Magnetic Relaxation and Quantum Tunneling of Magnetization

Abstract

Magnetic relaxation effect is becoming more and more evident and increasingly crucial in nanostructured magnetic systems as the size of the particles (or clusters) decreases. Therefore, to understand the physics of the magnetic relaxation is fundamentally important for both basic science and industrial applications (Dormann et al., 1997), which is actually one of the most important issues in the high-density magnetic recording media. It has been well known that magnetic relaxation effect is due to the magnetic moment flipping caused by thermal energy. The flipping frequency of the magnetic moment in a magnetic small particle, Γ, can be obtained from the Néel model (Néel, 1949a, 1949b): $$ \Gamma = u \exp \left( \{{\textbackslash}frac{{ - U}} {{k_B T}}} \right), $$ (5.1) where υ is the attempt frequency of the order of 1010 – 1013Hz (Johansson et al., 1993; Linderoth et al., 1993; Prene et al., 1993; Dickson et al., 1993), U = KV is the anisotropy energy barrier (K is anisotropy constant, and V is a volume), k B is Boltzmann constant, and T is the temperature in Kelvin. Actually the exponential relaxation law had been established before the Néel model (Becker and Döring, 1939). Since the anisotropy constant is a material property (also related to the shape and the size of the particles), it cannot be infinitely large. As recording density increases (V deceases), Γ will increase exponentially, and eventually the recorded information will become unstable.