Growth, structure and magnetic properties of FePt nanostructures on NaCl(001) and MgO(001)

A comparison of the structural and magnetic properties of FePt nanostructures grown at different temperatures on NaCl(001) and MgO(001) substrates is presented. A strong influence of the deposition temperature on the epitaxial growth as well as on the size distribution of FePt nanostructures grown on NaCl substrates is observed. In spite of a large lattice mismatch between FePt and NaCl, a 'cube-over-cube' growth of nanostructures with a narrow size distribution was achieved at 520 K. Moreover, the growth of FePt nanostructures on NaCl(001) is not preceded by the formation of a wetting layer as observed on MgO(001). The higher degree of L1(0) chemical ordering in FePt nanostructures grown on MgO(001) accompanied by the absence of L1(0) variants with an in-plane tetragonal c-axis indicates that the tensile epitaxial stress induced by the MgO substrate is a key factor in the formation of the L1(0) phase with an out-of-plane c-axis. Superparamagnetic behavior is revealed for the FePt nanostructures grown on NaCl(001) due to their small size and relatively poor chemical order.

Is plastic flow always controlled by dislocation mobility? An answer from in situ transmission electron microscopy straining tests

In situ transmission electron microscopy (TEM) straining experiments are used to illustrate in two extreme cases the possible role of dislocation nucleation and exhaustion as a controlling factor in plastic flow. In the first example (FeAl intermetallic compounds), a thermally activated dislocation exhaustion is responsible for an anomalous stress-temperature dependence and an associated small strain rate sensitivity, the latter being evidenced during in situ experiments through unstable localized slip. The second example (heavily drawn pearlite) shows specific dislocation loop nucleation processes that may account for the Hall-Petch law breakdown characteristic of fine scale nanostructures.