The effects of small metal additions (Co, Cu, Ga, Mn, Al, Bi, Sn) on the magnetocaloric properties of the Gd5Ge2Si2 alloy

The structural and magnetic properties of arc-melted and homogenized (1300 degrees C, 1 h) alloys of Gd5Ge1.9Si2X0.1 (X=Cu, Co, Ga, Mn, Al, Bi, or Sn) were investigated by powder x-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and magnetometry. The addition of Cu, Ga, Mn, and Al completely eliminated the large hysteresis losses present in the undoped Gd5Ge2Si2 alloy between 270 and 330 K, broadened the magnetic entropy change Delta Sm peak, and shifted its peak value from 275 to 305 K similar to that observed earlier for Gd5Ge1.9Si2Fe0.1. The addition of Bi or Sn had a negligible effect on either the alloy hysteresis losses or the characteristics of the Delta S-m vs T peak. The microstructure of the alloy doped with Cu, Co, Ga, Mn, or Al consisted of a majority phase (depleted of silicon) and a minor intergranular phase (rich in silicon and of the corresponding metal additive). For Bi or Sn doping, the microstructure consisted of only the Gd5Ge2Si2 phase. Low temperature x-ray diffraction data on an Fe-doped sample showed the same spectra at 245 and 300 K, consistent with the majority phase possessing an orthorhombic structure. Refrigeration capacity calculations show that Gd5Ge1.9Si2X0.1 (X=Fe,Cu,Co,Ga,Mn, or Al) alloys are superior magnetic refrigerants compared to the undoped Gd5Ge2Si2 alloy. (C) 2006 American Institute of Physics.