The archetype of geometrically frustrated compounds SrCr9pGa12-9pO19 is a kagomé bilayer of Heisenberg Cr3+ ions (S = 3/2) with antiferromagnetic interactions. We present an extensive gallium nuclear magneticresonance (NMR) study over a broad Cr-concentration range (0.72<=p<=0.95). This allows us to probe locally the susceptibility of the kagome bilayer and separate the intrinsic properties due to geometric frustration from those related to site dilution. Compared to the partial study on one sample, p = 0.90, presented in Phys. Rev. Lett. 85, 3496 (2000), we perform here a refined study of the evolution of all the magnetic properties with dilution, with a great emphasis on the lowest diluted p = 0.95 sample synthesized for this study. Our major findings are the following (1) The intrinsic kagomé bilayer susceptibility reaches a maximum at a temperature of 40-50 K. which we show here to be robust up to a dilution as high as 20%; this maximum is the signature of the development of short-range antiferromagnetic correlations in the kagomé bilayer. (2) At low T, a highly dynamical state induces a strong wipeout of the NMR intensity, regardless of dilution, (3) The low-T upturn of the macroscopic susceptibility is associated with paramagnetic defects, which stem from the dilution of the kagome bilayer. The low-T analysis of the p = 0.95 NMR line shape, coupled with a more accurate determination of the nuclear Hamiltonian at high T, allows us to discuss in detail the nature of the defect, Our analysis suggests that the defect can be associated with a staggered spin response to the vacancies of the kagomé bilayer. This, altogether with the maximum in the kagomé bilayer susceptibility, is very similar to what is observed in most low-dimensional antiferromagnetic correlated systems, even those with a short spin-spin correlation length. (4) The spin-glass-like freezing observed at Tg 2-4 K is not driven by the dilution-induced defects.
Susceptibility and dilution effects of the kagome bi-layer geometrically frustrated network. A Ga-NMR study of SrCr9pGa12-9pO19. A Ga-NMR study.
C Mondelli;
2002
Abstract
The archetype of geometrically frustrated compounds SrCr9pGa12-9pO19 is a kagomé bilayer of Heisenberg Cr3+ ions (S = 3/2) with antiferromagnetic interactions. We present an extensive gallium nuclear magneticresonance (NMR) study over a broad Cr-concentration range (0.72<=p<=0.95). This allows us to probe locally the susceptibility of the kagome bilayer and separate the intrinsic properties due to geometric frustration from those related to site dilution. Compared to the partial study on one sample, p = 0.90, presented in Phys. Rev. Lett. 85, 3496 (2000), we perform here a refined study of the evolution of all the magnetic properties with dilution, with a great emphasis on the lowest diluted p = 0.95 sample synthesized for this study. Our major findings are the following (1) The intrinsic kagomé bilayer susceptibility reaches a maximum at a temperature of 40-50 K. which we show here to be robust up to a dilution as high as 20%; this maximum is the signature of the development of short-range antiferromagnetic correlations in the kagomé bilayer. (2) At low T, a highly dynamical state induces a strong wipeout of the NMR intensity, regardless of dilution, (3) The low-T upturn of the macroscopic susceptibility is associated with paramagnetic defects, which stem from the dilution of the kagome bilayer. The low-T analysis of the p = 0.95 NMR line shape, coupled with a more accurate determination of the nuclear Hamiltonian at high T, allows us to discuss in detail the nature of the defect, Our analysis suggests that the defect can be associated with a staggered spin response to the vacancies of the kagomé bilayer. This, altogether with the maximum in the kagomé bilayer susceptibility, is very similar to what is observed in most low-dimensional antiferromagnetic correlated systems, even those with a short spin-spin correlation length. (4) The spin-glass-like freezing observed at Tg 2-4 K is not driven by the dilution-induced defects.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
