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Nickel-cobalt-titanate thin films - new sustainable magnetic oxides

Single phase nickel-cobalt-titanate thin films with a formula A1+2xTi1-xO3, where A is Ni2+,Co2+ and -0.25<x<1, were grown by pulsed laser deposition on sapphire substrates. There is a large window in which both Ni/Co ratio and x can be chosen independently. In the prototype ilmenite and corundum structures one third of the octahedra are vacant. The reported structure is obtained by filling vacant (x>0) or emptying filled (x<0) octahedra. When x = 1 all octahedra are filled. Two factors controlling the magnetism and crystal distortion are identified. First is a direct overlap between the adjacent cation d-orbitals resulting in a bond formation and magnetic interactions between the cations. This is most clearly revealed as a crystal distortion in the x approximately 0 compositions with approximately equal amounts of Ni and Co: the distortion of the x approximately 0 compound is a function of Ni/Co ratio. The second factor is x, which controls the cation shift towards a vacant octahedron. The displacement decreases and the symmetry increases with decreasing Ti content as was revealed by x-ray diffraction and Raman spectroscopy. When all octahedra are filled the cations prefer octahedron center positions. Also the number density of cations has increased by a factor of 50 percent when compared to the ilmenite structure. The number density ratios of Ni/Co cations between x=1 and x=0 compounds is 3. The Raman and x-ray diffraction data collected on samples with x = 1 or close to 1 are interpreted in terms of P63/mmc space group.