Susceptibility and dilution effects of the kagomé bilayer geometrically frustrated network: A Ga NMR study ofSrCr9pGa12−9p

Abstract

The archetype of geometrically frustrated compounds SrCr 9 p Ga 12Ϫ9 p O 19 is a kagomé bilayer of Heisenberg Cr 3ϩ ions (Sϭ3/2) with antiferromagnetic interactions. We present an extensive gallium nuclear magnetic resonance ͑NMR͒ study over a broad Cr-concentration range (0.72рpр0.95). This allows us to probe locally the susceptibility of the kagomé bilayer and separate the intrinsic properties due to geometric frustration from those related to site dilution. Compared to the partial study on one sample, pϭ0.90, presented in Phys. Rev. Lett. 85, 3496 ͑2000͒, we perform here a refined study of the evolution of all the magnetic properties with dilution, with a great emphasis on the lowest diluted pϭ0.95 sample synthesized for this study. Our major findings are the following ͑1͒ The intrinsic kagomé bilayer susceptibility reaches a maximum at a temperature of Ϸ40Ϫ50 K, which we show here to be robust up to a dilution as high as Ϸ20%; this maximum is the signature of the development of short-range antiferromagnetic correlations in the kagomé bilayer. ͑2͒ At low T, a highly dynamical state induces a strong wipeout of the NMR intensity, regardless of dilution. ͑3͒ The low-T upturn of the macroscopic susceptibility is associated with paramagnetic defects, which stem from the dilution of the kagomé bilayer. The low-T analysis of the pϭ0.95 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. ͑4͒ The spin-glass-like freezing observed at T g Ϸ2 -4 K is not driven by the dilutioninduced defects.

Effect of Chemical Disorder on Geometrically Frustrated Kagome Lattice

“…The disorder in this compound stems from the dilution of the Cr sublattice by nonmagnetic Ga 3+ ions. The magnetic susceptibility measurements (macroscopic, χ macro , in contrast to local measurements by NMR technique [10]) suggest the occurrence of spin glass state with spin freezing temperature T g ≈ 2-5 K [11][12][13][14][15]. All other experimental data confirms existence of a spin-liquid-like ground state [12,16,17].…”

“…exchange interactions other than nearest-neighbor interaction, magnetic anisotropy of Cr 3+ ions and dipolar anisotropy), usually suppressing spin liquid state, are very small compared to the Cr-Cr interactions in this kagome lattice. The results obtained in [10] show that NMR is a very powerful technique allowing to measure directly intrinsic frustration effects through determination of χ frustr . It was shown that a maximum in χ frustr occurs in SrGa 12−x Cr x O 19 at temperatures of about 0.1θ where θ is the Curie-Weiss temperature determined using χ macro at high-temperature region.…”

“…This Curie law contribution to the χ macro is an effect of the magnetic defects (χ def ) arising due to the dilution of the kagome lattice. Finally, according to [10]:…”

A comparison of the local magnetic susceptibility in rare earth pyrochlores

“…1), which is not observed in the Gd compound. This discrepancy could arise either because of a muon induced effect or possibly a small impurity phase detected by bulk susceptibility, but which contributes nothing to local probe measurements [21]. Alternatively, the contact hyperfine and/or dipolar field could be temperature dependent.…”

NMR and µSR in Highly Frustrated Magnets

“…Since 71 Ga(4f) nuclei are strongly coupled only to chromium ions within the kagomé bilayer one can single out just the intrinsic susceptiblity of the latter, whereas the macroscopic susceptibility detects also the contribution from defects. [6] …”