Design of a cryocooler based AC susceptometer

Abstract

An experimental set up for cryocooler based ac susceptometer is designed and fabricated. Hylum is chosen as the former material. Primary and secondary coils are winded over a single hylum former. Secondary coil, a set of two equal and oppositely winded coils, is first winded over the hylum former. Then the primary coil is winded over it. Copper wire (150 micron) is used for winding. The whole set up is then set in the cryocooler (HC-4E, Sumltomo cryogenics). An ac current is supplied to the primary coil from DSP Lock-in Amplifier (Model SR830), which produces the required magnetic field. This magnetic field magnetize the sample and an induced emf in secondary coil is observed. The effect of magnetic field in secondary coil is cancelled by the two oppositely winded coil system. So that the emf induced is only due to the change in moment of sample. This emf is then separated to in-phase and out-of phase voltage component in lock-in amplifier. The in-phase and out-of phase voltage component is converted to in-phase and out-of phase component of ac susceptibility by dividing a voltage factor V0. Temperature is lowered down to liquid helium temperature in cryocooler and data are recorded using lab view program. Then the in-phase and out-of phase component of ac susceptibility is plotted against varying temperature. This is the background plot for ac susceptometer.
In our project we have chosen La1-xSr xMnO3 (LSMO) as sample. Sample is prepared by auto combustion sol-gel method. Pellets are made and cut into rectangular shape. One such rectangular shaped cut sample is carefully placed in the centre of one of the secondary coil. Then the ac susceptibility measurement is done by retaining all the parameter such as applied voltage and frequency constant in lock-in amplifier. Data are plotted and the susceptibility plot with sample is obtained. The susceptibility plot for sample was obtained by deducting the background from the susceptibility plot with sample. The phase change in the LSMO system is studied using the Ac susceptometer.