High magnetic fields
High magnetic fields
Magnetic fields that are large enough to significantly alter the properties of objects that are placed in them. Valuable research is conducted at high magnetic fields. See Magnetism
Research and development efforts in magnets and magnet materials have led to gradual increases in the fields available for scientific research to fields near 20 tesla from superconducting magnets, 33 T in copper-core (resistive) magnets, and 45 T for hybrid magnets. Superconducting magnets have the advantage that they use no electrical power once the field is established and the temperature is maintained at liquid-helium temperatures of 4.2 K (-452°F) or below. The disadvantage is that there is a critical magnetic field, Hc2, determined by the type of conductor, that limits the attainable field to about 22 T in superconducting materials currently available. Resistive magnets, which consume enormous amounts of power and are very expensive to build and operate, are confined to a few central facilities worldwide. See Superconductivity
Advanced pulsed magnets that are not self-destructing provide fields beyond 70 T for about 0.1 s. Pulsed magnets using explosive magnetic flux compression have achieved fields above 500 T for periods of 10 microseconds. See Magnet
Research at very high magnetic fields spans a wide spectrum of experimental techniques for studies of materials. These techniques include nuclear magnetic resonance (NMR) in biological molecules utilizing the highest-field superconducting magnets, while the resistive magnet research is primarily in the investigation of semiconducting, magnetic, superconducting, and low-dimensional conducting materials.
Much of the progress in semiconductor physics and technology has come from high-field studies. For example, standard techniques for mapping the allowed electronic states (the Fermi surface) of semiconductors and metals are to measure the resistance (in the Shubnikov-de Haas effect) or magnetic susceptibility (in the de Haas-van Alphen effect) as a function of magnetic field and to observe the oscillatory behavior arising from the Landau levels of the electron orbits. Measurements at low fields are limited to low impurity concentrations since the orbits are large and impurity scattering wipes out the oscillations. At high fields of 20–200 T, the orbits are smaller, and higher impurity concentrations (higher carrier concentrations) have been studied. Another area in which very high magnetic fields have an important role is in high-temperature superconductors, which have great potential for high-field applications, from magnetic resonance imaging, to magnetically levitated trains, to basic science. See De Haas-van Alphen effect, Fermi surface, Semiconductor
Studies at high magnetic fields have played an important role in advancing understanding of magnetic materials. For example, in many organic conductors the conduction electrons (or holes) are confined to one or two dimensions, leading to very rich magnetic phase diagrams. High-field phases above 20 T include spin-density waves, a modulation of the electron magnetic moments that can propagate through the crystal, modifying the conduction and magnetic properties. Another area of interest is the magnetic levitation of diamagnetic materials (the most common materials). See Magnetic materials, Phase transitions, Spin-density wave