  NiCo2O4 (NCO) films grown on MgAl2O4 (001) substrates have been studied using
magnetometry, x-ray magnetic circular dichroism (XMCD) based on x-ray
absorption spectroscopy, and spin-polarized inverse photoemission spectroscopy
(SPIPES) with various thickness down to 1.6 nm. The magnetic behavior can be
understood in terms of a layer of optimal NCO and an interfacial layer (1.2+/-
0.1 nm), with a small canting of magnetization at the surface. The thickness
dependence of the optimal layer can be described by the finite-scaling theory
with a critical exponent consistent with the high perpendicular magnetic
anisotropy. The interfacial layer couples antiferromagnetically to the optimal
layer, generating exchange-spring styled magnetic hysteresis in the thinnest
films. The non-optimal and measurement-speed-dependent magnetic properties of
the interfacial layer suggest substantial interfacial diffusion.
