NUCLEAR-SPIN-LATTICE RELAXATION IN THE ONE-DIMENSIONAL FERRIMAGNET MN(HFAC)2NITIPR

Mn(hfac)2(NITiPr), where hfac is hexafluoroacetylacetonate and NITiPr is 2-isopropyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazoline-1-oxyl 3-oxide, is a one-dimensional ferrimagnet that orders ferromagnetically at ca. 8 K. NMR experiments performed on polycrystalline powders and single crystals in the paramagnetic phase show that the spin-lattice relaxation rates of both H-1 and F-19 in the range 8-90 MHz follow a nu-1/2 dependence down to about 20 MHz, while below the frequency dependence is reversed. The frequency dependence above 20 MHz is typical of a one-dimensional magnetic material, confirming that ferrimagnets are similar to ferromagnets and antiferromagnets under this respect. The breakdown of nu-1/2 dependence observed in the low-frequency range is related to a cutoff mechanism of spin-diffusive behavior, due to intrachain dipolar interactions. This result sets an upper limit to the interchain exchange coupling J' less-than-or-equal-to 6 x 10(-4) cm-1, which confirms the dipolar model for magnetic ordering previously suggested.