Ultra-High Density Arrays of Self-Organized and Single-crystalline 1D Nanomagnets – DensAr

The DensAr project concerns the synthesis and investigation of ultra-high density arrays of self-organized and single-crystalline 1D nanomagnets.

We have recently discovered a process by which we can directly grow on a metallic surface, arrays of Co nanowires (NWs) of 5nm in diameter, separated by a 2nm layer of stabilizing organic ligands. These nanowires are ferromagnetic at room temperature and organized in a hexagonal arrangement over large scales (1×1cm²), with densities of more than 10×1012 NWs/in². The process is based on colloidal chemistry and consists in the reduction of a Co molecular precursor in solution and at low temperature (<150°C) in the presence of a single-crystalline Pt film. The Co NWs are single-crystalline and grow epitaxially on the film. Thanks to the common orientation of the magnetization easy axes of the NWs, the NW arrays exhibit a high magnetic anisotropy perpendicular to the film. Currently, there are no other systems possessing ultra-high densities of single-crystalline nanomagnets which combine shape and magnetocrystalline anisotropies in the same direction. Indeed, all precedent studies on interacting nanomagnets involve polycrystalline elements, and/or arrays with limited periodic arrangements and densities, and/or random easy axis orientations. In the context of future hard disk drives of ultra-high densities, several fabrication routes are planned but up to now, a method for producing arrays of nanomagnets with densities superior to 1 Tbits/in² did not exist.

The objective of the DensAr project concerns the demonstration of the applicability of the material as a future ultra-high density magnetic recording medium. The project focuses on the synthesis and the study of the structural and magnetic properties of arrays of Co NWs. The proof of concept of the synthesis process has been shown and patented. The project is thus based on a detailed and extensive study of the fundamental mechanisms of the Co NW growth on substrates, i.e. the degrees of freedom of the synthesis (Co precursor/ligands concentration ratio, ligand nature, temperature, pressure, reaction time, substrate nature) that will modulate the system’s morphological characteristics thus allowing to reduce the different spatial distributions (diameter, length, aspect ratio, inter-NW distance). The structural properties (length, diameter, inter-NW distance, long range order, density, array roughness, epitaxy) will be investigated using state of the art techniques (X-ray diffraction, electron and atomic force microscopies and neutron scattering techniques) in order to fully describe the resulting system. A major effort will concern the study of the magnetic properties (anisotropy, coercivity, dipolar coupling, thermal stability, switching field distribution, magnetization reversal mechanisms, magnetic correlation lengths), both experimentally (magnetometry, neutron scattering techniques, ferromagnetic resonance), and theoretically (micromagnetic simulations). A special attention will be paid to the magnetic and structural features that fulfill the requirements for future ultra-high density magnetic media above the Tbit/in².

The DensAr project is a “Jeune Chercheur” project and consequently involves a single partner, the LPCNO, the Laboratory of Physics and Chemistry of Nano-Objects, and particularly the NanoMagnétisme and Nanostructures et Chimie Organométallique groups. They have complementary skills in the synthesis, structural and magnetic analyses of nano-objects. The consortium thus combines a high expertise on colloidal chemistry in solution and on thin films, as well as on structural investigations of nanosystems and on nanomagnetism. The gathering of specialists in each field, that have already proven their ability to work efficiently together in a friendly atmosphere, will guarantee the success of this enterprising project, that aims at providing new advanced and highly relevant nanomagnet arrays for industrial purposes.