Dilution in volborthiteS= 1/2 frustrated magnet: a  SR and NMR study

Abstract

Volborthite, Cu 3 V 2 O 7 (OH) 2 •2H 2 O, is a natural frustrated antiferromagnet (θ CW 130 K) with S = 1/2 spins (Cu 2+) sitting at the vertices of a Kagomélike lattice built on isosceles triangles. We report on the static (SQUID, 51 V NMR) magnetic properties of the pure and 5% Zn/Cu substituted compounds and on an extensive µSR study of the dilution effect (up to 15% Zn substitutions) on the spin dynamics. Although volborthite shares most of the unusual features already exhibited in Kagomé bilayer compounds, namely a dynamical state as T → 0 and a low temperature maximum in the local susceptibility, we found some surprising specificities. The T → 0 dynamical state is less robust against dilution and the low temperature local susceptibility studied by means of NMR also strongly depends on dilution. Such a sensitivity to dilution questions the role of the asymmetry of the exchange constants.

Ground State of the Kagomé-LikeS=1/2Antiferromagnet VolborthiteCu3V2O7<

“…No spurious phase was detected in X-ray measurements. SQUID and NMR measurements above 2 K and µSR experiments on these samples [12] were similar to the above described published data.…”

“…They thus probe symmetrically the magnetism of six Cu 2+ ions belonging to the same hexagon through an hyperfine coupling, estimated to be A ≃ 7.7 kOe from susceptibility versus NMR shift measurements (see Ref. [12]) above 90 K in agreement with the value estimated in Ref. [6].…”

“…In marked contrast with SCGO, T g for Volborthite is strongly affected by dilution. This reminds us of the drastic reduction upon dilution of the dynamical plateau value seen in µSR experiments [12] and suggests that the bilayer topology likely better accommodates defects than the single layer one in volborthite. Nonetheless, in both systems, the random dilution of the magnetic network reduces T g and hence simple magnetic dilution as a source of disorder cannot explain, alone, the spin-glass like transition.…”

Low temperature magnetization of theS=12kagome antiferromagnetZnCu3Bert1, Nakamae2, Ladieu3 et al. 2007Phys. Rev. B13281360View full textAdd to dashboardCite

“…This non trivial extended response of the system around a spin vacancy constitutes the second magnetic defect. In the closely related copper based anisotropic kagome structure of Volborthite 22 , the controlled magnetic dilution by Zn/Cu substitution indeed yields a Curie-like tail that scales with the Zn content 23 . We first consider the intermediate temperature range 1.5 K -10 K. As shown in the inset of Fig.1, 1/χ ≃ 1/χ d does not extrapolate to 0 when T → 0 as would be expected for free spins following a Curie law.…”

Magnetization “Steps” on a Kagome Lattice in Volborthite

“…3,4) Volborthite Cu 3 V 2 O 7 (OH) 2 •2H 2 O is a two-dimensional antiferromagnet comprising Cu 2+ ions arranged in a kagome-like lattice 18) and has been presumed to form a spin liquid on the kagome lattice without LRO down to 50 mK. [19][20][21][22] However, the recent study conducted by Bert et al revealed glass-like freezing at 1.2 K as well as a mixture of different spin configurations, 23) suggesting that the ground state of volborthite is also far from the true spin liquid state. The polycrystalline samples used in the previous study were prepared by precipitation and typically contained 1% of almost noninteracting spins in the kagome plane.…”