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Interplay between structure, microstructure and magnetic properties of the CoMn2O4 spinel material

Complex metal oxides, especially those crystallizing in the spinel-type family AB2O4, represent an important class of the functional materials, which as a result of their unique chemical, electric, magnetic and mechanical properties have wide range of potential applications ranging from energy storage and conversion to magnetism, electronics and catalysis.1 Among large number of different spinel materials, cobalt manganite (CoMn2O4) attracted great attention as new advanced anode material for lithium-ion batteries, electrocatalysts for oxygen reduction/evolution reactions and catalyst in CO oxidation. Majority of recent work appears to be strongly focused on electrochemical properties i.e. applications of the CoMn2O4 oxide as high capacity and high performance anodes LiB, while the structural and magnetic studies have been scarce in spite of few papers reporting on very intriguing and complex but still poorly understood magnetic behaviour.CoMn2O4 samples were prepared by thermal decomposition of single-molecular precursor {; ; [Co(bpy)3][Mn2(C2O4)3]·H2O}; ; n at varius temperature: 500, 700, 800 and 1000 °C. Rietvled structure refinement showed that the increase of decomposition temperature causes of the octahedral bond lengths d(Moct–O) and shortening of the tetrahedral once, d(Mtet–O). Decrease of metal oxygen interatomic distances resulted from thermally enhanced substitution of tetCo2+ by smaller tetMn3+ cations at A site (4a) of spinel structure. On the other hand, octahedral B site (8d) becomes partially occupied by octCo2+ as a consequence of transferred Mn cations from octahedral to tetrahedral site. Changes in the temperature of heating, besides influencing the structural features within crystal lattice, also had a strong impact on microstructural properties of prepared samples. Size-strain analysis performed during Rietveld refinement showed that average crystallite size of CoMn2O4 can be easily tuned in range of 8–40 nm.The temperature dependence of magnetization, M(T), of all prepared oxides was measured in different magnetic fields, in the temperature range 2–330 K. Two modes of measurement were applied: after cooling in zero-field (ZFC), and after cooling in magnetic field (FC) in which measurement is performed during heating. The field dependences of magnetization, M(H), i.e. magnetic hysteresis loops, were measured at several stable temperatures in fields up to 50 kOe. Our results show that it is possible to switch between the superparamagnetic and ferrimagnetic behaviour of CoMn2O4 and even to tailor the characteristic magnetic transition temperatures, i.e. the boundaries between the hard and soft magnetic behaviour. The work was financed by the Croatian Science Foundation grant no. IP-2014-09-4079. J. Hemberger, P. Lunkenheimer, R. Fichtl, H.-A. Krug von Nidda, V. Tsurkan and A. Loidl, Nature, 2005, 434, 364.

spinel structure; Rietveld refinement; temperature dependence of magnetization

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