Nearly-room-temperature ferromagnetism and tunable anomalous Hall effect in atomically thin Fe4CoGeTe2

TL;DR

This study addresses the challenge of achieving itinerant ferromagnetism near room temperature in two-dimensional van der Waals magnets by investigating Fe$_4$CoGeTe$_2$ (F4CGT), a Co-doped Fe$_5$GeTe$_2$. The authors combine thickness-controlled experiments on exfoliated F4CGT flakes with magnetotransport, magneto-optical measurements, and first-principles calculations to reveal nearly-room-temperature ferromagnetism persisting down to the bilayer and a tunable intrinsic anomalous Hall effect (AHE). They find $T_{ m C}$ is ~320–340 K for samples with $t \ge 12$ nm and ~284 K for bilayers, while the intrinsic AHC exhibits non-monotonic temperature and thickness dependence, explained by Berry curvature changes near the Fermi level as the band structure evolves with thickness. Theoretical results show a ferromagnetic ground state with in-plane anisotropy across ML, BL, and bulk, with Co substitutions at Fe(1) sites enhancing FM stability; these findings establish F4CGT as a platform for 2D spintronics and deepen understanding of Berry-curvature–driven AHE in ultrathin van der Waals magnets.

Abstract

Itinerant ferromagnetism at room temperature is a key ingredient for spin transport and manipulation. Here, we report the realization of nearly-room-temperature itinerant ferromagnetism in Co doped Fe5GeTe2 thin flakes. The ferromagnetic transition temperature TC (~ 323 K - 337 K) is almost unchanged when thickness is down to 12 nm and is still about 284 K at 2 nm (bilayer thickness). Theoretical calculations further indicate that the ferromagnetism persists in monolayer Fe4CoGeTe2. In addition to the robust ferromagnetism down to the ultrathin limit, Fe4CoGeTe2 exhibits an unusual temperature- and thickness-dependent intrinsic anomalous Hall effect. We propose that it could be ascribed to the dependence of band structure on thickness that changes the Berry curvature near the Fermi energy level subtly. The nearly-room-temperature ferromagnetism and tunable anomalous Hall effect in atomically thin Fe4CoGeTe2 provide opportunities to understand the exotic transport properties of two-dimensional van der Waals magnetic materials and explore their potential applications in spintronics.

Figures (4)

• Figure 1: Crystal structure and characterization of F4CGT thin flakes.a, Crystal structure of F4CGT. The blue and green balls represent Ge and Te atoms, respectively, and the red, purple and orange balls represent Fe/Co atoms at Fe(1), Fe(2) and Fe(3) sites, respectively. The Fe(1) and Ge positions are partially occupied (marked by the color difference). b, Atomic force microscope image of F4CGT thin flakes on a 285 nm SiO$_{2}$/Si substrate. The white scale bars represent 5 $\mu$m. Inset: optical picture of F4CGT thin flakes. c, Cross-sectional profile of the F4CGT thin flake along the blue line in b. d, Temperature dependence of $R_{\rm N}(T)$ for F4CGT with various thicknesses. Inset: the optical image of a typical F4CGT device covered with a h-BN layer. The white scale bar represents 15 $\mu$m.
• Figure 2: Nearly-room-temperature ferromagnetism in F4CGT thin flakes.a and b, Field dependence of $R_{yx}^{\rm A}(\mu_{0}H)$ measured at 2 K and 300 K for the F4CGT thin flakes with various $t$, respectively. The red and black lines represent the $R_{yx}^{\rm A}(\mu_{0}H)$ measured when decreasing and increasing fields. c, Arrott plots of a 2 nm device near 300 K. The red lines are linear fits at high-field region. d, Phase diagram of F4CGT as functions of $T$ and $t$. The $T_{\rm C}$ values (blue squares) are derived by linear interpolations of two adjacent Arrott plots. Vertical error bars represent the temperatures corresponding to two Arrott plots used to determine $T_{\rm C}$. Pink and blue regions mark the FM and PM states, respectively.
• Figure 3: Theoretical calculation results of F5GT and F4CGT.a-c, Band structures calculated with the spin-orbit coupling for the ground states (i.e., in-plane FM states) of ML F5GT, ML F4CGT and AA-stacking BL F4CGT with Co occupying at Fe(1) site, respectively. The points in red and blue in b and c represent the contributions from Co and Fe atoms, respectively. d, The in-plane anomalous Hall conductivity $\sigma_{xy}$ of ML and AA-stacking BL F4CGT as a function of $E-E_{\rm F}$. Here the magnetization is aligned to the $c$-axis and $E_{\rm F}$ is the Fermi level.
• Figure 4: Evolution of anomalous Hall effect with $T$ and $t$.a and b, Field dependence of $R_{yx}(\mu_{0}H)$ at different temperatures for the samples with $t$ = 2 nm and 50 nm, respectively. c-e, Temperature dependence of normalized $R_{yx}^{\rm{A,s}}$, $\sigma_{yx}^{\rm{A,s}}(T)$ and $S_{\rm H}(T)$ for the F4CGT samples with various $t$. These data are normalized by their values at $T$ = 2 K. f, Angular dependence of the $R_{yx}(\mu_{0}H)$ curves at $T$ = 5 K for sample with $t$ = 50 nm. g, The relationship of $\theta_{M}$ and $\theta_{H}$ at 5 K and 300 K for F4CGT samples with different $t$. The black dashed line marks $\theta_{M}$ = $\theta_{H}$ that corresponds to $K_{u}$ = 0. Inset shows the definition of $\theta_{M}$ and $\theta_{H}$. h, Temperature and thickness dependence of fitted $K_{u}$ using the Stoner-Wohlfarth model. Inset exhibits the fit of $\theta_{M}(\theta_{H})$ curve at 5 K for 50 nm thick sample and the black dashed line is same as that in inset of g.