Second project period 10/2010 - 03/2015

Main Objectives

• Magnetic coupling phenomena in film / nanostructure architectures
• Scaling of exchange bias effect in magnetic nanostructures

Description

The physical as well as chemical properties of nanoscale objects differ substantially from their bulk counterparts, thus resulting in a variety of applications in the field of catalyst, sensor technology, magnetic recording, biology, and medicine. In particular, one of the driving forces for the development of innovative magnetic nanomaterials is the sensor industry. In this regard, novel concepts for sensor media and architectures are required. One example are percolated perpendicular media (PPM) which consists of an exchange-coupled magnetic film grown on a substrate with densely distributed nonmagnetic defects acting as pinning sites for magnetic do-main walls [G6-1, G6-2].

Although a detailed theoretical description of PPM is provided in the literature, there are only a few experimental realizations of a percolated perpendicular medium. In one example, evenly distributed nonmagnetic defects in a magnetic matrix were achieved by co-deposition of magnetic material with nonmagnetic oxides [G6-3]. Another example concerns the direct deposition of Co/Pt multilayers onto anodized alumina substrates resulting in the formation of PPM with evenly distributed pinning centers [G6-4]. In this case, the pores serve as pinning sites for the domain wall propagation due to the variation of the magnetic parameters and magnetic film thickness around the perimeters of the pores.

A different template was created in collaboration with Prof. Ziemann (University of Ulm) by tailoring the size of polystyrene (PS) particles using isotropic plasma etching [G6-5]. In this case, plasma etching of densely packed particle arrays leads to arrays of spherical nanostructures with adjustable diameters while keeping the periodicity fixed. This approach will be extended in this project to various film systems including Co/Pt multilayers and CoPt alloys providing perpendicular magnetic anisotropy. This study includes a detailed investigation of the magnetization reversal in magnetic thin films grown on diluted particle arrays, which will improve the understanding of the modification of fundamental magnetic interactions (exchange and magnetostatic) as well as magnetic coupling phenomena on the nanoscale.

Another topic which involves densely packed particle arrays is the study of the exchange bias effect in nanoscale systems. The effect of exchange bias is vital for the performance of GMR and TMR sensors. The effect itself is a shift of the magnetization loop away from the zero field axis that can occur when a ferromagnetic (FM) layer is in contact with an antiferromagnetic (AFM) film, resulting in a pinning of a magnetization of the FM layer along the particular direction given by the AFM [G6-6].

In this project we concentrate on the detailed investigation of the exchange bias effect in a complex system realized by deposition of FM/AFM bilayers on curved surface of nonmagnetic nanoparticles [G6-7, G6-8]. In these systems, the fundamental magnetic interactions (exchange and magnetostatic) can be easily tuned by varying the particle diameter as well as the material properties of the deposited magnetic films. For instance, changing the size of the particles allows studying the scaling dependence of the integral magnetic properties such as coercive field, blocking temperature and exchange bias field.

A series of [Co(0.28nm)/Pd(0.9nm)]₈ multilayers will be prepared by magnetron sputter deposition onto SiO₂ and PS particle arrays with various sizes down to 10 nm. After the multilayer films deposition, an additional 1-nm-thick Co layer will be evaporated and then oxidized to form the AFM layer. The growth of the multilayer will be assisted by a Ta/Pd seed layer system. To prevent oxidation, the stack will be covered by an additional 2-nm-thick Pd layer. For studying the influence of the AFM layer on the magnetic properties of the FM, the same series will be prepared also without the AFM layer. In addition, continuous [Co/Pt]/CoO multilayer films will be grown under identical conditions on planar SiO₂ substrates for comparison.

Cooperative work is planned with the German subprojects G7 and G8 for sample characterization using SEM, TEM, and Raman spectroscopy. With subproject G2 we are providing measurement tools for the magnetic characterization of super paramagnetic particles. To investigate alternative approaches for template fabrication suitable for PPM media, a collaboration with the Chinese subprojects F1 and F2 is planned. Furthermore, a direct exchange of research results on exchange bias is envisioned with the Chinese subproject J1.

References