  Whether {\alpha}double prime-Fe16N2 possesses a giant saturation
magnetization (Ms) has been a daunting problem among magnetic researchers for
almost 40 years, mainly due to the unshakable faith of famous Slater-Pauling
(SP) curve and poor consistency on evaluating its Ms. Here we demonstrate that,
using epitaxy and mis-fit strain imposed by an underlying substrate, the
in-plane lattice constant of Fe16N2 thin films can be fine tuned to create
favorable conditions for exceptionally large saturation magnetization. Combined
study using polarized neutron reflectometry and X-ray diffraction shows that
with increasing strain at the interface the Ms of these film can be changed
over a broad range, from ~2.1T (non-high Ms) up to ~3.1T (high Ms). We suggest
that the equilibrium in-plane lattice constant of Fe16N2 sits in the vicinity
of the spin crossover point, in which a transition between low spin to high
spin configuration of Fe sites can be realized with sensitive adjustment of
crystal structure.
