Spin Dynamics Of The Magnetization Of The Magnetic Thin Film Systems In Nature

The magnetic properties of magnetic thin films have drawn much attention, in part motivated by potential applications such as spintronic devices and data storage devices. For example, the spin valve is a kind of multilayer film structure which is applied for the magnetic read heads of hard disk and magnetic sensors. The two ferromagnetic layers will reveal two kinds of magnetization status when the ferromagnet (FM)/antiferromagnet (AFM) bilayer films show exchange bias. Moreover, magnetic recording makes use of the magnetization properties of magnetic films achieving information recording and saving. Therefore, it is important to study the magnetization properties of magnetic films.The magnetization properties of magnetic films are dominated by the spin dynamical behavior in the magnetization process. Meanwhile, the direction and magnitude of magnetic anisotropy may change the dynamical behavior of magnetization reversal. In recent years, magnetic nanostripes has drawn much attention due to the racetrack memory, the shape anisotropy is paricularly important. Therefore, in this paper we mainly study the spin dynamical behavior during the magnetization process (especially during the magnetization reversal process) and the effect of magnetic anisotropy (especially the shape anisotropy) on the magnetization properties. We discussed these details by spin dynamic simulation method, and the micro interaction between spins was under consideration by this method. The spin dynamical behavior may be studied under ideal situation in order to discuss the magnetization properties of magnetic films. The third and forth parts are the main parts of this paper, as follow:1. We proposed a new analytical method to observe the dynamical behavior of magnetization reversal in exchange-bias system. In a FM/AFM exchange-bias system, there is a unidirectional anisotropy in ferromagnetic layer because of the pinning effect of aniferromagnetic layer. The barriers with the intrinsic potential energy in ferromagnetic layer may be asymmetric due to the unidirectional anisotropy. Base on Stoner-Wohlfarth model, we show that the asymmetric barriers may lead to four kinds of dynamical process underlying the hysteresis-loop measurement. These kinds of dynamical process are different in a topology-like property, which can be controlled by the orientation of the external field. In our study, a new analysis approach has been proposed to reveal the dynamical behaviors of magnetization reversal. With this approach, the coercivity, the exchange-bias field and the asymmetry of hysteresis loop can be quantitatively obtained.2. We studied the effect of shape anisotropy on magnetization properties in magnetic nanostripes by spin dynamic simulation method. In magnetic nanostripes system, demagnetizing field along the short axis is strong due to the large aspect ratio, it will lead to the whole spins lie along the long axis reducing a shape anisotropy along the long axis. Shape anisotropy and magnetocrystalline anisotropy may be collinear or noncollicear resulting in a clear competition and an enhancement between the magnetocrystalline and shape anisotropies. Thereby, the effective easy axis is changed, the effective easy axis is relevant to the magneocrystalline anisotropy and size of nanostripes.