Magnetic and transport properties of the Co‐doped manganite La 0.7 Sr 0.3 Mn 1– x Co x O 3 (0 ≤ x ≤ 0.5)

The effect of Co doping on the magnetic and transport properties of La 0.7 Sr 0.3 Mn 1– x Co x O 3 (0 ≤ x ≤ 0.5) is investigated. The Co doping at Mn sites dilutes the double‐exchange interaction between Mn 3+ and Mn 4+ ions and changes the long‐range ferromagnetic (FM) order of La 0.7 Sr 0.3 MnO 3 (LSMO) to the spin glass (SG) or cluster glass (CG) state for samples with x ≥ 0.1. For x ≥ 0.3, the paramagnetic (PM) metal to FM metal transition of LSMO disappears and the temperature dependence of resistivity ρ ( T ) follows semiconducting behavior in the whole measured temperature region with the resistivity increasing by orders of magnitude. An interesting result is that ρ ( T ) exhibits an obvious anomaly at T * ∼ 100 K, which is ascribed to the spin‐state transition of Co ions. For samples with x = 0.1 and 0.3, magnetoresistance (MR) effects are markedly enhanced in the low‐temperature region compared with undoped LSMO, which is suggested to originate from the appearance of spin‐dependent tunneling magnetoresistance. However, for samples with x = 0.5, the MR effect is suppressed over the entire temperature region measured and an obvious exchange anisotropy phenomenon, characterized by the shift of the hysteresis loop, is also observed, which is ascribed to the marked increase of the antiferromagnetic insulating phase. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)