Negative thermal expansion and antiferromagnetism in the actinide oxypnictide NpFeAsO

TL;DR

This study synthesizes and characterizes NpFeAsO, revealing antiferromagnetic order of neptunium without lattice distortion or iron magnetic order, and observes negative thermal expansion below the Néel temperature.

Contribution

It provides the first detailed experimental and theoretical analysis of NpFeAsO, highlighting its unique magnetic and thermal properties compared to related compounds.

Findings

Neptunium orders antiferromagnetically below 57 K.

No magnetic order observed on iron sublattice.

Lattice expands with decreasing temperature below T_N.

Abstract

A neptunium analogue of the LaFeAsO tetragonal layered compound has been synthesized and characterized by a variety of experimental techniques. The occurrence of long-range magnetic order below a critical temperature T_N = 57 K is suggested by anomalies in the temperature-dependent magnetic susceptibility, electrical resistivity, Hall coefficient, and specific heat curves. Below T_N, powder neutron diffraction measurements reveal an antiferromagnetic structure of the Np sublattice, with an ordered magnetic moment of 1.70(0.07) \mu_B aligned along the crystallographic c-axis. No magnetic order has been observed on the Fe sublattice, setting an upper limit of about 0.3 \mu_B for the ordered magnetic moment on the iron. High resolution x-ray powder diffraction measurements exclude the occurrence of lattice transformations down to 5 K, in sharp contrast to the observation of a tetragonal-to-orthorhombic distortion in the rare-earth analogues, which has been associated with the stabilization of a spin density wave on the iron sublattice. Instead, a significant expansion of the NpFeAsO lattice parameters is observed with decreasing temperature below T_N, corresponding to a relative volume change of about 0.2% and to an invar behavior between 5 and 20 K. First-principle electronic structure calculations based on the local-spin density plus Coulomb interaction and the local density plus Hubbard-I approximations provide results in good agreement with the experimental findings.