Browsing by Author "Hodges, JP"

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    Enhancement of the Curie temperature in NdBaCo2O5.5 by a-site Ca substitution
    (Americal Physical Society, 2012-08-24) Kolesnik, S; Dabrowski, B; Chmaissem, O; Avci, S; Hodges, JP; Avdeev, M; Świerczek, K
    Layered cobaltites RBaCo2O5.5 (R = rare earth and Y) undergo a sequence of magnetic and electronic transitions between antiferromagnetic/ferrimagnetic/paramagnetic and insulating/paramagnetic metallic states with respective transition temperatures T-N (similar to 230-260 K) < T-C (similar to 250-290 K) < T-MI (similar to 360 K). We have synthesized a Nd1-xCaxBaCo2O5.5 series (0 <= x <= 0.2) of cation-[(Nd,Ca)/Ba]-ordered and oxygen vacancy-ordered materials and investigated them by neutron diffraction, magnetization, and electronic and thermal transport. We observe that upon Ca doping, T-N is decreasing to 0 for x = 0.1 and T-C is increasing and coincides with T-MI for x > 0.12, which weakly changes with Ca substitution from similar to 360 to similar to 340 K. This is the largest enhancement of T-C observed for these cobaltites. Unlike hole doping by adding oxygen, Ca doping does not disrupt the cation and oxygen vacancy orderings up to x = 0.20. © 2012, American Physical Society.
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    Structural and magnetic properties of Nd1-xCaxBaCo2O5.5
    (American Physical Society, 2012-03-02) Chmaissem, O; Kolesnik, S; Dabrowski, B; Avci, S; Avdeev, M; Hodges, JP
    R1-xAxBaCo2O5.5 (R = rare earth, A = alkaline metal) is a relatively new class of complex oxide materials that exhibit a wide range of magnetic attributes in addition to metal/insulator switching properties, structural transitions and superstructure order parameters. In many ways, this family exhibits behaviors similar to those of the famous colossal magnetoresistive manganites; however, more complex properties have also been identified owing to the fact that the oxidation state of the cobalt ions often behave in unpredictable ways depending on the chemical composition of the investigated material and the corresponding Co local environment. Thus, Co^3+ and Co^4+ ions with high, intermediate and low spin states may be produced offering an additional degree of freedom to be accounted for when designing new materials with tunable magnetic properties. In this talk, I will discuss the effects of calcium substitution at the Nd sites and the various structural and magnetic models as determined by neutron powder diffraction and complementary magnetic measurements. © 2021 American Physical Society