Superexchange

Superexchange results in an indirect antiferromagnetic coupling of magnetic moments in a material.

The coupling takes place via a mediating, diamagnetic particles (eg, ligands). Here, the spin of an occupied metal orbital induced (usually a d- orbital) a " spin polarization " in a fully occupied, neighboring atomic orbital (usually a p- orbital ) of the ligand, in which according to the Pauli exclusion principle, the spins must have an antiparallel arrangement. This now leads to an anti-parallel coupling of the spins in another adjacent metal atom, and thereby an anti-ferromagnetic (partial) order. The super-exchange is only approximately linear or linear arrangement effectively ( ~ " 180 ° superexchange " ), because too large a deviation from linearity it is no longer one, but by several, but is magnetically independent mediating orbitals.

The name was coined in 1934 by Hendrik Anthony Kramers and deepened in 1950 by Nobel Prize winner for Physics Philip Warren Anderson. These authors have described not only the mechanism, but also indicated typical application systems:

Examples

Examples are oxides, which crystallize in the NaCl - type ( antiferromagnetic, see Figure 1) or spinels ( ferrimagnetic ).

Quantum mechanical perturbation theory results for the antiferromagnetic interaction of spins on neighboring Mn sites 1 and 2 the energy operator ( Hamiltonian ) where the so-called hopping energy between Mn and the oxygen atom, and U for Mn characteristic so-called " Hubbard energy. " The term is the scalar product of the spin vectors ( Heisenberg model ).

The super-exchange is responsible for ensuring that in the consideration of Manganchalkogenen (MnO, MnS, MnSe ) one finds that with increasing atomic number of an increase in the Néel temperature is observed. This is because the p orbitals of the heavier chalcogens gain in size and so a better overlap is ensured with the metal orbitals. The " hopping energy " becomes greater.