Magnetic behavior in Mg-stabilized bccβ-Gd andβ-Dy

Bcc β-Gd and β-Dy, stabilized by Mg additions, exhibit spin-glass-like behavior. Both systems show field-cooling effects in the magnetic susceptibility which is indicative of spin freezing reactions. The β-Gd alloys order ferromagnetically (<80 K) first on cooling before undergoing a Gabay-Toulouse-type reentrant spin-glass transition (<50 K) into a mixed ferromagnetic plus spin-glass phase. Low-field ac susceptibility measurements show both the Curie and spin freezing transitions. Low-temperature heat capacity (down to 1.5 K) shows evidence of both ferromagnetic and spin-glass excitations. A magnetic phase diagram predicts a pure spin-glass phase for Gd concentrations up to 66 at. % Gd. The β-Dy alloys exhibit a cusp in the ac susceptibility characteristic of spin-glass behavior. Field-cooled magnetic-susceptibility measurements suggest a close competition between antiferromagnetism and spin-glass behavior. The occurrence of the maximum in the magnetic susceptibility at 1.4 T is evidence that some atoms may order antiferromagnetically. A large linear heat-capacity term which is probably due to both the electronic specific heat γ and a spin-glass contribution plus the presence of large $T^2$ and $T^3$ terms support the mixed-state hypothesis. The metastable bcc Gd-Mg and Dy-Mg alloys are unique in that they have the highest concentration of magnetic atoms in a crystalline, metallic spin glass (>70 at. % Gd or Dy).