Fast Switching Of Nano-sized Magnetic Particles

Magnetization reversal of magnetic particles is an important issue in the application of such materials as magnetic data storage and magnetic field sensors. A number of data storage devices of high-capacity are made of magnetic data storage for the reason that the magnetic storage is more stable in the performance and the fabrication and manipulation of magnetic data storage are of great advantage, which allows the recording density as high as300Gbit/in~2 . The properties of magnetization reversal can be used not only in the application of data storage devices, but also in the application of magnetic field sensors and magnetic probes. And it plays a role which is becoming more and more important in our daily life. whatever it can be used to, these applications need the fast magnetization reversal. Especially in the growth of computing ability and of the speed of the central processing unit, the demand of the storage of high access speed is needed. Therefore, the study of magnetic properties of magnetic nano-particles is not only for theoretical interests but also valuable for future technological applications. Furthermore, in order to break the bottleneck of magnetic memory access speed, the requirements to the high quality of the magnetic storage material are needed.We study in details about the property of magnetization reversal of magnetic nano-particles by use of LLG equation. We find that the particle of biaxial anisotropy has more advantages to the magnetization reversal than that of uniaxial anisotropy.The property of magnetization reversal in two dimensional thin film is important, for the understanding of the magnetization dynamics in the three dimensional (3D) particles. For the 2D magnetic film, a bias field which is applied perpendicularly to the film plane can greatly shorter the reversal time, even if the magnetic field that applied anti-parallelly to the direction of initial magnetization. Meanwhile, the effect of the damping constant of the material to the magnetization reversal is much important. We find that a material of an appropriate damping constant can faster the magnetization reversal.The property of the magnetization reversal in three-dimensional biaxial particles is the main research work in this thesis. The form of external magnetic field plays an important role in the magnetization reversal. We adopt the square shaped pulse field because it can decrease the amplitude of the switching field and shorter the reversal time. Firstly, we study the magnetization reversal 3D particles because it is useful for the understanding of the magnetization reversal in the presence of pulse magnetic field. We apply the pulse field alone the +y-axis at first. It induces the demagneting field in the z-axis, and the demagneting field causes the magnetization precess. Thus, the magnetization reversal can be realized. Using this method, we can reverse the magnetization when the amplitude of the pulse field is far below the anisotropy field, and the reversal time is of the order of nano-second. For different amplitudes of the pulse magnetic field, there're different regions of pulse width which can cause the magnetization reverse. As the amplitude of the pulse increased, the region of the pulse width that can cause the magnetization reversal becomes narrower, but the reversal time is shorter. As a comparison, we also find that if the pulse magnetic field has a declining angle from the +y-axis and -x-axis, the reversal speed is faster. The magnitude of the angle is inversely proportional to the amplitude of the pulse field. The bigger the amplitude is, the smaller the angle is.Finally, we study the magnetization reversal under the unequal biaxial anisotropy. The biaxial anisotropy has different anisotropy properties along the x-axis and y-axis respectively. We find that the reversal properties of such a case are similar to cases of biaxial and uniaxial anisotropy. Thus the properties of the magnetization reversal in the case of unequal biaxial anisotropy seems to be a transition region between the case of uniaxial anisotropy and that of equal biaxial anisotropy.