Abstract
We present the results obtained by superconducting quantum interference device (SQUID) magnetometry, specific heat, and M\"ossbauer spectroscopy measurements carried out on ${\mathrm{Np}}_{2}{\mathrm{Ni}}_{17}$ polycrystalline samples. We show that long-range magnetic order, with a moment ${\ensuremath{\mu}}_{(2b)}\ensuremath{\sim}2.25$ ${\ensuremath{\mu}}_{B}$, occurs below ${\mathrm{T}}_{N}=17.5$ K on the Np $(2b)$ sites. A nontrivial situation is observed in that the other Np sites $(2d)$ do not take part to the order transition and carry only an induced moment of about 0.2 ${\ensuremath{\mu}}_{B}$ below ${\mathrm{T}}_{N}$. A combined analysis of the different experimental data sets allowed us to determine key parameters associated with the electronic structure of the system. The experimental results are discussed against first-principles electronic structure calculations based on the spin-polarized local spin density approximation plus Hubbard interaction.
Highlights
The coexistence of magnetic and nonmagnetic atoms of the same element in an ordered compound is unusual and generally associated with unconventional behavior, such as mixed valence in Ce intermetallics [1], spontaneous toroidal order [2], and magnetic instabilities related to topological frustration [3]
In the magnetically ordered phase below TN = 17.5 K, the Mossbauer absorption spectra are given by the sum of two components with equal total weight, one fully splitted and corresponding to a large value of the hyperfine field [Bhf = 483(1) T], the other showing a broad central absorption line indicating the presence of a second Np site on which the hyperfine field is one order of magnitude smaller [Bhf = 40(2) T]
Such a conclusion is corroborated by the variation of the magnetic entropy near the transition temperature, which confirms that only half of the Np atoms in the lattice takes part in the magnetic transition
Summary
The coexistence of magnetic and nonmagnetic atoms of the same element in an ordered compound is unusual and generally associated with unconventional behavior, such as mixed valence in Ce intermetallics [1], spontaneous toroidal order [2], and magnetic instabilities related to topological frustration [3]. Mossbauer and specific heat measurements we have determined the values of the magnetic moments on the two Np sites of Np2Ni17, and shown that only one of them takes part in the magnetic transition, whilst the other carries only a smaller (roughly one tenth) induced moment. This remarkable difference is explained in terms of the crystal field potential acting on the two sites, which stabilizes a magnetic doublet on the 2b and a nonmagnetic singlet on the 2d position
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