Study of magnetic materials for biomedical and other applications

We have studied different aspects of magnetic materials in bulk, nanoparticles, and thinfilm form with emphasis on their use in biomedical and technological applications. In this work: (1) We have synthesized several new Gd based compounds and alloys and have optimized their magnetic properties for the self-controlled hyperthermia applications. The self-controlled hyperthermia is a new non-invasive technique to employ heat treatment to cure cancerous cells without overheating the normal cells. The need for developing such materials was dictated by the lack of existing magnetic materials with magnetic ordering temperatures in the temperature range of (40-45)°C, which is the critical operating temperature range for the hyperthermia applications. (2) We have produced gold coated Fe-Au nanoparticles which are biocompatible and can easily be functionalized through gold surface for various technological applications, besides hyperthermia applications. Contrary to the previous reports of time dependent degradation of magnetic properties of the Fe-Au nanoparticles, our gold coated nanoparticles are quite robust and their magnetic properties remain unchanged under the ambient conditions. We have made a comprehensive study of the Fe-Au nanoparticles, and have observed that superparamgnetic Fe-Au nanoparticles can be produced with variable Fe content up to 30 at.% and the particle size remains nearly uniform (∼ 5 nm). When subjected to annealing at elevated temperatures, the magnetic core in the Fe-Au nanoparticles undergoes various interesting changes and the blocking temperature and magnetization increase when nanoparticles are annealed at elevated temperatures. The observation of the Verwey transition at ∼ 125K in the magnetization versus temperature data for samples annealed at 450°C and above indicates the formation of Fe3O4. The absence of any oxide peaks in the as-formed sample and presence of oxide peaks in the samples annealed at 450°C and above in the x-ray diffraction and x-ray photoemission data, as well as in the magnetic data, support the model that Fe-Au alloy core is protected by the Au shell in the as-formed state. Annealing at higher temperatures leads to the segregation of Fe and Au, and oxidation of Fe occurs when Au shell is punctured at the elevated temperatures. Also, we have studied the behavior of the as-formed and annealed Fe-Au nanoparticles in the a.c. field upto a frequency of 1 MHz and have demonstrated their suitability for hyperthermia appliations. (3) We have investegated the metal-organic interface for its impact on the magnetic properties by sputtering permalloy (Ni79Fe21) on the self assembled monolayers of polar [16-mercaptohexadecanoic acid (MHA)] and non polar [1-Octadecanethiol (ODT)] organic molecules. It has been observed that permalloy forms films exhibiting ferromagnetic properties for the 4 nm and higher thicknesses on the polar MHA molecules which offer better adhesion to permalloy, on the other hand, it forms scattered superparamgnetic clusters on the ODT molecules which offer poor adhesion. The systematic study of the deposition of permalloy with thickness varying from 2 nm to 70 nm on the self-assembled monolayes of MHA and ODT reveals that the effect of the underlaying organic surfaces decreases as the deposition thickness increases and inplane oriented magnetic thin films are produced for 12 nm thickness on both type of surfaces. The squareness of the magnetic hysteresis loop indicates that the best inplane oriented films are produced for the 20 nm thickness, and further increase in the thickness leads to randomization of the orientation of the deposited material on both type of surfaces. We also demonstrated that by sputtering permalloy on the prefabricated templates containing MHA and ODT patterns, small scale (micron size) templates can be made with magnetic and non-magnetic patterns. The dip-pen approach may be used to extend the pattering to submicron and nanoscale level.