Apparatus for nonresonant rf power absorption studies in high $T_c$ superconductors and CMR materials using rf oscillators

The design, fabication, and performance of an apparatus for measurement of nonresonant rf power absorption (NRRA) in superconducting and CMR samples are described. The system consists of an effective self-resonant LC tank circuit driven by a NOT gate (Logic gate). The samples under investigation are placed in the core of an inductive coil and nonresonant power absorption is determined from the measured shift in total current supplies to the whole oscillator circuit. A customized low temperature insert is used to integrate the experiment with a commercial oxford cryostat and temperature controller. The system makes use of a sensitive digital multimeter (Keithley 2002 model) and is capable of measuring NRRA in superconducting and colossal magnetoresistance samples of volume as small as $1X10^-^3$ $cm^3$ with a signal to noise ratio of 10. Further increase in the sensitivity of the experimental setup can be obtained by summing the results of repeated measurements obtained in the same temperature interval. The system has been tested for an IC 74LS04 oscillator at frequencies between 1 MHz and 25 MHz in the temperature range from 4.2 K to 400 K and in magnetic field from 0 to 1.4 T. The system performance is evaluated by measuring the NRRA in $YBa_2Cu_3O_7$ (YBCO) superconducting sample and $La_0_._7Sr_0_._3MnO_3$ (LSMO) colossal magnetoresistive (CMR) manganite samples at different rf frequencies. During a measurement all operation are controlled automatically by computer from a menu-driven software system, with user input required only on initiation of measurement sequence