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The design, fabrication and performance of an apparatus for the measurementof direct rf power absorption in colossal magnetoresistive (CMR) andsuperconducting samples are described. The system consists of a self-resonantLC tank circuit of an oscillator driven by a NOT logic gate. The samples underinvestigation are placed in the core of the coil forming the inductance L andthe absorbed power is determined from the measured change in the currentsupplied to the oscillator circuit. A customized low temperature insert is usedto integrate the experiment with a commercial Oxford Instruments cryostat andtemperature controller. The oscillator working in the rf range between 1 MHz to25 MHz is built around an IC 74LS04. The temperature can be varied from 4.2 to400 K and the magnetic field from 0 to 1.4 T. The apparatus is capable ofmeasuring direct power absorption in CMR and superconducting samples of volumeas small as 1/1000 cm^3 with a signal to noise ratio of 10:1. Further increasein the sensitivity can be obtained by summing the results of repeatedmeasurements obtained at a given temperature. The system performance isevaluated by measuring the absorbed power in La_0.7 Sr_0.3 MnO_3 (LSMO) CMRmanganite samples and superconducting Y Ba_2 Cu_3 O_7 (YBCO) samples atdifferent rf frequencies. All operations during the measurements are automatedusing a computer with a menu-driven software system, user input being requiredonly for the initiation of the measurement sequence.Comment: 32 pages including 14 figure

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