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We report Mossbauer spectroscopy, magnetization, and neutron-diffraction measurements on four samples (x=0.05, 0.10, 0.15, and 0.20) in the mixed system NpAs(1)-rSe(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 anti-ferromagnetic (AF) at T(m)(x) is from paramagnetism to an incommensurate phase, the second, at T(C)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 k in the range (0 0 0.14-0.24) that is shifted toward the centre 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 ( III ). 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 I k nature of the AF component for T < 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.

Additional information

Authors: BOMBARDI A, Commissariat a l'Energie Atomique, Grenoble (FR);BOURDAROT ET AL, Commissariat a l'Energie Atomique, Grenoble (FR);COLINEAU ET AL, JRC-Institute for Transuranium Elements, Karlsruhe, (DE);VOGT O, Laboratorium fur Festkorperphysik, ETH-Zurich (CH);MATTENBERGER K, Laboratorium fur Festkorperphysik, ETH-Zurich (CH)
Bibliographic Reference: ART 90300. Physical review B, Vol 62, No 22, 1-December (2000) pp.14920-14927
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