Fast-field-cycling NMR: Applications and instrumentation

Magnetic field cycling in nuclear magnetic resonance (NMR) experiments has been used since the early days of NMR. Originally such time-dependent magne tic field experiments were motivated to study cross relaxation, spin system thermodynamics and indirect detection of quadrupolar resonance. The first apparatus used mechanical or pneumatic systems to shoot the sample between two magnets, the typical "flying time" being a few hundreds of milliseconds . As a natural evolution of the experimental technique and the need to exte nd its application to samples with higher relaxation rates, faster magnetic field switching devices were developed during the last years. Special elec tric networks combined with sophisticated air core magnets allowed one to s witch magnetic fields between zero and fields of the order of 0.5 T in a fe w milliseconds. Today we refer to this new generation of instruments as "fa st-field-cycling" devices. The technique has been successfully used during the last years to obtain information on the molecular dynamics and order in different materials, ranging from organic solids, metals, polymers, liquid crystals, porous media to biological systems. At present it is also turnin g to be an important tool for the design of contrast agents for magnetic re sonance imaging. Fast field cycling was mainly oriented to T, relaxometry a s a unique technique offering a dynamic window of several decades, ranging from few kilohertz to several megahertz. However, there exist less conventi onal applications of the technique that can also provide relevant informati on concerning molecular dynamics, structure and molecular order. In this ar ticle we will briefly deal with basic aspects of the technique, its evoluti on, present-day relevant applications and the last improvements concerning specialized instrumentation.