NSRRC Activity Report 2022

076 NSRRC ACTIVITY REPORT 2022 They examined the dual nature of magnetic excitations in a van der Waals (vdW) metallic ferromagnet Fe 2.72 GeTe 2 using inelastic neutron scattering with the 4SEASONS spectrometer at J-PARC in Japan and SIKA at ACNS ANSTO in Australia and provided direct spectroscopic evidence on the coexistence of and interplay between local moments and itinerant electrons in a vdWmetallic ferromagnet Fe 2.72 GeTe 2 . Notably, the metallic ferromagnet can sustain controllable roomtemperature ferromagnetism down to the monolayer limit. Figure 1 illustrates findings on higher- energy magnetic excitations obtained using 4SEASONS. They discovered that ferromagnetic spin-wave excitations dispersed from the zone center at low energies due to local moments; they also noted a columnlike broad continuum at the zone boundary at high energies up to over 100 meV that resulted from itinerant electrons. Unlike the two-dimensional crystal structure, a three-dimensional structure was present at the low-energy mode, and the Fig. 2 : (a) Constant- Q cuts at (0, 0, 2) plotted with y axis in logarithmic scale. Purple diamonds indicate data points obtained using 4SEASONS with E i = 8.9 meV at 4 K. The data are multiplied by 8 for clarity. Red circles and blue squares indicate the data points obtained using SIKA with fixed E f = 3.5 meV at 4 and 250 K, respectively. Each solid line corresponds to a Gaussian function or Lorentzian function fitted to the data. The dashed line corresponds to a Gaussian function fitted to the data for the elastic part. (b) Constant- E scans at selected energies along the [1, 0, 0] direction at 4 K, with an energy interval of ±0.1 meV obtained using 4SEASONS with E i = 8.9 meV. The data are offset vertically for clarity. The solid lines correspond to Gaussian functions fitted to the data. (c) The low- E spin waves along various directions plotted as E vs Q 2 at 4 K. The dashed lines correspond to linear fitted functions. (d) Temperature dependence of the spin-wave stiffness for various directions. The dashed line indicates the Curie temperature T C . [Reproduced from Ref. 1] high-energy mode also had an out-of-plane dependence. Both modes persisted well above the Curie temperature of 160 K. Low-energy spin waves were investigated in detail using a cold neutron axis spectrometer SIKA at high resolution ( Fig. 2 ). Their neutron spectroscopic data revealed that the low-energy spin waves at 100 K were more coherent than those at 4 K, which demonstrates the weakening of 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 behavior is generally expected in heavy-fermion systems with heavy f electrons but is generally not clearly observed in materials with light d electrons. These findings elucidate the nature of magnetism in transition-metal compounds. In summary, by using both 4SEASONS and SIKA , Bao and coworkers successfully observed the localized mode and itinerant mode in one crystal sample of Fe 2.72 GeTe 2. The