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Magnetic systems studied by first-principles thermodynamics

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  • Design
  • Physics

Abstract

A major challenge in applying density functional theory (DFT) for real materials is that it has been originally designed to predict ground-state properties. Since finite temperature effects are crucial for practically all applications, a thermodynamic extension of DFT is indispensable. It has previously been shown that lattice and electronic excitations can provide extremely accurate thermodynamic predictions for nonmagnetic materials. However, for the vast variety of materials such as metallic alloys, where magnetic excitation processes are critical, this is not sufficient. So far practically no theoretical concepts to include magnetic excitations and to bridge between the complexity of real structural materials, magnetic theories which are designed to describe very particular model systems, and DFT calculations exist. In this work we have closed the gap between these fields. Finite temperature magnetism of real systems is commonly described using classical approaches such as classical Monte Carlo. These methods work well for temperatures well above the critical temperature, but fail for low temperatures, where spin-quantization becomes crucial. A key concern in this work is the correct incorporation of quantum effects into our models. We developed a hierarchy of numerically exact quantum Monte Carlo based methods and analytical (Greens functions) approaches to treat the magnetic free energy. The proposed approach allowed us to describe free energies with a hitherto not achievable accuracy and to reveal that spin quantum effects have a dramatic impact on free energies, heat capacities, and magnetizations, all the way up to the critical (Curie, Neel) temperature. We successful applied these methods to key materials in steel manufacturing like ferrite and cementite as well as various magnetic metals, providing a clear and systematic separation of different physically relevant contributions

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