Magnetic properties of NpM4Al8−x (M=Cr,Fe, Cu) intermetallics: Mössbauer effect and neutron diffraction studies

Mössbauer effect and neutron diffraction studies on the tetragonal NpM₄Al_8?x (M=Cr, Fe, Cu) of the 14/mmm space group are reported. In NpFe₄Al_8?x, both the Np and Fe sites order magnetically at close temperatures: the Np order ferromagnetically at 115(15) K (μ(Np)) ～ 0.6 μ_B and the Fe moments order antiferromagnetically at 130(10) K (μ(Fe) = 1.05(15) μ_B) with a ++?? sequence. In NpCr₄Al_8?x, the Np order magnetically around 50K. From the susceptibility data we conclude possible antiferromagnetic order of both Np and Cr sites. NpCu₄Al_8?x does not order magnetically down to at least 2 K. The magnetic hyperfine splitting observed below 45 K is explained by slow paramagnetic relaxation. The Np isomer shifts and also its magnetic behavior point to a trivalent ion (⁵I₄ Hund's rule ground state). The observed relaxation and magnetic phenomena are discussed in terms of crystalline electric field effects. In contrast to the isostructural rare-earth RM₄Al₈ compounds with practically independent order of R and M magnetic sublattices, we show that Np and M sublattices in NpM₄Al₈ are strongly coupled. This is caused by hybridization of both Np and M atoms with Al.