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

The existence of long-range magnetic order in low-dimensional magnetic systems, such as the quasi-two-dimensional van der Waals (vdW) magnets, has attracted intensive studies of new physical phenomena. The vdW ${\mathrm{Fe}}_N{\mathrm{GeTe}}_2$ ($N=3$, 4, 5; FGT) family is exceptional, owing to its vast tunability of magnetic properties. In particular, a ferromagnetic ordering temperature ($T_C$) above room temperature at $N=5$ (F5GT) is observed. Here, our study shows that, by nickel (Ni) substitution of iron in F5GT, a record high $T_C=478(6)\mathrm{K}$ is achieved. Importantly, pervasive, beyond room-temperature ferromagnetism exists in almost the entire doping range of the phase diagram of Ni-F5GT. We argue that this striking observation in Ni-F5GT can be possibly due to several contributing factors, including increased 3D magnetic couplings due to the structural alterations.

Figure 1

(a) Illustration of the AA-stacking order of both the Fe-rich and Ni-rich domains in Ni doped ${\mathrm{Fe}}_5{\mathrm{GeTe}}_2$ (Ni-F5GT) (green symbols, Fe; orange, Ni; purple, Te; blue, Ge) [44]. (b) The $(\begin{array}{ccc}0& 0& L\end{array})$-type of peaks of Ni-F5GT at select Ni doping levels $x$. (c) Effective layer spacing $d$ as a function of Ni doping (colored symbols) or Co doping (gray symbols [42]) in ${\mathrm{Fe}}_{5+\delta }{\mathrm{GeTe}}_2$. Vertical dashed lines indicate different regions in Ni-F5GT.

Figure 2

Nanoscale phase separation of Ni-F5GT at $x=0.36(2)$. (a) Energy-dispersive x-ray spectroscopy map showing both the Fe-rich (green) and Ni-rich (red) regions. (b)–(d) Atomic resolution HAADF STEM image demonstrating the two types of domains. The enlarged (c) Fe-rich and (d) Ni-rich domains show flat and rumpled atomic planes, respectively. The Te-Te planes in (b) are color coded based on the number and the rumpling of the atomic layers. Scale bars in (c),(d), $5\AA$.

Figure 3

Magnetization data of Ni-F5GT at select Ni doping level $x$. (a),(b) Temperature-dependent magnetization of Ni-F5GT (external in-plane magnetic field is 50 Oe). For clarity, the magnetization data at $x=0.97(1)$ and $x=1$ are multiplied by 200 and 2000, respectively. (c),(d) In-plane isothermal magnetization at $T=2\mathrm{K}$ (c) and $T=400\mathrm{K}$ (d), respectively. (e) Saturation moment per formula unit at 2 K under the magnetic field of 7 T. (f) Coercive fields extracted from the isothermal magnetization at $T=2\mathrm{K}$. In (e) and (f), the magnetic field is applied along the $ab$-plane (red symbols) or $c$ direction (green symbols).

Figure 4

Ni doping level $x$-dependent $T_C$ in Ni-F5GT. Empty symbols, $T_C$ for as-grown samples; solid symbols, $T_C$ after thermal cycling. Vertical dashed lines indicate different regions in Ni-F5GT.