  Vacancy is one of the most frequent defects in metals. We study the impacts
of magnetism on vacancy formation properties in fcc Ni, and in bcc and fcc Fe,
via density functional theory (DFT) and effective interaction models combined
with Monte Carlo simulations. Overall, the predicted vacancy formation energies
and equilibrium vacancy concentrations are in good agreement with experimental
data, available only at the high-temperature paramagnetic regime. Effects of
magnetic transitions on vacancy formation energies are found to be more
important in bcc Fe than in fcc Fe and Ni. The distinct behaviour is correlated
to the relative roles of longitudinal and transversal spin excitations. At
variance with the bcc-Fe case, we note a clear effect of longitudinal spin
excitations on the magnetic free energy of vacancy formation in fcc Fe and Ni,
leading to its steady variation above the respective magnetic transition
temperature. Below the N\'{e}el point, such effect in fcc Fe is comparable but
opposite to the one of the transversal excitations. Regarding fcc Ni, although
neglecting the longitudinal spin excitations induces an overestimation of the
Curie temperature by 220 K, no additional effect is visible below the Curie
point. The distinct effects on the three systems are closely linked to DFT
predictions of the dependence of vacancy formation energy on the variation of
local magnetic-moment magnitudes and orientations.
