Key Lab of Microstructure and Property of Advanced Material, Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing 100124, People's Republic of China.
Phys Chem Chem Phys. 2018 Jul 4;20(26):18117-18126. doi: 10.1039/c8cp01495g.
MnFe(P,Ge) is a promising magnetocaloric material for potential refrigeration applications near room temperature. However, its relatively large hysteresis and large temperature/field range of two-phase [paramagnetic (PM) and ferromagnetic (FM)] coexistence displayed in the cyclic first order magnetic transition (FOMT) cause energy losses and reduce the energy conversion efficiency. In this work, we explore the underlying causes of phase coexistence, hysteresis and structural transformation based on determination of the Ge distribution in MnFeP1-xGex (0.10 < x < 0.50) materials. We find that all the samples crystallize in the Fe2P-type structure [P6[combining macron]2m (No. 189), Z = 3] and Ge displays a strong preference for the 2c site. First principles total energy calculations confirm this site preference of Ge, and Ge entering the 2c site changes the electronic structures and enhances the Fe and Mn 3d exchange splitting across the Fermi level as well as the FM exchange interactions, consequently leading to a linear increase in the transition temperature with increasing Ge content. Scanning electron microscopy and energy-dispersive spectroscopy reveal the inhomogeneous distribution of Ge in grains, which makes the grains with larger Ge content transform from the PM to the FM phase first when cooling and thus causes the phase coexistence. Maximum entropy method electron-densities show that weakening the coplanar Fe-P/Ge(2c) and Mn-P(1b) bonding strengths across the PM to FM phase transition can release some 3d-electrons to enhance the Fe-Mn FM exchange interaction and result in coupling between the magnetic and structural degrees of freedom. This provides first direct evidence for the dominant role of Fe-Mn exchange interaction in the ferromagnetic ordering and may provide a method to observe the exchange interaction. Diminishing the variances in covalent bonding strengths across the FOMT gives rise to an exponential decay in the heat hysteresis when increasing the Ge occupancy at the 2c site. To the best of our knowledge, this is the first time a relationship between the variances in covalent bonding strengths and hysteresis is proposed. This material thus provides an example of a FOMT and hysteresis driven by reversible weakening and strengthening of covalent bonds. Based on these, a strategy of designing better magnetocaloric materials is suggested.
MnFe(P,Ge) 是一种很有前途的磁热材料，可用于室温附近的潜在制冷应用。然而，在循环一级磁转变 (FOMT) 中表现出的较大磁滞和较大的温场/场共存范围（顺磁 (PM) 和铁磁 (FM) 共存）导致能量损失并降低了能量转换效率。在这项工作中，我们基于 MnFeP1-xGex (0.10 < x < 0.50) 材料中 Ge 分布的测定，探索了相共存、磁滞和结构转变的根本原因。我们发现所有样品都结晶为 Fe2P 型结构 [P6[combining macron]2m（No. 189），Z = 3]，Ge 强烈倾向于占据 2c 位。第一性原理全能量计算证实了 Ge 的这种位置偏好，并且 Ge 进入 2c 位会改变电子结构并增强 Fe 和 Mn 3d 交换分裂跨越费米能级以及 FM 交换相互作用，从而导致过渡温度随 Ge 含量的增加呈线性增加。扫描电子显微镜和能量色散光谱揭示了 Ge 在晶粒中的不均匀分布，这使得晶粒中 Ge 含量较大的部分在冷却时首先从 PM 相转变为 FM 相，从而导致相共存。最大熵法电子密度表明，在 PM 到 FM 相变过程中减弱共面 Fe-P/Ge(2c) 和 Mn-P(1b) 键合强度可以释放一些 3d 电子来增强 Fe-Mn FM 交换相互作用，并导致磁和结构自由度之间的耦合。这为 Fe-Mn 交换相互作用在铁磁有序中的主导作用提供了第一个直接证据，并可能为观察交换相互作用提供一种方法。在 FOMT 过程中，减小共价键合强度的方差会导致当 2c 位 Ge 占据率增加时热滞后呈指数衰减。据我们所知，这是首次提出共价键合强度方差与磁滞之间的关系。因此，这种材料提供了一个由可逆减弱和增强共价键引起的一级磁转变和磁滞的例子。基于这些，提出了一种设计更好的磁热材料的策略。
