In the local or itinerant extreme, magnetic excitations can be described by the Heisenberg model which treats electron spins as localized moments, or by the itinerant-electron model where the exchange interaction between electrons leads to unequal numbers of electrons with up and down spins. However, it has been elusive when both local moments and itinerant electrons are present in the intermediate range. Using inelastic neutron scattering, we provide direct spectroscopic evidence on the coexistence of and interplay between local moments and itinerant electrons in a van der Waals metallic ferromagnet Fe$_{2.72}$GeTe$_{2}$, which can sustain tunable room-temperature ferromagnetism down to the monolayer limit. We find that there exist ferromagnetic spin-wave excitations dispersing from the zone center at low energies resulting from local moments, and a column-like broad continuum at the zone boundary at high energies up to over 100 meV resulting from itinerant electrons. Unlike the two-dimensional crystal structure, the low-energy mode exhibits a three-dimensional nature, and the high-energy mode also has an out-of-plane dependence. Both modes persist well above the Curie temperature of 160 K. Our neutron spectroscopic data reveal that the low-energy spin waves at 100 K are more coherent than those at 4 K, which is evidence of the weakening of the Kondo screening at high temperatures. These results unambiguously demonstrate the coexistence of local moments and itinerant electrons, and the Kondo effect between these two components in Fe$_{2.72}$GeTe$_{2}$. Such behaviors are generally expected in heavy-fermion systems with heavy $f$ electrons but rarely clearly observed in materials with light $d$ electrons. These findings shed light on the understanding of magnetism in transition-metal compounds.
Magnetic anisotropy in the near-stoichiometric van der Waals ferromagnet Fe$_3$GeTe$_2$
Leibniz Institute for Solid State and Materials Research（莱布尼茨固体与材料研究所）；Institute for Solid State and Materials Physics, TU Dresden（德累斯顿工业大学固体与材料物理研究所）；Department of Condensed Matter and Materials Physics, S. N. Bose National Centre for Basic Sciences（S.N. 玻色基础科学国家中心凝聚态与材料物理系）；School of Physical Sciences, Indian Association for the Cultivation of Science（印度科学促进协会物理科学学院）
Disentangling the unusual magnetic anisotropy of the near-room-temperature ferromagnet Fe$_{4}$GeTe$_{2}$
Leibniz Institute for Solid State and Materials Research（莱布尼茨固体和材料研究所）；Institute for Solid State and Materials Physics, TU Dresden（德累斯顿工业大学固体和材料物理研究所）；S. N. Bose National Centre for Basic Sciences（S.N. 玻色基础科学国家中心）；Max Planck Institute for Chemical Physics of Solids（马克斯·普朗克固体化学物理研究所）；Saha Institute of Nuclear Physics（萨哈核物理研究所）
High-Temperature ferromagnetism from site-selective filling in (Fe,Ni)$_{6-δ}$GeTe$_2$
Yale University（耶鲁大学）；University of California, Berkeley（加州大学伯克利分校）；Lawrence Berkeley National Lab（劳伦斯伯克利国家实验室）；Cornell University（康奈尔大学）；University of Southern California（南加州大学）；Boston College（波士顿学院）
$μ$SR measurements of 3D Ising ferromagnetism in bulk Fe$_3$GeTe$_2$ crystals
Memorial University（纪念大学）；PSI Center for Neutron and Muon Sciences（瑞士保罗谢尔研究所中子与μ子科学中心）；Durham University（杜伦大学）；University of Warwick（华威大学）；Clarendon Laboratory, Department of Physics, University of Oxford（牛津大学物理系克拉伦登实验室）
Dichotomy of flat bands in the van der Waals ferromagnet Fe$_5$GeTe$_2$
Rice University（莱斯大学）；University of Washington（华盛顿大学）；Oak Ridge National Laboratory（橡树岭国家实验室）；Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室）；Kyung Hee University（庆熙大学）
High-spin orbital interactions across van der Waals gaps controlling the interlayer ferromagnetism in van der Waals ferromagnets
我们研究了什么样的相互作用控制范德华铁磁体（如Fe 3-xGeTe 2，Cr2 Ge 2 Te 6，CrI 3和VI 3）中层间耦合的符号和强度，发现范德华能隙中的高自旋轨道相互作用是理解其铁磁性的关键。Fe_（3-x）GeTe_2Cr_2Ge_2 Te_6和Cr_I_3的层间铁磁耦合是由高自旋双轨道双电子不稳定性控制的，而VI_3的层间铁磁耦合是由高自旋四轨道双电子不稳定性控制的。这些相互作用解释了范德瓦尔斯铁磁体中许多看似令人困惑的观测结果。
Thermal and Magnetoelastic Properties of the van der Waals Ferromagnet Fe$_{3-δ}$GeTe$_2$: Anisotropic Spontaneous Magnetostriction and Ferromagnetic Magnon Excitations
Contrasting anisotropic electron-phonon-spin coupling in Fe$_{3}$GeTe$_{2}$ and Fe$_{5}$GeTe$_{2}$: A helicity-resolved Raman study
Indian Institute of Technology Kharagpur（印度理工学院卡拉格普尔分校）；Jawaharlal Nehru University（贾瓦哈拉尔·尼赫鲁大学）；Institute of Nuclear Physics, Polish Academy of Sciences（波兰科学院核物理研究所）；Sungkyunkwan University（成均馆大学）
Suppression of local magnetic moment formation and paramagnetic exchange interactions in monolayer Fe$_3$GeTe$_2$
Moscow Institute of Physics and Technology（莫斯科物理技术学院）；M. N. Mikheev Institute of Metal Physics of Ural Branch of Russian Academy of Sciences（俄罗斯科学院乌拉尔分校 M.N. 米凯耶夫金属物理研究所）；Radboud University, Institute for Molecules and Materials（拉德堡德大学分子与材料研究所）
Mechanical control of physical properties in the van der Waals ferromagnet Cr2Ge2Te6 via application of electric current
Cr2 Ge 2 Te 6是一种范德华铁磁体，居里温度为66 K。在这里，我们报告了一个快速的变化，在应用小直流电流，一个巨大的，但各向异性的磁电效应，和一个尖锐的，晶格驱动的量子开关表现在I-V特性的大块单晶Cr2 Ge 2 Te 6的磁基态。在这些观察到的现象的核心是一个新发现的，强烈的各向异性磁弹性耦合，使强烈的各向异性响应的晶格应用的电流和/或磁场，因此在Cr2 Ge 2 Te 6的奇异现象。磁性半导体中这种罕见的机械可调性为独特的功能材料和器件提供了诱人的前景。
Spin Reorientation Driven Renormalization of Spin-Phonon Coupling in Fe$_4$GeTe$_2$
S. N. Bose National Centre for Basic Sciences（S.N. 玻色基础科学国家中心）；Uppsala University（乌普萨拉大学）；Bose Institute（玻色研究所）；Indian Institute of Technology Madras（马德拉斯印度理工学院）；Saha Institute of Nuclear Physics（萨哈核物理研究所）
