Abstract
The spin transition, or spin crossover, is a manifestation of electronic instability induced by
external constraints such as pressure1. Among known examples that exhibit spin transition,
3d ions with d6 electron configurations represent the vast majority, but the spin transition
observed thus far has been almost exclusively limited to that between high-spin (S = 2) and
low-spin (S = 0) states2-9. Here we report a novel high-spin to intermediate-spin (S = 1) state
transition at 33 GPa induced by pressurization of an antiferromagnetic insulator SrFeO2
with a square planar coordination10. The change in spin multiplicity brings to
ferromagnetism as well as metallicity, yet keeping the ordering temperature far above
ambient. First-principles calculations attribute the origin of the transition to the strong inlayer
hybridization between Fe dx
2
-y
2 ��� O p��, leading to a pressure-induced electronic
instability toward the depopulation of Fe dx
2
-y
2 ��� O p�� antibonding states. Furthermore, the
ferromagnetic S = 1 state is half-metallic due to the inception of half-occupied spin-down
(dxz, dyz) degenerate states upon spin transition. These results highlight the square-planar
coordinated iron oxides as a new class of magnetic and electric materials and provide new
avenues toward realizing multi-functional sensors and data-storage devices.