Field demagnetizing

The intercepts of the plots in Fig. 5.10 (right) correspond to H0, and are equal to 34.3+0.5 and 32.7 2.0 T at 77 and 300 K, respectively. These values are in agreement with the bulk fields of metallic iron after corrections for the influence of the demagnetizing field (0.7 T) in isolated spherical single domain particles [28]. 

The second one is the demagnetizing field, which is determined by the shape of the crystal. This component of the anisotropy is equal to zero for a sphere and increases with the elongation of the crystal. 

T. M. Barbara, Cylindrical demagnetization fields and microprobe design in highresolution NMR, J. Magn. Reson., Series A, 1994, 109, 265-269. 

The micromagnetic structure is directly related to the microstructure and chemical inhomogenities in the layer. The materials used and the deposition technology as well as the parameters play an important role. Thin-film growth, nudeation processes in relation to the deposition parameters, are very important for understanding the thin film microstructure. The relationships between sfd and recording properties are not necessarily valid for media with perpendicular anisotropy as the demagnetizing field can be more important than sfd. 

Fig. 9. The relation of an LMR and PMR transition (a) initial state of magnetization (b) the magnetization and corresponding demagnetization fields of a ...

This critical field called coercivity ff. or switching field Ff., is also equal to FF. If a field is applied in between 0 and 90° the coercivity varies from maximum to zero. In the case of this special example the applied field Ha = Hs = Hc = Hk. Based on the classical theory, Stoner-Wohlfarth (33) considered the rotation unison for noninteracted, randomly oriented, elongated particles. The anisotropic axis can be due to the shape anisotropy (depending on the size and shape of the particle) or to the crystalline anisotropy. In the prolate ellipsoids b is the short axis and a the longest axis. The demagnetizing factors are IV (in the easy direction) and The demagnetizing fields can then be calculated by Hda = — Na Ms, and Hdb = — Nb Ms. The shape anisotropy field is Hd = (Na — Nb)Ms. Then the switching field Hs = Hd = (Na — Nb)Ms. 

Agreement between theory and experiment becomes quantitative, if the demagnetizing field effects are also taken into account. This leads to values of the anisotropy field, HK=29 G and the demagnetizing field, Hdem=-39 G. The nonlinear dependence of the resonance field on the resonance frequency disappears... 

The operating state of a permanent magnet lies in the second quadrant of a B-H hysteresis loop since the magnet is always subject to its own demagnetizing field. Apart from its coercivity and remanence, a permanent magnet material is rated by its maximum energy product ... 

The precessional motion can be maintained by a suitable radio frequency field superimposed on the steady field. For example, in Fig. 9.38(b), when a steady field Hz is applied along the z axis and a radiofrequency field //,., is applied in the x-y plane and rotates in the same sense and at the same frequency as the precession, resonance occurs. Gyromagnetic resonance as outlined above is in principle the same as ferrimagnetic resonance referred to earlier (Section 9.3.1), except that in the former case the material is magnetically saturated by a strong applied field. In practice the steady field, which determines the Larmor frequency, is made up of the externally applied field, the demagnetizing field and the anisotropy field, and is termed the effective field He. Figure 9.39 shows the He values at which resonance occurs in some of the important communications and radar frequency bands. 

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