Ong Quy Khac, Lin Xiao-Min, Wei Alexander
Department of Chemistry, 560 Oval Drive, Purdue University, West Lafayette, IN 47907-2084.
J Phys Chem C Nanomater Interfaces. 2011 Jan 24;115(6):2665-2672. doi: 10.1021/jp110716g.
Core-shell Fe@Fe(3)O(4) nanoparticles exhibit substantial exchange bias at low temperatures, mediated by unidirectionally aligned moments at the core-shell interface. These spins are frozen into magnetic alignment with field cooling, and are depinned in a temperature-dependent manner. The population of such frozen spins has a direct impact on both coercivity (H(C)) and the exchange-bias field (H(E)), which are modulated by external physical parameters such as the strength of the applied cooling field and the cycling history of magnetic field sweeps (training effect). Aging of the core-shell nanoparticles under ambient conditions results in a gradual decrease in magnetization but overall retention of H(C) and H(E), as well as a large increase in the population of frozen spins. These changes are accompanied by a structural evolution from well-defined core-shell structures to particles containing multiple voids, attributable to the Kirkendall effect. Energy-filtered and high-resolution transmission electron microscopy both indicate further oxidation of the shell layer, but the Fe core is remarkably well preserved. The increase in frozen spin population with age is responsible for the overall retention of exchange bias, despite void formation and other oxidation-dependent changes. The exchange-bias field becomes negligible upon deliberate oxidation of Fe@Fe(3)O(4) nanoparticles into yolk-shell particles, with a nearly complete physical separation of core and shell.
核壳结构的Fe@Fe(3)O(4)纳米颗粒在低温下表现出显著的交换偏置，这是由核壳界面处单向排列的磁矩介导的。这些自旋通过场冷被冻结成磁排列，并以温度依赖的方式解钉扎。这种冻结自旋的数量对矫顽力（H(C)）和交换偏置场（H(E)）都有直接影响，它们会受到外部物理参数的调制，如施加的冷却场强度和磁场扫描的循环历史（训练效应）。核壳纳米颗粒在环境条件下的老化导致磁化强度逐渐降低，但H(C)和H(E)总体保持不变，同时冻结自旋的数量大幅增加。这些变化伴随着从明确的核壳结构到包含多个空隙的颗粒的结构演变，这归因于柯肯达尔效应。能量过滤和高分辨率透射电子显微镜都表明壳层进一步氧化，但铁核保存得非常好。尽管形成了空隙和其他与氧化相关的变化，但随着年龄增长冻结自旋数量的增加是交换偏置总体保持的原因。当将Fe@Fe(3)O(4)纳米颗粒有意氧化成蛋黄壳颗粒，使核和壳几乎完全物理分离时，交换偏置场变得可以忽略不计。
