CTS-0408933 S. Hariharan, U of South Florida
Intellectual merit of the proposed research
Magnetocaloric effect has been observed in a number of materials where the intimate coupling between the magnetic order and structural properties result in changes in lattice temperature under application and removal of an external magnetic field. Promising materials such as Gd5Si1.5Ge2.5 and Gd3Ga5O12 exhibiting large MCE have been discovered. However, the cooling produced by these materials is restricted to regions close to the magnetic ordering temperatures typically lower than 20K. Other drawbacks include the need for large external fields and the difficulty in processing them into thin film forms required for most electronic device applications. Other than the standard magnetic ordering temperatures like the ferromagnetic Curie (Tc) and antiferromagnetic Neel (TN) temperatures in bulk materials, there is a versatile order-disorder transition temperature in nanoparticles that is determined by the average particle size. This is the blocking temperature (TB) above which nanoparticles are known to exhibit superparamagnetism (disordered state) and below which the size-dependent anisotropy energy (KV) overcomes the thermal energy (kBT) resulting in freezing of the particle moments (partially ordered state). The key advantage is that TB can be varied over the intermediate temperature range from 20 to 300K and this can be used for magnetic refrigeration in this temperature range. We propose to investigate the entropy change across the blocking transition through systematic MCE measurements in nanoparticle assemblies of monodisperse Fe, ?-Fe2O3 and other ferrites. It has been theoretically proposed that the entropy change can be maximized when the inter-particle interactions are increased. Using Langmuir-Blodgett (LB) technique, we will use self-assembled, ordered nanoparticle multilayers with different surfactant coatings to tune the inter-particle interaction strength. A very sensitive RF susceptibility method will be used to monitor the magnetic anisotropy in different systems and correlating these measurements with the MCE measurements. The LB deposited films automatically provide us a simple test device scheme to directly measure the temperature changes. Once the choice material, particle size, optimum temperature range are identified, we will test the cooling efficiency by directly depositing the films on a high thermal conductivity substrate like Kapton and attaching a thermocouple to this device. Our proposed study has the potential to contribute to fundamental understanding of MCE in nanoparticles and also advance the state-of-the-art in magnetic refrigeration.
Broader impacts of the proposed research
The proposed project will lead to understanding functional response in new materials and devices that could essentially impact a broad base of refrigeration technology. Magnetic refrigeration is environmentally friendly and does not use ozone-depleting, global-warming volatile liquid refrigerants. A notable aspect of the proposed project is the capacity to provide hands-on experience in topical research areas. Students will acquire specialized skills in areas like nanotechnology, cryogenics, electronic instrumentation, materials synthesis and processing, radio frequency (RF) and thermal management techniques that are sought after by a large number of employers. Postdoctoral scientists, graduate and undergraduate students in the PI's group will particularly benefit from working in the topical area of nanoscience and nanotechnology that are expected to become as socially transforming in the future as the development of running water, electricity, antibiotics and microelectronics. The PI currently has a diverse group of students that includes two women graduate students. In addition, the PI will make all efforts to recruit minority students particularly from the large Hispanic community base in South Florida. The collaborations identified and supported by letters documented with this proposal will lead to enhanced networking and partnerships through further possibilities of multidisciplinary research. The PI will broaden the scope of integrating research and education through curriculum enhancement targeted for USF Physics department students pursuing their MS and PhD degrees in the Applied Physics program.