Polarized neutron diffraction

Abstract

Abstract. This lecture addresses the different applications of polarized neutron diffraction, with or without subsequent polarization analysis, to the determination of magnetic structures, to the separation of magnetic and nuclear contributions and to magnetization distributions mapping. The first sections introduce some of the fundamental equations. Then the problems of magnetic domains, encountered in crystals and of particular importance as soon as polarized neutrons are concerned, are presented. The final sections discuss some of the different ways to use polarized neutron and are illustrated by examples. PREAMBLENeutron scattering has progressed over the last sixty years to become an invaluable tool to probe experimentally condensed matter. As far as magnetism is concerned, this technique has been recognized from the early days as unique. The most widespread use of this tool is of course the determination of magnetic structures using unpolarized beams, that is the determination of the directions in which moments point in a magnetically ordered material. Since the first experiment on MnO by Shull and Smart in 1949 [1] which was an experimental proof of the antiferromagnetism predicted by Néel, thousands of structure determinations have been reported and have revealed much more complicated magnetic arrangements than the simple ferromagnetic or antiferromagnetic cases. The comprehension of the stability of these new structures was and is still at the origin of numerous theoretical developments, leaving the study of magnetic structures completely open.From a theoretical point of view, the neutron spin and the possibility of using polarized neutron beam was taken into account from the very beginning [2-6]. Complete equations of polarization analysis in a neutron scattering experiment were derived as early as 1963 [7,8]. But in comparison to those theoretical advances, experimental possibilities were much longer to appear. In 1959 Nathans et al. demonstrated the huge sensitivity of polarized neutrons to weak magnetic signals, allowing very precise magnetization distribution maps to be drawn [9]. Then it took ten years to Moon, Riste and Koehler to operate in Oak Ridge the first triple axis machine with both incident polarized neutrons and longitudinal polarization analysis capabilities, giving access to many new pieces of information [10]. Another twenty years were necessary until Tasset et al. generalized longitudinal polarization analysis to a completely spherical one [11]. During this time, Maleyev also showed that polarization of neutron beams could allow very precise measurements of the energy exchange in a scattering experiment, leading to the development of the spin echo technique by Mezei [12] and to the numerous applications this technique has today. This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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