EFFECT OF SAMPLE PREPARATION ON THE MAGNETIC AND MAGNETOCALORIC PROPERTIES OF AMORPHOUS GD70NI30

Certain applications of magnetic materials require the knowledge of th e magnetocaloric effects, i.e., the adiabatic change of temperature or the isothermal change of the entropy upon application or removal of m agnetic field. While the isothermal magnetic entropy change can be cal culated from magnetic data only, the calculation of the adiabatic temp erature change requires the knowledge of the field-dependent specific heat as well. To compare magnetocaloric effect results obtained from m agnetic-only, thermal-only, or combined methods requires the evaluatio n of the mutual reliability of all methods. In crystalline materials i t usually does not present a problem to perform thermal and magnetic m easurements on the same sample or at least on samples of identical sta te. For amorphous materials, unfortunately, thermal measurements are e xtremely difficult to perform on the ribbons themselves so usually pel lets pressed from powdered ribbons, with silver as a binder, are appli ed. After the magnetic entropy change calculated from thermal measurem ents performed on such a composite did not agree with the data obtaine d from magnetic measurements performed on the original ribbon, magneti c measurements were carried out on the pellet and its predecessor powd ers. Comparison of the results revealed that the pellet's properties d o not reflect the properties of the original ribbon. However, comparis on of magnetic and thermal measurements carried out on the pellet itse lf show good agreement if proper numerical methods are used for evalua tion. The study concludes that amorphous ribbons are far more competit ive for certain applications than the thermal measurements, performed on the pellet only, would indicate. (C) 1998 American Institute of Phy sics.