Department of Physics, University of Idaho, Moscow, ID 83844, USA.
Nanoscale. 2013 Sep 7;5(17):7872-81. doi: 10.1039/c3nr02247a.
Cr-doped core-shell iron/iron-oxide nanoparticles (NPs) containing 0, 2, 5, and 8 at.% of Cr dopant were synthesized via a nanocluster deposition system and their structural and magnetic properties were investigated. We observed the formation of a σ-FeCr phase in 2 at.% of Cr doping in core-shell NPs. This is unique since it was reported in the past that the σ-phase forms above 20 at.% of Cr. The large coercive field and exchange bias are ascribed to the antiferromagnetic Cr2O3 layer formed with the Fe-oxide shell, which also acts as a passivation layer to decrease the Fe-oxide shell thickness. The additional σ-phase in the core and/or Cr2O3 in the shell cause the hysteresis loop to appear tight waisted near the zero-field axis. The exchange interaction competes with the dipolar interaction with the increase of σ-FeCr grains in the Fe-core. The interaction reversal has been observed in 8 at.% of Cr. The observed reversal mechanism is confirmed from the Henkel plot and delta M value, and is supported by a theoretical watermelon model based on the core-shell nanostructure system.
Cr 掺杂核壳结构铁/氧化铁纳米颗粒（NPs）（分别含有 0、2、5 和 8 原子百分比的 Cr 掺杂剂）通过纳米团簇沉积系统合成，并对其结构和磁性进行了研究。我们观察到在 2 原子百分比的 Cr 掺杂的核壳 NPs 中形成了 σ-FeCr 相。这是独特的，因为过去曾报道过σ相在超过 20 原子百分比的 Cr 中形成。大矫顽力和交换偏置归因于与 Fe-氧化物壳形成的反铁磁 Cr2O3 层，它也作为钝化层来减小 Fe-氧化物壳的厚度。核心中的额外 σ 相和/或壳中的 Cr2O3 导致磁滞回线在零场轴附近出现紧腰。随着 Fe 核中 σ-FeCr 晶粒的增加，交换相互作用与偶极相互作用竞争。在 8 原子百分比的 Cr 中观察到了反转。从 Henkel 图和 delta M 值证实了观察到的反转机制，并得到了基于核壳纳米结构系统的理论西瓜模型的支持。
