Transition metal dichalcogenides (TMDCs) have emerged as a promising class of materials for spintronic applications due to their unique electronic and spin properties. Their atomically thin structure, strong spin-orbit coupling (SOC), and valley-dependent optical selection rules make them ideal candidates for exploring spin and valley physics. This article focuses on the interplay between SOC, valley polarization, and spin transport in TMDCs, along with proximity effects induced by ferromagnetic materials, which are critical for advancing spintronic technologies.

TMDCs, with the general formula MX2 where M is a transition metal (Mo, W) and X is a chalcogen (S, Se, Te), exhibit strong SOC due to the heavy transition metal atoms. The SOC splits the valence and conduction bands, leading to spin-polarized valleys at the K and K' points of the Brillouin zone. In monolayer TMDCs, the lack of inversion symmetry combined with SOC results in a large valley-dependent spin splitting, typically ranging from 100 to 500 meV depending on the material composition. For instance, monolayer MoS2 exhibits a spin splitting of approximately 150 meV in the valence band, while WSe2 shows a larger splitting of around 400 meV. This spin-valley locking enables selective optical excitation of spin-polarized carriers in specific valleys using circularly polarized light, a phenomenon known as valley polarization.

Valley polarization is a key feature of TMDCs that can be exploited for spintronic applications. Under circularly polarized excitation, one valley (K or K') becomes preferentially populated with carriers of a specific spin orientation. The degree of valley polarization can exceed 90% at low temperatures, though it decreases at higher temperatures due to intervalley scattering. The valley lifetime, which determines how long the polarized state persists, ranges from picoseconds to nanoseconds depending on the material quality and external conditions. For example, in high-quality WSe2 monolayers encapsulated in hexagonal boron nitride (hBN), valley lifetimes can extend to several nanoseconds. The ability to maintain valley polarization is crucial for designing valleytronic devices, where information is encoded in the valley degree of freedom.

Spin transport in TMDCs is influenced by SOC, defects, and interactions with the environment. The strong SOC in TMDCs leads to spin-flip scattering, which can limit spin diffusion lengths. However, recent studies have shown that spin lifetimes in TMDCs can be enhanced through careful engineering of the material and its surroundings. For instance, spin lifetimes in monolayer MoS2 have been measured to be around 1-10 ns at room temperature, with spin diffusion lengths of several micrometers in high-quality samples. The presence of defects or impurities can significantly reduce these values, highlighting the importance of defect passivation techniques such as hBN encapsulation or chemical functionalization.

Proximity effects with ferromagnetic materials offer a powerful way to manipulate spin and valley properties in TMDCs. When a TMDC is placed in contact with a ferromagnetic substrate or layer, exchange interactions can induce spin splitting and modify the valley polarization. For example, coupling monolayer MoSe2 to a ferromagnetic insulator like EuS has been shown to induce an exchange field of up to 14 T, leading to a measurable shift in the valley polarization. Similarly, proximity coupling to ferromagnetic metals such as cobalt or nickel can introduce spin-dependent scattering and modify the spin transport characteristics. These effects enable the design of hybrid spintronic devices where the magnetic properties of the ferromagnet are used to control the spin and valley states in the TMDC.

The interplay between SOC, valley polarization, and proximity effects can be harnessed for various spintronic applications. One promising direction is the development of spin-valley transistors, where the spin and valley degrees of freedom are manipulated by external electric or magnetic fields. Another application is the integration of TMDCs with ferromagnetic electrodes to create spin valves or magnetic tunnel junctions with high spin injection efficiency. The strong SOC in TMDCs also makes them suitable for spin-orbit torque devices, where spin currents generated in the TMDC can switch the magnetization of an adjacent ferromagnetic layer.

Despite the progress, challenges remain in achieving efficient spin injection, long spin diffusion lengths, and room-temperature operation in TMDC-based spintronic devices. The interface quality between TMDCs and ferromagnetic materials plays a critical role in determining the performance of these devices. Imperfections at the interface can lead to spin scattering and reduced efficiency. Advances in material synthesis, interface engineering, and device fabrication are essential to overcome these challenges and realize the full potential of TMDCs for spintronics.

In summary, TMDCs offer a rich platform for exploring spin-orbit coupling, valley polarization, and spin transport, with significant implications for spintronic applications. The strong SOC and valley-dependent properties enable novel device functionalities, while proximity effects with ferromagnetic materials provide additional control over spin and valley states. Continued research into material quality, interface engineering, and device design will be crucial for advancing TMDC-based spintronics and unlocking new opportunities in information processing and storage.