  We study the crystal structure of Pd(100) ultrathin films, which show
ferromagnetism induced by the quantum confinement effect, using in-situ X-ray
crystal truncation rod measurement and density functional calculation. The
energy gain due to the appearance of ferromagnetism in Pd results in flatter
and uniform film growth of ferromagnetic Pd films compared with paramagnetic
Pd. In addition, ferromagnetic Pd films expand the lattice constant in order to
suppress the increase in kinetic energy of electrons accompanied by the
occurrence of exchange splitting. Although the traditional theory of magnetism
in metals indicates that the increase in density of states that induces
ferromagnetism (Stoner criterion), our present finding reveals a mechanism of
modulation in the density of states via the appearance of ferromagnetism, i.e.,
the inverse mechanism of Stoner's theory.
