Faske Tom, Radulov Iliya A, Hölzel Markus, Gutfleisch Oliver, Donner Wolfgang
Fachbereich Material- und Geowissenschaften, Technische Universität Darmstadt, Alarich-Weiss-Strasse 2, D-64287 Darmstadt, Germany.
J Phys Condens Matter. 2020 Mar 13;32(11):115802. doi: 10.1088/1361-648X/ab5a99. Epub 2019 Nov 22.
Spin fluctuations are a crucial driving force for magnetic phase transitions, but their presence usually is indirectly deduced from macroscopic variables like volume, magnetization or electrical resistivity. Here we report on the direct observation of spin fluctuations in the paramagnetic regime of the magnetocaloric model system LaFeSi in the form of neutron diffuse scattering. To confirm the magnetic origin of the diffuse scattering, we correlate the temperature dependence of the diffuse intensity with ac magnetic susceptibility and x-ray diffraction experiments under magnetic field. Strong spin fluctuations are already observable at 295 K and their presence alters the thermal contraction behavior of LaFeSi down to the Curie temperature of the first-order magneto-structural transition at 190 K. We explain the influence of the spin fluctuation amplitude on the lattice parameter in the framework of the internal magnetic pressure model and find that the critical forced magnetostriction follows Takashi's spin fluctuation theory for itinerant electron systems.
自旋涨落是磁相变的关键驱动力，但其存在通常是从诸如体积、磁化强度或电阻率等宏观变量中间接推导出来的。在此，我们报告了在磁热模型系统LaFeSi的顺磁区中以中子漫散射形式对自旋涨落的直接观测。为了证实漫散射的磁起源，我们将漫散射强度的温度依赖性与交流磁化率以及磁场下的x射线衍射实验相关联起来。在295 K时就已经可以观测到强烈的自旋涨落，并且它们的存在改变了LaFeSi直至190 K的一级磁结构转变的居里温度时的热收缩行为。我们在内部磁压力模型的框架内解释了自旋涨落幅度对晶格参数的影响，并发现临界强迫磁致伸缩遵循Takashi关于巡游电子系统的自旋涨落理论。
