Internal magnetic field

The mixed derivative of an external and an internal magnetic field (nuclear spin) is the NMR shielding eonstant, [Pg.239]

Mossbauer spectroscopy is a specialist characterization tool in catalysis. Nevertheless, it has yielded essential information on a number of important catalysts, such as the iron catalyst for ammonia and Fischer-Tropsch synthesis, as well as the CoMoS hydrotreating catalyst. Mossbauer spectroscopy provides the oxidation state, the internal magnetic field, and the lattice symmetry of a limited number of elements such as iron, cobalt, tin, iridium, ruthenium, antimony, platinum and gold, and can be applied in situ. 

Powder spectra of paramagnetic compounds measured with applied fields are generally more complicated than those shown in Fig. 4.14. Large internal fields at the Mossbauer nucleus that are temperature- and field-dependent give rise to this complication. If, however, the measurement is performed at sufficiently high temperature, which is above ca. 150 K, the internal magnetic fields usually collapse due to fast relaxation of the electronic spin system (vide infra, Chap. 6). Under... 

On the other hand, internal magnetic fields at the iron nucleus arising from the magnetic moments of unpaired valence electrons can be much stronger than any applied field and their effect can easily exceed the quadrupole interaction. For instance, Mossbauer nuclei in magnetic materials such as metals or oxides may experience fields of 30-50 T even without applied field. Similarly, the typical... 

Fig. 4.17 Magnetic hyperfine pattern of a powder sample with randomly distributed internal magnetic field (a), and with (b) an applied magnetic field (0q = 90°), and (c) an applied magnetic field (00 = 0°)...

Marathe, V.R., Trautwein, A. Calculation of charge density, electric field gradient and internal magnetic field at the nuclear site using molecular orbital cluster theory. In Thosar,... 

T on diluted in iron. The internal magnetic field at the tungsten site in alloys of 0.5-5% mngsten in iron turned out to be —70.8 2.5 T. This value, together with another value (63 1.3 T [230]) obtained from NMR data,... 

Much of the Pt Mossbauer work performed so far has been devoted to studies of platinum metal and alloys in regard to nuclear properties (magnetic moments and lifetimes) of the excited Mossbauer states of Pt, lattice dynamics, electron density, and internal magnetic field at the nuclei of Pt atoms placed in various magnetic hosts. The observed changes in the latter two quantities, li/ (o)P and within a series of platinum alloys are particularly informative about the conduction electron delocalization and polarization. 

NMR Pore Size Measurements Using an Internal Magnetic Field in Porous Media... 

The essence of the DDIF method is to first establish a spin magnetization modulation that follows the spatial variation of the internal magnetic field within the individual pore. Such modulation is created by allowing spins to precess in the internal magnetic field. Then the diffusion-driven time-evolution (often decay) of such a modulation is monitored through a series of signal measurements at various evolution times tD. The time constant of this decay corresponds to the diffusion time of a molecule (or spin) across the pore and thus is a direct measure of the pore size. 

The muon spin relaxation technique uses the implantation and subsequent decay of muons, n+, in matter. The muon has a polarized spin of 1/2 [22]. When implanted, the muons interact with the local magnetic field and decay (lifetime = 2.2 ps) by emitting a positron preferentially in the direction of polarization. Adequately positioned detectors are then used to determine the asymmetry of this decay as a function of time, A t). This function is thus dependant on the distribution of internal magnetic fields within a... 

The observed Schottky-specific heat is due to changes in internal energy that occur when nearly adjacent energy levels are occupied. The degenerate energy levels may be caused by external or internal magnetic fields. 

Magnetic contributions to the Gibbs energy due to an internal magnetic field are present in all magnetically ordered materials. An additional energetic contribution... 

In the case of iron, magnetism is due to the unpaired electrons in the 3d-orbitals, which have all parallel spin. These electrons interact with all other electrons of the atom, also the s-electrons that have overlap with the nucleus. As the interaction between electrons with parallel spins is slightly less repulsive than between electrons with anti parallel spins, the s-electron cloud is polarized, which causes the large but also highly localized magnetic field at the nucleus. The field of any externally applied magnet adds vectorially to the internal magnetic field at the nucleus. 

Because the interactions measured in Mossbauer experiments are products of atomic and nuclear factors, experiments on iodine isotopes have yielded values of the change of nuclear radius between the ground state and the excited state, AR/R, quadrupole moment values Q, and magnetic moment values, fi, as well as electric field gradients and internal magnetic fields. 

Maruthe,V.R. Trautwein, A. (1983) Calculation of charge density, electric field gradient and internal magnetic field using molecular orbital cluster theory. In Thosar, B.V. (ed.) Advances in Mossbauer spectroscopy. Elsevier, Amsterdam, 398-449 Matijevic, E. Cimas S. (1987) Formation of uniform colloidal iron(lll) oxides in ethylene... 

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