Er₅Si₄ belongs to the R₅(Si_xGe_1-x)₄ family (R=rare earth), which has been intensively studied in recent years due to the appearance of a giant magnetocaloric effect. This effect is associated with a simultaneous 1st-order magnetostrutural transition which causes drastic changes on the crystal lattice, magnetic state and Fermi surface, inducing large entropy changes. However in Er₅Si₄ the two transitions are decoupled: the 1st-order structural transition occurs at T_S =190K, from the orthorrombic (O) to the monoclinic (M) phase below T_S, and the 2nd-order magnetic transition occurs at T_N =30K from PM to AFM below T_N. We can therefore separately study the physical effects of each transformation.

Here we report detailed measurements of the thermopower S(T), electrical resistivity ρ(T) and magnetization on a polycrystal of Er₅Si₄ between 10 and 300 K. Particular attention is given to the corresponding behaviour near the structural (T_S) and magnetic (T_N) transitions.

The structural transition produces a large thermopower step (ΔS), associated with the change in crystal symmetry (O→M) and Fermi surface modification. As a consequence we have from S~-10 μVK^-1 below T_S and S~-4 μVK^-1 above T_S. The relative changes of the ρ(T) is however considerably smaller (Δρ/ρ~10%). Large thermal hysteresis (ΔT~10K) is also associated with the structural transition.

The magnetic transition T_N produces no discontinuities on S and ρ, but a pronounced decrease with temperature occurs in |S| and ρ within the AF-phase. This effect is analyzed in terms of electron-spin scattering within the Kasuya mean field treatment. At low temperatures ρ reaches its residual value imposed by electron impurity scattering, whereas S gets fairly small and changes from negative to positive (with dS/dT>0).Important spin fluctuation effects are observed in dρ/dT and dS/dT near T_N, extending in the PM phase up to ~2T_N.

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