Room temperature ferromagnetism in CoxTi1-xO2 powders made by sol-gel method: a NMR study

Co_xTi_1-xO₂ (x=2-4 at%) powders is fabricated by sol-gel method. Titanium isopropoxide and cobalt acetate are dissolved in 2methoxyethanol and ethylene glycol, following by heating, hydrolyzation, aging and drying, thus forming the precursors. A series of annealing process with same annealing temperature (450°C) and time (one hour) by different oxygen partial pressure is performed for these precursors in order to test the effect of oxygen on the formation of magnetic semiconductor. X-ray diffraction is used to determine the crystal structure and vibrating sample magnetometer is utilized to examine the magnetic properties. Results show that the sample (x=4 at%) annealed in air exhibits an evident hysteresis loop with small magnetic moment of 0.028 μ_B/per Co atom and has not saturated yet in the magnetic field of 1.2 Tesla. This means that paramagnetism exists in the sample besides ferromagnetism. On the other hand, samples annealed in vacuum have enhanced ferromagnetism in which the magnetic moment per Co atom increases and the oxygen partial pressure is reduced. Also, NMR spectrum of the vacuum-annealed samples is done and a good ratio of signal to noise is acquired which exhibits peaks located from 218 MHz to 228 MHz. This clearly indicates that cobalt clusters exist in the sample.