Domain wall resonance

Interface polarization Dipole stretching Ferroelectric hysteresis Electric domain wall resonance Electrostriction Kezoelectricity Nuclear magnetic resonance Ferromagnetic resonance Ferrimagnetic resonance... 

The real part is the magnetic permeability whereas the imaginary part is the magnetic loss. These losses are quite different from hysteresis or eddy current losses, because they are induced by domain wall and electron-spin resonance. These materials should be placed at position of magnetic field maxima for optimum absorption of microwave energy. For transition metal oxides such as iron, nickel, and cobalt magnetic losses are high. These powders can, therefore, be used as lossy impurities or additives to induce losses within solids for which dielectric loss is too small. 

There is another, quite distinct, resonance phenomenon concerned with domain wall movements occurring at approximately one-tenth of the ferrimagnetic resonance frequency. To understand this Bloch wall motion needs to be... 

Many of the specific applications of ferrites depend on their behaviour at high frequencies. When subjected to an ac field, ferrite permeability shows several dispersions as the field frequency increases, the various magnetisation mechanisms become unable to follow the field. The dispersion frequency for each mechanism is different, since they have different time constants. Fig. 4.59. The low-frequency dispersions are associated with domain wall dynamics and the high-frequency dispersion, with spin resonance the latter, usually in the GHz range, is discussed in Section 4.6.2. 

At very high frequencies, domain walls are unable to follow the field and the only remaining magnetisation mechanism is spin rotation within domains. This mechanism eventually also shows a dispersion, which always takes the form of a resonance. Spins are subjected to the anisotropy field, representing spin-lattice coupling as an external field is applied (out of the spins easy direction), spins experience a torque. However, the response of spins is not instantaneous spins precess around the field direction for a certain time (the relaxation time, r) before adopting the new orientation. Fig. 4.62. The frequency of this precession is given by the Larmor frequency ... 

Dillon, J. F. Jr Earl, H. E. Jr (1959). Domain wall motion and ferrimagnetic resonance in a manganese ferrite. Journal of Applied Physics, 30, 202-13. 

Vella-Coleiro, G. P., Smith, D. H. Van Uitert, L. G. (1972). Resonant motion of domain walls in yttrium gadolinium iron garnets. Journal of Applied Physics, 43, 2428-30. 

EPR experiments on La2Cu04. (6 0.015) powders, obtained from crushed single crystals, were performed by Szymczak et al. (1995). The authors observed the typical powder spectrum of isolated Cu + centers with g =2.355 and gj =2.098. This resonance absorption was associated with Cu + ions located in domain walls and grain boundaries. The spectrum showed a hyperfine-like structure with a hyperfine constant... 

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