  We report on novel magnetic and electronic properties of SrCo6O11 that
exhibits a unique stepwise magnetization and its relevant magnetotransport
phenomena investigated by the site-selective 59Co nuclear magnetic resonance
(NMR) at zero and applied magnetic fields. This compound is composed of three
Co sites in the unit cell, i.e., Co(1) in the metallic Kagome layer, a Co(2)
dimerized pillar between the layers and Co(3) in the triangular lattice.
Zero-field NMR spectra have revealed that large local moments at the Co(3)
sites are magnetically ordered without any trace of bulk magnetization M at
zero field. The field-swept NMR spectra show that the internal hyperfine field
at the Co(1) site is derived from fully polarized moments Ms at the Co(3) sites
in the "1"-plateau state at fields higher than 2.5 T, whereas it is partially
cancelled out in the "1/3"-plateau state in which one-third of Ms is induced at
intermediate fields once a small field is applied. It has been clarified from a
microscopic point of view that the local moments at Co(3) site undergo a
field-induced ferrimagnetic (up-up-down)-to-ferromagnetic (up-up-up)
transition, which is consistent with the evidence obtained from the recent
neutron diffraction experiment. The Co(1) Kagome layer and the dimerized pillar
Co(2) site between the layers are of nonmagnetic origin, suggesting that the
nearly quasi-2D metallic conductivity is dominated by nonmagnetic Co(1) and
Co(2) sites. Consequently, unique magneto-transport phenomena observed in
SrCo6O11 are demonstrated owing to the interaction between the conduction
electrons at the Co(1) and Co(2) sites and the local moments at Co(3) sites.
