Room Temperature Ferromagnetism and Enhanced DC Conductivity in Cd_0.9Cr_0.1S Nanocrystalline Thin Films

This study presents a comparative investigation of the structural, DC electrical, and magnetic properties of pure cadmium sulfide (CdS) and 10% chromium-doped CdS (Cd0.9Cr0.1S) nanocrystalline thin films deposited by thermal evaporation. X-ray diffraction analysis confirmed the polycrystalline hexagonal (wurtzite) structure of both films with no detectable secondary phases. Upon Cr doping, the full width at half maximum (FWHM) of the principal diffraction peaks increased systematically, leading to a reduction in average crystallite size from 19.8 nm to 13.9 nm, accompanied by an increase in lattice strain (from 2.48 to 3.52) × 10−3 and dislocation density (from 2.55 to 5.17 line/nm2) × 10−3. Temperature-dependent DC conductivity measurements revealed thermally activated conduction in both films, following the Arrhenius relation. Chromium doping significantly reduced the activation energy from 0.43 eV to 0.30 eV and enhanced room temperature conductivity by approximately three orders of magnitude (from 8.3 × 10−7 S/cm to 3.4 × 10−4 S/cm), attributed to the introduction of impurity states and increased carrier concentration. Most notably, magnetic measurements demonstrated a clear transition from diamagnetic behavior in pure CdS to well-defined room-temperature ferromagnetism in the (CdS)0.9Cr0.1 film, with saturation magnetization (Ms) of 1.11 emu/cm3, remanent magnetization (Mr) of 0.31 emu/cm3, and coercivity (Hc) of 220 Oe. The observed ferromagnetism is explained by carrier-mediated exchange interactions through bound magnetic polaron formation, supported by the high density of structural defects and localized states. The combination of enhanced electrical transport and room-temperature ferromagnetic ordering positions Cr-doped CdS thin films as promising candidates for spintronic and optoelectronic device applications.