Revealing room temperature ferromagnetism in exfoliated Fe₅GeTe₂ flakes with quantum magnetic imaging

Abstract

Van der Waals material Fe5GeTe2, with its long-range ferromagnetic ordering near room temperature, has significant potential to become an enabling platform for implementing novel spintronic and quantum devices. To pave the way for applications, it is crucial to determine the magnetic properties when the thickness of Fe5GeTe2 reaches the few-layers regime. However, this is highly challenging due to the need for a characterization technique that is local, highly sensitive, artifact-free, and operational with minimal fabrication. Prior studies have indicated that Curie temperature T C can reach up to close to room temperature for exfoliated Fe5GeTe2 flakes, as measured via electrical transport; there is a need to validate these results with a measurement that reveals magnetism more directly. In this work, we investigate the magnetic properties of exfoliated thin flakes of van der Waals magnet Fe5GeTe2 via a quantum magnetic imaging technique based on nitrogen vacancy in diamond. Through imaging the stray fields, we confirm room-temperature magnetic order in Fe5GeTe2 thin flakes with thickness down to 7 units cell. The stray field patterns and their response to magnetizing fields with different polarities is consistent with previously reported perpendicular easy-axis anisotropy. Furthermore, we perform imaging at different temperatures and determine the Curie temperature of the flakes at ≈300 K. These results provide the basis for realizing a room-temperature monolayer ferromagnet with Fe5GeTe2. This work also demonstrates that the imaging technique enables a rapid screening of multiple flakes simultaneously as well as time-resolved imaging for monitoring time-dependent magnetic behaviors, thereby paving the way towards high throughput characterization of potential 2D magnets near room temperature and providing critical insights into the evolution of domain behaviors in 2D magnets due to degradation.

Pervasive beyond Room-Temperature Ferromagnetism in a Doped van der Waals Magnet

“…Given its ability to detect magnetic field with high-sensitivity and high spatial resolution, the NV center in diamond realizes a powerful quantum sensor of local magnetic field and hence provides an enabling tool for advancing condensed matter physics and materials science [32]. In particular, the implementation of NV sensing in the form of wide-field magnetic imaging has been applied to study novel electrical flow in graphene [33], vdW superconductors [34], as well as 2D magnets both at low-temperatures [35] as well as at room-temperature [27]. Hence, QMI provides a highly suitable tool for our investigation.…”

“…The setup of QMI is described previously [27] and summarized in Figure 1c. An antenna for delivering microwave (MW) necessary for NV measurement is fabricated on the diamond.…”

“…As in Ref. [27], optically detected magnetic resonance (ODMR) was performed with a camera to image the stray magnetic field generated by CFGT flakes. We adopted the configuration where an out-of-plane (z-direction) bias field B0 is applied, which enables us to image the distribution of the out-of-plane component of the stray field Bz(x,y).…”

“…We adopted the configuration where an out-of-plane (z-direction) bias field B0 is applied, which enables us to image the distribution of the out-of-plane component of the stray field Bz(x,y). [27] To help inform what kind of pattern of stray-field distribution one anticipates to observe, we show the simulated Bz(x,y) generated by a square magnetic structure with a uniform out-ofplane (Figure 2a) and in-plane 2D magnetization (Figure 2b). The former generates a Bz(x,y) that switches sign at the boundary, where the latter generates a dipole-like pattern that is antisymmetric along the magnetization direction.…”

“…The family of iron-based vdW metals FenGeTe2 provides a promising platform for engineering high Tc 2D magnet, where Tc can be raised by increasing Fe content. For ultrathin flakes (≲20 nm), Tc is enhanced from ~ 130 K in Fe3GeTe2 [16], to 270 K in Fe4GeTe2 [25] and 270-310 K in Fe5GeTe2 [26,27]. While Fe5GeTe2 system has a stacking related transition near 570K, the material appears to be thermally and structurally stable [28].…”