Abstract
Altermagnetism, the third magnetic phase beyond ferromagnetism and antiferromagnetism, holds great promise for spintronics but also faces challenges in materials discovery and mechanism exploration. Here, through high-throughput screening of 170 pentagonal 2D materials, 4 altermagnetic semiconductors (MnS2, CoS2, MnC2, and CoPSe) are identified with sizable spin splitting (109-172 meV). It is shown that MnS2 is the first-of-kind altermagnetic second-order topological insulator (AMSOTI), as evidenced by a nontrivial real Chern number (nu R = 1) and spin-polarized corner state. Using ab inito quantum transport simulation, a MnS2-based altermagnetic tunneling junction (AMTJ) is designed and a giant tunneling magnetoresistance (TMR) of 1.5 x 105% at the Fermi level, which surpasses MF2 (M = Co and Ni)-based AMTJ by three orders of magnitude. In addition, by applying a twisting angle of 90 degrees to the AMTJ without altering the Neel${\rm N}\acute{\rm {e}}{\rm el}$ order, a significant TMR of 1.2 x 107% as driven by the structural changes is observed. This work unveils pentagonal 2D materials as a compelling material platform for exploring the physics and device applications of altermagnets.