In addition, magnetic resonance imaging (MRI), which is often mentioned in people's daily life, is an imaging device for medical examination, which makes use of the phenomenon of nuclear magnetic resonance.
The classical phenomenological description of magnetic vibration (except cyclotron vibration) is that atoms, electrons and nuclei all have angular momentum, and the ratio of their magnetic moment to the corresponding angular momentum is called magnetic rotation ratio γ. The magnetic moment m is acted by the moment MBsinθ(θ is the angle between m and b) in the magnetic field b, which makes the magnetic moment precess around the magnetic field. The angular frequency of precession ω=γB, ωo is called larmor frequency. Due to the damping effect, this precession will soon decay, that is, when m reaches parallel to b, the precession will stop. However, if a high-frequency magnetic field b(ω) is applied in the vertical direction of the magnetic field B (the angular frequency is ω), the moment generated by the action of b(ω) will make M leave B, which is contrary to the damping effect. If the angular frequency of high-frequency magnetic field is equal to the larmor (angular) frequency ω =ωo of magnetic moment precession, the effect of b(ω) is the strongest, and the precession angle of magnetic moment m (the included angle between m and b) is also the largest. This phenomenon is called magnetic vibration.
Magnetic vibration can also be described by quantum mechanics: the constant magnetic field B splits the ground state energy level of the magnetic spin system, and the split energy level is called Zeeman effect (see Zeeman effect). When the spin quantum number S= 1/2, the splitting distance E=gμBB, g is the Lund factor, μ is the Bohr magneton, and e and me are the charge and mass of the electron. When a high frequency magnetic field b(ω) perpendicular to B is applied, its optical quantum energy is к ω. If it is equal to the Zeeman level gap, ω= gμbb =ωγB, that is, ω=γB(ω= h/2π, h is Planck constant), the spin system will absorb this energy and jump from the low energy level to the high energy level (excited state), which is the so-called * * vibration transition between magnetic Zeeman levels. The magnetic oscillation condition ω=γB described by quantum is the same as that described by phenomenology.
When m is the magnetic moment of atoms (ions) in paramagnetic body, this magnetic vibration is paramagnetic vibration. When m is the magnetization (magnetic moment per unit volume) in a ferromagnetic body, this magnetic * * * vibration is ferromagnetic * * * vibration. When M=Mi is the magnetization of the ith magnetic sub-lattice in a ferrimagnet or an antiferromagnet, this magnetic * * vibration is a ferrimagnetic * * vibration or an antiferromagnet * * vibration generated by I coupled magnetic sub-lattice systems. When m is the nuclear magnetic moment in matter, it is the nuclear magnetic vibration. These kinds of magnetic * * * vibrations are all generated by the spin magnetic moment, which can be described by the classical phenomenological spin magnetic equation dM/dt=γMBsinθ[ the corresponding vector equation is dM/dt =γ(M×B)].
Cyclotron vibration caused by charged particles in a constant magnetic field. Let a charged particle with a charge of Q and a mass of M move in a constant magnetic field B with a velocity of V. When the magnetic field B is perpendicular to the velocity V, the charged particle will be subjected to Lorentz force generated by the magnetic field, which makes the charged particle rotate around the magnetic field B with a velocity of V. The angular frequency of rotation is called the cyclotron angular frequency. If a high-frequency electric field E (ω) is applied to the plane perpendicular to B (ω is the angular frequency of the electric field), ω=ωc, the charged particles will be periodically accelerated by the electric field E(ω). Because it is similar to cyclotron, it is called cyclotron vibration. It is also called diamagnetism because it is similar to diamagnetism without high frequency electric field. When V is perpendicular to B, the equation describing this * * * vibration motion is d(mv)/dt=q(vB). If described by a quantum mechanical image, cyclotron vibration can be regarded as Landau energy level transition caused by the motion state of charged particles in a magnetic field under the action of a high-frequency electric field, and the conditions for satisfying the * * * vibration transition are as follows:
Magnetic vibration
ω=ωc .
There is generally a process of energy transfer and redistribution between the spin (magnetic moment) system itself (carrier system when cyclotron vibrates) and the lattice system, which is called magnetic vibration relaxation process. In the case of spin magnetic oscillation, magnetic relaxation includes spin-spin (S-S) relaxation in spin (magnetic moment) system and spin-lattice (S-L) relaxation between spin system and lattice system. The relaxation time from one equilibrium state to another is called relaxation time, which is a measure of energy transfer rate or loss rate. * * * The vibration linewidth indicates the energy level width, and the relaxation time indicates the energy state lifetime. The linewidth of magnetic resonance is closely related to the magnetic relaxation process (time). According to the uncertainty principle, the product of energy level width and energy state lifetime is constant, that is, the linewidth of magnetic resonance is inversely proportional to relaxation time (energy transfer speed). Therefore, magnetic vibration is an important method to study the magnetic relaxation process and magnetic loss mechanism.
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