Evolution of magnetic structures inNpAs1−xSexsolid solutions

Abstract

We report M\ossbauer spectroscopy, magnetization, and neutron-diffraction measurements on four samples $(x=0.05,0.10,0.15,$ and $0.20)$ in the mixed system ${\mathrm{NpAs}}_{1\ensuremath{-}x}{\mathrm{Se}}_{x}.$ The evolution of magnetic phase diagram vs the temperature T and the Se concentration x has been determined. Up to $x=0.15,$ three distinct phase transitions are present. The first antiferromagnetic (AF) at ${T}_{N}(x)$ is from paramagnetism to an incommensurate phase, the second, at ${T}_{C}(x),$ is to a ferromagnetic (FM) phase, the third at ${T}_{m}(x)$ is from pure FM to a mixed phase where AF and FM ordering coexist along perpendicular directions. The temperatures of the transitions are functions of Se concentration x. The incommensurate structure is described by a longitudinal amplitude modulated sinusoidal wave with a wave vector $\stackrel{\ensuremath{\rightarrow}}{k}$ in the range $(000.14\char21{}0.24)$ that is shifted toward the center of the Brillouin zone with increasing x. A squaring up of this wave, if present, is too small to be detected. At ${T}_{C}(x)$ all Np sites become equivalent and the moment directions become $〈111〉.$ Surprisingly, at ${T}_{m}(x)$ AF interactions again become important and cause the moments to rotate and to align along $〈221〉$ directions. Measurements in an external magnetic field up to 4.8 T have allowed us to determine the $1\stackrel{\ensuremath{\rightarrow}}{k}$ nature of the AF component for $Tl{T}_{m}$ and to establish the moment directions. Increasing Se content reduces the incommensurate phase to a small range of T and reduces the AF interactions. At $x=0.20$ pure FM behavior, as shown by the magnetization measurements, is recovered. The saturated Np moments at low T decrease slightly with increasing x.