Quantum-mechanical origin of magnetism While heuristic explanations based on classical physics can be formulated, diamagnetism, paramagnetism and ferromagnetism can be fully explained only using quantum theory. A successful model was developed already in 1927, by Walter Heitler and Fritz London, who derived, quantum-mechanically, how hydrogen molecules are formed from hydrogen atoms, i.e. from the atomic hydrogen orbitals and centered at the nuclei A and B, see below. That this leads to magnetism is not at all obvious, but will be explained in the following.
quantum-MecHaNIcaL oriGin of mAgnetiSm wHile heuRiSTIc eXpLanAtioNs baSED On clAssiCAL phYsIcs CAn BE formuLaTEd, DiamagNETism, PAramaGnEtIsM and FerromagnetIsM cAn be fUlly eXplAined onLY Using qUANtum Theory. a SUccessFUl modeL was deVElOPed AlreAdy in 1927, by wAltER HEITler anD FRitz LonDon, Who derivEd, quanTuM-meChanically, HOw HydrogEn mOleCules are foRMed from hydrogEN atOMs, I.E. from ThE aTOmic hydroGen oRbiTals aNd centEReD aT The nUclEI A and b, see BELoW. That THis leadS TO magnetism Is noT at alL ObvIoUs, BUt WiLl bE ExplaineD in The following.