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

Abstract

Mossbauer effect and neutron diffraction studies on the tetragonal NpM4Al8−x (M=Cr, Fe, Cu) of the 14/mmm space group are reported. In NpFe4Al8−x, both the Np and Fe sites order magnetically at close temperatures: the Np order ferromagnetically at 115(15) K (μ(Np)) ∼ 0.6 μB and the Fe moments order antiferromagnetically at 130(10) K (μ(Fe) = 1.05(15) μB) with a ++−− sequence. In NpCr4Al8−x, the Np order magnetically around 50K. From the susceptibility data we conclude possible antiferromagnetic order of both Np and Cr sites. NpCu4Al8−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.