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

The coexistence of ferromagnetic: and paramagnetic phases in the 4f-electron systems Gd-5(S1.5Ge2.5) and Gd-5(Si2Ge2) during the magnetic-martensitic phase transition have been observed. A de magnetic field reversibly changes both the magnetic and crystal structures of the alloys above their respective Curie temperatures. A negative imaginary component of the uc 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.