Bernardi M I B, Mesquita A, Béron F, Pirota K R, de Zevallos A O, Doriguetto A C, de Carvalho H B
Instituto de Física de São Carlos, USP - Universidade de São Paulo, 13560-970 São Carlos, SP, Brazil.
Phys Chem Chem Phys. 2015 Feb 7;17(5):3072-80. doi: 10.1039/c4cp04879b. Epub 2014 Dec 16.
Ceria (CeO2) is a promising dilute magnetic semiconductor. Several studies report that the intrinsic and extrinsic structural defects are responsible for room temperature ferromagnetism in undoped and transition metal doped CeO2 nanostructures; however, the nature of the kind of defect necessary to promote and stabilize the ferromagnetism in such a system is still a matter of debate. In the work presented here, nanorods from the system Ce1-xCuxO2-δ with x = 0, 0.01, 0.03, 0.05 and 0.10, with the more stable {111} surface exposed were synthesized by a microwave-assisted hydrothermal method. A very careful structure characterization confirms that the Cu in the samples assumes a majority 2+ oxidation state, occupying the Ce (Ce(4+) and Ce(3+)) sites with no secondary phases up to x = 0.05. The inclusion of the Cu(2+) in the CeO2 structure leads to the introduction of oxygen vacancies in a density proportional to the Cu(2+) content. It is supposed that the spatial distribution of the oxygen vacancies follows the Cu(2+) distribution by means of the formation of a defect complex consisting of Cu(2+) ion and an oxygen vacancy. Superconducting quantum interference device magnetometry demonstrated a diamagnetic behavior for the undoped sample and a typical paramagnetic Curie-Weiss behavior with antiferromagnetic interactions between the Cu(2+) ions for the single phase doped samples. We suggest that the presence of oxygen vacancies is not a sufficient condition to mediate ferromagnetism in the CeO2 system, and only oxygen vacancies in the surface of nanostructures would lead to such a long range magnetic order.
二氧化铈（CeO₂）是一种很有前景的稀磁半导体。多项研究报告称，本征和非本征结构缺陷是未掺杂和过渡金属掺杂的CeO₂纳米结构中室温铁磁性的原因；然而，在这样一个系统中促进和稳定铁磁性所需的缺陷种类的本质仍是一个有争议的问题。在本文所展示的工作中，通过微波辅助水热法合成了具有x = 0、0.01、0.03、0.05和0.10的Ce₁₋ₓCuₓO₂₋δ系统的纳米棒，其暴露的是更稳定的{111}表面。非常仔细的结构表征证实，样品中的Cu呈现主要的2 +氧化态，在x = 0.05之前占据Ce（Ce(4 +)和Ce(3 +)）位点且无第二相。CeO₂结构中Cu(2 +)的掺入导致以与Cu(2 +)含量成比例的密度引入氧空位。据推测，氧空位的空间分布通过由Cu(2 +)离子和一个氧空位组成的缺陷复合体的形成遵循Cu(2 +)分布。超导量子干涉仪磁力测量表明，未掺杂样品表现出抗磁性行为，而单相掺杂样品表现出典型的顺磁居里 - 外斯行为，Cu(2 +)离子之间存在反铁磁相互作用。我们认为，氧空位的存在不是介导CeO₂系统中铁磁性的充分条件，只有纳米结构表面的氧空位才会导致这种长程磁有序。
