Unusual magnetic behavior of TiO2-doped CuFeO2 crystals (CuFe1−xTixO2) grown by the optical floating zone method

Abstract

Synthetic Ti-doped delafossite (CuFe_1−xTi_xO₂) was prepared by solid state reaction of Fe₂O₃, CuO and TiO₂, and crystals were successfully prepared by the optical floating zone method using a modified experimental protocol that produced crystals in <10% of the time used in previous work. The effects of Ti on the microstructure and magnetic properties of CuFeO₂ were investigated. The nonlinear change of cell size may be the result of Ti⁴⁺ reduction to Ti³⁺. However, the Fe oxidation state was not affected by Ti doping, although some oxidation of Cu⁺ to Cu²⁺ was observed with the highest Ti contents. The two characteristic magnetic transition temperatures of CuFeO₂ occurred at 17 K (T_N1) and 11 K (T_N2) in the undoped crystal, and these increased to 51 K and 20 K, respectively, in the crystal with x = 0.007, although there was no major change in the two magnetic transition temperatures in crystals with other doping concentrations. In an applied magnetic field of 500 Oe, the magnetization of the x = 0.007 sample was 22 times that of CuFeO₂, and its magnetization curve shows the coexistence of regions with ferrimagnetic and antiferromagnetic properties. Furthermore, magnetic hysteresis loops revealed that the saturation magnetization (M) of the sample x = 0.007 is 4.15 emu g⁻¹, but at higher Ti concentrations the M–H curves returned to pairwise linear temperature dependence. Overall, crystals of synthetic delafossite with greatly affected magnetic properties can be prepared by doping with a limited range of Ti contents.