Transport properties in the pseudobinary Er5Si4 compound

Joao P. Araujo 2Andre M. Pereira 2Jose Teixeira 2M. E. Braga 2F Correia 2R P. Pinto 2J. B. Sousa 2L. Morellon 1C. Magen 1P. A. Algarabel 1Ricardo M. Ibarra 1

1. Universidad de Zaragoza-CSIC, Facultad de Ciencias, Pedro Cerbuna 12, Zaragoza 50009, Spain
2. Departamento de Fisica and IFIMUP, Universidade do Porto (UP), Rua do Campo Alegre, 687, Porto 4169-007, Portugal

Abstract

Er5Si4 belongs to the R5(SixGe1-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 TS =190K, from the orthorrombic (O) to the monoclinic (M) phase below TS, and the 2nd-order magnetic transition occurs at TN =30K from PM to AFM below TN. 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 (TS) and magnetic (TN) 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 TS and S~-4 μVK-1 above TS. 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 TN 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 TN, extending in the PM phase up to ~2TN.

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Presentation: oral at E-MRS Fall Meeting 2005, Symposium B, by Andre M. Pereira
See On-line Journal of E-MRS Fall Meeting 2005

Submitted: 2005-05-05 16:07
Revised:   2009-06-07 00:44
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