On the room temperature ferromagnetism in Co-doped TiO2 films

We report detailed studies of magnetization (M) as a function of magnetic field H (up to 50 kOe) and temperature T (5–380 K) in $\mathrm{x\%}$ ${\mathrm{Co/TiO}}_2$ (rutile) films for $\text{x=1,}$ 4, 6, 10, and 12. These films, prepared by sputtering, show room temperature ferromagnetism (RTFM). Measurements of M vs T at $\text{H=50\hspace{0.17em}}\mathrm{Oe}$ reported here in zero-field-cooled (ZFC) and field-cooled (FC) cases show M peaking at $T_p\text{≃30\hspace{0.17em}}\mathrm{K}$ for 1% Co to $T_p\text{≃125\hspace{0.17em}}\mathrm{K}$ for 12% Co doped films whereas bifurcation of the FC and ZFC data is observed near 300 K. For ${\mathrm{T<T}}_p,$ the coercivity $H_c$ rises sharply. These observations and the electron magnetic resonance data suggest that the observed RTFM is at least partly due to magnetic nanoparticles, most likely of Co, that are undetected in standard X-ray diffraction (XRD) for lightly doped samples. XRD studies show that a fraction of Co does substitute for Ti in ${\mathrm{Co}}_x{\mathrm{Ti}}_{\text{1−x}}{\mathrm{O}}_2.$ However, because of interference from Co nanoparticles, the RTFM of ${\mathrm{Co}}_x{\mathrm{Ti}}_{\text{1−x}}{\mathrm{O}}_2$ in these samples could not be established.