Magnetic field effects saturated systems

DNP at very low magnetic fields is attractive for two reasons. First, the electron saturation frequency at low fields is in the radiofrequency range, where it is much easier to obtain high-power radiation sources, amplifiers and transmission equipment. In fact, the first experimental verifications of the Overhauser effect were conducted between 1 and 5 mT,2/85/86 likely due to the ease of constructing a suitable magnet and equipment to perform ESR saturation. The second reason for adding DNP to a low-field system is to help overcome the limited thermal polarization at low... 

A systematic study of the Eu/Yb and Eu/Ba alloys has been made [52, 53]. In the ytterbium system, the Curie temperature falls from 90 to 5 K and the saturation field also falls from 265 to 160 kG as the ytterbium content increases from 0 to 92 at. %. The relationships are linear apart from a discontinuity at 50 at. % where there is a phase change. Similarly for barium the Curie temperature falls from 90 to 40 K and the field from 265 to 206 kG as the barium content rises to 50 at. %. However, the chemical isomer shift is not significantly altered. The sign of the magnetic field is known to be negative from neutron diffraction data. Calculations suggest that a contribution of —340 kG to the field in europium metal arises from core polarisation, that +190 kG comes from conduction-electron polarisation by the atoms own 4/-electrons, and that —115 kG comes from conduction-electron polarisation, overlap, and covalency effects from neighbouring atoms. 

One can see that with lowering temperature the model curves acquire the additional contributions to coercivity near cp = 0 and jt/2 while for intermediate angles their variation is small. This behavior is similar to the experimental curves evolution for 20,40 and 115 K shown on the Fig. 3.36. The absence of the effect of potential (3.114) on the FMR line positions may be understood if we suppose that strong (close to saturation one) magnetic field completely polarizes the system so that the above shallow energy profile disappears. 

According to the collision complex model, the saturation effect is observed when the magnetic field is high enough so that W E, H) K(E). Notice that the systems like NO2 and glyoxal, which exhibit intermolecular magnetic quenching, show a saturation effect. 

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