Altermagnetic-Like Behavior and Enhanced Coercivity in Ferrimagnets at a Critical Point of an Extended N\'eel-Diagram

We generalize the classic N\'eel diagram for ferrimagnets within a mean-field framework and reveal a critical point at which full magnetic compensation is maintained below the Curie temperature and extends past the nominal compensation point. Ferrimagnets tuned to this critical point display altermagnetic-like features and markedly enhanced coercive fields. We show that proximity to this regime requires minimizing the net local moment while balancing exchange interactions with respect to the number of equivalent atoms in each sublattice. The resulting extended N\'eel diagram provides practical design principles for engineering ferrimagnets near the critical point via targeted chemical substitution that combines atoms with robust and weak local moments, as demonstrated through density functional theory and Monte Carlo simulations for GdCo$_5$-type ferrimagnets.