. Recent reports of ferromagnetism in gold nanoparticles are quite surprising since Au has completely filled 5d orbitals. Various explanations have been advanced to explain the unusual magnetic characteristics of nanostructured Au particles, but considerable controversy remains to this date regarding whether ferromagnetism is intrinsic or somehow related to the medium in which the particles are supported. In this effort, pulsed laser deposition (PLD) will be used to achieve dispersed Au nanoparticles with tight control of size and shape. PLD of Au in nonmagnetic oxide thin films (MgO, Al2O3, and ITO) is expected to provide simultaneous synthesis and in-situ passivation of the nanoparticles, while preventing agglomeration. The proposed experimental approach is expected to enable the synthesis of pristine Au nanoparticles for various characterization and physical property measurements. A thorough understanding of the origin of ferromagnetic behavior, if it truly exists in Au nanoparticles, is the expected outcome and will serve as a catalyst for the development of new theories to explain this surprising phenomenon.
NON-TECHNICAL SUMMARY. The discovery that ferromagnetism is an intrinsic property of Au nanoparticles will be transformative, if confirmed, in the sense that is will revolutionize the current understanding of the physics of nanomagnetic materials. Ferromagnetism in Au nanoparticles, coupled with their already established unique optical properties, may have an enormous industrial impact, from data storage to biomedical applications. One graduate student will contribute to this research. Additionally, summer students will be participants in the outreach program, Success Through Enthusiasm and Awareness of Materials Engineering Research (Project STEAMER) at VPI.
Project Outcomes for the General Public 1.0 Background Noble metals like gold, silver and platinum have always been considered at be non-magnetic materials. Recent experiments on gold nanoparticles and thin films of ~ 100 nanometers (nm) have suggested that these materials may become magnetic when their dimensions are reduced to the nanoscale. The objective of this project was to explore this intriguing phenomenon to determine whether magnetism is a feature of nanoscale gold. Currently, the scientific community is divided between two competing explanations for this unusual behavior. The first explanation suggests that magnetism is an intrinsic property of the materials once their dimensions are reduced to a few nanometers. The second explanation suggests that the magnetism is attributed to coating of these materials with a specific type of polymer. In both cases the large fraction of atoms at the nanoparticle surface has been suggested to play a role in the magnetic characteristics. The objective of this project is to test the hypothesis that Au becomes magnetic when its dimensions are reduced to the nanoscale. We prepared Au nanoparticles without the polymer layers to determine if magnetism is intrinsic or induced. We embedded Au nanoparticles into nonmagnetic thin films, which allowed us to mimic bare gold surfaces without exposure to the environment and then measured the magnetic properties of the samples. 2.0 Overview of Results A magnetometer is most commonly used to determine the magnetic characteristics of materials. A typical non-magnetic material has a well-defined response. The response may be paramagnetic or diamagnetic. Figure 1 is a magnetic measurement on a bulk Au sample. The negative slope of the line and lack of hysteresis or separation in the curve is a clear sign that the material is non-magnetic as expected. A paramagnetic material would also have a linear magnetic response, but the slope would be positive (i.e. the curve would increase from left to right) and paramagnetism is a function of temperature. The main objective of this study was to explore magnetic properties of nanostructured gold. The magnetic properties of our samples were measured from ~ -270 °C to ~ 27 °C. The extreme low temperature measurements allow us to understand the physics related to the magnetic characteristics. Figure 2b is the raw data and figure 2a was plotted after correction of the background. The Au samples were deposited on alumina (Al2O3) substrates were used, which is responsible for the diamagnetism observed at high field values, whereas the ferromagnetic part is detectable in low field only. In order to remove the contribution of the substrate to the ferromagnetic hysteresis loops, the linear diamagnetic signal was subtracted from our original result, which is shown in figure 2b. The magnetometry results for the Au alloy samples indicate all samples are ferromagnetic. Both figures are very different from figure 1. Broadening of the curves (hysteresis) and flattening of the curves (saturation) at high magnetic fields are not signatures of non-magnetic materials. We also measured the composition of the samples to ensure that the magnetic response was not due to the presence of magnetic impurities. Our measurements have confirmed that no impurities above the detection limit of the instrument were found. Furthermore, impurities like Fe in low concentrations could not be responsible for such a large signal. 3.0 Summary and Conclusions We made several Au nanostructured samples to and tested their magnetic properties. The samples showed clear signs of ferromagnetism up to room temperature. Ferromagnetic impurities were ruled out as the source of the magnetic signal by careful compositional analysis. Further study is needed to understand the fundamental reasons why Au becomes magnetic when its dimensions are reduced to the nanoscale.
