The effect of spin fluctuations and atomic vibrations on the magnetic phase transition and the phase stability of iron at finite temperatures

Ning Wang, ICAMS, RUB, Bochum, Germany

A challenging problem in the computer-aided materials design is to perform realistic simulations at operating conditions for real-life applications, i.e., thermal excitations should be taken into account. The situation becomes even more complicated for the magnetic transition metals as both the spin fluctuations, the atomic vibrations and their couplings should be treated properly. Here we present a spin-lattice fluctuation theory combining the conventional spin fluctuation theory and the semi- empirical tight-binding model. This model is then solved with an efficient Hamiltonian Monte Carlo algorithm in order to evaluate thermal-equilibrium properties. As an application to iron, the calculated phonon spectra for fcc and bcc lattices are in a surprisingly good agreement with the experimental data in a wide range of temperatures, and the spin-fluctuation induced phonon softening is found to be the driving force for the transformation back from fcc to bcc at the high temperature. Besides, the atomic vibrations are found to have considerable effect on melting of both magntic long-range and short-range orders with increasing temperatures, which demonstrates the insufficiency of the widely-used rigid lattice spin-interaction models to perform realistic simulations. Furthermore, we find that the atomic vibrations are crucial to stabilize the high-temperature BCC phase of iron dynamically. The proposed model can be straightforwardly applied to other magnetic transition metals and also structures with defects such as point defects, surfaces and dislocations, and can be used as a new simulation tool for computer-aided design of microstructures of magnetic transition metals

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