Magnetic structure of TbBaCo 2 O 5.4 perovskite
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Q. Huang NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8562, and Department of Materials and Nuclear Engineering, University of Maryland, College Park, Maryland 20742
J.W. Lynn NISTCenter for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8562
H. Szymczak Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland (Received 18 October 2001; accepted 26 January 2002)
In accordance with magnetization studies, the fast-cooled TbBaCo2O5.4 is characterized by spontaneous magnetization around 0.18 B per cobalt ion, which develops below TN ⳱ 245 K. The neutron diffraction study of this compound revealed that magnetic moments of Co3+ ions adopting intermediate spine state are ordered antiferromagnetically. Both magnetization and neutron diffraction study showed that there is a spin reorientation process in the wide temperature range. The crystal and magnetic structures are discussed.
I. INTRODUCTION
The discovery of giant magnetoresistance (GMR) in hole-doped rare-earth (RE) manganites Ln1−xAxMnO3 (Ln, lanthanide; A, divalent metal) has attracted considerable attention to the complex oxides of 3d transition metals.1–3 Among the perovskite oxides, other than manganites, cobaltites have been found to show interesting magnetic and transport properties, as well as the GMR effect.4–6 Besides nearly stoichiometric La1−xSrxCoO3 perovskites, both GMR properties and metal–insulator transition have been recently observed in oxygen-deficient cobaltites LnBaCo2O5+␥ (Ln ⳱ Eu, Gd, Tb; ␥ ≈ 0.5)7–9 with perovskitelike structure related to YBaFeCuO5 one. The oxygen vacancies are ordered in such a way that Co ions present in both CoO6-octahedral and CoO5square pyramidal coordinations in an ordered manner.10 According to Refs. 7–10, these compounds show an antiferromagnet–ferromagnet (AFM-FM) first-order phase transition at Ti ∼ 230 to 260 K. The AFM-FM transition leads to a drop of resistivity as well as GMR properties. It is worthwhile to point out that the spontaneous magnetization above Ti is too low for parallel ordering of magnetic moments of Co3+ ions in the case of the intermediate spin state (IS; t52ge1g; S ⳱ 1). According to the calculation in Ref. 11, the spin magnetic moment of the intermediate spin configuration of Co3+ 838
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J. Mater. Res., Vol. 17, No. 4, Apr 2002 Downloaded: 29 Nov 2014
is around 2 B per Co3+ ion for the LaCoO3, whereas observed magnetic moment for LnBaCo2O5.5 taken from magnetization data is only 0.18 B per Co3+ ion. The nature of the FM state and an origin of the field-induced AFM-FM transition in LnBaCo2O5.5 double perovskites are matters for discussion. The most likely explanation supposes that LnBaCo2O5.5 undergoes a transition from a collinear AFM state into a weak FM state (Morintype transition).12 Another approach primarily associates complicated magnetic phase transitions in LnBaCo2O5.5 with the spin-state transitions f
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