title: Watermelon-like iron nanoparticles: Cr doping effect on magnetism and magnetization interaction reversal.
abstract: 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.
authors.affiliation: Department of Physics, University of Idaho, Moscow, ID 83844, USA.
authors.forename: Maninder
authors.initials: M
authors.lastname: Kaur
authors.forename: Qilin
authors.initials: Q
authors.lastname: Dai
authors.forename: Mark
authors.initials: M
authors.lastname: Bowden
authors.forename: Mark H
authors.initials: MH
authors.lastname: Engelhard
authors.forename: Yaqiao
authors.initials: Y
authors.lastname: Wu
authors.forename: Jinke
authors.initials: J
authors.lastname: Tang
authors.forename: You
authors.initials: Y
authors.lastname: Qiang