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Nanoscale. 2013 Sep 7;5(17):7872-81. doi: 10.1039/c3nr02247a.

Watermelon-like iron nanoparticles: Cr doping effect on magnetism and magnetization interaction reversal.

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Department of Physics, University of Idaho, Moscow, ID 83844, USA.


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.

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