Magnetic Property Of Finite Nanodot Arrays

Periodic arrays of single-domain magnetic dots have attracted much attention due totheir possible applications as future high-density magnetic data storage (1 Tb/in~2) andmagnetic sensing devices. In these systems, both the dipolar interaction between particlesand the applied field orientation can strongly affect magnetization processes. Especially insmall-sized arrays, some interesting phenomena and special properties are found.Therefore, the study of magnetic properties of magnetic nanoparticles is not only fortheoretical interests but also valuable for technological applications.We study the hysteresis loops for finite N×N arrays with N=3, ...10 in terms ofthe Landau-Lifshitz-Gilbert equation. Each particle has a perpendicular anisotropy and themagnetic dipolar interaction between particles is considered within all of the array. Themagnetic moments can be oriented any direction in 3D space.First, we investigate the dipolar interaction. When an external magnetic field appliesnormally to the array plane, the component of the magnetization that parallel to the appliedfield seems sensitive to the strength of dipolar interaction. The magnetization hysteresisshows a single loop with stepwise shape at weak dipolar interaction region. If the strengthof the dipolar interaction increases, the hysteresis loop will divide into one main loop andseveral minor loops at moderate dipolar interaction region. While the hysteresis loop closesas the dipolar interaction is quite strong. These changes are associated with the sharpchanges of the system's energy, which correspond to transition from one configuration ofthe moment orientation to another. When an external magnetic field applies parallel tothe array plane, generally, the system becomes anhysteresis and seems to be insensititve tothe dipolar interaction strength .The remanence and the critical field refers to the field thatmakes the magnetization transit from positive value to negative value, are studied atvarious strength of the dipolar interaction.Then, we study the direction of the magnetizing field varies from in-plane orientation to out-of-plane orientation. We find that the magnetizing pattern is sensitive to thedirection of the external magnetic field. The hysteresis loops show strongly size-dependenteffect due to the competition of dipolar interaction, perpendicular anisotropy and Zeemanenergy. Both the remanence and the switching field show the anisotropic property whichdepends on the magnetization process.