M A T E R I A L S F O R T O M O R R O W ' S I N N O V A T I V E A N D S U S T A I N A B L E I N D U S T R Y

S C I E N T I F I C H I G H L I G H T S

8 0 H I G H L I G H T S 2 0 2 3 I

Synchrotron techniques show effects of pressure on van der Waals magnetism in Fe3GeTe2

The effects of high pressure on the magnetic and structural properties of van der Waals (vdW) ferromagnet Fe3GeTe2 were studied using synchrotron Mössbauer source spectroscopy, X-ray powder diffraction and Raman spectroscopy. The findings highlight pressure as a driving force for a possible magnetic quantum criticality in layered vdW magnetic systems.

Following the discovery of graphene the strongest and thinnest two-dimensional (2D) vdW material, numerous efforts have been made to search for new 2D vdW materials with outstanding optoelectronic and magneto- optical properties for ultrathin, ultracompact and low- power nanodevice applications. The recent discovery of ferromagnetism in 2D vdW magnetic systems has provided unprecedented opportunities for the external field manipulation of magnetic and spin transport phenomena down to the monolayer limit [1]. Their magnetic, structural and electronic properties exhibit much stronger responses to external stimuli like pressure, magnetic and electric field, and light, unlike their conventional 3D counterparts. Among layered 2D vdW magnetic materials, Fe3GeTe2 (FGT) is especially interesting for spintronics applications since it possesses itinerant ferromagnetism (FM) with a high Curie temperature of TC ~220 K in its bulk stoichiometric form [2], and the variation in

thermodynamic parameters leads to a variety of novel phenomena such as the tunnelling spin-valve behaviour, the anomalous Hall effect, heavy fermion states and chiral spin structures.

A strong suppression of ferromagnetism and TC of FGT has recently been reported, pointing to the pressure- induced instability of the magnetic state [3]. However, the microscopic origin of this phenomenon remains unclear. Since the low-dimensional nature of FGT is expected to facilitate pronounced quantum fluctuations, one of the possible scenarios involves applying an external stimulus, such as pressure, to drive the system into a quantum critical regime.

This work investigated the magnetic and structural properties of single crystalline FGT in the powder form over the 0 20 GPa pressure and 10 300 K temperature ranges by means of synchrotron Mössbauer spectroscopy (SMS), X-ray powder diffraction (PXRD), and Raman spectroscopy measurements. SMS spectra of Fe3GeTe2 under various pressures were measured at low (e.g., T = 10 K) and high (e.g., T = 300 K) temperatures at beamline ID18, as shown in Figure 60. Below the TC ~220 K, for pressures up to 11 GPa, the spectra measured at 10 K are well fitted by the model containing two sextet components, corresponding to the Fe1 (4e) and Fe2 (2c) inequivalent sites in the hexagonal structure of P63/mmc symmetry. The hyperfine magnetic fields of the Fe1 and Fe2 sites are reduced progressively under pressure. A critical pressure for a full suppression of the FM state in Fe3GeTe2 is evaluated

Fig. 60: a) The synchrotron Mössbauer source spectra of Fe3GeTe2, measured at selected pressures and temperatures. The sextet components in the ferromagnetically ordered phase (red and blue) and the doublet components in the

pressure-induced paramagnetic phase (orange and cyan) correspond to two nonequivalent Fe sites. b) The unit cell of a Fe3GeTe2 hexagonal crystal structure.