Loss of long-range magnetic order in a nanoparticle assembly due to random anisotropy

Abstract

We have used soft x-ray photoemission electron microscopy (XPEEM) combined with x-raymagnetic circular dichroism (XMCD) and DC SQUID (superconducting quantuminterference device) magnetometry to probe the magnetic ground state in Fe thin filmsproduced by depositing size-selected gas-phase Fe nanoparticles with a diameter of 1.7 nm(∼200 atoms) onto Si substrates. The depositions were carried out in ultrahigh vacuum conditions andthicknesses of the deposited film in the range 5–50 nm were studied. The magnetometry data areconsistent with the film forming a correlated super-spin glass with a magnetic correlation length∼5 nm. The XPEEM magnetic maps from the cluster-assembled films were compared to thosefor a conventional thin Fe film with a thickness of 20 nm produced by a molecular beamepitaxy (MBE) source. Whereas a normal magnetic domain structure is observedin the conventional MBE thin film, no domain structure could be observed inany of the nanoparticle films down to the resolution limit of the XMCD basedXPEEM (100 nm) confirming the ground state indicated by the magnetometrymeasurements. This observation is consistent with the theoretical prediction that anarbitrarily weak random anisotropy field will destroy long-range magnetic order.