Superparamagnetic Behavior of Granular Co-C Films Consisting of Nanocrystalline Cobalt Encapsulated in Carbon
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Superparamagnetic Behavior of Granular Co-C Films Consisting of Nanocrystalline Cobalt Encapsulated in Carbon Hao Wang*1, S.P. Wong*, W.Y. Cheung*, N. Ke*, M.F. Chiah*, G.H. Wen** and X.X. Zhang** *Department of Electronic Engineering and Materials Science and Technology Research Center, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, China 1 Institute of Thin Films and Nanomaterials, Department of Mathematics and Physics, Wuyi University, Jiangmen, Guangdong, 529020, China **Department of Physics, Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China ABSTRACT Nanocomposite Co-C thin films of about 15 nm thick were prepared by pulsed filtered vacuum arc deposition. The films were characterized by x-ray photoelectron spectroscopy, nonRutherford backscattering spectrometry, x-ray diffraction, magnetic force microscopy and magnetic measurements. The as-deposited films were amorphous. After annealing at 350oC for one hour in vacuum (< 10-3 Pa), the films were found to consist of nanocrystalline Co grains encapsulated in carbon. The superparamagnetism of the annealed Co36C64 film was demonstrated by the measurement of DC susceptibility and magnetic hysteresis using a SQUID magnetometer. The superparamagnetic relaxation blocking temperature was marked to be about 12K by the peak of the zero-field-cooled magnetization under a field of 100 Oe. The magnetic properties of these annealed granular Co-C films transform from superparamagnetism to ferromagnetism when the Co concentration increases. INTRODUCTION Magnetic thin films of cobalt-based nanocrystals encapsulated in carbon have recently attracted increasing attention due to their great potential for application as ultra-high-density [14] and extremely high-density [5] magnetic recording media. The most important advantage of carbon encapsulation is the increase of the effective distance of neighboring magnetic grains so that the inter-grain exchange coupling can be weakened or eliminated. This is an important issue in ongoing attempt to reduce the media noise. On the other hand, carbon encapsulation can also provide protection for air-sensitive grains against degradation. It is well known that an assembly of uniaxial single-domain grains becomes superparamagnetic at some finite temperature. When the thermal energy can overcome the anisotropy barriers, the magnetic moment can jump between different equilibrium states. However, when the system is cooled below a characteristic blocking temperature TB as described by Neel and Brown [6, 7], the magnetic moments will be frozen and history-dependent magnetic properties appear. The study of superparamagnetic relaxation behavior in systems of fine magnetic particles for ultra-high-density and extremely high-density data storage technologies is of crucial importance, because relaxation is associated with the reduction of the coercivity and
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