Magnetization reversal pinning

Fig. 15. Coercivity versus maximum applied field at r = 200K for a NdFeB (20nm)/Fe(0.5nm) sample. The result indicates that magnetization reversal in the easy direction is controlled by a domain-wall pinning mechanism. (By courtesy of Dr. D. Sellmyer, Behlen Lal5. of Physics, Univ. Nebraska, Lincoln, NB, USA).

Note that the expulsion and exclusion responses are equal for T close to Tc. This "reversible" region extends from Tr down to an "irreversibility" temperature T which is a function of the applied field, varying as (H )2/ (35). The lack of reversibility at temperatures below Tirr indicates pinning of the magnetic flux the... 

As the applied field is removed the route the M-H curve takes depends upon the extent to which the vortices are pinned . With no pinning the magnetization remains negative and fully reversible, that is in the opposite sense to the applied field. 

Figure 11. Pinning and nucleation. Pinning means that coercivity is created by trapping the domain wall at pronounced inhomogenities (pinning centers). In the absence of pinning centers, the coercivity is determined by the reversal field at which the original magnetization configuration becomes unstable (nucleation).

In this section, we concentrate on the fundamental impact of particle size reduction on magnetization processes in individual particles. Although not directly related to coercivity, the classical effect of single domain particle formation is described. At small particle size, reversal by coherent rotation tends to be favoured with respect to nucleation/pinning-depinning finally thermal activation effects and macroscopic quantum tunnelling are discussed. 

Figure 20. Example of double barrier magnetic tunnel junctions used in thermally assisted switching double free layer and weakly pinned storage layer (left) and minor TMR loops of double barriers junctions (right). Solid squares prior to pulse heating, open triangles after heating pulse and reversing the pinned layer in a 60 Oe easy axis external field. The junction area is 1 pm2, and a TMR of 25% and a resistance x area product of300 Opm2 are obtained.

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