Bias in Fe @ Cr Core - Shell Nanoparticles

We have used X-Ray Magnetic Circular Dichroism (XMCD) and magnetometry to study isolated Fe@Cr core-shell nanoparticles with an Fe core diameter of 2.7 nm (850 atoms) and a Cr shell thickness varying between 1 and 2 monolayers. The addition of Cr shells significantly reduces the spin moment but does not change the orbital moment. At least two Cr atomic layers are required to stabilize a ferromagnetic/antiferromagnetic interface and generate the associated exchange bias and increase in coercivity. Main Text of Communication Gas-phase synthesis of nanoparticles offers the new capability to engineer nanoparticles at the atomic level and then use them to produce novel materials by depositing them onto surfaces, either alone or in conjunction with a molecular beam of a matrix material. In particular recent developments have enabled the synthesis of alloy and core-shell nanoparticle building blocks in which there is independent control over core size, shell thickness and flexible choice of element in either. In the work reported here we have exploited this flexibility to study how the magnetic behaviour of Fe nanoparticles evolves as they are coated with one or two monolayers of Cr. The motivation for doing this is to understand the nature of the magnetic interface as a function of Cr shell thickness and to search for the onset of the Exchange Bias phenomenon. Exchange Bias (EB) is the term used to describe unidirectional anisotropy, or the horizontal shift in the magnetisation loop in systems containing a ferromagnetic(F)/antiferromagnetic(AF) interface when they are field-cooled below the Néel temperature of the AF material. It was first observed over 50 years ago in 20 nm diameter Co nanoparticles, partially oxidized to form an AF CoO shell. The original explanation was given in terms of a flat F/AF interface with a layered antiferromagnet whose spins are pinned in a single direction by the rest of the AF material at the interface. Rotating the F material by an external field produces a shift in the magnetic loop given by the difference in exchange between parallel and antiparallel spin alignments at the interface. The