Anisotropy magnetization distribution, magnetic

In a polycrystalline material each crystallite experiences its own effective field, which is determined in part by magnetic anisotropy. Therefore, because of the different orientations of the crystallites, there will be a distribution of resonant frequencies in consequence, narrow linewidths are associated with low anisotropies. Magnetic inhomogeneities - inclusions and pores - also disturb the local effective fields and so lead to line broadening. The effect of porosity on resonance linewidth can be expressed by the relationship... 

AJ Rondinone, ACS Sarnia, ZJ Zhang. Superparamagnetic relaxation and magnetic anisotropy energy distribution in CoFeoOa spinel ferrite nanocrystaUites. J Phys Chem B 1999 103 6876. 

In contrast, soft magnetic solids and paramagnetic systems with weak anisotropy may be completely polarized by an applied field, that is, the effective field at the Mossbauer nucleus is along the direction of the applied field, whereas the EFG is powder-distributed as in the case of crystallites or molecules. In this case, first-order quadrupole shifts cannot be observed in the magnetic Mossbauer spectra because they are symmetrically smeared out around the unperturbed positions of hyperfine fines, as given by the powder average of EQ mj, d, in (4.51). The result is a symmetric broadening of all hyperfine fines (however, distinct asymmetries arise if the first-order condition is violated). 

Anisotropy Non-uniform distribution of electrons about a group which can lead to non-uniform localised magnetic fields within a molecule. The phenomenon leads to unexpected chemical shifts -particularly in 1H NMR - in molecules where steric constraints are present. 

French workers have studied the 1H- and 13C-NMR parameters of disubstituted selenophenes.37 38 The proton chemical shifts are discussed in terms of magnetic anisotropy and electric field effects of the substituents in order to study the conformational equilibrium of the carbonyl group. The relationship between the H- and 13C-chemical shifts and 7t-electron distribution calculated by the PPP method are examined. Shifts and coupling constants are discussed in additivity terms. 

In early work, Spiesecke and Schneider (59) pointed out that inductive effects alone cannot account for a- and -signal shifts. They held diamagnetic neighbor-anisotropy effects (63) arising from anisotropic electron-charge distributions responsible for the deviations in the electronegativity correlations. For bonds with conical symmetry they applied McConnell s magnetic point-dipole approximation (64) for the estimation of this contribution, Act ... 

ENDOR experiments can be performed in liquid solution, in which only the isotropic hfc s (Ajso) are detected. They are proportional to the spin density at the respective nucleus. Erom the assigned isotropic hfc s a map of the spin density distribution over the molecule can be obtained. In frozen solutions and powders the anisotropic hf interactions can also be determined. Eurthermore, the method allows the detection of nuclear quadrupole couplings for nuclei with 1 1. For dominant g anisotropy as found in many metal complexes the external magnetic field can be set to several specific g values in the EPR, thereby selecting only those molecules that have their g tensor axis along the chosen field direction. In such orientation-selected spectra only those hf components are selected that correspond to this molecular orientation ( single crystal-like ENDOR ). 

Anisotropy is also associated to a magnetic field, whose modulus is Ba = 2 KaIMs, Ms being the crystal magnetization. The precession frequency of the magnetic moment into this field corresponds to the frequency of the peak characterizing the distribution of the transition frequencies. 

Thermodynamic perturbation theory is used to expand the Boltzmann distribution in the dipolar interaction, keeping it exact in the magnetic anisotropy (see Section II.B.l). A convenient way of performing the expansion in powers of is to introduce the Mayer functions fj defined by 1 +fj = exp( cOy), which permits us to write the exponential in the Boltzmann factor as... 

Since the application of pressure may modify strongly the charge density distribution in a solid, and therefore affects the orbital more than the spin moment, magnetic form factors and magnetic anisotropy may become much more pressure-dependent than usually assumed. 

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