Signatures of ferromagnetic order and spin-triplet pairing state in two-dimensional niobium diselenide

The co-existence of ferromagnetism and superconductivity becomes possible through unconventional superconducting pairing state. Though ferromagnetic superconductivity has been reported in heavy fermions, ZrZn₂, and rare-earth and hydroxide compounds, the nature of Cooper pairs for coupling the two competing orders is still under debate. Here, we investigate the role of disorder on atomically thin niobium diselenide (NbSe₂) intercalated with dilute cobalt (Co) atoms and show that such systems spontaneously display ferromagnetism below the superconducting transition temperature (T_C). We elucidate the origin of the superconductivity-triggered ferromagnetism by characterizing the tunnelling magnetoresistance (TMR) via vertical magnetic tunnel junctions (Co/BN/Co-doped NbSe₂), which shows a bistable state below T_C and deep inside the superconducting gap, suggesting a Ruderman–Kittel–Kasuya–Yosida (RKKY)-induced ferromagnetism mediated by Cooper pairs. We then performed non-local spin valve measurements to directly resolve the pairing state in the undoped NbSe₂ channel at 50 mK, which shows unambiguous Hanle precession signals with estimated spin diffusion length up to micrometre, indicating the presence of intrinsic spin-triplet state in NbSe₂ and consequently, the crucial role of triplet Cooper pairs for generating a ferromagnetic order in superconducting condensate. Our discovery opens the door for engineered ferromagnetic superconductors in 2D form.