Magnetic fields diamagnetism

We have seen that the magnetic field R, required to obtain the resonance crmdition for nucleus i at a particular irradiating rf field (Ri) is not equal to the applied static field Rq, but is instead R, = Ro(l — a) [see Eq. (20.9)], where the nuclear screening constant, a, depends on the chemical structural environment of nucleus /. The local electron density in the vicinity of the nucleus shields it from the applied field Bo by producing small local magnetic fields (diamagnetic currents). Any structural feamre that alters the electronic environment of a nucleus will affect its screening constant a and lead to an alteration in its resonance frequency or chemical shift 5,. 

The two primary causes of shielding by electrons are diamagnetism and temperature-independent paramagnetism (TIP). Diamagnetism arises from the slight unpairing of electron orbits under the influence of the magnetic field. This always occurs so as to oppose the field and was first analysed by Lamb [7]. A simplified version of his fomuila. 

It is a white crystalline, brittle metal with a pinkish tinge. It occurs native. Bismuth is the most diamagnetic of all metals, and the thermal conductivity is lower than any metal, except mercury. It has a high electrical resistance, and has the highest Hall effect of any metal (i.e., greatest increase in electrical resistance when placed in a magnetic field). 

The static magnetic susceptibilities of the aligned CNTs with keeping tube s cylindrical direction parallel x and perpendicular x to the magnetic field were measured by SQUID as shown in Fig. 4 [31]. The CNTs are diamagnetic with... 

It is perfectly diamagnetic, i.e. it completely excludes applied magnetic fields. This is the Meissner effect and is the reason why a superconductor can levitate a magnet. 

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