Tunneling Magnetoresistance in Noncollinear Antiferromagnetic Tunnel Junctions

Antiferromagnetic (AFM) spintronics has emerged as a subfield of spintronics driven by the advantages of antiferromagnets producing no stray fields and exhibiting ultrafast magnetization dynamics. The efficient method to detect an AFM order parameter, known as the Néel vector, by electric means is critical to realize concepts of AFM spintronics. Here, we demonstrate that noncollinear AFM metals, such as ${\mathrm{Mn}}_3\mathrm{Sn}$, exhibit a momentum dependent spin polarization which can be exploited in AFM tunnel junctions to detect the Néel vector. Using first-principles calculations, we predict a tunneling magnetoresistance (TMR) effect as high as 300% in AFM tunnel junctions with ${\mathrm{Mn}}_3\mathrm{Sn}$ electrodes, where the junction resistance depends on the relative orientation of their Néel vectors and exhibits four nonvolatile resistance states. We argue that the spin-split band structure and the related TMR effect can also be realized in other noncollinear AFM metals like ${\mathrm{Mn}}_3\mathrm{Ge}$, ${\mathrm{Mn}}_3\mathrm{Ga}$, ${\mathrm{Mn}}_3\mathrm{Pt}$, and ${\mathrm{Mn}}_3\mathrm{GaN}$. Our work provides a robust method for detecting the Néel vector in noncollinear antiferromagnets via the TMR effect, which may be useful for their application in AFM spintronic devices.

Figure 1

(a)–(d) The top view of atomic and magnetic structures of AFM ${\mathrm{Mn}}_3\mathrm{Sn}$ for Néel vectors (green arrows) oriented at $\alpha =0°$ (a), 60° (b), 120° (c), and 180° (d). $({\mathrm{Mn}}_1,{\mathrm{Sn}}_1)$ and $({\mathrm{Mn}}_2,{\mathrm{Sn}}_2)$ layers are located at $z=c/4$ and $z=3c/4$, respectively. (e)–(f) The corresponding band structures along high symmetry lines in the Brillouin zone indicating in color the in-plane spin expectation values $⟨{\sigma }_x⟩$ (e) and $⟨{\sigma }_y⟩$ (f). The color scale for the spin values is shown on the right.

Figure 2

(a)–(d) The top view of atomic and antiferromagnetic structures of bulk ${\mathrm{Mn}}_3\mathrm{Sn}$ for $\alpha =0°$, 60°, 120° and 180°. The black dashed lines in (b) indicate the mirror planes perpendicular to $ab$ plane for $\alpha =60°$. (e) and (f) are the in-plane spin $⟨{\sigma }_x⟩$ and $⟨{\sigma }_y⟩$ projected on the Fermi surface (band 1 in Fig. S1) of bulk ${\mathrm{Mn}}_3\mathrm{Sn}$.

Figure 3

(a) Schematic of the ${\mathrm{Mn}}_3\mathrm{Sn}/\text{Vacuum}/{\mathrm{Mn}}_3\mathrm{Sn}$ AFMTJ for the parallel (${\alpha }_L=0°$, ${\alpha }_R=0°$) and antiparallel (${\alpha }_L=0°$, ${\alpha }_R=180°$) Néel vectors. (b)–(e) ${\mathit{k}}_{\Vert }$-resolved transmission $T({\mathit{k}}_{\Vert })$ in the 2D Brillouin zone for ${\alpha }_L=0°$ and ${\alpha }_R=0°$ (b), 60° (c), 120° (d), and 180° (e).

Figure 4

(a) The calculated tunneling conductance $G$ per lateral unit cell area (left axis) and resistance-area ($RA$) product (right axis) for a ${\mathrm{Mn}}_3\mathrm{Sn}/\text{Vacuum}/{\mathrm{Mn}}_3\mathrm{Sn}$ AFMTJ as a function of $a_R=0°$, 60°, 120°, and 180°. The black and red lines are guides to the eyes. (b) Schematic of the three-terminal AFM spintronic device, where the Néel vector of the bottom noncollinear AFM electrode ${\mathrm{Mn}}_{3}X$ ($X=\mathrm{Sn}$, Ge, Ga) can be switched by the electric current driven SOT and read out through the TMR effect in a ${\mathrm{Mn}}_{3}X/\text{Tunnel}$ $\mathrm{barrier}/{\mathrm{Mn}}_3\mathrm{X}$ AFMTJ.