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Influence of the Ge distribution on the first order magnetic transition of the MnFe(P,Ge) magnetocaloric material†
Physical Chemistry Chemical Physics ( IF 2.9 ) Pub Date : 2018-06-01 00:00:00 , DOI: 10.1039/c8cp01495g Zhen-Lu Zhang 1, 2, 3, 4, 5 , Dan-Min Liu 1, 2, 3, 4, 5 , Wei-Qiang Xiao 1, 2, 3, 4, 5 , Hui Li 1, 2, 3, 4, 5 , Shao-Bo Wang 1, 2, 3, 4, 5 , Yun-Tian Liang 1, 2, 3, 4, 5 , Hong-Guo Zhang 3, 4, 6, 7, 8 , Shan-Lin Li 1, 2, 3, 4, 5 , Jun-Jie Fu 1, 2, 3, 4, 5 , Ming Yue 3, 4, 6, 7, 8
Physical Chemistry Chemical Physics ( IF 2.9 ) Pub Date : 2018-06-01 00:00:00 , DOI: 10.1039/c8cp01495g Zhen-Lu Zhang 1, 2, 3, 4, 5 , Dan-Min Liu 1, 2, 3, 4, 5 , Wei-Qiang Xiao 1, 2, 3, 4, 5 , Hui Li 1, 2, 3, 4, 5 , Shao-Bo Wang 1, 2, 3, 4, 5 , Yun-Tian Liang 1, 2, 3, 4, 5 , Hong-Guo Zhang 3, 4, 6, 7, 8 , Shan-Lin Li 1, 2, 3, 4, 5 , Jun-Jie Fu 1, 2, 3, 4, 5 , Ming Yue 3, 4, 6, 7, 8
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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 [P![[6 with combining macron]](https://www.rsc.org/images/entities/char_0036_0304.gif)
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.
中文翻译：
Ge分布对MnFe（P，Ge）磁热材料的一阶磁跃迁的影响†
MnFe（P，Ge）是一种有前景的磁热材料，可用于室温附近的潜在制冷应用。但是，其在循环一阶磁跃迁（FOMT）中显示的相对较大的磁滞和较大的两相[顺磁性（PM）和铁磁性（FM）]的温度/场范围共存会导致能量损失并降低能量转换效率。在这项工作中，我们基于确定MnFeP 1- x Ge x（0.10 < x <0.50）材料中Ge的分布，探索了相共存，滞后和结构转变的根本原因。我们发现所有样品均以Fe 2 P型结构[ P 2 m（No. 189），ž= 3]，Ge对2c网站表现出强烈的偏爱。第一原理总能量计算证实了Ge的这种位点偏好，Ge进入2c位点改变了电子结构并增强了费米能级上的Fe和Mn 3d交换分裂以及FM交换相互作用，因此导致了线性增加。随着Ge含量的升高转变温度。扫描电子显微镜和能谱分析表明，Ge在晶粒中的分布不均匀，这使得Ge含量较大的晶粒在冷却时首先从PM向FM相转变，从而引起相共存。最大熵方法的电子密度表明，削弱从PM到FM相变的共面Fe–P / Ge（2c）和Mn–P（1b）的键合强度可以释放一些3d电子，从而增强Fe–Mn FM交换相互作用并导致磁性和结构自由度之间的耦合。这为铁锰交换相互作用在铁磁有序中的主导作用提供了直接的直接证据，并可能提供观察交换相互作用的方法。当增加2c处的Ge占有率时，减小整个FOMT上共价键强度的差异会导致热滞后的指数衰减。据我们所知，这是首次提出共价键强度变化与磁滞之间的关系。因此，这种材料提供了FOMT和由共价键的可逆弱化和增强引起的磁滞现象的示例。基于这些，提出了设计更好的磁热材料的策略。
更新日期：2018-06-01
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.
中文翻译：
Ge分布对MnFe（P，Ge）磁热材料的一阶磁跃迁的影响†
MnFe（P，Ge）是一种有前景的磁热材料，可用于室温附近的潜在制冷应用。但是，其在循环一阶磁跃迁（FOMT）中显示的相对较大的磁滞和较大的两相[顺磁性（PM）和铁磁性（FM）]的温度/场范围共存会导致能量损失并降低能量转换效率。在这项工作中，我们基于确定MnFeP 1- x Ge x（0.10 < x <0.50）材料中Ge的分布，探索了相共存，滞后和结构转变的根本原因。我们发现所有样品均以Fe 2 P型结构[ P
2 m（No. 189），ž= 3]，Ge对2c网站表现出强烈的偏爱。第一原理总能量计算证实了Ge的这种位点偏好，Ge进入2c位点改变了电子结构并增强了费米能级上的Fe和Mn 3d交换分裂以及FM交换相互作用，因此导致了线性增加。随着Ge含量的升高转变温度。扫描电子显微镜和能谱分析表明，Ge在晶粒中的分布不均匀，这使得Ge含量较大的晶粒在冷却时首先从PM向FM相转变，从而引起相共存。最大熵方法的电子密度表明，削弱从PM到FM相变的共面Fe–P / Ge（2c）和Mn–P（1b）的键合强度可以释放一些3d电子，从而增强Fe–Mn FM交换相互作用并导致磁性和结构自由度之间的耦合。这为铁锰交换相互作用在铁磁有序中的主导作用提供了直接的直接证据，并可能提供观察交换相互作用的方法。当增加2c处的Ge占有率时，减小整个FOMT上共价键强度的差异会导致热滞后的指数衰减。据我们所知，这是首次提出共价键强度变化与磁滞之间的关系。因此，这种材料提供了FOMT和由共价键的可逆弱化和增强引起的磁滞现象的示例。基于这些，提出了设计更好的磁热材料的策略。
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