Giant internal magnetic pressure and compressibility anomalies

We show that the kinetic-energy increase associated with magnetic ordering is responsible for the anomalously large lattice constants and small bulk moduli of the magnetic members of the $3d$ transition series. The effect follows from the itinerant model of magnetism and does not involve spin-orbit coupling. For Fe and Ni, this effect is demonstrated directly using self-consistent spin-polarized energy-band calculations. A simple, approximate expression for the magnetic pressure is obtained from the Stoner model. This expression gives results for Fe and Ni which are in good agreement with the detailed calculations and is used to obtain estimates of the size of the effect in Cr, Mn, and Co.