Direct dipolar field contribution

In eq. (4.25), is the orientational order parameter (relative to the external magnetic field) of the vector mn, and < > indicates averaging over internal vibrations. The coefficients sfp in eq. (4.26) are Saupe orientational order tensor elements relative to the external magnetic field in a molecular-fixed frame. Consequently, it is necessary to know the tensor in order to be able to determine experimentally the direct dipolar contribution from... 

The characteristic line profile caused by the contribution of the dipolar field iJjip, which depends on the orientation of the magnetization, to the resonance field at a non-cubic lattice site can be used to dismantle the easy direction of magnetization. An early NMR example was the A1 line shape analysis in ferromagnetically ordered RAI2 (R = rare earth) by Kaplan et al. (1973). 

We start with a calculation of the ordering temperature solely based on the dipolar interaction. According to calculations by Smit et al. (1987) the contributions to the dipolar fields due to adjacent R layers are insignificant hence, a 2D calculation is sufficient to obtain an idea of the importance of the dipolar interaction. For our cluster calculations (following the method described by Macisaac et al. 1992), we used the ordered magnetic moment obtained from neutron diffraction or - if no data were available - the value calculated from the CEF parameters. The size and direction of the moment together with the lattice constants used for our calculations are fisted in table 3. 

In Equation (I). i2 ikT is the average component in the direction of the field of the permanent dipole moment of the molecule. In order that this average contribution should exist, the molecules must be able to rotate into equilibrium with the field. When the frequency of the alternating electric field used in the measurement is so high that dipolar molecules cannot respond to it, the second term on the right of the above equation decreases to zero, and we have what may he termed the optical dielectric constant f,.t defined by the expression... 

Figure 14.11 Basic 2D pulse sequence used for the measurement of residual dipolar local fields. After the excitation pulse the spins are allowed to evolve for some time tj (indirect dimension) under influence of the relevant spin interactions before crosspolarisation takes place. The direct detection during time t2 then takes place on the 13C side typically under proton dipolar decoupling (DD). The basic scheme can be extended by various spin manipulation techniques (not shown) during time tj. For instance, the heteronuclear dipolar contribution can be removed by a decoupling pulse...

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