TAILIEUCHUNG - Physics of Magnetism Magnetic Materials 2011 Part 7

Tham khảo tài liệu 'physics of magnetism magnetic materials 2011 part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 100 CHAPTER 11. MAGNETIC ANISOTROPY Figure . Sucksmith-Thompson plot for NdisFe77Bg measured at room temperature. After Durst and Kronmiiller 1986 . where Jr is the remanence in the hard direction and where the factor a Js dr Zs has been introduced to simulate perfect magnetic alignment of the powder particles. An example of a modified Sucksmith-Thompson plot obtained on a single crystal of Nd2Fei4B by Durst and Kronmuller 1986 is shown in Fig. . The values of K and K2 derived from the intercept and slope in this plot are equal to and MJ m 2 respectively. The variation of the anisotropy energy with the direction of the magnetization in cubic materials is commonly expressed in terms of direction cosines. Let OA OB OC be the cube edges of a crystal and let the magnetization be in the direction of OP. Furthermore oti cos POA 2 cos POB and 0t3 cos POB. The anisotropy energy per unit volume of the material if it is magnetized in the direction OP is given by Ean K K4 áịaị a2a2 3 k6 ia2a3 11-9 The constant K has been included for completeness although it is rarely used. In many textbooks the constants and Ẫ6 are represented as Ẵ J and 2- Note that odd powers of Ct are absent in Eq. because a change in sign of any of the as should bring the magnetization vector into a direction that is equivalent to the original direction. Furthermore the second-order terms can be left out of consideration since a2 a2 a2 1. The anisotropy constants can most conveniently be determined by measuring the energy of magnetization f H dJ along different crystal axes of a single crystal. These determinations include measurements of the J H curve starting from the demagnetized state up to magnetic saturation. Subsequently the area between this curve and theJ-axisis determined. Examples of such measurements were already displayed in Fig. . The energies required for magnetizing cubic materials to saturation in the various crystallographic directions can be derived from Eq.

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