IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Magnetic Interfaces and Nanostructures Friday Sessions
       Session MI+SS-FrM

Paper MI+SS-FrM8
Relating Magnetic and Structural Changes of Thin FeNi Alloy Films on Cu(100)

Friday, November 2, 2001, 10:40 am, Room 110

Session: Magnetic Thin Films and Surfaces II
Presenter: M. Hochstrasser, Lawrence Livermore National Laboratory
Authors: M. Hochstrasser, Lawrence Livermore National Laboratory
J.G. Tobin, Lawrence Livermore National Laboratory
S.A. Morton, University of Missouri-Rolla
G.D. Waddill, University of Missouri-Rolla
F.O. Schumann, Freie Universität Berlin, Germany
N.A.R. Gilmann, The Pennsylvania State University
R.F. Willis, The Pennsylvania State University
Correspondent: Click to Email
At a concentration of around 65% Fe, bulk FeNi alloys exhibit the "Invar effect", a sudden arresting of the Wigner-Seitz cell volume and a zero expansion coefficient. Simultaneously, the crystal structure changes from face-centered cubic to body-centered cubic while the Curie temperature goes to zero. This structural transformation can be arrested in ultrathin alloys films grown epitaxially on a Cu(001) substrate. Theoretical work predicts that the fcc phase can exist in two possible states: a ferromagnetic high volume state or a antiferromagnetic low volume state (2@gamma@ state model) and a volume change between the paramagnetic and the high spin state of ~7%, and 1% change between a non-collinear equilibrium state and the high spin state. Experimental work shows a lattice expansion increasing linearly up to 3% at 65% Fe content followed by a sudden relaxation of 2% with increasing Fe content. The initial volume increase is associated with increasing magnetization/magnetic moment & spin alignment in the Ni-rich alloys. As the alloy is cooled below T@sub C@ (or a strong external magnetic field is applied), an increasing alignment of the magnetic moments causes the nearest-neighbor spins to push apart producing an internal pressure which expands the lattice. With increasing Fe content, this effect increases due to the increasing number of Fe nearest neighbors with the larger atomic magnetic moments. Eventually, a critical limit is reached (~65% Fe), when a magnetic/lattice volume instability develops. W ith x-ray magnetic dichroism the changes in the elemental magnetic moments were tracked. Spin polarized photoemission studies record a sudden decrease in the "mean-field" exchange splitting of the d-states with increasing Fe content through the critical "Invar transition". Angle-resolved photoemission imaging of states at the Fermi level reveal a much smaller splitting of the sp-states, which also tracks the changing magnetization with changing composition.