This structure utilized a single ferromagnetic material to inject electron spin into the LED, while a commercial SOT technology was employed to electrically control the magnetization direction of the ferromagnetic material. Magneto-optic Kerr effect (MOKE) microscope has observed a switching of magnetic domains (Fig. 1(d)). Based on this design, the switching degree of magnetization could be precisely regulated by finely controlling the amplitude and duration of the injected current pulse, thereby achieving multi-level circular polarization degree (P_c) modulation (Figs. 1(e) and 1(f)). Under a small external magnetic field of 10 mT at room temperature, reversible switching of P_c from +30.5% to −31% was achieved by applying a current pulse of 30 mA to change the magnetization direction of ferromagnetic materials (Fig. 1(g)). Under zero magnetic field, partial magnetization switching could still be achieved to control P_c and obtain the intermediate P_c around 12% (Fig. 1(h)). The robustness of the device was also demonstrated by repeating the switching cycles (Fig. 1(i)). This multi-level P_c modulation was expected to be applied to multi-level coding in optical communication, improving data transmission efficiency. In addition, the quantum-dot-based spin-LED shortened the distance between the spin injector and the active region to within 100 nm, greatly improving the carrier spin polarization and enhancing the ability to control the polarization of light. The stable two-state switching that could be fulfilled at zero external magnetic field hinted at its potential applications in the future.

In summary, a control of the helicity of emitted light was realized at room temperature and zero external magnetic field. An electrically switched magnetization was combined with photon spin to achieve nonvolatile spin optical information conversion, processing, and storage. This technology may also be applied to lasers. Integrating spin injectors directly into the optical cavity and close to the semiconductor optical gain region of spin lasers will minimize absorption and magnetic circular dichroism, which could improve spin injection efficiency^[9]. By utilizing the SOT spin injection concept, modulation of optical polarization can be achieved at a constant carrier density, thereby decoupling from the optical intensity. This is crucial for future high-speed and secure data transmission. In a word, this kind of spin optical device based on spin injectors and the SOT effect is expected to achieve ultrafast polarization modulation, providing a new solution for multi-level coding in optical communication and improving data transmission efficiency. However, the modulation bandwidth has not yet been studied, and further improvement of polarization is still challenging. Meanwhile, new materials and structures may also be employed^[10−12], utilizing unique electronic structures and external electric field effects to achieve more efficient electrical control of light polarization.

The data presented in Fig. 1 is based on experimental results in Ref. [8]. The circular polarization modulation was realized in a platform of GaAs-based LED system, as shown in (Fig. 1(a)). The LED device consisted of multiple layers designed to optimize spin injection and light emission. Starting with a p-GaAs (001) substrate, it was sequentially layered with a 300 nm thick p-GaAs layer, a 400 nm thick p-Al_0.3Ga_0.7As layer for p-type doping, and a 30 nm thick GaAs layer with Be δ-doping, which contained InAs quantum dots (QDs) serving as the active region for light emission. This was followed by a 50 nm thick n-GaAs layer. The device employed Ti/Au electrodes, while SiO₂ insulating layers ensured electrical isolation and guided the injection current. A spin injector was integrated on top of the n-GaAs layer, responsible for injecting spin-polarized carriers into the active region. Besides the common spin injection tunnel junction, MgO/CoFeB/Ta, a significant innovation involved incorporating a 3-nm thick layer of Cr into the injector. Fig. 1(b) displays a cross-sectional high-resolution scanning transmission electron microscopy (HR-STEM) image of the spin injector, providing detailed insights into its multilayer structure. By employing an annealing process, the CoFeB layer crystallized and formed a good epitaxial relationship with MgO and GaAs, improving the interface quality. Meanwhile, B atoms diffused into the Ta layer, which facilitated the establishment of perpendicular magnetization anisotropy at the MgO/CoFeB interface. The chemical composition of the CoFeB layer was optimized to achieve complete overlap between Co and Fe elements, avoiding the Fe enrichment and further improving the interfacial properties. The additional Cr layer offered advantages involving the following aspects: (1) It effectively reduced channel resistance to enhance the breakdown current of the Schottky barrier, which could address the semiconductor shunting issue. (2) The large spin Hall angle θ_SH of Cr (approximately −0.1 measured by the tunneling spin Hall spectroscopy) and the same sign of θ_SH with Ta could enhance the SOT for magnetization switching (Fig. 1(c)). (3) Although the generated orbital current by Cr has an opposite sign to its spin Hall angle, the SOT effect could also be enhanced through a negative orbital-to-spin current conversion process facilitated by the inserted 3-nm thick Ta layer.

The revolution in information sharing is fundamentally supported by the highly efficient processing, storage, and transmission of data^[1]. For the latter, energy consumption continuously increases with the rapid development of information and communication technology^[2]. This highlights the importance of developing an integrated multifunctional single-chip platform that combines magnetic storage, optical transmission, and CMOS (complementary metal oxide semiconductor) processing to achieve higher energy efficiency in optical communication. As an intrinsic feature of the photon, spin angular momentum offers a solution to realize this idea. The direct coupling of photon and electron spin serves as an effective source of spin information, surmounting the constraints posed by spin imbalance which typically restrict the scale to typical spin diffusion length^[3]. As developed spin-polarized lights can be applied in fields such as quantum information processing, bioimaging, and three-dimensional (3D) display^[4-6]. At present, the foundation of information transfer and processing lies mostly in controlling the intensity of emitted light and charge current. Nevertheless, robust information storage and magnetic random-access memories were achieved through the utilization of carrier spin and the corresponding magnetization in ferromagnets^[7]. The gap that needs to be bridged between photonics, electronics, and spintronics is the electrically controlled magnetization to modulate the circular polarization of emitted light. The solution to this problem may pave the way for more advanced and energy-efficient information technologies. Recently, Pambiang et al. employed electrically controlled magnetization in the modulation of photon spin to develop the spin light emitting diode (LED) at room temperature and zero external magnetic field^[8] (Nature 2024, https://doi.org/10.1038/s41586-024-07125-5). The experimental setup consisted of a semiconductor LED and a spin injector, where the spin injector generated spin-polarized carriers to regulate the circular polarization of the emitted light. Spin−orbit torque (SOT) effect was introduced to switch the magnetization state of the spin injector, thereby control the spin orientation of injected carriers and alter the helicity of the light emitted from the LED.