Inducing high-$T_c$ ferromagnetism in the van der Waals crystal ${\mathrm{Mn}(\mathrm{ReO}}_4{)}_2$ via charge doping: A first-principles study

Magnetic van der Waals (vdW) materials, which can serve as ideal platforms to study low-dimensional magnetism and possess potential applications in next-generation spintronic devices, have attracted intensive attention recently. Here, based on density functional theory calculations we predict that the electron-doped $\mathrm{Mn}{\left({\mathrm{ReO}}_4\right)}_2$ (MRO) with an undulating layered structure is a ferromagnetic (FM) vdW material. Our calculations show that the magnetic interaction between two nearest-neighboring Mn atoms in the undoped MRO is very weak due to the long distance between the Mn atoms separated by the nonmagnetic ${\mathrm{ReO}}_4$ tetrahedra. With moderate electron doping from charge injection or K intercalation, the Re atoms become spin polarized. Thus, an effective FM coupling between the Mn atoms can be induced via the Mn-O-Re antiferromagnetic superexchange, while the calculated $T_c$ can exceed room temperature. The electron-doped MRO not only takes the advantage of strong FM interaction as double perovskites, but also avoids the shortage of the antisite disorder. The calculated small cleavage energy and stable phonon spectra indicate that MRO can be exfoliated to thin films, which facilitates its surface modification. Our paper on MRO, thus, provides a template to realize ferromagnetism in vdW materials with multitype transition metals.

Figure 1

(a) Side view and (b) top view of the crystal structures of MRO in which the ${\mathrm{MnO}}_6$ octahedra (purple) are connected by the ${\mathrm{ReO}}_4$ tetrahedra (gray). (c) Side view of the crystal structure of K-intercalated MRO. The K atoms locate at the coordinate of (0, 0, 0.5). (d) Top view of the $2\times 2\times 1$ supercell of MRO with an antisite defect highlighted by green circles.

Figure 2

Three typical collinear and one noncollinear spin configurations of Mn atoms in bulk MRO: (a) $A$-type antiferromagnetic ($A$-AFM) state with intralayer FM and interlayer AFM couplings, (b) intralayer stripe AFM, (c) intralayer zigzag AFM, and (d) intralayer noncollinear AFM state with interlayer FM coupling, respectively.

Figure 3

Total and local density of states for (a) the noncollinear AFM state of MRO and (b) the FM state of KMRO calculated with GGA $+ U$. The Fermi level sets to zero. Spin densities for the ferromagnetic states of (c) MRO and (d) KMRO. The kermesinus and green isosurfaces show the spin-up and spin-down densities, respectively. (e) Differential charge densities with the intercalation of K ions (${\rho }_{\mathrm{KMRO}}\text{−}{\rho }_{\mathrm{MRO}}\text{−}{\rho }_{\mathrm{K}}$). The yellow and cyan isosurfaces show the electron accumulation and depletion areas, respectively.

Figure 4

Curie temperature ($T_c$) and Heisenberg exchange interactions $J_n{S}^2$ calculated with different Hubbard $U$ values. The inset shows the schematic of the magnetic couplings $J_n$ ($n=1–3,c$) among ${\mathrm{Mn}}^{2+}$ ions.

Figure 5

(a) Cleavage energy of MRO calculated with different methods to include the vdW interactions. (b) Phonon spectra of the MRO monolayer.