Structure and magnetic properties of Fe nanoparticles embedded in a Cr matrix

The structure of 2 nm diameter (340 atoms) Fe nanoparticles embedded in a Cr matrix was determined using X-ray Absorption Fine Structure (EXAFS) and the magnetic properties studied by Superconducting Quantum Interference Device (SQUID) magnetometry. The thin films were produced by the co-deposition of pre-formed gas-phase Fe clusters synthesised by a gas aggregation source with an atomic vapour of Cr produced by an MBE source. The behaviour was studied as a function of Fe nanoparticle volume fraction in the range 5-20% and was compared to previous results on ferromagnetic nanoparticles in antiferromagnetic matrices. EXAFS showed that the atomic structure in the Cr-embedded Fe nanoparticles is the same as the bulk bcc structure. Whereas alloying between the nanoparticles and matrix material has previously been shown to be very pronounced for Co nanoparticles in antiferromagnetic Mn, it was found that any alloying between Fe nanoparticles and Cr matrix material is limited. For dilute samples of Fe nanoparticles in Cr the measured saturation magnetisation (M) was 1μ/Fe atom, which is significantly less than the bulk M value of 2.22μ/Fe atom indicating that the surface of Fe nanoparticles is either antiferromagnetic or non-magnetic. An increase in the volume fraction produces an increase in the value of M and at a volume fraction of 20%, M exceeds the value of bulk Fe showing that some Cr spins provide a ferromagnetic contribution. After field cooling below 30 K, all films show Exchange Bias (EB) and an increase of coercivity, which are both much larger for the most concentrated sample. The Cr spins at the surface of the Fe particles play a key role in determining the overall magnetic behaviour.