SPIN-STATE AND HYPERFINE INTERACTION IN LACOO3 - NMR AND MAGNETIC-SUSCEPTIBILITY STUDIES

Co-59 and La-139 NMR measurements have been performed to study the mic roscopic magnetic properties closely related with the spin state of a trivalent cobalt ion in LaCoO3. The temperature-independent Co-59 and La-139 Knight shifts below similar to 30 K clearly revealed the presen ce of a nonmagnetic ground state at low temperatures. We calculated th e temperature dependence of the magnetic susceptibility chi and the Co -59 Knight shift K-59 in a single ion model with the low-spin (1)A(1) ground state above which the high-spin T-5(2) State is located. Then, both the trigonal crystal-field and spin-orbit interactions are taken into account. From this analysis, we infer the presence of an antiferr omagnetic exchange interaction between the high-spin states. The chara cteristic temperature dependence of the Co-59 hyperfine coupling const ant is also discussed.