Magnetic and EPR Characterization of Ni(core)/NiO(shell) Nanoparticles

Core/shell nanoparticles have interesting applications in various fields [1-4]. Among these systems, ferromagnet(core)/antiferromagnet(shell) are of particular importance because of their potential use as MRI contrasting agents, high density magnetic recording devices, etc. [3, 4]. Here, Ni(core)/NiO(shell) nanoparticles of different core sizes ranging from 8 nm to 22 nm have been synthesized and characterized by TEM, magnetic susceptibility and electron paramagnetic resonance (EPR) spectroscopy, as ``free'' (non-agglomerated) particles and agglomerated clusters. Using EPR at 9.1 GHz it is shown that the temperature dependence of the g-value and the EPR linewidth are similar for both the free particles and the agglomerated clusters. This suggests that at this magnetic field ($\sim $0.3 T) the EPR signal arises mostly from the saturated magnetic moment. EPR measurements at multiple fields/frequencies provide further insight on the microscopic magnetic structure in the free particles and the agglomeration effects. [1]. M. A. Hines, P. Guyot-Sionnest, \textit{J. Phys. Chem.}, $100$, 468 (\textbf{1996}). [2]. Z.C. Xu, Y.L. Hou, S.H. Sun, \textit{J. Am. Chem. Soc.}, $129$, 8698 (\textbf{2007}). [3]. V. Skumryev, S. Stoyanov, Y. Zhang, G. Hadjipanayis, D. Givord, J. Nogues, \textit{Nature} $423$, 850 (\textbf{2003)}. [4]. A. H\"{u}tten, D. Sudfeld, I. Ennen, G. Reiss, W. Hachmann, U. Heinzmann, K. Wojczykowski, P. Jutzi, W. Saikaly, G. Thomas\textit{ J. Biotechnology} $112$, 47-63 (\textbf{2004}).