Đang chuẩn bị nút TẢI XUỐNG, xin hãy chờ
Tải xuống
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 4.3. ANTIFERROMAGNETISM 31 and hence the susceptibility by 2 . . 9 X x XIIcos a XX sin . 4.3.31 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. 4.3.33 with Xpoly XII x at r Tn 4.3.34 and Xpoly - x at T 0. 4.3.35 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 ụ.ữHdM. 4.3.36 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- 4.3.37 It can be seen in Fig. 4.3.2 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. 4.3.2. With increasing field strength the bending of the two sublattice moments into the field direction becomes stronger until both sublattice .
