Observation of two magnetic phases in single-crystalline rutile (TiO2) implanted by cobalt ions

Full text: Recently we have shown that single-crystalline rutile TiO2 heavily implanted by Co ions reveals high Curie-temperature anisotropic ferromagnetism [1,2]. However, the origin of the ferromagnetism has not been clarified yet. In this work we have implanted the rutile substrates with variable dose of the magnetic ion to study nucleation of ferromagnetism. Substrates with (100) and (001) crystallographic orientation of the surface were irradiated by Co ions with the energy 40 keV at the constant current density of 8 μA/cm2 in the dose range of 0.125-2.0x1017 ion/cm2. Magnetic properties were investigated by using differential thermo-magnetic analysis and coil magnetometric technique. The structure, surface morphology and element composition of the implanted samples were studied by XRD, atomic force microscopy (AFM) and scanning electron microscopy (SEM) techniques. Our magnetic measurements have shown that in the dose range of 0.25-0.7x1017 ion/cm2 the Co:TiO2 system reveals superparamagnetic behavior with magnetic-nonmagnetic transition at temperature of about 600 degree Celsius. The observed superparamagnetism we regard to nucleation and growth of Co nanoparticles in the irradiated layer of rutile. Ferromagnetic response appears at the dose above 0.75x1017 ion/cm2. It is characterized by lower transition temperature, Tc∼400 degree Celsius, well defined hysteresis loop, the remanence-to-saturation ratio as high as 0.3. The ferromagnetism coexists with the superparamagnetism in the dose range of 0.75-1.25x1017 ion/cm2 - the composite system shows two magnetic transition temperatures, 400 and 600 degree Celsius. At highest implantation doses the high-temperature magnetic phase is not observed, only transition to the ferromagnetic phase with low temperature takes place. From hysteresis measurements we established strong anisotropy of the low-temperature ferromagnetic phase in the plane of the sample surface. We attribute the observed dynamics of the two magnetic phase formation to the 2D magnetic percolation [3] in the Co-implanted layer. The work was supported by the RFBR grant No 04-02-97505 and RAS Programme 'New materials and Structures' [1] R.I. Khaibullin, L.R. Tagirov, B.Z. Rameev et al., J. Phys.: Cond. Matter (2004) v.16, L443. [2] B. Aktas, F. Yildiz, B. Rameev et al., Phys. Status Solidi C (2004) v.1, 3319. [3] E. Gu, S. Hope, M. Tselepi, and J.A.C. Bland, Phys. Rev. B (1999) v.60, 4092