Coercivity and Exchange Bias in Ti-Doped Maghemite Nanoparticles

Ti-doped maghemite nanoparticles of average crystallite size 12.9 nm were synthesized using the sol–gel method. The XRD profile mainly showed the presence of maghemite phase with very small phases of TiO₂ (rutile and anatase). Magnetization hysteresis loops of the nanoparticles were obtained between −4 T to +4 T at temperatures of 2, 10, 30, 50, 70, 100, 150, 200, and 300 K under field cooling (FC) of 1, 2, 3, and 4 T and zero-field cooling conditions (ZFC). The coercivity displayed nonmonotonic field dependence while it decreased sharply with temperature and vanished at 150 K at all fields. Horizontal hysteresis loop shifts were observed in the 2–150 K temperature range in both the ZFC and FC conditions. The exchange bias effect became negligible in both ZFC and FC states above 50 K. Magnetization vs. applied field measurements were conducted in both ZFC and FC cooled conditions at several temperatures in the range of 2–400 K, with spin freezing being observed below 50 K. The exchange bias effect obtained below 50 K is suggested to be attributed to the competing roles of the long-range dipolar and short-range exchange coupled interactions.