β -NMR Investigation of the Depth-Dependent Magnetic Properties of an Antiferromagnetic Surface

By measuring the prototypical antiferromagnet α-Fe2O3, we show that it is possible to determine the static spin orientation and dynamic spin correlations within nanometers from an antiferromagnetic surface using the nuclear spin polarization of implanted 8Liþ ions detected with β-NMR. Remarkably, the firstorder Morin spin reorientation in single crystal α-Fe2O3 occurs at the same temperature at all depths between 1 and 100 nm from the (110) surface; however, the implanted nuclear spin experiences an increased 1=T1 relaxation rate at shallow depths revealing soft-surface magnons. The surface-localized dynamics decay towards the bulk with a characteristic length of ϵ ¼ 11 1 nm, closely matching the finite-size thresholds of hematite nanostructures.