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magnetic resonance, in physics and chemistry, phenomenon produced by simultaneously applying a steady magnetic field and electromagnetic radiation (usually radio waves) to a sample of atoms and then adjusting the frequency of the radiation and the strength of the magnetic field to produce absorption of the radiation. The resonance refers to the enhancement of the absorption that occurs when the correct combination of field and frequency is reached. The procedure is analogous to tuning a radio dial exactly to a desired station.
Several distinct kinds of magnetic resonance exist. In cyclotron resonance the magnetic field is adjusted so that the frequency of revolution of a charged particle around the field lines is exactly equal to the frequency of the radiation. This principle is used to produce beams of energetic particles in particle accelerators.
Other magnetic resonance phenomena depend on the fact that both the proton and electron exhibit intrinsic spin about their own axes and thus act like microscopic magnets. Electron paramagnetic resonance (EPR) arises from unpaired electron spins in liquids or solid crystals. Because of their own magnetism, the spins line up with the external magnetic field. For a given magnetic field the spins can be made to "flip" to the opposite direction when they absorb radiation at a corresponding "resonant" frequency. From the point of view of quantum mechanics, the spin flips can be considered as transitions between states that become separated in energy when the magnetic field is applied. The effect is related to the splitting of spectral lines when an atom is subjected to a magnetic field (see spectrum; Zeeman effect).
Nuclear magnetic resonance (NMR) is analogous to EPR; however NMR is produced by the much smaller magnetism associated with unpaired nuclear spins. The NMR resonant frequency (usually that of protons in complex molecules) is slightly shifted by interactions with nearby atoms in the sample, thus providing information about the chemical structure of organic molecules and other materials. NMR is now extensively employed in medicine, although the use of the word "nuclear" is avoided, the preferred name being magnetic resonance imaging (MRI). The technique provides high-quality cross-sectional images of internal organs and structures.
Magnetic resonance can also occur without an external magnetic field from interactions of the electron and nuclear spins; such resonance produces the fine and hyperfine structure of atomic spectra.
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