Dilute magnetic semiconductors (DMS) based on II-VI compounds represent a unique class of materials where magnetic ions are substitutionally doped into a non-magnetic semiconductor host. These materials, such as Mn-doped CdTe or Fe-doped ZnSe, exhibit intriguing magnetic and optical properties due to the interaction between localized magnetic moments and charge carriers. Unlike their III-V counterparts, II-VI DMS typically display paramagnetic or spin-glass behavior rather than long-range ferromagnetic ordering, primarily due to weaker sp-d exchange interactions. This article explores the fundamental aspects of II-VI DMS, focusing on their magnetic behavior, crystal field effects, doping limitations, and distinctive optical phenomena such as giant Zeeman splitting.

The magnetic properties of II-VI DMS are governed by the exchange interaction between the localized d-electrons of transition metal ions (e.g., Mn²⁺, Fe²⁺) and the delocalized sp-band electrons of the host semiconductor. In materials like Mn:CdTe, the Mn²⁺ ions substitute for Cd²⁺ sites, introducing localized magnetic moments. The sp-d exchange interaction in II-VI materials is generally weaker than in III-V DMS, leading to paramagnetic behavior at higher temperatures and spin-glass transitions at low temperatures. The spin-glass state arises from competing ferromagnetic and antiferromagnetic interactions between randomly distributed magnetic ions, resulting in a frozen, disordered magnetic configuration below a critical temperature. For example, in Cd₁₋ₓMnₓTe, spin-glass transitions are observed at temperatures below 5 K for x ≈ 0.17, while paramagnetic behavior dominates at higher temperatures.

Crystal field effects play a crucial role in determining the electronic and magnetic properties of II-VI DMS. The tetrahedral coordination of the host lattice splits the d-orbitals of the transition metal ions into higher-energy e_g and lower-energy t₂ states. The occupancy and splitting of these states influence the magnetic anisotropy and the strength of the exchange interaction. In Mn-doped II-VI semiconductors, the high-spin d⁵ configuration of Mn²⁺ results in a half-filled t₂ shell, leading to a relatively isotropic magnetic response. However, for ions like Fe²⁺ (d⁶), the crystal field splitting and Jahn-Teller distortions can introduce additional anisotropy, affecting the spin dynamics and magnetic susceptibility.

Doping limits in II-VI DMS are constrained by several factors, including solubility limits, lattice strain, and the formation of secondary phases. For instance, in Zn₁₋ₓMnₓSe, the maximum Mn concentration achievable without phase separation is typically around x ≈ 0.30. Beyond this limit, the strain induced by the size mismatch between Mn²⁺ and Zn²⁺ ions promotes the precipitation of MnSe clusters or other secondary phases. Similarly, in Cd₁₋ₓMnₓTe, the solubility limit is approximately x ≈ 0.70, but high Mn concentrations often lead to increased disorder and inhomogeneous magnetic properties. These doping constraints directly impact the strength of the sp-d exchange interaction and the resulting magnetic behavior.

Optical properties of II-VI DMS are profoundly influenced by the exchange interaction between magnetic ions and band carriers. One of the most striking phenomena is giant Zeeman splitting, where an external magnetic field induces large energy shifts in the band edges due to the exchange interaction. In Mn-doped II-VI materials, the giant Zeeman splitting can be orders of magnitude larger than in non-magnetic semiconductors. For example, in Cd₁₋ₓMnₓTe, the conduction and valence band splitting can exceed 100 meV at low temperatures under a magnetic field of a few tesla. This effect arises from the strong exchange coupling between the Mn²⁺ d-electrons and the sp-band electrons, leading to a large effective g-factor for the carriers. The giant Zeeman splitting has been exploited in magneto-optical devices and spin-polarized carrier injection studies.

The contrast between II-VI and III-V DMS is particularly evident in their magnetic ordering. III-V DMS, such as Ga₁₋ₓMnₓAs, often exhibit ferromagnetism with Curie temperatures reaching up to 200 K, driven by hole-mediated exchange interactions. In contrast, II-VI DMS rarely show ferromagnetic ordering unless co-doped with additional charge carriers or subjected to specific growth conditions. The absence of ferromagnetism in II-VI materials is attributed to the lack of sufficient free carriers to mediate long-range magnetic interactions and the weaker p-d exchange compared to the sp-d exchange in III-V systems. Instead, II-VI DMS are characterized by their rich paramagnetic and spin-glass physics, making them ideal for studying fundamental magnetic interactions in semiconductors.

The optical response of II-VI DMS also differs from III-V materials due to their direct bandgap and stronger excitonic effects. In Mn-doped ZnSe or CdTe, the exciton binding energy is enhanced by the exchange interaction with Mn²⁺ ions, leading to pronounced excitonic features in absorption and photoluminescence spectra. The exciton-Mn interaction also gives rise to magnetic polaron formation, where the localized spins align around the exciton, further modifying the optical properties. These effects are less prominent in III-V DMS due to their typically lower exciton binding energies and different exchange mechanisms.

In summary, II-VI dilute magnetic semiconductors offer a fascinating platform for exploring the interplay between magnetism and semiconductor physics. Their paramagnetic and spin-glass behaviors, governed by weaker sp-d exchange interactions, contrast sharply with the ferromagnetism observed in III-V DMS. Crystal field effects and doping limits further shape their electronic and magnetic properties, while phenomena like giant Zeeman splitting highlight their unique optical characteristics. Although II-VI DMS may not exhibit the high-temperature ferromagnetism desired for spintronic applications, their rich physics continues to provide valuable insights into the fundamental mechanisms of magnetic semiconductors. Future research may explore novel doping strategies or hybrid structures to enhance their magnetic properties while preserving their exceptional optical responses.