Magnetic behaviour of NpAs2 from Mössbauer spectroscopy

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Abstract

The Mössbauer hyperfine spectra of the 60 keV resonance of 237Np in powder and single crystal absorbers of NpAs2 were measured between 4.2 and 60 K. Below 18 K a simple magnetic plus quadrupole pattern is seen in accordance with a ferromagnetic spin structure in tetragonal NpAs2. The isomer shift favors the 4+ charge state, the hyperfine field of 288 T implies a moment of 1.5μB at the Np ion. The large reduction compared to the free ion values points towards a strong mixing of the electronic ground state by crystalline field interactions. Above 18 K the spectrum changes into a complex hyperfine pattern indicating a sinusoidally modulated spin structure. Near 54 K a transition into the paramagnetic state is observed. Both magnetic transitions (18 and 54 K) exhibit a feature typical for a first-order character.

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    1

    Supported by CNRS.

    2

    On leave from Technical University Munich, Fed. Rep. Germany.

    3

    Centre d'Etudes Nucléaires de Fontenay-aux-Roses, France.

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