The occurrence of bcc-fcc ($\alpha -\gamma$) and fcc-bcc ($\gamma -\delta$) phase transitions in magnetic iron stems from the interplay between magnetic excitations and lattice vibrations. However, this fact has never been confirmed by a direct dynamic simulation, treating noncollinear magnetic fluctuations and dynamics of atoms, and their coupling at a finite temperature. Starting from a large set of data generated by ab initio simulations, we derive noncollinear magnetic many-body potentials for bcc and fcc iron, describing fluctuations of atomic coordinates in the vicinity of near perfect lattice positions. We then use spin-lattice dynamic simulations to evaluate the difference between the free energies of bcc and fcc phases, assessing their relative stability within a unified dynamic picture. We find two intersections between the bcc and fcc free energy curves, which correspond to the $\alpha -\gamma$ bcc-fcc and $\gamma -\delta$ fcc-bcc phase transitions. The maximum bcc-fcc free energy difference over the temperature interval between the two phase transitions is 2 meV per atom, in agreement with other experimental and theoretical estimates.

中文翻译：

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磁性铁中结构相变的动态模拟

bcc-fcc的发生（$\alpha -\gamma$）和fcc-bcc（$\gamma -\delta$磁铁中的相变源于磁激发和晶格振动之间的相互作用。然而，这一事实从未被直接的动态模拟所证实，该模拟处理了非共线的磁性波动和原子的动力学，以及它们在有限温度下的耦合。从头开始生成的大量数据开始通过模拟，我们得出了bcc和fcc铁的非共线磁性多体势，描述了接近完美晶格位置附近原子坐标的波动。然后，我们使用自旋晶格动力学模拟来评估bcc和fcc相的自由能之间的差异，并在统一的动态图中评估它们的相对稳定性。我们发现bcc和fcc自由能曲线之间有两个交点，分别对应于$\alpha -\gamma$ 密件抄送-fcc和 $\gamma -\delta$fcc-bcc相变。与其他实验和理论估计一致，在两个相变之间的温度间隔内，最大bcc-fcc自由能差为2 meV /原子。