Nuclear Magnetic Resonance in High Neel Temperature Garnets

Abstract

V51 and Fe57 NMR of Y3−2xCa2xFe5−xVxO12 garnets for x between 0.2 and 1.5 have been studied at 4.2°K by pulsed NMR method. This garnet system is known to retain a high Neel temperature for the increase of x right up to x = 1.5. The center of gravity of the V51 NMR line increases from 9.5 MHz to 15.6 MHz with increasing x from 0.2 to 1.5. The hyperfine field of V ion nuclei between 8.5 and 14 kOe is what one would expect in its order of magnitude for the supertransferred hyperfine fields at the diamagnetic V5+ ion nuclei on tetrahedral sites in iron garnets. The Fe57 NMR spectra of octahedral Fe3+ consist of several peaks with nearly equal spacing. Frequencies of these peaks are approximately given by νn = νc + nνT, with n = 0, 1, 2, 3, ⋯·, where νc ≃ 75.7 MHz and νT ≃ 2.2 MHz. Relative intensities of these peaks are calculated by a model in which Fe and V are distributed at random on the tetrahedral sites. Similar calculation is made for V51 NMR intensity distribution. From the comparison between the calculated and the observed NMR spectra, it is suggested that there exists a certain degree of short range ordering in the distribution of V ions on the tetrahedral sites. The NMR frequency of about 64.5 MHz for the tetrahedral site Fe57 is almost constant up to x = 1.5, which is in contrast with the NMR frequency of 62.5 MHz for the tetrahedral Fe57 of Y3−xCaxFe5−xSixO12 with x = 1.5. If we assume that the decrease of core polarization hyperfine fields for the both garnets are roughly the same, this result leads to a conclusion that the supertransferred hyperfine field at tetrahedral Fe3+ in Ca3Fe3.5V1.5O12 is larger then that in Y1.5Ca1.5Fe3.5Si1.5O12.