Abstract:
This thesis presents the design and fabrication of a high-temperature AC susceptometer aimed at measuring the linear and non linear ac-susceptibility (χ) of magnetic materials with their transition temperature above the room temperature. The work involves both linear and non-linear harmonic analysis, with a primary focus on understanding the behaviour of first and higher harmonics in the material as it transitions through its Curie point. The initial stages of the project involved preliminary experiments conducted on using an existing ac-χ set up which was designed and fabricated for the measurement of linear and non linear susceptibility at cryogenic temperatures, down to 77 K. Using this set up, linear and non linear ac-χ measurements were recorded for two ferromagnetic samples, Ni and CrO2. Ni was measurened in two morphology, Ni Foam and Ni Plate. Notabaly, CrO2 is a half metallic ferromagnet with TC ∼ 393 K and Ni is well known elemental ferromagnets with Tc ∼ 627 K. A significant portion of the work was dedicated to the design and fabrication of the high temperature setup. This project required multiple iterations to design a heater capable of achieving the necessary high temperatures. Both Nichrome and Kanthal wires were tested for high temperature heater, wound in non inductive geometry. A dimmerstat was used to regulate the power input, and Lakeshore temperature controller for measuring temperature. After several trials, the final heater used was a Nichrome wire wound inductively on a ceramic rod. This heater reached a temperature of around 750 K under vacuum condition, exceeding the requirements for the Nickel sample. Once the heater was optimized, the final step was the integration of all components into a cohesive high-temperature susceptometer system. This included the fabrication of key parts such as the secondary coil, sample rod, and the inner and outer Dewar systems, which were assembled from metal, hylum plates, rods, and teflon. After assembling all the components and setting up the system, a final set of measurements was conducted on a Nickel foam. A fast temperature scan, in which data is recorded while heating the sample from below its TC (without controlling the temperature) was conducted so as to measure the temperature variation of both first and the third harmonic of ac susceptibility of Nickel. This showed clear paramagnetic to ferromagnetic transition, in both first and third harmonics in the vicinity of TC. However, the measurement could not be extended to take reasonable number of data points in the paramagnetic region, but still confirming the functionality of the setup. While sample holder needs to be modified for controlled measurements, this work lays the groundwork for future studies in high-temperature magnetism, opening avenues for exploring new materials and refining measurement techniques at elevated temperatures.
