從超薄膜到自組排列奈米顆粒: 低維度磁性系統之成長、晶格結構與磁性分析

Abstract

In low-dimensional magnetic systems, due to the finite size, the symmetry breaking and the large ratio of surface to bulk atoms, many interesting physics, including the crystalline structure and magnetic properties etc., can be found or manipulated by tuning the alloy composition or deposition coverage, adding the capping layer, and choosing the proper substrate for small lattice mismatch etc.. As the film thickness increases above 8 ML, the CoxNi1−x/Cu3Au(100) alloy ultrathin films clearly exhibited progressively more relaxed structure. Due to the strain relaxation, both the 1st and the 2nd spin-reorientation transitions (SRT) occurred within 20 ML. The thickness region with perpendicular magnetization was strongly reduced by increasing Co concentration. By combining both alloy and strain relaxation effects, the SRT boundaries in the phase diagram can be described in a phenomenological model on the basis of magnetoelastics. Face-centered cubic (fcc) Mn, which exists at 1400 K in bulk material, can be successfully grown on Cu3Au(100) at 300 K (RT) and 100 K (LT), because of the small lattice mismatch at the interface. Mn films deposited at RT and LT demonstrate very different behaviors in the crystalline structure, morphology and magnetism. Both the RT and LT-Mn films proceed a thickness-dependent structural transition from face-centered cubic (fcc) to face-centered tetragonal (fct) at 12-14 and 8 ML, respectively. Significant exchange bias is observed in Fe/RT-Mn bilayers and monotonously increases with Mn thickness. The exchange bias coupling in Fe/LT-Mn is much weaker as compared with Fe/RT-Mn and drastically varies with Mn film thickness. Both the RT and LT-Mn/Cu3Au(100) films are concluded to be antiferromagnetism. Fe films grown on 15, 9 and 6 ML Mn/Cu3Au(100) revealed a structural transition from face-centered tetragonal (fct) to body-centered tetragonal (bct) during 3.7-6.9 ML, corresponding to the spin reorientation transition (SRT) from polar to longitudinal direction. Therefore we may prepare polar magnetized 6 ML Fe grown on 6 and 9 ML Mn/Cu3Au(100). After the polar field-cooling, a significant enhancement in the coercive field and a small bias field were observed. Thus the AFM-Mn/Cu3Au(100) ultrathin films were proved to have the capability of providing polar exchange bias coupling. The structural and magnetic properties of Fe/FexMn1−x bilayers prepared by epitaxial growth on Cu3Au(100) are investigated. For FexMn1−x with x=54-83%, the periodical oscillations of medium electron diffraction (MEED) persist up to 15 monolayer (ML). After field-cooling, the large exchange bias up to 200-300 Oe is measured at 100 K in 21 ML Fe/15 ML FexMn1−x for x=0-54%, indicating the antiferromagnetic properties of the single crystalline FexMn1−x films and the significant exchange bias coupling in the Fe/FexMn1−x bilayers. Co nanoparticle chains are grown by vapor deposition over a single-crystalline Al2O3 layers on NiAl(100) with such features as self-limiting size distribution with the average size of 2.7 nm, well-ordered alignment, and high thermal stability. We attribute these features to peculiar one-dimensional long stripes with, 4 nm inter-distance on the surface of the ultrathin Al2O3 template. This also provides a natural explanation why several different metals (Fe, Cu, Mn) we tried all show the same kind of spectacular alignment. The ferromagnetism of Fe nanoparticle assembly on Al2O3/NiAl(100) is observed above 150 K with the coverage larger than 5 monolayer (ML). Cu capping layer induces an enhancement of the Curie temperature (TC) in both Fe and Co magnetic nanoparticle assembly. The TC of Fe nanoparticle assembly with 2 ML and 6 ML Cu capping layer is enhanced by 20 K and even higher, indicating the critical effects of metallic capping layer in such magnetic nanostructures as nanoparticle assembly.