Magnetisation concept

To keep track of the populations of the different levels and how pulses affect the magnetisation, the most complete and powerful formalism is to use the density operator or density matrix (Farrar 1990, 1990a). This approach will not be adopted here but ideas that have come out of this can be extremely informative, allowing an understanding of some of the more complex pulse sequences adopted today. A central idea is that of coherences and coherence order (Munowitz 1988, Ernst, Bodenhausen and Wokaun 1988, Keeler 1990). Transverse magnetisation is a concept that can be readily visualised and can be represented by a classical vector. As the if pulses are applied as... 

The transverse magnetisation we observe directly in an NMR experiment is known as single quantum coherence. Multiple quantum coherence, however, cannot be directly observed because it induces no signal in the detection coil. For multiple quantum coherence to be of use to us, it must be transferred back into signal quantum coherence by the action of rf pulses. The concept of coherence is developed further in Chapter 5. 

Fig. 4.10, instead of in the form of latent heat at the transition. This results in a magnetisation curve with small variations at low temperatures and a large decrease as T approaches Tq, Fig. 4.11. The concepts of reduced magnetisation and reduced temperature, m = M(T)fM(0), and t = T/Tq, respectively, are used to compare materials with different spontaneous magnetisations and Curie points. M T) represents the magnetisation value at temperature T, and M(0) the value at 0 K. Since in ferromagnets magnetisation is always maximum for 0 K, a decreasing curve is obtained.

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