Atom-selective spin-polarized transport in a charge-ordered altermagnet
Altermagnets provide a promising platform for spin-polarized transport without net magnetization, but their transport properties are usually discussed in terms of momentum-space spin splitting. Here, using first-principles calculations and quantum transport simulations, we show that the charge-ordered altermagnet $α$-Fe$_2$PO$_5$ exhibits a distinct form of real-space spin selectivity despite weak altermagnetic spin splitting near the Fermi level. The charge order creates inequivalent Fe$^{2+}$ and Fe$^{3+}$ sites within each sublattice, while the puckered C-type antiferromagnetic stacking suppresses inter-sublattice transport. As a result, electron and hole doping activate spin-polarized transport predominantly through Fe$^{3+}$- and Fe$^{2+}$-based channels, respectively. These atom-selective channels carry opposite spin polarizations on the two antiferromagnetic sublattices, giving rise to a globally compensated charge current with hidden Néel spin character. We further propose an all-in-one $α$-Fe$_2$PO$_5$ tunnel junction, where matching or mismatching atom-selective conduction channels yields orders-of-magnitude conductance modulation. Our findings establish a real-space design principle for atomically controlled spin functionality and spintronic devices.