Flat-band tuning and emergent itinerant magnetism in Sr(Co _{1− <i>x</i>} Pd _<i>x</i> ) ₂ As ₂

The interplay between magnetism and flat-band (FB) instability is a central theme in quantum materials research. A striking example is the emergence of magnetic order in a nominally nonmagnetic compound when a flat band is tuned near the Fermi energy ([Formula: see text]). In this study, we investigate this phenomenon in the Pauli paramagnet [Formula: see text], where an FB associated with Co [Formula: see text] orbitals lies close to [Formula: see text]. Remarkably, a minute substitution of the nonmagnetic element Pd onto the Co site ([Formula: see text]2%) induces antiferromagnetic order with a transition temperature as high as [Formula: see text] K. Temperature- and magnetic-field-dependent magnetic and transport measurements, complemented by zero-field neutron diffraction, reveal a helical magnetic order for [Formula: see text] in Sr(Co_1-<i>x</i>Pd_<i>x</i>)₂As₂, transitioning to a complex ferromagnetic state at higher Pd concentrations. Spectroscopic evidence and theoretical band structure calculations demonstrate that electron doping shifts the flat band closer to [Formula: see text], significantly enhancing the Stoner parameter. This enhancement drives a strong ferromagnetic instability, leading to helical magnetic ordering dominated by in-plane ferromagnetic interactions. The emergence of robust magnetic ordering through substitution with nonmagnetic elements is a unique phenomenon that underscores the pivotal role of flat-band instability in tuning magnetism in itinerant systems.