-orbitronics) instead exploits spin-orbit coupling (SOC) in nonmagnetic materials to generate, detect, or exploit spin-polarized currents.
Positioning itself at the forefront of spin-orbitronics, the MOSES project aims to develop innovative spintronic devices using materials (Si, Ge, and SiGe) and fabrication processes fully compatible with mainstream silicon technology, in which SOC could be exploited for the efficient generation, detection, and manipulation of spin currents.
MOSES is based on a nonlocal spin injection/spin detection scheme developed by the PI’s unit, where a very efficient spin injection at the source electrode is achieved by optical orientation, a phenomenon consisting in the SOC-mediated angular momentum transfer from circularly polarized light to the photoexcited conduction electrons. The SOC-mediated spin-to-charge conversion, known as the inverse Spin-Hall effect, is used for spin detection. We have recently demonstrated that such an approach can induce a significant spin voltage, the spin counterpart of an electrostatic voltage, at distances of a few μm, compatible with multiterminal spintronic devices. MOSES will extend the functionality of the demonstrated spin injection/detection scheme by adding electrical modulation of spin currents. In our most recent work, we have investigated a route to spin current modulation. We propose to selectively control the spin voltage for opposite spin currents by exploiting the spin accumulation resulting from SOC-induced charge-to-spin conversion (Spin-Hall effect, SHE). SHE is achieved by flowing a charge current in a nm-thick heavy metal film in the vicinity of the channel where the spin current is confined. Preliminary work suggests that detectable modulation of the spin current is possible. MOSES will therefore investigate device geometry and materials to enhance such spin-by-spin modulation. Adding such functionality to our platform is clearly a high-risk, high-reward proposition.
Finalità
MOSES aims to demonstrate spin-by-spin modulation for devices operating at room temperature. This result would be a crucial step towards the demonstration of complex CMOS-compatible devices that combine channeling and distribution of spin currents in semiconductors with enhanced functional blocks for spin-charge conversion. In this context, MOSES starts from a solid scientific foundation, but is also well aligned with cutting-edge research trends that are struggling to find innovative solutions to enhance spin-dependent effects in solids.
Although the focus of the MOSES project is to advance the state of the art in the rapidly growing field of spin transport in the presence of external stimuli from a perspective that privileges fundamental research, the investigation of spin-by-spin modulation will also open up exciting perspectives for spintronic applications. An immediate application would be the realization of lateral spin valves that could be exploited to partially block the flow of a spin population when a charge current is flowing between the source and the drain. Therefore, the demonstration of even small capabilities to control the diffusion of spin-polarized electrons will have as an immediate application the possibility of realizing spin injection schemes in semiconductors with competitive efficiencies with respect to existing electrical spin injection devices, with the further advantages of reduced power consumption and scalability to dense arrays of individually addressable devices.
Risultati attesi
The goal of the MOSES project is to achieve electrical control of a spin current by exploiting the spin accumulation electrically induced by SHE at the interface of a high-Z material and a semiconductor.
Such an ambitious goal requires the development of a technological platform with the following characteristics
- High spin current injection efficiency
- Efficient charge-spin interconversion
- Long spin diffusion length in a quantum confined 2D gas of spin carriers
These requirements will be met by developing the elctrodes and the active layer functional blocks, which we will bring together to realize a spin device fabricated using materials and processes compatible with mainstream microelectronics technology.
Stato dell’arte
Spintronics, the active control and manipulation of the spin degree of freedom in solid-state systems, has shown great potential in consumer electronics and has sparked a technological revolution in mass storage media. However, existing spin-based devices such as read heads and memory cells use spin only as an internal variable, while the terminal quantities for each individual functional element are still based on charge, requiring frequent charge-spin conversions. Consequently, a much sought-after goal in spintronics is to develop implementations in which information is manipulated directly using the spin degree of freedom. This need led Datta and Das to propose the seminal concept of the spin transistor. Their original concept exploits the SOC acting on a spin-polarized two-dimensional electron gas flowing between ferromagnetic source and drain electrodes in the presence of a transverse gate-controlled electric field. In the rest frame of the carriers, this field is relativistically transformed into a spin-preceding magnetic field, a phenomenon known as the Rashba effect, allowing the electrical control of a spin current. Although such spin manipulation in a spin-orbit field has been demonstrated in a nonlocal measurement experiment, the signal levels remain small due to limited spin polarization lifetimes and low spin injection and detection efficiencies at the source/drain contacts. The spin lifetime is reduced by momentum scattering in a strong spin-orbit field, which translates into a wobbling magnetic field in the rest frame of the carrier, randomizing the spin precession. For this reason, implementations of the spin transistor concept based on the Rashba effect can only operate at cryogenic temperatures, where momentum scattering is mitigated.
Modulation of pure spin currents can also be achieved thanks to the spin-mixing conductance at the interface between the 2D channel carrying the spin current and a magnetic material. A modulation of the nonlocal resistance is obtained at relatively high temperatures as a function of the orientation of the material magnetization with respect to the polarization of the spin current. However, magnetization reversal requires either an external magnetic field or intense local electric currents and is a power- and time-consuming process, which limits the switching time of the spin current.
MOSES will contribute to advancing the state of the art in this field by exploring an alternative way to modulate the spin current by exploiting, instead of the spins “frozen” inside a magnetic material, those that can be electrically accumulated at the interface between a high-atomic number (high-Z) material and a semiconducting channel carrying the spin current.
In fact, spin accumulation can be achieved thanks to SOC-mediated spin-charge interconversion phenomena, which are now widely studied in spintronics and require high-Z metals for better conversion efficiencies. For example, the spin-Hall effect (SHE) allows the conversion of a charge current into a transverse pure spin current, while the inverse spin-Hall effect (ISHE) induces a transverse charge current when a spin current is flowing in the solid. The most common techniques used to induce spin currents in semiconductors, namely precession-induced spin pumping and spin injection from ferromagnetic materials , suffer from detrimental interfacial effects. To overcome this problem, MOSES will employ the optical orientation technique using circularly polarized light . The advantage over purely electrical spin injection schemes is that spin-polarized electrons are directly promoted into the conduction band of the semiconductor, resulting in orders of magnitude larger spin currents.
MOSES PI has demonstrated the potential of this approach by realizing a ferromagnet-free platform where large spin currents are optically generated at room temperature and detected in a nonlocal geometry by ISHE . Furthermore, we have shown that the finite spin diffusion length in a semiconductor can be exploited to realize a scheme where spin transport is modulated at room temperature by a modest electric field. The materials of choice for such a platform are group IV semiconductors (Si, Ge, SiGe alloys). Indeed, they combine their compatibility with mainstream silicon technology with a centrosymmetric lattice, which excludes parity-forbidden spin relaxation phenomena, and a relatively small SOC, which protects the spin lifetime from momentum scattering, resulting in spin diffusion lengths in the μm range, comparable to the typical size of multi-terminal spin devices .
These premises give MOSES the ability to detect very weak spin-by-spin modulations even at room temperature and provide the best basis for the success of the project.
Riferimento: PRIN 2022 – Codice progetto: 20222LPKZR – CUP: F53D23001000006
Investimento totale del progetto: 189.475€
Partner/proponente: Politecnico di Milano (Coordinatore), Università degli Studi Roma Tre, Consiglio Nazionale delle Ricerche
Coordinatore dell’UdR Università degli Studi Roma Tre: Giovanni Capellini
