Strain-engineered high-temperature ferromagnetic oxygen-substituted ${\mathrm{NaMnF}}_3$ from first principles

Using first-principles calculations, we investigated the magnetic, electronic, and structural properties of oxygen-substituted ${\mathrm{NaMnF}}_3$ $\left({\mathrm{NaMnF}}_{1.5}{\mathrm{O}}_{1.5}\right)$ with in-plane biaxial strain. For simplicity, a structure containing an oxygen octahedron is used to explore the underlying physical mechanism. We found that the oxygen octahedron induces a transition from an insulating antiferromagnet to a high-temperature half-metallic ferromagnet. More importantly, the Curie temperature can be significantly enhanced and even might reach room temperature by applying tensile strain. The changing trends of exchange coupling constants with the increasing biaxial tensile strain can be attributed to the cooperative effects of Jahn-Teller distortion and rotation distortion. It is expected that these findings can enrich the versatility of ${\mathrm{NaMnF}}_3$ and make it a promising candidate for spintronic applications.

Figure 1

Crystalline structures. (a) Side and (b) top views of orthorhombic perovskite ${\mathrm{NaMnF}}_3$ ($Pnma$). (c) Side and (d) top views of ${\mathrm{NaMnF}}_{1.5}{\mathrm{O}}_{1.5}$. Green, purple, argenteous, and red balls represent Na, Mn, F, and O atoms, respectively. The corner-sharing octahedra are shown in purple.

Figure 2

Four considered magnetic configurations: (a) FM, (b) C-AFM, (c) A-AFM, (d) G-AFM of ${\mathrm{NaMnF}}_3$. Four considered magnetic configurations: (e) FM, (f) C-AFM, (g) A-AFM, (h) G-AFM of ${\mathrm{NaMnF}}_{1.5}{\mathrm{O}}_{1.5}$. The purple and green octahedra denote up and down spins of magnetic Mn atoms, respectively. Mn1, Mn2, Mn3, and Mn4 denote four locations of Mn atoms in ${\mathrm{NaMnF}}_3$, as well as in ${\mathrm{NaMnF}}_{1.5}{\mathrm{O}}_{1.5}$ crystal cell. The exchange paths are indicated by black dashed arrows.

Figure 3

(a) In-plane biaxial tensile strain dependencies of the magnetic-exchange interactions ($J_{ij}$). The gray horizontal dashed line denotes the boundary of magnetic interaction. (b) Evolution of Curie temperature ($T_C$) as a function of in-plane biaxial tensile strain.

Figure 4

(a) The Mn-O-Mn bond angles ${\theta }_1$ and ${\theta }_2$ from top view and ${\theta }_3$ along $b$-axis. For the oxygen-dominated octahedra, the in-plane Mn-O bonds are denoted by $l_x$ and $l_y$, and out-of-plane Mn-O bond is denoted by $l_z$. (b) Mn-O-Mn bond angles as a function of biaxial tensile strain. Evolution of the normal Jahn-Teller (JT) distortion mode (c) $Q_2$ and (d) $Q_3$.

Figure 5

The total density of states (TDOS) of ${\mathrm{NaMnF}}_{1.5}{\mathrm{O}}_{1.5}$ at (a) 0 strain and (b) biaxial tensile strain of 10%. The symbols ${\mathrm{\Delta }}_{sf}$ and ${\mathrm{\Delta }}_{\downarrow }$ represent the spin-flip excitation energy and spin-down gap, respectively. The Fermi level is set at zero. (c) The schematic of the electronic structures restructuring models at zero strain and biaxial tensile strain of 10%. The white plane denotes the Fermi surface. Spin-up and spin-down states are marked by red and blue, respectively.

Figure 6

The projected density of states (PDOS) on the Mn-$3d$ orbitals in ${\mathrm{MnO}}_6$ octahedra (Mn2) at (a) 0 strain, biaxial tensile strain (b) 2%, (c) 4%, (d) 6%, (e) 8%, and (f) 10%. The gray perpendicular dashed lines denote the Fermi level.