Boson Fields in Ordered Magnets

Spin wave theory of magnetism reveals two severe shortcomings. First, it considers non quantized classical spins and, second, the predicted temperature power functions for the thermal decrease of the magnetic order parameter hold asymptotically at T = 0 only. As experiments unambiguously show the dynamics is different for magnets with integer and half-integer spin and the "critical" power functions at T = 0 of type ΔM ≈ $T^{ε}$ or at T = $T_{c}$ of type ≈$(T_{c}-T)^β$ hold over a finite temperature range, independent of spin structure. The finite critical range unequivocally indicates that the dynamics of the spins is controlled by a field of freely propagating bosons instead by exchange interactions. Consequently, field theories are necessary for description of the thermodynamics of ordered magnets. The experimental indications will be discussed that the field quanta are essentially magnetic dipole radiation emitted by the precessing magnetic moments. Since integer and half-integer spins precess differently the generated field quanta and the dynamics of the field are correspondingly different.