Crystal-facet-oriented altermagnets for detecting ferromagnetic and antiferromagnetic states by giant tunneling magnetoresistance

Emerging altermagnetic materials with vanishing net magnetizations and unique band structures have been envisioned as an ideal electrode to design antiferromagnetic tunnel junctions. Their momentum-resolved spin splitting in band structures defines a spin-polarized Fermi surface, which allows altermagnetic materials to polarize current as a ferromagnet, when the current flows along specific directions relevant to their altermagnetism. Here, we design an altermagnet/insulator barrier/ferromagnet junction, renamed as an altermagnetic tunnel junction (ATMTJ), using RuO₂/TiO₂/CrO₂ as a prototype. Through first-principles calculations, we investigate the tunneling properties of the ATMTJ along the 001 and 110 directions, which shows that the tunneling magnetoresistance (TMR) is almost zero when the current flows along the 001 direction, whereas it can reach as high as 6100% with current flows along the 110 direction. The spin-resolved conduction channels of the altermagnetic RuO₂ electrode are found responsible for this momentum-dependent (or transport-direction-dependent) TMR effect. Furthermore, this ATMTJ can also be used to readout the N\'eel vector of the altermagnetic electrode RuO₂. Our work promotes the understanding toward the altermagnetic materials and provides an alternative way to design magnetic tunnel junctions with ultrahigh TMR ratios and robustness of the altermagnetic electrode against external disturbance, which broadens the application avenue for antiferromagnetic spintronic devices.