Quasi-ferromagnet spintronics in the graphene nanodisc-lead system

A zigzag graphene nanodisc can be interpreted as a quantum dot with an internal degree of freedom. It is well described by the infinite-range Heisenberg model. We have investigated its thermodynamical properties. 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 susceptibility. We have also analyzed how thermodynamical properties are affected when two leads are attached to the nanodisc. It is shown that lead effects are described by the many-spin Kondo Hamiltonian. There appears a new peak in the specific heat, and the multiplicity of the ground state becomes just one half of the system without leads. Another lead effect is to enhance ferromagnetic order. Being a ferromagnet, a nanodisc can be used as a spin filter. Furthermore, since the relaxation time is finite, it is possible to control the spin of the nanodisc by applying an external spin current. We then propose a rich variety of spintronic devices made of nanodiscs and leads: spin memory, spin amplifier, spin valve, spin-field-effect transistor, spin diode and spin logic gates such as spin-XNOR gate and spin-XOR gate. Graphene nanodiscs could well be basic components of future nanoelectronic and spintronic devices.