Solution-based synthesis of nickel-substituted Co₂TiO₄ and its consequences on the magnetic property

The current manuscript deals with the magnetic property changes induced by substituting nickel in place of cobalt in Co₂TiO₄ by preparing the samples from a single-step calcination process. Monophasic cubic spinels bearing the compositions Co_1.80Ni_0.20TiO₄, Co_1.40Ni_0.60TiO_4, and CoNiTiO₄ were formed, and higher nickel contents of more than 50 mol% in Co_2–2xNi_2xTiO₄ resulted in biphasic products consisting of spinel and NiO. The successful structural refinements of the X-ray diffraction patterns in the Fd-3 m space group revealed a gradual decrease in the lattice parameter with increasing nickel content. Raman spectra augmented the stabilization of cubic spinel, exhibiting all the fingerprint modes. Optical transitions corresponding to Ti³⁺, Co²⁺, and Ni²⁺ were noted in the UV–visible spectra of nickel-substituted samples. XPS analysis of CoNiTiO₄ confirmed the presence of Co²⁺, Ni²⁺, Ti⁴⁺ (77%), and Ti³⁺ (23%). Both the temperature and field-dependent magnetic measurements were performed on the nickel-substituted samples. While Co_1.80Ni_0.20TiO₄, Co_1.40Ni_0.60TiO₄ remained paramagnetic at 100 K, CoNiTiO₄ showed ferromagnetic ordering. At 2 K, all three samples showed ferromagnetic hysteresis. In the temperature-dependent measurements, all samples show divergence in the zero field-cooled and field-cooled conditions, showing negative magnetization. Higher magnetic fields shifted the temperature at which the divergence occurred in the case of CoNiTiO₄. The spin-only magnetic moment values decreased due to the gradual increase in nickel content, which had weaker magnetizing power than cobalt.