Superconducting quantum interference device (SQUID) magnetometry is a non-destructive technique that reveals detailed information about the electron spin interactions in many types of materials. This project will involve a state-of-the-art SQUID magnetometer and Magnetic Property Measurement System (MPMS), which is a critical tool for characterizing several types of materials currently being investigated by researchers within the Laboratory for Surface Science & Technology (LASST) and other University of Maine (UMaine) laboratories. Specific measurement capabilities include DC and AC magnetic susceptibility, magnetoresistivity, van der Paaw conductivity, and Hall mobility. State-of-the-art MPMS capabilities will be especially valuable to several research programs at UMaine pertaining to (i) surface magnetism in nanoparticles, (ii) magnetic anisotropies in sedimentary rocks, (iii) electrical transport in physical and chemical sensing devices, (iv) optical properties of nanostructures in high magnetic fields, and (v) magnetic nanoparticle based biosensors. The MPMS will serve as a focal point for training undergraduates, graduate students, postdocs, and visiting scientists in magnetic materials, nanotechnology, biophysics, and materials science. This instrument is a critical tool for expanding the capacity of UMaine research into magnetic aspects of nanotechnology, biophysics, sensor technology, and materials science. As no SQUID magnetometer currently exists in the State of Maine, the instrumentation will provide access for research projects from interested parties throughout the state, including non-Ph.D. granting institutions and small Maine businesses. The instrument is relatively easy to operate and provides direct information on electron spin interactions, and thus it will be a powerful tool to teach physics and nanotechnology concepts to several different constituents participating in UMaine outreach activities, including K-12 students and teachers, the general public, under-represented groups, and industry partners.
Layman Summary: Superconducting quantum interference device (SQUID) magnetometry is a non-destructive technique that reveals detailed information about the electron spin interactions in many types of materials. Knowledge of electron interactions in materials is extremely important in building the next generation of computers, electronics, and contrast agents in biological magnetic screening techniques (i.e. MRI). To gain the necessary information, a system with control over both the magnetic field strength and temperature is critical. To this end, a SQUID/Magnetic Property Measurement System (MPMS) is ideal for these measurements. This project will purchase a state-of-the-art MPMS system and will be especially valuable to several research programs at UMaine pertaining to surface magnetism in nanoparticles, magnetic anisotropies in sedimentary rocks, electrical transport in physical and chemical sensing devices, and magnetic nanoparticle based biosensors. The proposed MPMS will serve as a focal point for training undergraduates, graduate students, postdocs, and visiting scientists in magnetic materials, nanotechnology, biophysics, and materials science. As no SQUID magnetometer currently exists in the State of Maine, the instrumentation will provide access for research projects from interested parties throughout the state, including non-Ph.D. granting institutions and small Maine businesses. The instrument is relatively easy to operate and provides direct information on electron spin interactions, and thus it will be a powerful tool to teach physics and nanotechnology concepts to several different constituents participating in UMaine outreach activities, including K-12 students and teachers, the general public, under-represented groups, and industry partners.
The goals of project 1040006 can be sumarized in the following statement: A superconducting quantum interference device (SQUID) magnetometer was purchased to answer crucial questions about the magnetic/electrical properties of a host of materials systems for many research groups at the University of Maine. Towards this goal, the following research projects were targeted: (i) ligand exchanged quantum dots, (ii) doped quantum dots, (iii) magnetite nanocrystals, (iv) high surface area gold nanostars, (v) Au/iron oxide core/shell nanoparticles, (vi) Ru/RuO2 nanowires, (vii) igneous and metamorphic rocks, (viii) photomagnetic properties of nanostructures, and (ix) magnetoresistance and Hall conductivity measurements. This work has trained five graduate students, and one undergraduate student. All the PIs have been involved with several tours of 'nanotechnology research' to different constituents. These outreach groups include the local middle and high schools; several Maine legislators; participants in the UMaine Engineering EXPO; the UMaine Sensors REU program; and the UMaine Physics department "Summer Camp" (for young children). The tours highlight various nanotechnology research at UMaine and showcase the SQUID magnetometer. The results obtained for the materials systems mentioned above have revealed information about intrinsic magnetization, particle-particle interactions, and surface contributions to magnetization. Examples of four specific results are follows: Example #1: External magnetic fields were used to promote ordering of magnetite nanocrystal arrays. Field strengths were varied up to 1 Tesla on either Si(111) or silica wafers. Both dc and ac magnetometry was used to probe how the magnetic interactions are affected with particle ordering. DC temperature dependent magnetization provides evidence that the splitting of the ZFC/FC data changes when samples are prepared under differing magnetic field strengths. Both the superparamagnetic transition temperature and coercive fields are unaffected, which suggests that the nature of particle coupling is differing. AC susceptibility confirms our DC measurements. The models used to fit our AC measurements indicate that the nanocrystal assemblies are strongly interacting at large deposition magnetic fields. Example #2: CdSe quantum dots (QDs) were synthesized chemically and ligand exhange experiments were performed. To date, we have undertaken much effort towards elucidating the magnetic properties of native capped CdSe QDs with a direct comparison to the typical magnetic response of doped QDs (both copper and cobalt doped CdSe). Both phosphorus and amine (natively) coated QDs exhibit a temperature independent magnetic susceptibility, suggestive of some sort of Pauli paramagnetism, and a low temperature (less than 50 K) Curie tail. This tail is not due to magnetic impurities, as much effort was undertaken by our group of devising proper sample holders and background subtraction methods. Currently, all of our experiments have suggested only paramagnetism is present in these systems, irrespective of ligand. This is contrary to published reports of ferromagnetism inCdSe QDs. We have also developed chemical methods in which we can selectively control the number of ligand molecules on the QD surface. Using titration methods, we can slwoly add more molecules onto the QD surface, and track the effect on the magnetic susceptibility. For QD with very few surface ligands, the observed magnetic susceptibility is close to the bulk CdSe value. This is signifcant, as we believe we are finally seeing proof that the surface molecules are indeed responsible for induced paramagnetism in QDs. Example #3: We have been studying both Co and Cu doped CdSe QDs. The Co doped particles have interesting structural properties, are superparamagnetic with a blocking temperature of ~5K. This is the largest reported transition temperature for Co doped CdSe. The magnetization curves exhibit hysteresis with coercive field on the order of ~50 Oe. The copper doped CdSe QDs are an interesting system. They do not dope as Cu(II), and are therefore magnetically silent. The optical properties of these materials are fascinating with a blue optical absorption edge and a ~1 eV Stokes shift resulting in a deep red emission. Example #4: We have been investigating the effect of strain of the magnetic properties of QDs. In experiments performed by my group at the Canadian Light Source, we compared the Se L3-x-ray absorption edge for wurtzite, cubic, and a pseudo-cubic CdSe QDs. An interesting observation was that the wurtzite and cubic data looks indentical, but the psuedo-cubic edge was shifted to higher energy, similar to data for CdSe/ZnS core/shell materials (materials that posses a large lattice mismatch and have a large lattice strain). When observed via magnetometry, the data for the wurtzite and cubic were similar to the results reported above (namely, paramagnetic). The pseudo-cubic CdSe QD exhibited differences in the zero field and field cooled behavior in the magnetic susceptibility and looked to be superparamagnetic. At low temperature, we observed magnetic hysteresis. These results are quite exciting and we are in the process of following them up in more detail and for reproducibility.
