The mechanism of antiferromagnetism in chromium

It is shown that two special properties of Cr are needed to explain its antiferromagnetism. One special property is the well known sensitivity to antiferromagnetic spin-density waves due to nesting of the Fermi surface. A second new special property comes from first-principles total-energy calculations on bcc Cr, which show that, although the lowest energy state is nonmagnetic, a small expansion of the lattice brings a second-order transition into a type-1 antiferromagnetic phase with rapidly rising local moments. The combined properties provide a mechanism for stabilization of the unusual antiferromagnetic ground state, since a spin-density wave which modulates the moments of the antiferromagnetic phase can be used to compensate the strain energy of the lattice expansion. This combined mechanism also explains various properties of Cr, such as the great sensitivity of the antiferromagnetism to pressure, that are otherwise puzzling.