  The p-electrons of carbon in the interstitial compound Fe3C hybridize with
the Fe d-band and enhance the valence electron concentration of Fe from 8 to
8.67. At this concentration, substitutional 3d transition metals and alloys
exhibit strong moment-volume coupling phenomena and associated magnetovolume
instabilities, otherwise known as the Invar effect. For this reason, Fe3C is
also expected to incorporate a strong magnetovolume instability, and therefore,
we examine the pressure dependence of the of the K edge x-ray magnetic circular
dichroism in Fe3C at ambient temperature and pressures up to 20 GPa. We and
clear evidence for a high-moment to low-moment transition at about 10 GPa.
