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The time differential perturbed angular correlation (TDPAC) technique was used to study the temperature dependence of electric field gradient (EFG) in LaCoO 3 perovskite using 111 In → 111 Cd and 181 Hf → 181 Ta nuclear probes. The radioactive parent nuclei 111 In and 181 Hf were introduced into the oxide lattice through a chemical process during sample preparation and were found to occupy only the Co sites in LaCoO 3 . The PAC measurements with 111 Cd and 181 Ta probes were made in the temperature range of 4.2-1146 K and 4.2-1004 K, respectively. No long-range magnetic order was observed up to 4.2 K. The EFGs at 111 Cd and 181 Ta show very similar temperature dependences. They increase slowly between 4.2 and about 77 K and then decrease almost linearly with increasing temperature until about 500-600 K, where a broad peak-like structure is observed, followed by linear decrease at still higher temperatures. These discontinuities at about 77 K and 500-600 K have been interpreted as thermally activated spin state transitions from the low-spin (t 6 2g e 0 g ) ground state configuration to the intermediate-spin (t 5 2g e 1 g ) state and from the intermediate-spin to the high-spin (t 4 2g e 2 g ) state of the Co 3+ ion, confirming previous observation in other recent studies. An indication of a Jahn-Teller distortion, which stabilizes the intermediate-spin state with orbital ordering, is also pointed out.

The local magnetic interaction at the transition metal sites in RMO 3 (R = La, Nd; M = Cr, Fe) compounds has been investigated by perturbed angular correlation (PAC) technique using 181 Hf→ 181 Ta probe nuclei. The present measurements cover a temperature range from 10 K to 1000 K. Above the respective Néel temperature, each compound shows a unique quadrupolar frequency that decreases linearly with temperature. These interactions were assigned to the radioactive probe substituting Cr or Fe sites. Below T N , a combined electric plus magnetic hyperfine interaction was observed. The magnetic interaction revealed that the super transferred hyperfine fields on 181 Ta at the Cr sites in (La,Nd)CrO 3 extrapolated to 0 K, are much smaller than the corresponding values at Fe sites in (La,Nd)FeO 3 . This difference was attributed to different distribution of d electrons in Cr 3+ (3d 3 ) and Fe 3+ (3d 5 ) ions in each compound. As the fields for Nd compounds are smaller than those for La compounds, the role of rare-earth ions in the magnetism of these oxides is also discussed.