Abstract
Several experimental works currently demonstrate that metallic nano-oxides and carbon nanomaterials expected to be diamagnets, in fact, behave as ferromagnets at room temperature. More than scientifically intriguing, this unconventional and unexpected ferromagnetism pave the way for innovation products and novel nanotechnological applications, gathering the magnetism to interesting functionalities of these nanomaterials. Here, we investigate the non-conventional ferromagnetism observed at high temperatures in nanocrystalline cerium dioxide (CeO2or nanoceria) thin films that are optically transparent to visible light. Nanoceria exhibits several concrete applications in catalytic processes, photovoltaic cells, solid-state fuel cells, among others, which are mostly due to natural presence of oxygen vacancies and easy migration of the oxygen through the structure. The ferromagnetism in non-stoichiometric nanocrystaline ceria can be consistently described by ab initio electronic structure calculations, which support that oxygen vacancies cause the formation of magnetic moments and can provide a robust interconnectivity within magnetic polarons theoretical framework. Additionally, we present a conceptual model to account the oxygen transport to the non-conventional ferromagnetism at temperatures well above room temperature. The approach is complementary to the thermally-activated effective transfers of charge and spin around oxygen vacancy centers.
Highlights
Excellent performance of CeO2 - based materials for extensive applications has attracted much attention for decades[1,2,3,4,5,6]
We demonstrate that room temperature ferromagnetism in nanoceria, arising from a defect chemistry dominated by VO centers with polaronic-like character, is consistently described by electronic band structure calculations
We settled down that the migration, of the VO centers and oxygen diffusion, are both crucial to understand this unconventional ferromagnetism at high temperatures
Summary
Excellent performance of CeO2 - based materials for extensive applications has attracted much attention for decades[1,2,3,4,5,6] Their physical properties come mostly from point defects consisting on missing ions (vacancies), excess ions (interstitial) or foreign kind ions (substitutional dopants). The electron trapped, by its self-induced short-range forces, in a region of the order of a lattice constant, give rise to a so-called small polaron. Such a polaron bound to a charged vacancy is called a bound magnetic polaron, since the electrons (in this case, left behind by vacant oxygen) are localized on spin polarized Ce 4f sites[10,11]. Isolated point defects and their configurations often occur in the metallic oxides together with a local magnetic moment, a direct or indirect magnetic coupling mechanism between magnetic moments is not at all evident for the stabilization of the long-range ferromagnetic order[13]
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