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

Abstract

Recent studies on the orthorhombic Gd5(Si0.1Ge0.9)4 compound show, upon heating, a ferromagnetic to antiferromagnetic-like (AFM*) transition at TS=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 TN=125 K. We report thermopower (S) measurements for the Gd5(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 ρ(T) behavior upon thermal cycling. In spite of this, the thermopower data show a common intrinsic behavior both in the ferromagnetic phase (T<TS=78 K) and above ∼230 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 TS to TN, 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 μV K−1 at ∼95 K. Then S(T) rises with increasing slope as T approaches TN, where dS/dT exhibits a singularity. S(T) hysteresis sets in at TS, reaches a maximum around 100 K, does not disapear at TN but persists in the PM phase up to ∼230 K. The later feature correlates well with a similar effect observed in ρ(T). The striking differences between S(T) and ρ(T) behavior under thermal cycling are analyzed.