Ab initio study on the coupling of lattice and magnetic degrees of freedom and the role of interfaces in magneto-caloric materials

在这个项目中，我们的主要重点将是开发和采用一个完整的第一性原理为基础的方法来研究磁热效应（MCE）接近的技术和科学的相关制度的一阶磁结构转变。在与我们的合作伙伴的协调行动中，我们的目标是深入了解晶格和磁自由度在这个转变中的耦合。识别和理解这些耦合关键取决于有限温度的精确处理，这对第一原理方法是一个挑战，例如在这个项目中应用的密度泛函理论。在过去的几年里，我们的小组已经开发和应用了各种方法，将用于推导MCE系统中的温度依赖熵。然而，进一步的方法开发，特别是对磁场和磁激发的结合，以及在无序系统中的声子的测定将是必要的。相应地，在这个项目中的调查将从建立Heusler合金Ni 2 MnGa和Ni 2 MnSn，其中MCE的基础知识已经知道。当进入更复杂的系统时，我们的理论研究是由我们的实验合作伙伴项目的意图触发的，该项目在结构上联合收割机结合了Ni54.5Mn20.5Ga25和Ni47Co3.1Mn36.6Sn13.3的薄膜，它们分别显示出常规和逆MCE。因此，我们将系统地研究非化学计量的Ni-Mn-Ga和Ni-Co-Mn-Sn Heusler合金。我们的建议的第二个中心目标是研究的接口的影响，发生在实验研究的多层系统。为此，我们的从头算为基础的振动和磁激发机制的调查将扩展从块体到超晶格结构包含两个Heusler合金之间的界面，这样获得的见解将有助于知识驱动的磁热材料的改进，用于创新的冷却设备在未来的应用。

In this project, our prime focus will be to develop and employ a complete first-principles based methodology to study the magneto-caloric effect (MCE) close to the technologically and scientifically relevant regime of first order magneto-structural transitions. Within a concerted action with the partners of our package, we aim at an in-depth understanding of the coupling of lattice and magnetic degrees of freedom at this transition.Identifying und understanding these couplings critically depends on an accurate treatment of finite temperatures, which is a challenge for first principles approaches, such as density functional theory applied in this project. A variety of methods that have been developed and applied over the last couple of years in our group will be used to derive temperature dependent entropies in MCE systems. However, further method development in particular towards the incorporation of magnetic fields and magnetic excitations as well as the determination of phonons in disordered systems will be necessary.Correspondingly, the investigations in this project will start with the established Heusler alloys Ni2MnGa and Ni2MnSn, for which the basics of the MCE are already known. When going to more complex systems, our theoretical research is triggered by the intension of our experimental partner project to structurally combine thin films of Ni54.5Mn20.5Ga25 and Ni47Co3.1Mn36.6Sn13.3, which show conventional and inverse MCE, respectively. We will therefore systematically investigate offstoichiometric Ni-Mn-Ga and Ni-Co-Mn-Sn Heusler alloys. A second central goal of our proposal is to study the influence of the interfaces, occuring in the experimentally investigated multilayer systems. For this purpose our ab initio based investigations of vibrational and magnetic excitation mechanisms will be extended from bulk to superlattice structures containing an interface between two Heusler alloys.The such obtained insights will contribute to a knowledge-driven improvement of magneto-caloric materials, to be used for innovative cooling devices in future applications.