Ferromagnetic resonance study in exchange-coupled core/shell Ni/NiO Nanoparticles
Ashish Chhaganlal Gandhi1*, Jauyn Grace Lin1
1Center for condensed matter sciences, National Taiwan Univeristy, Taipei, Taiwan
* presenting author:Ashish Gandhi, email:acg.gandhi@gmail.com
Ferromagnetic resonance (FMR) is commonly used to study the magnetic anisotropy in nano-magnetic materials. In FMR experiments, the magnetic interfacial interaction in exchange-coupled ferromagnetic (FM)/ antiferromagnetic (AFM) core/shell nanoparticles (Nps) can be measured by the processing of FM magnetization. In particular, a sudden drop in the resonance field (Hr) and enhanced line-width (ΔHr) observed in low temperature from exchange-coupled nanostrucutres is assumed to originate from the change of anisotropy in the FM layer. However, a rise of Hr at low temperature has been reported due to either interfacial perpendicular magnetic anisotropy or the presence of inhomogeneity at the interface of FM and AFM. To address above issue we have synthesized Ni/NiO core/shell (fixed core diameter ~ 30 nm with varying shell thickness from 8 to 12 nm) Nps. The FMR spectra of all Ni/NiO core/shell Nps are deconvoluted using sum of three Gaussian function. The resulting effective g-factor at room temperature for the 30nm/12nm Ni/NiO Nps are g₁ = 2.12, g₂ = 2.88, and g₃ = 8.71. The value of g₁ can be assigned to uncompensated Ni-moment at the interface of Ni/NiO, g₂ to metallic Ni-core and g₃ = 8.71 to inhomogeneously broadened asymmetric line originating from inter-particle interactions. In our temperature dependent data we observed a drop and then rising behavior of g2/g3 below Néel temperature of NiO. The effect becomes more pronounced with the increase of shell thickness, which can be explained by considering the presence of frozen spin. The magnetic frozen spins could be caused by Ni2+ vacancies (as confirmed from static magnetic measurements). The details about the analysis of FMR results will be discussed further at the time of presentation.

Acknowledgement
The financial support of this project is partly from National Taiwan University with the project no. 104R4000.

Contact person: jglin@ntu.edu.tw (JGL)


Keywords: Ni/NiO core/shell, Exchange bias, Ferromagnetic Resonance