  Paramagnetic (PM) properties of columnar cobaltosic oxide (Co3O4)
nanoparticles, about 75 nm in diameter, have been investigated by magnetization
measurements at T > TN = 39 K. In zero or low applied field, the effective PM
moment per formula unit (FU), Mu_eff, enhances significantly from the bulk
value of 4.14 Mu_B/FU. It decreases asymptotically from 5.96 Mu_B/FU at Happ =
50 Oe down to 4.21 Mu_B/FU as the applied field increases to Happ = 10 kOe. The
field dependent PM properties are explained by a structural inversion, from the
normal spinel (spin-only moment ~ 3.9 Mu_B/FU) to the inverse spinel structure
(spin-only moment ~ 8.8 Mu_B/FU). The structural inversion is reversible with
the variation of the applied field. The lattice structure becomes almost 100%
normal spinel in the high field exceeding 10 kOe, as revealed by the magnitude
of the effective PM moment. The reversible, field dependent structure inversion
is an important property with promising application potential. It is
interesting for the future investigations.
