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Physics of Magnetism Magnetic Materials 2011 Part 10

Tham khảo tài liệu 'physics of magnetism magnetic materials 2011 part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | SECTION 14.2. SURVEY OF MATERIALS 151 Figure 14.2.3. Domain structure in rolled polycrystalline Ni-Fe alloys. From Chikazumi et al. 1957 . Figure 14.2.4. Dependence on annealing temperature of the thermomagnetic anisotropy constants obtained in Fe-Ni alloys of various concentrations. From Ferguson 1958 . For a polycrystalline alloy the magnetization directions of the various domains have a random orientation meaning that the induced anisotropy directions in the various domains will also have a random orientation. This results in a constricted hysteresis loop as shown by means of curve B in Fig. 14.2.2. At this stage it is good to recall that the main effect of a magnetic field when applied during annealing is to destroy the domain pattern and to align the local magnetization in the field direction across the whole sample. The value of the thermomagnetic anisotropy constant is generally of the order of a few hundred Jm-3. As can be seen in Fig. 14.2.4 it increases with Fe concentration as a result of an increased number of aligned atom pairs. The value of Ku is generally higher the lower the annealing temperature. More details can be found in the review of Ferguson 1958 . It is important to bear in mind that the thermomagnetic anisotropy is generated by annealing treatments performed below the Curie temperature. Because the pair formation requires diffusion of atoms and because diffusion is a thermally activated process too low 152 CHAPTER 14. SOFT-MAGNETIC MATERIALS Figure 14.2.5. Schematic representation of slip-induced directional order in a simple alloy a Before slip b after slip on horizontal planes. From Chin and Wemick 1980 . annealing temperatures lead to poor results. That means that large Kn values can only be generated in alloys with sufficiently high Curie temperatures. This is the case for Ni-Fe alloys with Ni content near 65 because at this composition the Curie temperature reaches its maximum in this binary alloy system. Thermomagnetic treatments .