Nuclear Magnetic Resonance

The meaning of the Nuclear Magnetic Resonance (NMR) thesedays has less importance in view of its first success in determination of nuclear moments and relaxation times, but it has rather shifted to the application as a highresolution technique in

The NMR technique is pricipally based on the fact that in a steady magnetic field nuclei possessing a magnetic moment like 1H, 19F, 13C, 29Si, 27Al, 63Cu etc. build up a macroscopic magnetisation parallel to this field; this magnetisation is made up of the parallel components of the nuclear moments which rotate around the field axis with a special frequency. Short pulses carrying this resonance frequency of the nuclear moments generate a torsion of this macroscopic magnetisation. After this torsion the moment induces a signal in a receiver coil while returning to the equilibrium state.

On closer inspection this relaxation process is characterized by two different time scales: The magnetisation components perpendicular to the orientation of the steady field decay by the so-called transversal relaxation with a time constant T2 while simultaneously the macroscopic magnetisation parallel to the steady field is restored by the so-called longitudinal relaxation with the time constant T1. In general these relaxation processes strongly depend on the local environment like the surrounding atom shells or the neighbouring molecules of the nuclear moments. Therefore the extraction of T1 and T2 from the NMR experiment allows one to draw conclusions concerning the surrounding matter.

Our interest in NMR in the solid state stems from its useful application in order to investigate the structure of electrons and their correlations in metals. We are running a home-built spectrometer within the temperature range 1.5 K up to 500 K. The magnet allows field sweep up to 11 Tesla and is specified for a field homogenity of 10ppm. The upper limit of the nuclear frequency amounts 500 MHz.

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