☆ 4.7 Article

Structural, optical and magnetic properties of α- and β-MnO2 nanorods

APPLIED SURFACE SCIENCE (2021)

Journal

APPLIED SURFACE SCIENCE

Volume 557, Issue -, Pages -

Publisher

ELSEVIER

DOI: 10.1016/j.apsusc.2021.149693

Keywords

Hydrothermal synthesis technique; Nanorods; alpha- and beta-MnO2; Spin-glass; Remnant magnetization measurement; ac Susceptibility

Funding

University Grant Commission (UGC) New Delhi, India

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Abstract

In this study, various polymorphs of MnO2 were synthesized using a hydrothermal technique, and their characteristics were analyzed through different experimental methods such as FTIR, XRD, and FESEM. It was found that beta-MnO2 exhibited high specific capacitance and showed spin-glass/cluster-glass behavior, while a high concentration of Mn3+ ions was observed in alpha-MnO2 and alpha beta-MnO2.

Herein, we synthesise various polymorphs of MnO2 through hydrothermal technique. Formation of pure tetragonal structured alpha-, beta- and mixture of alpha and beta phase of MnO2 are identified through Fourier transform infrared spectroscopy (FTIR) and X-Ray diffraction (XRD).Rietveld refinement reviles 73% of alpha-MnO2 and 27% of beta-MnO2 present in alpha beta-MnO2. From Field emission scanning electron micrographs (FESEM), average diameter of alpha- and beta-MnO2 nanorods is found to be 36 nm and 23 nm, respectively, while in alpha beta-MnO2, nanorods of two different sizes, 28 and 116 nm are observed. Among all polymorphs, small size of nanorods in beta-MnO2 results high specific capacitance. Through UV-visible absorption spectra, the optical band gap of alpha-, beta- and alpha beta-MnO2 nanorods are estimated to be 1.53, 1.30 and 1.45 eV, respectively. XPS confirms the presence of large concentration of Mn3+ in alpha- and alpha beta-MnO2 nanorods results high effective magnetic moment in comparison to the theoretical one which is almost same in beta-MnO2. Henceforth, we evaluate the accurate composition of MnO2 as alpha-MnO1.48, beta-MnO1.86 and alpha beta-MnO1.15. Spin-glass/cluster-glass behavior is confirmed from ac susceptibility and remanent magnetization measurements in beta-MnO2 and alpha beta-MnO2 which is absent in alpha-MnO2 depending on a critical concentration of Mn3+ ions.

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Non-equilibrium magnetic response in concentrated spin-glass Au0.89Fe0.11 alloy

Sudip Pal, Y. Thiebes, R. Niewa, A. K. Nigam, S. B. Roy

Summary: We conducted a detailed study on the dc magnetization and ac susceptibility of the ZFC and FC state of polycrystalline Au0.89Fe0.11 alloy. The temperature dependence of ZFC and FC magnetization shows a distinct peak at T-f = 33 K, indicating the cooperative freezing of finite size spin clusters. A weak thermomagnetic irreversibility is observed between ZFC and FC magnetization at T-tr, slightly below T-f. At lower temperatures (T-sh < T-tr), strong thermomagnetic irreversibility is observed in the ZFC and FC magnetization curves. The AC susceptibility recorded after cooling in the presence of a field exhibits significant dispersion below T-f, and the FC state of Au0.89Fe0.11 alloy shows a pronounced memory effect in the dc magnetization, indicating that it is not an equilibrium state.

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS (2023)