Tham khảo tài liệu 'physics of magnetism magnetic materials 2011 part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | SECTION . ANTIFERROMAGNETISM 31 and hence the susceptibility by 2 . . 9 X x XIIcos a XX sin . In a polycrystalline sample one has crystallites with all orientations relative to the field. Since the number of orientations lying within dot of the inclination is proportional to sin a da we have for the susceptibility of a piece of polycrystalline material or for a powder sample This leads to 1 . 9 Xpoly 3X11 fX-L. with Xpoly XII x at r Tn and Xpoly - x at T 0. The above results for the magnetic susceptibilities are generally found to be in qualitative agreement with the properties observed for polycrystalline samples of several simple antiferromagnetic compounds. A sharp maximum in the susceptibility at the Néel temperature or equivalently a sharp minimum in the reciprocal susceptibility are generally considered as experimental evidence for the occurrence of antiferromagnetic ordering in a given material. Let us consider the effect of an external field H on a magnetic material for which the magnetization is equal to zero before a magnetic field is applied. The work necessary to generate an infinitesimal magnetization is given by 9W ikoHdM. The total work required to magnetize a unit volume of the material is w . For antiferromagnetic materials and comparatively low magnetic fields we may substitute M H into this equation. After carrying out the integration one finds for the free energy change of the system AF -W x 2- It can be seen in Fig. that x Xll below the Néel temperature Tn- This means that the application of a magnetic field to a single crystal of an antiferromagnetic material will always lead to a situation in which the two sublattice moments orient themselves perpendicular to the direction of the applied field or nearly so as shown in the right part of Fig. . With increasing field strength the bending of the two sublattice moments into the field direction becomes stronger until both sublattice .