Van Vleck paramagnetism in undoped and Lu-doped bulk ceria

The magnetic properties of undoped, bulk CeO₂ are not fully understood. In contrast to nanocrystalline ceria that exhibits paramagnetism attributed to Ce³⁺ at grain surfaces, bulk ceria is weakly paramagnetic, despite the absence of magnetic ions. In the present work, the magnetic susceptibility of bulk ceria ceramics doped with Lu³⁺, which has neither spin nor orbital angular momentum, was measured in order to assess the relative contributions of the crystal lattice, residual Ce³⁺ and oxygen vacancies to the overall bulk magnetization. We observed a magnetic response consisting of two parts: temperature independent (5-300 K) magnetic susceptibility, and Curie-Weiss paramagnetism. The temperature independent susceptibility decreases linearly with Lu content, and becomes diamagnetic at 30 mol% Lu. The Curie-Weiss magnetism visible at low temperatures was identified as resulting from a few ppm of Fe contaminant. However, Fe contamination does not contribute to the temperature independent paramagnetism. No contribution from Ce³⁺ could be detected. The fact that the magnetization decreases with Lu content, even though the concentration of oxygen vacancies, and the lattice defects associated with them, increases, indicates that neither is coupled to the magnetic field. Weak, temperature-independent paramagnetism in non-metals is usually attributed to a second order, Van Vleck-type magnetization. However, Van Vleck paramagnetism requires that the population of the first excited state be constant within the range of temperatures investigated. We discuss possible modifications of the large band gap electronic structure of undoped ceria which could account for our observations.