Magnetic Films on Self-assembled Nanospheres

<正>1 Results Nanoparticle media using arrays of monodisperse nanoparticles with high magneticanisotropy are assumed to be the ideal future magnetic recording media. However,key requirements like control of the magnetic anisotropy orientation along with magnetic domain isolation have not been achieved so far. Here, we report on a combination of a two-dimensional topographic pattern formed of self-assembled nanoparticles with sizes as small as 20 nm and magnetic multilayer film deposition[1]. The so formed nanostructures on top of a sphere are monodisperse, reveal a uniform magnetic anisotropy and are magnetically exchange isolated(see Fig.1 at page 710). This system is distinct from the classical nanostructure geometries: Neither extrinsic properties nor the intrinsic properties are uniform in space. The film is extended over a wide region of the sphere and thus shows substantial curvature. The film thickness varies and so do the intrinsic magnetic properties most notable the magneto-crystalline anisotropy, which is a key factor affecting the fundamental nature of the reversal process[2]. Co/Pd multilayers containing thin Co layers of 0.3 nm thickness result in an anisotropy direction pointing perpendicular to the particle surface. This has a drastic impact on the switching mechanism which differs remarkably from a Stoner-Wohlfarth behavior. For Co/Pt and Co/Pd multilayer film deposition, the anisotropy direction depends critically on the Co layer thickness, thus, changing the orientation from parallel to perpendicular to the particle surface below a critical thickness of about 0.8 nm. Increasing the Co thickness allows the creation of systems with a spin reorientation transition across the cap. Experimental results will be compared to micromagnetic simulations. Furthermore, the magnetic nanopattern is used to study the size-dependent scaling of exchange bias in nanostructures. [Pd/Co]-CoO and [Pt/Co]-IrMn layers with perpendicular magnetic anisotropy were deposited onto different arrays of monodisperse PS nanospheres with a diameter ranging from 58 to 320 nm[3]. Below the blocking temperature we find for both systems a strong increase of the exchange bias field compared to continuous films. Interestingly, the exchange bias field increases drastically with decreasing particle size and shows a strong dependence on the applied cooling fields accompanied by a strong training effect.