Giant tunneling magnetoresistance in insulated altermagnet/ferromagnet junctions induced by spin-dependent tunneling effect

The nondegenerate energy bands along a specific path of altermagnet enable the realization of magnetic tunnel junctions (MTJs) with high tunnel magnetoresistance (TMR). Herein, we propose a MTJ based on insulated altermagnet/ferromagnetic $\mathrm{MTJ}\ensuremath{-}\mathrm{Mn}{\mathrm{F}}_{2}/\mathrm{Cr}{\mathrm{O}}_{2}$. In addition to effectively simplifying the device configuration, high TMR is attained in the $[110]$ and $[111]$ crystal directions due to the spin-dependent tunneling effect in the real space of $\mathrm{Mn}{\mathrm{F}}_{2}$. The increase in the thickness of the $\mathrm{Mn}{\mathrm{F}}_{2}$ layer leads to an increase in electron spin filtering efficiency (SFE). When the thickness of the $\mathrm{Mn}{\mathrm{F}}_{2}$ layer is three layers, the SFE is as high as 85.65% and the TMR is as high as ${10}^{4}%$. In addition, we find that the interface type has a significant impact on the magnetoresistance performance of the tunnel junction. Compared with F interface, the O interface leads to a further increase in the SFE of $\mathrm{Mn}{\mathrm{F}}_{2}$ (99.56%) and conductance of MTJ, and thus a significant enhancement of the TMR (up to ${10}^{5}%$). The analysis of electronic states in real space reveals that the coordination of oxygen atoms with magnetic atoms in the current direction gives rise to the endowed electronic states and an increase in spin polarizability in the vicinity. The results provide insight into the development of ultrafast and efficient spintronic devices with altermagnet.