d-band filling dictates magnetic stability in Mn- and Co-substituted FeRh alloys

The composition-dependent magnetic properties of B2-ordered \zfr~alloys with substitutional disorder on the Fe sublattice are investigated using first-principles calculations within the coherent potential approximation. By systematically substituting Mn and Co on the Fe sublattice, we establish $d$-band filling as the primary control parameter governing magnetic stability in this itinerant system. Mn substitution (hole doping) shifts the Fermi level into the minority-spin bonding states, driving a collapse of spin polarization (crossing zero at $x \approx 0.5$) and the emergence of competing antiferromagnetic interactions ($η_\mathrm{Mn} < 0$). Even though the ferromagnetic configuration remains energetically well separated from the G-type AFM-II configuration across the studied range ($ΔE$ up to $\sim$0.35~eV/atom), this exchange competition drives an ``itinerant magnetic softness'' that suppresses the Curie temperature by $\sim$450~K -- a finite-temperature instability set by the near-cancellation of competing exchange interactions rather than by AFM--FM energy proximity. In contrast, Co substitution (electron doping) acts as a ``magnetic hardener'' by pinning the Fermi level within the majority-spin pseudogap, preserving high spin polarization ($|P| \approx 0.75$) and stabilizing ferromagnetic exchange across the full composition range. These results show that tuning the Fermi level relative to the pseudogap provides a systematic, microscopic framework for controlling magnetic stability in B2-ordered itinerant magnets, distinct from simple magneto-volume models.