NONCONTACT THERMOACOUSTIC METHOD TO MEASURE THE MAGNETOCALORIC EFFECT

We report here the measurement of the magnetocaloric effect in a magnetic material with the use of noncontact type detection. The technique is based on the thermoacoustic principle. A sample with a periodically changing surface temperature induces exponentially decaying pressure waves which can be detected by a sensitive microphone. A magnetic material placed within a gas-tight cell at an appropriate dc magnetic field and temperature will exhibit a periodically changing surface temperature due to the magnetocaloric effect when subjected to a small ac field superimposed over the dc field. The microphone response, detected via a lock-in amplifier can be correlated to the magnitude of the temperature changes after suitable calibration. The knowledge of the magnetic equation of state allows the determination of the total magnetocaloric effect ΔT for an arbitrary field step from zero to the final value, using the magnetization data of the sample as well. Gadolinium has been used for calibration, and new results are presented for a gadolinium-dysprosium and a gadolinium-erbium alloy. © 1995 American Institute of Physics.