Intrinsic Ferromagnetism and Surface Oxidation in Nb2FeC: Revealed by Synchrotron-Based X-Ray Spectro- and Microscopy

Abstract

A-site substitution in M_n+1AX_n (MAX) phases with transition metals carrying partially filled 3d orbitals offers a pathway to induce magnetism in otherwise nonmagnetic layered carbides/nitrides. Here, we investigate Nb₂FeC, obtained by replacing the nonmagnetic Al site in Nb₂AlC with Fe, using synchrotron-based spectroscopy and microscopy. Structural analysis confirms hexagonal P6₃/mmc phase with alternating Fe and Nb₂C layers along the c-axis. Surface-sensitive X-ray absorption reveals a thin Fe-oxide overlayer from ambient exposure, while Fe in the bulk reveals alloying-driven hybridization between Fe and Nb states. Magnetization measurements establish robust ferromagnetism with a Curie temperature near 283 K. Importantly, Fe L_3,2-edge Scanning Transmission X-ray Microscopy (STXM) combined with X-ray Magnetic Circular Dichroism (XMCD) directly demonstrates intrinsic bulk ferromagnetism, even though conventional surface-sensitive XMCD shows no signal. The STXM-XMCD dichroic response remains nearly unchanged across regions of varying thickness, indicating that ferromagnetic (FM) correlations are confined within the two-dimensional Fe layers and coupled only weakly across Nb₂C spacers. Thickness-dependent absorption spectra further reveal Fe─Nb hybridization, underscoring the role of A-site substitution in tuning magnetic interactions. These findings identify Nb₂FeC as a rare/near room-temperature FM MAX phase and highlight A-site engineering as a versatile strategy for designing layered magnetic carbides for spintronic applications.