Polymorphic Spin Ordering in a Single-Crystalline Cobalt-Doped Fe3GaTe2.

A single-crystalline system typically stabilizes a unique state for spin ordering below a critical temperature. Certain materials exhibit multiple magnetic states, often driven by structural phase transitions under varying thermodynamic conditions. Recently, van der Waals magnets have demonstrated subtle interlayer exchange interactions, offering a promising approach to control spin and correlated states. Here, we report the emergence of three distinct magnetic states─ferromagnetic ordering and both collinear and noncollinear antiferromagnetic orderings─in a layered single-crystalline magnet, cobalt-doped Fe3GaTe2 ((Co, Fe)3GaTe2). These three magnetic phases can be observed in a single material, a phenomenon we designate as polymorphic spin ordering in the material. The introduction of 16% Co-doping in Fe3GaTe2 modulates the interlayer magnetic interaction, enabling multiple spin orderings within the same lattice system with three critical temperatures: a Curie temperature for a ferromagnetic state (Tc = 210 K) and two Néel temperatures for the collinear (TN1 = 110 K) and noncollinear (TN2 = 30 K) antiferromagnetic states. Our findings, supported by magnetic force microscopy, first-principles calculations, and circular dichroism angular photoemission spectroscopy, reveal varying spin ordering and abrupt changes in the topological band structure and Berry curvature within single-crystalline (Co, Fe)3GaTe2.