Antiferromagnetic Order
A classical example of magnetic structure
determination by neutrons is shown in figure
(1) for MnO2 which has the NaCl structure.
At 80 K there are extra neutron reflections
not present at 293 K.
The reflections at 80 K may be classified in
terms of a cubic unit cell of lattice constant
8.85 Å.
At 293 K the reflections correspond to an fcc
unit cell of lattice constant 4.43 Å.
But the lattice constant determined by x-ray
reflection is 4.43 Å at both temperatures, 80
K and 293 K.
We conclude that the chemical unit cell has
the 4.43 Å lattice parameter, but that at 80 K
the electronic magnetic moments of the Mn+2
ions are ordered in some nonferromagnetic
arrangement.
If the ordering were ferromagnetic the
chemical and magnetic cells would give the
same reflections.
The spin arrangement shown in figure (2) is
consistent with the neutron diffraction results
and with magnetic measurements.
The spins in a single [111] planes are
parallel, but spins in adjacent [111] planes
are antiparallel.
