THERMOMAGNETIC PROPERTIES OF AMORPHOUS RARE-EARTH ALLOYS WITH FE, NI,OR CO

The magnetic properties and field-dependent specific heat of melt-spun amorphous RE(70)TM(30) (RE=Gd, Tb, Dy, Ho and Er; TM=Fe and Ni) and G d65Co35 alloys were investigated as potential magnetic refrigerants. E ssentially zero magnetic hysteresis was observed in all the Gd-TM allo ys at temperatures from 5 K up to the ordering temperatures. The coerc ive force of the RE(70)TM(30) alloys depended mainly on the RE species and increased according to the order of RE=Gd<Ho<Er<Dy<Tb. The magnet ic susceptibility of most of the alloys showed apparently normal Curie -Weiss behavior above the ordering temperatures, The heat capacity mea surements in zero field and applied fields of 4 and 8 T indicated that the magnetic transition in these alloys are significantly broadened. The maximum adiabatic temperature changes for Er70Fe30, Gd70Ni30 and G d65Co35 amorphous alloys in a field change of 8 T are 4.0, 3.4, and 3. 0 K, respectively. Mossbauer spectroscopy revealed that Fe atoms in th e amorphous RE(70)Fe(30) alloys carry a small magnetic moment that may complicate the magnetic ordering in the alloys, A simple model assumi ng a Gaussian distribution of ordering temperatures around the apparen t Curie temperature was constructed to attempt to reconcile the differ ences in the observed magnetic properties of these amorphous alloys, T he broad magnetic transition is attributed to the fluctuation of the e xchange integral caused by the structural disorder in amorphous alloys . The calculated susceptibility, magnetization, and heat capacity agre ed reasonably well with the experimental data and show that the magnet ic susceptibility and magnetization are only weakly affected by the di stribution of ordering temperatures, but the heat capacity is much mor e sensitive to such a distribution. To effectively screen out magnetic refrigerants with sharp magnetic transitions and correspondingly larg e adiabatic temperature changes from those with broadened transitions and small adiabatic temperature changes, the field-dependent heat capa city measurement technique is a powerful tool to use. (C) 1996 America n Institute of Physics.