Cation ratio and oxygen defects for engineering the magnetic transition of monodisperse nonstoichiometric zinc ferrite nanoparticles

非化学计量比锌铁氧体中阳离子比与氧缺陷对其磁性转变的调控研究

Monodisperse nonstoichiometric zinc ferrite nanoparticles with a tunable size of 4.1–32.2 nm are fabricated via thermal decomposition. An extrinsic impurity phase of the ZnO component is present in the zinc ferrite nanoparticles with a size of <10 nm, but this phase can be eliminated after the air annealing treatment. The atom ratio of Zn/Fe and concentration of oxygen vacancies decrease as the particle size of zinc ferrite increases, causing magnetic transition from superparamagnetism to ferromagnetism. The X-ray magnetic circular dichroism spectra reveal that the spin magnetic moments of Fe^(3+)are reduced, and the orbital magnetic moments are frozen with the increasing atom ratio of Zn/Fe. Therefore,saturation magnetization decreases. The saturation magnetizations of all the zinc ferrite nanoparticles decrease after the air annealing treatment, suggesting that oxygen vacancies considerably influence the magnetic properties. The air annealing treatment can minimize the number of oxygen defects,which trigger some of the Fe^(3+)–OV–Fe^(3+)ferrimagnetic couplings to transfer into the Fe^(3+)–O^(2-)–Fe^(3+)antiferromagnetic couplings. This work provides new insights regarding the magnetic performance of spinel ferrites by tuning the stoichiometric ratio and oxygen defects.

本文采用热解法制备了粒径在4.1–32.2 nm范围内可调的单分散非化学计量比锌铁氧体纳米颗粒.当颗粒尺寸小于10 nm时,样品中含有少量的非本征ZnO杂质相,且空气退火后该杂质相消失.锌铁氧体中锌/铁原子比与氧缺陷浓度均随着颗粒尺寸的增大而降低,导致其从超顺磁性转变为铁磁性.磁性圆二色谱表明,随着Zn/Fe比的增加,Fe^(3+)的自旋磁矩减小,轨道磁矩冻结,饱和磁化强度降低.经过空气退火,所有样品的饱和磁化强度降低,表明氧缺陷(OV)对其磁性有很大影响.空气退火会降低氧缺陷含量,部分Fe^(3+)–O_(V)–Fe^(3+)铁磁耦合转变为Fe^(3+)–O^(2-)–Fe^(3+)反铁磁耦合.该工作通过调控化学计量比和氧缺陷实现了对锌铁氧体磁性的调节,为理解和调控铁氧体的磁学性质提供了新的思路.