Antiferromagnetic-ferromagnetic crossover in UO2-TiOx multi-phase systems

An antiferromagnetic(AF)-weakly ferromagnetic(WF) crossover has been found for UO2-TiOx multi-phase systems, (1 - y)UO2 + yTiO(x) (y = 0.05-0.72. x = 0. 1.0, 1.5 and 2.0), when these mixtures are heat treated at high temperat ure in vacuum. From the powder X-ray diffraction and electron-microprobe an alyses, their phase assemblies were as follows: for x 0. 1.0 and 1.5. a het erogeneous two-phase mixture of UO2 + TiOx; for x = 2.0. that of UO2 + UTi2 O6 for y < 0.67. showing characteristic microstructures. and for y > 0.67 t hat of UTi2O6 + TiO2 (plus residual minor UO2). Magnetic susceptibility (y) of the present UO2 powder, vas confirmed to exhibit an antiferromagnetic s harp drop at T-N ( = 30.5 K). In contrast, chi of these multi-phase systems was found to exhibit a sharp upturn at the respective TN, while their TN v alues remained almost constant with varying y. This chi upturn at TN is mos t pronounced for UO2 + Ti-oxide (titania) systems (x = 1.0. 1.5 and 2.0) ov er the wide mixture ratio above y similar to 0.10. These observations indic ate that an AF-WF crossover is induced for these multi-phase systems, plaus ibly due to the interfacial magnetic modification of UO2 in contact with th e oxide partners. (C) 2001 Elsevier Science B.V. All rights reserved.