All-temperature magnon theory of ferromagnetism

Abstract

We present an all-temperature magnon formalism for ferromagnetic solids. To ourknowledge, this is the first time that all-temperature spin statistics have been calculated.The general impression up to now is that the magnon formalism breaks down atthe Curie point as it introduces a series expansion and unphysical states. Ourtreatment is based on an accurate quantum mechanical representation of theHolstein–Primakoff transformation. To achieve this end, we introduce the ‘Kubo operator’.The treatment is valid for all 14 types of Bravais lattices, and not limited to simple cubicunit cells. In the present work, we carry out a zeroth-order treatment involvingall possible spin states, and leaving out all unphysical states. In a subsequentpaper we will show that the perturbed energy values are very different, but themagnetic properties undergo only small modifications from the zeroth-order results.