First-principles prediction of altermagnetism in transition metal graphite intercalation compounds

We report the emergence of altermagnetism, a magnetic phase characterized by the coexistence of compensated spin ordering and momentum-dependent spin splitting, in graphite intercalation compounds (GICs), a prototypical material system long investigated for its tunable electronic and structural properties. Through first-principles calculations, we demonstrate that vanadium-intercalated stage-1 graphite compounds, exhibit inherent altermagnetic properties. The hexagonal crystal system and antiferromagnetic ordering of V atoms generate a magnetic space group that enforces alternating spin polarization in momentum space while maintaining zero net magnetization. The calculated band structure reveals robust altermagnetic signatures: along the high-symmetry direction, we observe a pronounced spin splitting of ~270 meV with alternating spin polarization. Crucially, the spin splitting exhibits minimal sensitivity to spin-orbit coupling (SOC) effect, highlighting the dominance of exchange interactions over relativistic effects. From Monte Carlo simulations, we predict a magnetic transition temperature ($T_m$ ) of ~228 K, indicating stable magnetic ordering above liquid nitrogen temperatures. The combination of symmetry-protected spin textures, SOC-independent splitting, and elevated $T_m$ temperature makes V-GICs as a promising candidate for spintronic applications, particularly for zero-field spin-polarized current generation and topologically robust spin transport. As the first demonstration of carbon-based alternating magnetic systems, this work offers a design paradigm for engineering spin-polarized quantum states governed by crystalline symmetry constraints.