Large magnetoresistance in carbon-coated Ni/NiO nanoparticles

We report here a large magnetoresistance (MR) observed in carbon-coated Ni/NiO nanostructures synthesized by a chemical method. The crystalline nature and particle size of the graphitic-carbon-coated Ni/NiO nanostructure was investigated by X-ray diffraction study and field emission scanning electron microscope images. The Raman spectroscopy confirms the presence of graphite layer over the Ni/NiO nanostructure. The field-cooled (FC) magnetic hysteresis curves show exchange bias effect suggesting possible Ni/NiO core–shell structure. The temperature-dependent magnetization data show bifurcation in FC–zero-field-cooled curves, indicating the superparamagnetic behaviour and competing ferromagnetic (FM) and antiferromagnetic interactions in the nanocomposite. MR studies show a large negative MR of ∼\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim $$\end{document}20% at 18 K and ∼\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim $$\end{document}4.2% at room temperature, revealing significant enhancement of FM interactions at low temperatures and spin-dependent tunnelling of current through the nanocomposite.