A thorough study of the magnetic and transport properties of La0.5Sr0.5Co1−xFexO3 (0⩽x⩽0.6) compounds has been made. The Fe substitution destroys the metallic state and the resistivity increases by orders of magnitude even with a very small extent of Fe substitution. The charge localization due to Fe substitution is likely to have its origin in the electronic configuration rather than in its ionic size. The hole-poor regions, corresponding to the Fe-rich regions, would also dilute the magnetic lattice and thereby prevent the occurrence of long-range order. Spin-glass behavior was observed for x⩾0.5 compositions and is ascribed to the frustration of random competing exchange interactions, namely the ferromagnetic double-exchange interaction between Co3+ and Co4+, and the antiferromagnetic interactions like Co-O-Fe and Fe-O-Fe. A dynamic scaling analysis of ac susceptibility data using conventional critical slowing down indicates a finite spin-glass phase-transition temperature Tg≈85 K and a dynamic exponent zv≈12.4, for x=0.5 composition.
We studied single-crystalline Pr 0.5 Sr 0.5 MnO 3 by means of measurements of magnetic susceptibility and specific heat at ambient pressure (P), and electrical resistivity (!) in hydrostatic pressures up to 2 GPa. This material displays ferromagnetic (FM) order, with Curie temperature T C ! 255 K. A crystallographic transformation from I4/mcm to Fmmm is accompanied by the onset of antiferromagnetism (AFM), with Néel temperature T N !"161 K. The effect of pressure is to lower T C , and raise T N at the approximate rates of -3.2 K/GPa, and 14.2 K/GPa, respectively.Although the value of T N increases with P, due to the enhancement of the superexchange interactions, the AFM-Fmmm state is progressively suppressed, as pressure stabilizes the FM-I4/mcm phase to lower temperatures. The ! vs T data suggest that the AFM phase should be completely suppressed near 2.4 GPa.
The magnetic and transport properties of La 0.5 Sr 0.5 (Fe x Co 1-x )O 3 (0 ≤ x ≤ 0.2) compounds were investigated. T C decreased slightly and the resistivity increased, even with a very small extent of Fe substitution. The Fe substitution induced additional antiferromagnetic Fe-O-Fe interactions that suppress strongly the ferromagnetism and conductivity, thus causing a separation between two phases at around 124 K for both x = 0.1 and 0.2 compositions. Spin-glass behavior was observed for the Fe-doped samples. The resistivity, ( ) T ρ , can be described by the Mott's variable range hopping model in the low temperature region.
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