Felix Bloch introduced the concept of a magnon in 1930 to explain the reduction of the
spontaneous magnetization in a
ferromagnet. At
absolute zero temperature (0 K), a
Heisenberg ferromagnet reaches the state of lowest energy (so-called
ground state), in which all of the atomic spins (and hence
magnetic moments) point in the same direction. As the temperature increases, more and more spins deviate randomly from the alignment, increasing the internal energy and reducing the net magnetization. Viewing the perfectly magnetized state at zero temperature as the
vacuum state of the ferromagnet, shows the low-temperature state with a few misaligned spins as a gas of quasiparticles, in this case magnons. Each magnon reduces the total spin along the direction of magnetization by one unit of \hbar (the reduced Planck constant) and the magnetization by \gamma\hbar, where \gamma is the
gyromagnetic ratio. This leads to Bloch's law for the temperature dependence of spontaneous magnetization: : M(T) = M_0 \left[1 - \left(\frac{T}{T_\text{c}}\right)^{3/2}\right] where T_\text{c} is the (material dependent) critical temperature, and M_0 is the magnitude of the spontaneous magnetization.
Theodore Holstein and
Henry Primakoff, and then
Freeman Dyson further developed the quantitative theory of magnons, quantized
spin waves. Using the
second quantization formalism they showed that magnons behave as weakly interacting quasiparticles obeying
Bose–Einstein statistics for
bosons.
Bertram Brockhouse achieved direct experimental detection of magnons by inelastic
neutron scattering in ferrite in 1957. Magnons were later detected in
ferromagnets,
ferrimagnets, and
antiferromagnets. The fact that magnons obey Bose–Einstein statistics was confirmed by 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 \hbar \omega , then the Stokes line becomes more intense, as follows from Bose–Einstein statistics.
Bose–Einstein condensation of magnons was proven in an antiferromagnet at low temperatures by Nikuni
et al. and in a ferrimagnet by Demokritov
et al. at room temperature. In 2015 Uchida
et al. reported the generation of spin currents by surface plasmon resonance. == Paramagnons ==