Evolution of magnetic structures in NpAs_1-xSe_x solid solutions

We report Mössbauer spectroscopy, magnetization, and neutron-diffraction measurements on four samples (x=0.05,0.10,0.15, and 0.20) in the mixed system NpAs1−xSex. 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 TN(x) is from paramagnetism to an incommensurate phase, the second, at TC(x), is to a ferromagnetic (FM) phase, the third at Tm(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 →k in the range (000.14–0.24) 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 TC(x) all Np sites become equivalent and the moment directions become ⟨111⟩. Surprisingly, at Tm(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→k nature of the AF component for T