The magnetization process of single crystals of the metallic kagomé ferromagnet Co 3 Sn 2 S 2 was carefully measured via magnetization and AC susceptibility. Field-dependent anomalous transitions observed in low fields indicate the presence of an unconventional magnetic phase just below the Curie temperature, T C . The magnetic phase diagrams in low magnetic fields along different crystallographic directions were determined for the first time. The magnetic relaxation measurements at various frequencies covering five orders of magnitude from 0.01 to 1000 Hz indicate markedly slow spin dynamics only in the anomalous phase with characteristic relaxation times longer than 10 s.

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We report quasi-two-dimensional (Q2D) itinerant electron magnetism in the layered Co-based shandites. Comprehensive magnetization measurements were performed using single crystals of Co 3 Sn 2−x In x S 2 (0 ≤ x ≤ 2) and Co 3−y Fe y Sn 2 S 2 (0 ≤ y ≤ 0.5). The magnetic parameters of both systems; the Curie temperature T C , effective moment p eff and spontaneous moment p s ; exhibit almost identical variations against the In-and Fe-concentrations, indicating significance of the electron count on the magnetism in the Co-based shandite. The ferromagnetic-nonmagnetic quantum phase transition is found around x c ∼ 0.8. Analysis based on the extended Q2D spin fluctuation theory clearly reveals the highly Q2D itinerant electron character of the ferromagnetism in the Co-based shandites.