Atomic-Scale Segregation and Fluctuations in Chemical Ordering FePt Thin Films
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Atomic-Scale Segregation and Fluctuations in Chemical Ordering FePt Thin Films Karen L Torres1, Chandan Srivastava1, Richard L Martens2, and Gregory B Thompson1 1 Department of Metallurgical and Materials Engineering, The University of Alabama, PO Box 870202, Tuscaloosa, AL, 35487-0202 2 The Central Analytical Facility, The University of Alabama, PO Box 870164, Tuscaloosa, AL, 35487-0164 ABSTRACT A series of atom probe and transmission electron microscopy (TEM) studies have been performed to quantify minute compositional fluctuations in Fe55Pt45 thin films during the A1 to L10 phase transformation. The atom probe specimens were analyzed in an Imago Local Electrode Atom Probe (LEAP®) at a target evaporation of 0.5%, a pulse fraction of 20% and a temperature of 120K. We noted a propensity of fracture failures in the LEAP with this material at lower temperatures. The atom probe reconstruction showed small levels of Pt segregation at grain boundaries in the as-deposited films. Fresnel-contrast TEM imaging confirmed high density fluctuations in these boundaries. Upon annealing at 600oC for 30 minutes, the film transformed from A1 to L10 and the grain boundaries become Fe enriched as compared to the as-deposited film.
INTRODUCTION During the past decade, the magnetic bit for areal storage density has decreased to a size that is rapidly approaching the superparamagnetic barrier. Superparamagnetism results from the following fundamental consideration: the energy that maintains the direction of magnetization in a grain of a magnetic material is proportional to the volume of the grain, i.e. KuV where Ku is the uniaxial magnetic anisotropy and V is the volume of the grain. As the bit density increases, V becomes smaller. The KuV product must remain larger than about 40 times kBT for media to be thermally stable and useful for information storage. The thermal stability of very small magnetic crystals or grains can be improved if the material has a larger Ku value. The L10 phase of FePt has been identified as a candidate material for next-generation media because of its high Ku [1]. When this intermetallic is sputter-deposited as a polycrystalline thin film, a metastable solid solution face-centered-cubic phase (A1) is formed [2]. By annealing at temperatures in excess of 500°C, the crystalline lattice atomistically orders into the desired hard magnetic L10 phase. Unfortunately, this annealing step results in grain growth destroying the narrow nano-granular size distribution needed for small, uniform bits. Moreover, grain coarsening has been observed to correlate with a higher degree of atomistic order [3]. There have been limited experimental reports on possible atomistic compositional fluctuation that could contribute to the ordering and grain growth in this phase transformation. This proceeding paper provides atom probe (AP) and transmission electron microscopy (TEM) studies of compositional fluctuations that develop in the deposition and phase transformation of a Fe55Pt45 thin film.
EXPERIMENTAL DETAILS FePt
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