59Co and 139La NMR studies of the spin state in LaCoO3

Evolution of the ferromagnetic and nonferromagnetic phases with temperature in phase-separated La1−xSrxCoO3 by high-field La139 NMR

“…As T is raised, Co (III) ions in the LS paramagnetic PM phase undergo a spinstate transition to an IS state, between 30 and 100 K, corresponding to the energy gap of 180 K between t 2g 6 e g 0 (LS) and t 2g 5 e g 1 (IS) states. 59 Co NMR experiments on undoped LaCoO 3 have shown a temperature-dependent resonance shift with a low-temperature (LS) value of 2.1%, attributed to a Van Vleck orbital contribution, rising to over 5% at the maximum around 100 K. 17 The much smaller shifts for 139 La, found in the present experiments, are consistent with the small shifts due to transferred hyperfine coupling found in the undoped material at temperatures above 40 K. 19 As mentioned in Sec. III A above, the magnetic susceptibility shows a peak at 100 K, reflecting the LS to IS transition, with Curie-Weiss behavior at higher temperatures.…”

“…The quadrupolar coupling frequency, obtained by fitting the I =7/2 spec-trum, using standard second-order expressions and assuming axial symmetry, 18 is v Q = 1.7 MHz, in agreement with the value obtained by Itoh and Natori. 19 A spectrum obtained at 120 K showed the same structure as the 20 K spectrum with a small Knight shift. The 139 La Knight shift as a function of temperature has a form similar to that of the magnetic susceptibility, rising from zero for T below 20 K to a maximum of 0.56% at 115 K. 19 The form of the susceptibility curve as a function of temperature has been discussed by Yamaguchi et al 13 using a mean-field approach.…”

“…19 A spectrum obtained at 120 K showed the same structure as the 20 K spectrum with a small Knight shift. The 139 La Knight shift as a function of temperature has a form similar to that of the magnetic susceptibility, rising from zero for T below 20 K to a maximum of 0.56% at 115 K. 19 The form of the susceptibility curve as a function of temperature has been discussed by Yamaguchi et al 13 using a mean-field approach. Recent electronic structure calculations 20 have provided a description of the temperature induced LS-IS transition and the associated changes in the magnetic properties revealed by experiment.…”

“…19 The hyperfine frequency contribution per Co neighbor, B / ប, may be estimated using the static hyperfine coupling in the ferromagnetic phase in Sr-doped material. (This implicitly assumes that Co 3+ and Co 4+ ions involved in double exchange have similar transferred hyperfine couplings at the La sites.)…”

59CoNMR studies of metallicNaCo2Ray1, Ghoshray2, Ghoshray3 et al. 1999Phys. Rev. B

“…It is worthwhile to mention that 59 Co NMR in perovskites such as LaCoO 3 , PrCoO 3 , etc., exhibits similar large temperatureindependent shifts which accounted for Van Vleck temperature-independent susceptibility. 16,25 In the present case, the temperature-independent shifts in principle are the sum of K vv and K s as described in Eq. ͑4.4͒.…”