The Changing Shape of Magnetic Refrigeration: an investigation of adaptive magnetic materials

现代冷却几乎完全基于压缩/膨胀制冷循环，这是一项自一个多世纪前发明以来几乎没有改变的技术。这是一个高能耗的行业，每年消耗数十亿千瓦时。然而，现代制冷已接近其基本性能极限，远低于热力学可能的极限。此外，用作制冷剂的液体化学品，最终逃逸到环境中，是臭氧层消耗和全球变暖的气体，或危险化学品。近年来，磁制冷已成为一种有前途的新型环保固态制冷技术。在过去的十年里，磁性冰箱的原型已经被展示出来。它们已经被证明比传统的冰箱更节能，并且可以在室温附近的宽温度范围内工作。但大多数原型机使用昂贵的稀土金属，如钆作为制冷剂，迫切需要替代品。已经发现了几个有前途的磁性材料族，但到目前为止，这个过程还只是一个启发式的过程。在本提案中，我们打算建立一个从头算量子材料建模工具来改变这一过程，并促进其在磁性材料研究小组中的应用。在最合适的材料中，支撑磁性质的相互作用必须精细地平衡，我们的建模将需要能够跟踪和指示它们的温度依赖性，它们如何随成分和结构变化和/或添加掺杂剂而变化。在磁性制冷剂中，当施加磁场使固体升温时，材料中随机定向的磁矩排列。通过使用传热流体（如水或空气）去除这些热量，然后去除磁场，磁性材料可以降低其温度。被冷却物体的热量随后与传热流体一起被提取，循环完成。当磁场作用在材料上时，熵和温度的变化描述了磁热效应，这个提议将在定量的基础上确定它和它背后的磁相互作用。我们对几种材料的结果将根据现有的大量实验数据进行测试。这项工作的一个特别新颖和雄心勃勃的部分将是研究如何纳米结构一个大的磁热效应。为此，我们将研究一些稀土过渡金属异质结构，并对其效果进行优化。当施加磁场时，这种产生强烈变暖效应的物理现象还有另一个有趣的方面。它可以解释为什么一些最有前途的材料在磁场存在下也会显著改变形状。这种磁塑性、“磁形状记忆”效应具有多种潜在的技术应用，如微泵、声纳和磁机械传感器。我们将采用我们的理论纳米结构模型来研究材料中边界缺陷的磁相互作用的强度和各向异性，以及它们如何导致缺陷本身在磁场作用下移动。将对Ni-Mn-Ga Heusler合金进行试验，并根据合金成分的变化对效果进行优化。

Modern cooling is based almost entirely on a compression/expansion refrigeration cycle - a technology more or less unchanged since its invention over a century ago. It is a high-energy demand industry which consumes billions of kWh every year. Yet, modern refrigeration is close to its fundamental performance limit which is well below what is thermodynamically possible. Furthermore, the liquid chemicals used as refrigerants, which eventually escape into the environment, are ozone layer depletive and global warming gases, or hazardous chemicals.Recently magnetic refrigeration has emerged as a promising way for a new and environmentally friendly solid state cooling technology. Prototype magnetic fridges have been demonstrated during the last decade. They have been proven to be much more energy efficient than conventional fridges and can span a broad temperature range around room temperature. But most prototypes use expensive rare earth metals such as gadolinium as the refrigerant and alternatives are urgently required. Several families of promising magnetic materials have been discovered but up to now this process has been a heuristic one. In this proposal we intend to establish an ab-initio quantum materials modeling tool to transform this process and to facilitate its application by groups working with magnetic materials. In the most suitable materials the interactions that underpin the magnetic properties have to be delicately poised and our modeling will need to be able to track and indicate their temperature dependence, how they vary with compositional and structural changes and/or when dopants are added. In a magnetic refrigerant randomly oriented magnetic moments in the material align when a magnetic field is applied making the solid warm up. By removing this heat using a heat transfer fluid, like water or air, and then removing the field allows the magnetic material to lower its temperature. The heat from the object being cooled is then extracted with the heat transfer fluid and the cycle completed. The changes in entropy and temperature that happen when a magnetic field is applied to a material describe the magnetocaloric effect and this proposal will determine it and the magnetic interactions behind it on a quantitative basis. Our results for several classes of materials will be tested against the extensive experimental data available. A particularly novel and ambitious part of the work will be to investigate how to nanostructure a large magnetocaloric effect. To this end we will study some rare earth - transition metal heterostructures and optimise the effect.This physics which produces a strong warming effect when a magnetic field is applied has another intriguing facet. It can explain how some of the most promising materials also change their shape significantly in the presence of a magnetic field. Such magnetoplastic, 'magnetic shape memory' effects have diverse potential technological applications, such as micropumps, sonars and magnetomechanical sensors. We will adapt our theoretical nanostructural modeling to investigate the strengths and anisotropies of the magnetic interactions across a boundary defect in the material and how they lead to the defect itself moving as a magnetic field is applied. A test case of a Ni-Mn-Ga Heusler alloy will be undertaken and the effect will be optimised as the composition of the alloy is varied.