Metastable Phase Formation and Stimulated Transitions in Metallic Nanometer Films
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L11.57.1
Metastable Phase Formation and Stimulated Transitions in Metallic Nanometer Films Dirk C. Meyer 1, Alexandr A. Levin 1, Stefan Braun2 , Andre Gorbunov 3, Michael Mertig 3, Wolfgang Pompe 3 and Peter Paufler 1 1 Institut für Strukturphysik, Technische Universität Dresden, D-01062 Dresden, Germany 2 Fraunhofer-Institut Werkstoff- und Strahltechnik Dresden, D-01277 Dresden, Germany 3 Institut für Werkstoffwissenschaft, Technische Universität Dresden, D-01062 Dresden, Germany ABSTRACT Due to the non-equilibrium nature of deposition techniques, thin films can exhibit an energetic state far from thermodynamic equilibrium. Energy supply can stimulate a transition into other metastable states. Examples of metastable phase formation presented in this work are thermally stimulated solid state reactions in metallic nanometer Al/Co/Ni multilayers and phase formation and transition in metallic alloy films of the elemental materials system Fe-Cr. An interesting application illustrates the technical potential of metastable nanometer films. INTRODUCTION Nanometer multilayers (MLs) due to intrinsic gradients of the chemical potential and alloy films mixed under compulsion are generally far from thermodynamic equilibrium. Energy supply can initiate structural relaxation and/or solid state reactions in the films driven by those gradients (e.g. chemical composition, interface energy, strains, geometrical kinetic constrains and other factors specific for the film materials). The reaction kinetics illustrates Ostwald’s rule of successive transformations, which states that the first state to appear is the least thermodynamically stable and it is successively replaced by the more stable states (cf., e.g.[1]). It is of importance, that under ambient conditions lifetimes can achieve the order of 100 years and more, allowing for technical application of intermediate phases associated with these stages and having interesting physical properties. The intermediate (‘metastable’) stages agree with local minima of the corresponding thermodynamic potential of the system whereas the stable state of the system is due to its minimum. At fixed pressure, the equilibrium agrees with the minimum of the Gibbs free energy G of the system. As illustrated in Fig. 1, during a change of state an energy barrier ∆G has to be overcome. By thermal activation, the motion of atoms is stimulated reflecting an increase of amplitude of fluctuations ∆G of free energy of the system facilitating overcoming of the barrier and turning the system into a metastable state with a smaller value of the free energy. These processes can also be activated by energy conversion following any absorption processes of the electromagnetic field energy or of the kinetic energy of particles during bombardment. Thereby at the same time any kinds of thermally stimulated processes are to be taken into account. This applies to diffusion as well as to restoration and recrystallization processes. Independent of the kind of solids, by varying of these processes manifold reactions can be t
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