TECHNICAL: Surfactant-coated nanoparticles have tremendous advantages due to their monodispersity and ability to form ordered arrays. Nanoscale magnetic measurements have demonstrated that uniformity in the particle size and spacing affects the collective dynamical response and the length scale of magnetic order. The surfactant controls the particle size distribution and provides mobility needed for self-assembly, but it also limits the range of interesting magnetic nanostructures that can be prepared. The typical 2-4 nm separation between cores means that interparticle coupling is almost purely magnetostatic. The surfactant provides at best a temporary, semi-permeable barrier to oxidation of materials such as iron and cobalt. Assemblies have the mechanical consistency of wax, making them unsuitable for most device applications. The surfactant makes it difficult to achieve good electrical contact, and varying amounts of surface coverage causes particle-to-particle differences in the apparent tunneling barriers. The project explores processing methods to prepare monodisperse metallic nanopillars in ordered arrays. The pillars will be made by reactive ion etching (RIE) a nanoparticle mask. In the simpler form the mask will be nonmagnetic and the magnetic particles will be created by sputter deposition on top of the dense pillar array. Direct patterning of magnetic multilayers with high density methanol-based plasma RIE will also be investigated to determine the minimum feature size and the effect of this dry etching process on magnetic response. The research focuses on systems where magnetic imaging and neutron reflectivity techniques will reveal temperature and field-dependent magnetic correlation lengths, which provides a deeper understanding of magnetic nanoparticle interactions. Manganese phosphide has a first order ferromagnetic to paramagnetic transition, so that the strength of magnetostatic interactions between MnP nanoparticles should change sharply with temperature. Ferromagnetic Co nanoparticles coupled to an antiferromagnetic (AF) IrMn layer may show differences in collective correlations due to exchange bias effects, depending on the AF domain size. Model soft nanocrystalline materials, with Fe pillars in a FeBSi matrix, will also be used to test the degree of exchange coupling between the nanoparticles in the array, which should change abruptly at the Curie temperature of the matrix, enabling quantitative comparison of exchange and magnetostatic contributions to the composite material. Finally the RIE of magnetic materials will be used to prepare some multilayer nanopillars where the small feature size leads to novel quantum confinement and spin accumulation effects. The intellectual merit of this project is in the development of a versatile, scalable process to prepare uniform magnetic nanoparticles and nanoparticle arrays in an inorganic matrix. The nanoscale characterization tools will reveal new features concerning the development of long-range magnetic order, and will clarify the interpretation of macroscopic measurements. NON-TECHNICAL: The work will have broad impact in numerous ways. It will form the thesis project for a graduate student and undergraduate research projects for several students. Hands-on demonstrations and laboratory experiments related to the magnetic phase transition in MnP will be developed for middle school students, and undergraduate physics laboratories.
This project is jointly supported by DMR?s Metals program and Condensed Matter Physics program.
This project focused on the development of methods to make and study uniform magnetic nanostructures using self-assembly. Here spherical particles on the order of 10 nm in diameter were coated with a surfactant and packed like tiny ping pong balls into a single layer, with a small space around each particle core due to the soft surfactant coating. The major scientific outcome as a result of this project is the new capability to transfer the pattern of the particle cores into the material underneath by a gas phase etching process. This is significant because it could enable inexpensive nanopatterning of thin films for many different types of material. We found that nanohole arrays in silicon could be filled directly with gold, creating arrays of gold nanodots that could be useful for plasmonic sensors. We also found that coating the patterned substrate with most magnetic metals or alloys led to non-preferential deposition, and to make arrays of separated nanodots a thick film had to be deposited and then etched back. While the magnetic properties are not yet optimized, this approach could be used in the preparation of data storage media. The results of this project included nine publications in scientific journals, plus one manuscript in preparation and one currently under review, and one provisional patent application, and well as extended discussions with researchers in the magnetic recording industry. This project was important for the Ph.D. thesis research and training of one graduate student, and for the training of several undergraduates, including one female minority engineer.
