Magnon

Condensed matter physics
Phases Phase transition QCP
States of matter Solid Liquid Gas Plasma Bose–Einstein condensate Bose gas Fermionic condensate Fermi gas Fermi liquid Supersolid Superfluidity Luttinger liquid Time crystal
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Quasiparticles Phonon Exciton Plasmon Polariton Polaron Magnon Roton
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Scientists Ginzburg Leggett Parisi
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A magnon is a

behavior.

Brief history

The concept of a magnon was introduced in 1930 by

vacuum state

of the ferromagnet, the low-temperature state with a few misaligned spins can be viewed as a gas of quasiparticles, in this case magnons. Each magnon reduces the total spin along the direction of magnetization by one unit of ${\displaystyle \hbar }$ (reduced Planck's constant) and the magnetization by ${\displaystyle \gamma \hbar }$, where ${\displaystyle \gamma }$ is the

${\displaystyle M(T)=M_{0}\left[1-\left({\frac {T}{T_{c}}}\right)^{3/2}\right]}$

where ${\displaystyle T_{c}}$ is the (material dependent) critical temperature, and ${\displaystyle M_{0}}$ is the magnitude of the spontaneous magnetization.

The quantitative theory of magnons, quantized

The fact that magnons obey the Bose–Einstein statistics was confirmed by the light scattering experiments done during the 1960s through the 1980s. Classical theory predicts equal intensity of Stokes and anti-Stokes lines. However, the scattering showed that if the magnon energy is comparable to or smaller than the thermal energy, or ${\displaystyle \hbar \omega <k_{B}T}$, then the Stokes line becomes more intense, as follows from Bose–Einstein statistics.

Paramagnons are magnons in magnetic materials which are in their high temperature, disordered (paramagnetic) phase. For low enough temperatures, the local atomic magnetic moments (spins) in ferromagnetic or anti-ferromagnetic compounds will become ordered. Small oscillations of the moments around their natural direction will propagate as waves (magnons). At temperatures higher than the critical temperature, long range order is lost, but spins will still align locally in patches, allowing for spin waves to propagate for short distances. These waves are known as a paramagnon, and undergo diffusive (instead of ballistic or long range) transport.

The concept was first proposed based on the spin fluctuations in transition metals, by Berk and Schrieffer^[10] and Doniach and Engelsberg,^[11] to explain additional repulsion between electrons in some metals, which reduces the critical temperature for superconductivity.

Properties

Magnon behavior can be studied with a variety of scattering techniques. Magnons behave as a Bose gas with no chemical potential. Microwave pumping can be used to excite spin waves and create additional non-equilibrium magnons which thermalize into phonons. At a critical density, a condensate is formed, which appears as the emission of monochromatic microwaves. This microwave source can be tuned with an applied magnetic field.

See also

• Magnonics
• Holstein-Primakoff transformation
• Surface magnon polariton

References