Mossbauer and magnetic susceptibility studies of HoCoO, have shown that there is coexistence of low-spin Co(III) ions and high-spin Co'+ ions; Co(III) being more predominant at low temperatures. The population of Co(III) and Co'+ equalizes above a particular temperature with these ions occupying alternate oxygen octahedra, leading to an ordered phase. The ordering transition is evidenced by the temperature variation of Lamb-Mossbauer factor, x-ray Debye-%aller factor, and inverse susceptibility.Electrical-conductivity data reflect these changes in the spin-state equilibria and show that at around 1080 K, HoCo03 becomes metallic. At this temperature, a first-order localized electron-collective electron transition seems to occur. Co(II) and Co'+ are not formed by electron transfer from Co'+ to Co(III) as in LaCoO, . This behavior is correlated with the variation of covalency in the cobaltates.
IVTossbauer studies show that cobalt ions in LaCo0 3 exist predominantly in the low-spin Co 111 state at low temperatures and partially transform to the high-spin Co 3+ state up to 200 K. Above 200 K, Co 11 and Co 4+ ions are formed by the transfer of d electrons from Co 3+ to Co 111 ; Co 3+ ions completely disappear at the first-order localized-to~itinerant electron transition temperature.
The resistivity, magnetoresistance (MR) and magnetization of double-layered manganite La 1.2 Sr 1.8-x Ca x Mn 2 O 7 (0.0 ≤ x ≤ 0.4) are studied to investigate the effect of substitution of Ca 2+ for Sr 2+ on the transport and magnetic properties. The resistivity data suggest that Ca doping results in the decrease of insulator-tometal transition temperature, T IM . The conduction mechanism above T IM is found to be of Mott variable range hopping type. The highest MR ratios (∆ρ/ρ 0 ) of 73% (H = 1.5 T) and 93% (H = 4 T) are observed for x = 0.3 sample at ~25 K. The variation of MR with temperature and applied magnetic field is discussed. A secondary transition at T * is observed in all the samples, which may be due to the twodimensional short-range ferromagnetic ordering.
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