Thermopower behavior in the Gd5(Si0.1Ge0.9)4 magnetocaloric compound from 4 to 300 K

Recent studies on the orthorhombic Gd-5(Si0.1Ge0.9)(4) compound show, upon heating, a ferromagnetic to antiferromagnetic-like (AFM(*)) transition at T-S=78 K, coupled with a first-order structural martensitic transformation keeping the orthorhombic symmetry but producing a large increase in the interlayer Si(Ge) distances leading to covalent bond-pair breaking. A second-order AFM(*)-->(paramagnetic)PM transition occurs at T-N=125 K. We report thermopower (S) measurements for the Gd-5(SixGe1-x)(4) series, performed on an x=0.1 sample, from 4 to 300 K, with increasing and decreasing temperatures through successive thermal cycling. Resistivity measurements show a systematic increase in the residual resistivity and a dramatic change in the rho(T) behavior upon thermal cycling. In spite of this, the thermopower data show a common intrinsic behavior both in the ferromagnetic phase (T<T-S=78 K) and above similar to230 K, i.e., independent of the number of thermal cycles, increasing or decreasing temperature, and of the particular residual resistivity. Also no drastic differences are seen in S(T) from T-S to T-N, upon thermal cycling. The structural transition is marked (upon heating) by a sharp increase in the S magnitude, reaching a deep negative minimum of -23 muV K-1 at similar to95 K. Then S(T) rises with increasing slope as T approaches T-N, where dS/dT exhibits a singularity. S(T) hysteresis sets in at T-S, reaches a maximum around 100 K, does not disapear at T-N but persists in the PM phase up to similar to230 K. The later feature correlates well with a similar effect observed in rho(T). The striking differences between S(T) and rho(T) behavior under thermal cycling are analyzed. (C) 2002 American Institute of Physics.