Magnetic Relaxation(redirected from Relaxation (NMR))
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The relaxation or approach of a magnetic system to an equilibrium or steady-state condition as the magnetic field is changed. This relaxation is not instantaneous but requires time. The characteristic times involved in magnetic relaxation are known as relaxation times. Relaxation has been studied for nuclear magnetism, electron paramagnetism, and ferromagnetism.
Magnetism is associated with angular momentum called spin, because it usually arises from spin of nuclei or electrons. The spins may interact with applied magnetic fields, the so-called Zeeman energy; with electric fields, usually atomic in origin; and with one another through magnetic dipole or exchange coupling, the so-called spin-spin energy. Relaxation which changes the total energy of these interactions is called spin-lattice relaxation; that which does not is called spin-spin relaxation. (As used here, the term lattice does not refer to an ordered crystal but rather signifies degrees of freedom other than spin orientation, for example, translational motion of molecules in a liquid.) Spin-lattice relaxation is associated with the approach of the spin system to thermal equilibrium with the host material; spin-spin relaxation is associated with an internal equilibrium of the spins among themselves. See Magnetism, Spin (quantum mechanics)
a stage of relaxation. In magnetic relaxation, the system of spin magnetic moments of the atoms and molecules of a medium takes part in the process by which the medium achieves thermodynamic equilibrium. In many cases, the interaction between the spins (between the magnetic moment of the spins) is much stronger than the other interactions in which the spins participate, such as the interaction of the spins with the phonons of a crystal. Equilibrium is thus often reached more rapidly in the spin system than in the medium as a whole—that is, than for the other internal degrees of freedom. Magnetic relaxation therefore proceeds in stages. The last and longest stage generally corresponds to the achievement of equilibrium between the spins and other degrees of freedom—for example, between the spin system and phonons, which are the quanta of vibrations of the crystal lattice. Each stage of magnetic relaxation is described by its own relaxation time, for example, spin-spin and spin-lattice relaxation times are used with crystals.
In media that have a magnetic structure—ferromagnetic and antiferromagnetic materials—magnetic relaxation occurs through the collision of spin waves (magnons) with each other and also with phonons, dislocations, impurity atoms, and other crystal defects.
In solids, magnetic relaxation depends essentially on the structure of the solid. Determining factors here include the character of the crystal lattice (single crystal or polycrystal), the presence of impurities and dislocations, and the domain structure. A decrease in the number of crystal defects and in the crystal temperature generally results in an increase in the magnetic relaxation time.
The magnetic relaxation of nuclear spins (nuclear magnetic moments) has its own specific characteristics, which are due to the especially weak interaction of the nuclear spins with the other degrees of freedom of the medium.
Magnetic relaxation plays a role in the processes of magnetization and alternating magnetization (seeMAGNETIC VISCOSITY) and determines the width of nuclear magnetic resonance lines, electron paramagnetic resonance lines, ferromagnetic resonance lines, and antiferromagnetic resonance lines. The properties of ferromagnetic and antiferromagnetic materials in high-frequency electromagnetic fields depend substantially on magnetic relaxation. In many cases, magnetic relaxation sets limits to the use of materials. For example, it imposes restrictions on the conditions governing the use of magnetic thin films in technology and on the speed of magnetic elements in electronic computer storage devices. Magnetic relaxation times are among the parameters of a solid that are altered comparatively readily by industrial processes, such as alloying and hardening.
M. I. KAGANOV