压力诱导下铁基二元晶态合金的因瓦行为

Under high pressure, for the iron-based binary crystalline alloys with different chemical compositions, taking into account electron-phonon coupling, magnon-phonon coupling and electron-phonon-magnon coupling, the elementary excitation spectrum, the elementary excitation state density and electronic structure, etc. are studied by using first principles density functional theory (DFT) and the Green function theory in different crystal structures at different temperatures. By combining the chemical composition with the average number of effective valence electrons, the physical origin which the Invar alloy is sensitive to chemical composition will be revealed. Thereby, it may be identified whether the magnon-phonon coupling or the electron-phonon coupling in the iron-based binary crystalline alloys plays a major role in the low-energy phonon softening. Furthermore it may be confirmed whether the phonon excitation under magnon-phonon coupling is the 'hidden' excitation as predicted by Ishikawa. Finally, it will be confirmed whether the Invar effect orginates from the electron-phonon-magnon coupling. Through deeply researching on the pressure-induced continuous magnetic and structual phase transition process, and the magnetic collapse when ferromagnetic state disappears above the critical pressure, we will theoretically illustrate the physical mechanisms of the high-spin states to the low-spin states transition and the magnetovolumic effect， and clarify the origin of the 'hidden' excitation. Theoretical studies devoted to the law of variations of the spontaneous volume magnetostriction and the magnetization with temperature and pressure will be carried out, and the physical mechanisms of the abnormal elasticity and magnetism in Invar alloys will be revealed. It is meaningful to reveal the ferromagnetic origin of the 3d transition metals and their alloys.

高压下，对于不同化学组分的铁基二元晶态合金，考虑电子-声子耦合、磁振子-声子耦合、电子-声子-磁振子三者耦合，并在不同的晶体结构下利用第一性原理与格林函数理论研究不同温度下的元激发谱、元激发态密度、电子结构等。结合化学组分与平均有效价电子数的关系，揭示因瓦合金对组分敏感的物理根源。从而在铁基二元晶态因瓦合金判断磁振子-声子耦合与电子-声子耦合哪一项对低能声子软化起主要作用，并且要确认磁振子-声子耦合下声子激发是否也为Ishikawa所预期的"潜藏"激发，最终确认因瓦效应是否源于电子-声子-磁振子三者耦合。深入研究压力诱导连续磁相变和晶体结构相变过程及在临界压力以上铁磁态消失的磁性坍塌现象，并从理论上阐明高自旋态-低自旋态转变的物理机制和磁体积效应，进而明确"潜藏"激发的根源。理论上研究自发体积磁致伸缩与磁化强度随温度与压力的变化规律，并揭示因瓦合金的弹性反常与磁性反常的物理机制。

高压下铁基晶态因瓦合金的反常行为的研究丰富了探索因瓦效应机理的途径。铁基晶态因瓦合金处于特殊的磁临界状态，这种磁临界状态下体系的晶格动力学稳定性对压力极为敏感。我们采用CALYPSO晶体结构预测方法和软模相变理论对铁基晶态合金（Fe-Ni、Fe-Pt、Fe-Pd、Fe-Mn、Fe-N、Fe-Ge等）进行了压力诱导下的晶体结构预测及计算。从而，解释了压力诱导下铁基晶态因瓦合金结构相变机理。⑴在Fe3Pt晶态合金的高压物性的研究中，从理论上发现，压力小于26.95GPa的铁磁态下L12-Fe3Pt的自发磁化诱导了体系横向声学支声子软化，表明体系中存在很强的自发体积磁致伸缩。特别是在铁磁性坍塌临界压力41.9GPa至磁性完全消失57.25GPa的压力区间，晶格动力学稳定性对压力更加敏感。压力大于57.25GPa时，压力诱导了体系声子谱的稳定。在铁磁性坍塌临界压力附近体系的微观磁结构不稳定性导致了弹性模量出现了急剧震荡现象和费米能级处的电子自旋极化率也对压力非常敏感。⑵γˊ-Fe4N晶态合金高压物性研究中，利用软模相变理论对于γˊ-Fe4N，在10GPa下的声学支声子的M点处软化现象的处理，发现了动力学稳定的高压新相P2/m-Fe4N。压力小于1GPa时高压新相P2/m-Fe4N是热力学稳定的相，且磁矩与γˊ-Fe4N的磁矩几乎相同。对比没有考虑磁性的γˊ-Fe4N的声子谱，得出压力小于1GPa时，自发磁化诱导了铁磁相γˊ-Fe4N基态晶格动力学稳定。⑶利用费曼图技术及松原格林函数理论，推导出多体耦合下体系的元激发谱及元激发衰减。如磁振子-声子耦合下的磁振子谱及其衰减和不同模式声子谱和对应的声子衰减及声子态密度等。总之，通过铁基晶态合金的物性研究揭示了因瓦合金的弹性反常与磁性反常，上述反常行为源于很强多体耦合作用，如电子-声子耦合、磁振子-声子耦合或者电子-声子-磁振子耦合。