Magnetostructural Coupling in Itinerant Magnets

第1部分：非技术性某些材料在靠近强磁体时，其温度会发生很大变化。近年来，人们已经认识到这种效应可以用于冷却，例如空调或制冷，并且这些磁冷却技术甚至可以潜在地取代涉及压缩/膨胀循环的更传统的冷却方式。这种对已知和新材料结构的基础研究，因为它们暴露于温度和磁场强度的变化，是在固态和材料化学计划的资助下进行的。该项目的目标是确定已知材料如何在有用的设备中发挥作用，并为寻找新的和改进的材料提供信息，这些材料可以进一步提高冷却效率。据估计，冷却技术消耗的能源占总能源消耗的五分之一。更好的材料和更好地理解冷却过程，这两者都是该项目的目标，因此，美国在这一重要的新兴技术方面处于领先地位，这符合国家利益。第二部分：技术磁性材料的温度变化由于施加的磁场早在1881年就被注意到了，然而，对利用这种效应在室温附近使用磁热材料进行冷却的巨大兴趣是最近的。虽然顺磁性材料可以用于在非常低的温度下冷却，但对于接近室温的应用和对于适度磁场的应用，在接近其铁磁居里温度的材料中观察到最强的效果。该项目中感兴趣的系统是居里温度接近室温的3d过渡金属基铁磁体。拟议的工作的主要重点是材料的结构类型来自二硼化铝结构的aristotype，其特点是他们的能力，主机两个磁性原子。通常，两个磁性原子之一是锰。在许多磁热中，磁转变似乎与结构变化有关。为了进一步研究这一点，本研究通过回答以下两个问题来指导：（i）当铁磁转变通过磁场或通过转变温度冷却时，晶体结构如何变化，以及磁热冷却的含义是什么？以及（ii）是否可以将变磁跃迁（即自旋态跃迁）设计到材料中，从而增强与打开磁场相关的熵变？研究的实验方面侧重于已知的磁热材料以及新材料的发现。采用基于密度泛函理论的方法来协同设计材料成分。

Part 1: Non TechnicalCertain materials show large changes in their temperature when they are brought close to strong magnets. In recent years, there has been a recognition that such effects can be employed in cooling, for example, in air-conditioning or refrigeration, and that these magnetic cooling technologies can potentially even replace more conventional ways of cooling involving compression/expansion cycles. This fundamental study of the structure of known and new materials as they are exposed to changes in the temperature and the strength of the magnetic field is carried out with funding from the Solid State and Materials Chemistry program. The goal of this project is to identify how known materials can be expected to perform in useful devices, as well as inform the search for new and improved materials that could lead to further improvements in efficient cooling. Estimates are that cooling technologies consume as much as a fifth of the total energy used. Better materials and a better understanding of processes for cooling, both of which are target goals of this project, are therefore of national interest in terms of the US staying at the forefront of this important emerging technology. Part 2: TechnicalChanges in the temperature of magnetic materials due to applied magnetic fields were noted as long ago as 1881, the huge interest in the utility of this effect in employing magnetocaloric materials for cooling near room temperature is however more recent. While paramagnetic materials can be employed for cooling at very low temperatures, for applications near room temperatures and for applications of modest magnetic fields, the strongest effects are observed in materials close to their ferromagnetic Curie temperatures. The systems of interest in this project are 3d transition-metal based ferromagnets with Curie temperatures near room temperature. The main focus of the proposed work is on materials with structure types derived from the aluminum diboride structural aristotype, and are characterized by their ability to host two magnetic atoms. Typically, one of the two magnetic atoms is manganese. In many magnetocalorics, the magnetic transition appears to be associated with a structural change. To investigate this further, this research is guided by answering the following two questions: (i) How do the crystal structures change as ferromagnetic transitions are traversed with magnetic field or by cooling through the transition temperature, and what are the implications for magnetocaloric cooling? and (ii) Can metamagnetic transitions (i.e. spin-state transitions) be designed into materials in such a way that the entropy change associated with turning on a magnetic field is enhanced? The experimental aspect of the research focuses on known magnetocaloric materials as well as the discovery of new ones. Density functional theory-based methods are employed to co-design material compositions.