Unusual magnetic behavior in Gd5(Si1.5Ge2.5) and Gd5(Si2Ge2)

Abstract

The coexistence of ferromagnetic and paramagnetic phases in the $4f$-electron systems ${\mathrm{Gd}}_5{(\mathrm{S}\mathrm{i}}_{1.5}{\mathrm{Ge}}_{2.5})$ and ${\mathrm{Gd}}_5{(\mathrm{S}\mathrm{i}}_2{\mathrm{Ge}}_2)$ during the magnetic-martensitic phase transition have been observed. A dc magnetic field reversibly changes both the magnetic and crystal structures of the alloys above their respective Curie temperatures. A negative imaginary component of the ac magnetic susceptibility for both alloys has been observed in the ferromagnetic state. The results are discussed in terms of indirect Ruderman-Kittel-Kasuya-Yosida and direct superexchange interactions, and anomalies of the relaxation process.