  The trigonal zigzag nanodisk with size $N$ has $N$ localized spins. We
investigate its thermodynamical properties with and without external leads.
Leads are made of zigzag graphene nanoribbons or ordinary metallic wires. There
exists a quasi-phase transition between the quasi-ferromagnet and
quasi-paramagnet states, as signaled by a sharp peak in the specific heat and
in the susceptability. Lead effects are described by the many-spin Kondo
Hamiltonian. A new peak emerges in the specific heat. Furthermore, the band
width of free electrons in metallic leads becomes narrower. By investigating
the spin-spin correlation it is argued that free electrons in the lead form
spin-singlets with electrons in the nanodisk. They are indications of many-spin
Kondo effects.
