Shift of the blocking temperature of Co nanoparticles by Cr capping

We have studied the effect of Cr capping on the magnetic properties of Co nanoparticles (NPs). The NPs have an average diameter of 2.2 nm. The blocking temperature TB of the bare Co particles is 13.2 K. By capping with a thin Cr layer up to a thickness of tCr = 0.52 nm, we first observe a decrease of TB up to tCr = 0.14 nm, followed by an increase of TB for larger thicknesses 0.14 nm ≤ tCr ≤ 0.52 nm. X-ray magnetic circular dichroism measurements at the resonant Co and Cr L3 edges confirm a magnetic polarization of Cr which is opposite to the magnetization of Co. The antiparallel alignment of Co and Cr spins at the Co/Cr interface can explain the decrease at low capping layer thickness. However, for larger Cr capping layer thicknesses, the Cr film bridges the Co NPs, mediating interparticle exchange coupling and enhancing dipolar coupling that leads to an increase of the blocking temperature.