Demagnetization process

Figure 4.1 Schematic diagram showing the thermodynamics of the demagnetization process. By changing the magnetic field from the initial value Hl to the final one Hf, with Hf < Hj a gain in magnetic entropy, ASm, occurs during the isothermal process from...

A change in dB/dt can be produced either by a demagnetization process or by a vibration in a non-perfectly homogeneous field. To reduce the effect of eddy currents, low-resistivity (plastic) MCs have been built (in which the sample under study is immersed in the mixture, see Fig. 6.15 showing the electrical feedthrough) and even complete plastic DR [34],... 

It would appear that by a finite number of successive applications of the magnetization-demagnetization process shown in Fig. la, 0 K could be achieved. The only thing that could prevent this goal, as required by the third law, is if, as the temperature is reduced, the two curves approach each other, as shown in Fig. lb. In this case, the temperature change for the isentropic demagnetization approaches zero as T approaches 0 K ... 

In Section 3 the Zeeman polarization-enhanced method is discussed. The method demonstrates its efficiency when spin-lattice relaxation time of the NQR spin-system is long enough to ensure the feasibility of the adiabatic demagnetization process. 

Then the sample is removed from the field of the magnets. To preserve the proton polarization, the demagnetization process is carried out adiabatically dB/dt < 7h 1oc/ where 7h is the gyromagnetic ratio for protons, Bioc is the mean local dipole field at the sites of protons). The value of the permanent magnetic field inside the sample slowly decreases finally reaching the value oiB 2tivn/7h-The entropy of the Zeeman proton reservoir at this moment is determined by expression... 

Figure 20.1. The magnetization process (a) demagnetized state (b) unsaturated state (c) saturated state. When an external magnetic field is applied, domain rotation and domain wall motion occur simultaneously or sequentially. Domain configuration can be found by minimizing the total energy related to magnetization. (From Ref 1, with permission from Elsevier.)...

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. 

Sm2Fei7Nx powders are produced commercially since 1999 by the reduction and diffusion process [136]. The currently mass-produced powder has a BHimx of 290 kJ/m3 with /ioMt = 1.35 T and fi(iHc = 1.1 T. Milling and surface treatment have been optimized to improve the squareness of the demagnetization curve and resistance to corrosion [137]. 

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