  The design, fabrication and performance of an apparatus for the measurement
of direct rf power absorption in colossal magnetoresistive (CMR) and
superconducting samples are described. The system consists of a self-resonant
LC tank circuit of an oscillator driven by a NOT logic gate. The samples under
investigation are placed in the core of the coil forming the inductance L and
the absorbed power is determined from the measured change in the current
supplied to the oscillator circuit. A customized low temperature insert is used
to integrate the experiment with a commercial Oxford Instruments cryostat and
temperature controller. The oscillator working in the rf range between 1 MHz to
25 MHz is built around an IC 74LS04. The temperature can be varied from 4.2 to
400 K and the magnetic field from 0 to 1.4 T. The apparatus is capable of
measuring direct power absorption in CMR and superconducting samples of volume
as small as 1/1000 cm^3 with a signal to noise ratio of 10:1. Further increase
in the sensitivity can be obtained by summing the results of repeated
measurements obtained at a given temperature. The system performance is
evaluated by measuring the absorbed power in La_0.7 Sr_0.3 MnO_3 (LSMO) CMR
manganite samples and superconducting Y Ba_2 Cu_3 O_7 (YBCO) samples at
different rf frequencies. All operations during the measurements are automated
using a computer with a menu-driven software system, user input being required
only for the initiation of the measurement sequence.
