Testing of an AC Magnetic Susceptometer

Abstract

Knowledge of strongly correlated electron materials may provide new or enhanced technologies that use these materials such as magnetic refrigeration or magnetic resonance imaging. These materials are unique since they may have ferromagnetic, antiferromagnetic, or superconducting states. A susceptometer, an instrument capable of measuring magnetic susceptibility or a material’s response to an applied magnetic field, can be used to find the critical temperature at which the material becomes superconducting or magnetic phase transitions take place. The design of the instrument is based on Faraday’s Law of Induction. A 500 Hz AC current of 100 A is applied to a primary coil. A magnetic field of about .1 Oe is generated and polarizes the sample. This induces a voltage onto two inner coils that are oppositely wound to minimize the background signal. The susceptometer system is placed in a cryocooler that creates a temperature environment from 10 K to 300 K. To test the susceptometer, a 122 mg gadolinium polycrystal was measured. A sharp change was found near 293 K at which gadolinium is known to transition to a ferromagnetic state. At around 210 K, there is a peak at which the signal begins to decrease linearly with temperature, which qualitatively agrees with literature data. In the near future, the susceptometer will be used to find the critical temperature of Gd and Nd nanoparticles that are produced through the inverse micelle method.