Magnetic properties of Sn- and Mn-incorporated Co₂TiO₄ from single-step calcination

This report deals with the rapid synthesis of cobalt titanate spinels (Co₂TiO₄ (CTO), Co₂Sn_0.50Ti_0.50O₄ (CSTO), and Co₂Mn_0.50Ti_0.50O₄ (CMTO) by the single-step calcination of hydroxide precursors and their extensive characterization. The cubic unit cell expanded and contracted when Ti in CTO was partially replaced with Sn and Mn, respectively. The Raman spectra confirmed the cubic spinel structure and showed a systematic shift with the inclusion of tin and manganese. The broadening of Raman bands suggested cation disorder. The absorbance spectra of CTO and CSTO proved the existence of cobalt in the +2 and +3 states with optical bandgaps of 0.6 and 1.1 eV, respectively. X-ray photoelectron spectroscopic (XPS) analysis confirmed the presence of cobalt in both the +2 and +3 states with titanium in the +3 state in CTO. While cobalt and titanium existed in the +2 and +3 states in CMTO, mixed-valence (+3 (minor) and +4 (major amounts) states) was deduced for manganese. All these samples showed an exchange-bias effect and a long-range ferrimagnetic ordering with T_N of 50 (CTO), 46 (CSTO), and 54 K (CMTO). These transitions have been independently verified from heat capacity measurements performed at zero fields and applied fields of 500 and 1000 Oe. The compensation of magnetic moments from the tetrahedral and octahedral units of the spinel structure was not observed in any of these samples, perhaps because of the asymmetric distribution of cations among the available crystallographic sites.