• PDF / 173,016 Bytes
• 6 Pages / 612 x 792 pts (letter) Page_size
• 16 Downloads / 262 Views

DOWNLOAD

REPORT

---

1228-KK11-36

Electron magnetic resonance studies on nanowire and nanoparticle arrays O. K. Amponsah1, R. R. Rakhimov1, Yu Barnakov1, R. A. Lukaszew 2, J. C. Owrutsky3, M. Pomfret3 and N. Noginova1 1 Center for Materials Research, Norfolk State University, Norfolk, VA 2 College of William & Mary, Williamsburg, VA 3 Naval Research Lab, Washington DC ABSTRACT Arrays of magnetic nanowires and well-oriented chains of superparamagnetic nanoparticles were fabricated using polymer and alumina membrane templates. The systems were characterized by SQUID and studied by electron magnetic resonance methods. Comparative analysis of the obtained results for different geometries and sizes of the magnetic inclusions is presented. INTRODUCTION Composites of magnetic materials with high anisotropy and certain values of permeability are of great interest for a number of applications, including magnetoelectronic devices, magnetic memory, and metamaterials [1-3]. Recent efforts have been devoted to the fabrication and magnetic studies of ferromagnetic nanowires [48]. Well-aligned arrays of ferromagnetic nanowires (nanorods) can be obtained using electrodeposition in porous membranes [4-6]. Another method of template-based fabrication involves formation of nanowires through thermal annealing from nanoparticles embedded in an anodic alumina membrane [7, 8]. Commonly, nanowires produced by both techniques are polycrystalline. As it has been shown in literature [5, 6], imperfections and inhomogeneities play a significant role in the dynamic behavior of such systems, and an idealized model of perfect long cylinders does not describe well the experimental data. Instead, another model has been suggested in [5, 6], where a nanowire is considered as a chain of a large number of ferromagnetic spheres or ellipsoids coupled through dipolar interactions. This model has shown much better agreement with experimental data for Ni nanowires of 35 and 55 nm diameter. However, if the diameter of a wire becomes small enough, one has to consider a chain of particles that are superparamagnetic rather than ferromagnetic, which can have static magnetic and magnetic resonance properties different from the ferromagnetic ones. In this work we investigate the following questions: a) How does a significant decrease in the nanowire diameter affect the overall magnetic resonance behavior? b) Does a chain of superparamagnetic particles (well separated from each other but still coupled with dipolar interaction) behave similarly to ferromagnetic nanowires, or does it have its own specific behavior? In what follows we restrict ourselves to a qualitative picture only. The full quantitative analysis of the obtained data will be published elsewhere.

EXPERIMENTAL The following systems were studied in the experiment: a) Arrays of nanorods of Ni with nominal diameters of 30 nm and 10 nm in porous polymer membranes; b) Chains of iron oxide nanoparticles embedded in anodic alumina membrane before and after thermal annealing. Ni nanorods were fabricated through electroplating in