The Seebeck effect and the thermal diffusivity of La0.67Ca0.33MnO3−δ were investigated in the temperature range 35<T<300 K and in particular in the regime of the high magnetoresistance and maximum resistivity ρmax. We find a sign change of the thermoelectric power S(T) and a step in heat conductivity near ρmax which we attribute to a temperature-induced metal–insulator transition at about 80 K, suggesting that the large giant magnetoresistance found in these substances is due to a field-induced metal–insulator transition.
The heat diffusivity D, the electrical resistivity p and the thermoelectric power S of tetragonal Μn2-x Crx Sb single crystals with 0.02 < x < 0.12 have been measured in the temperature range of 90 K< Τ < 350 K and along different crystal axes. The heat conductivity κ and σ = 1/p are anisotropic along the crystal axes a, c with a ratio of 2:1. κ(T), p(Τ) and S(Τ) show anomalies at the spin reorientation temperature TRm of the matrix (M) Μ n 2 -C r x S b , b u t a l s o a t t h a t o f t h e c o h e r e n t f e r r o m a g n e t i c p r e c i p i t a t e (P) Mn1.037Cr0.11Sb, ΤRP which is assigned to variations in the spin dependent scattering at the Μ/Ρ interfaces via changes in the relative orientation of the magnetization vectors of matrix and precipitate (spin valve effect).
The thermoelectric power S of (Fe 1Àx Mn x ) 75 P 15 C 10 ferromagnetic metallic amorphous alloys with x 0, 0.05, 0.20, 0.30 has been measured in the temperature range 30 K`T`350 K. For all compounds, the sign of S is negative, suggesting that the transport properties are dominated by electrons. A humplike structure at low temperatures which is superimposed to a linear rise and seemingly correlated to magnetic order suggests in particular magnon drag added on diffusion thermopower as the main contributions to S(T).
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