D. Bono

Ground State of the Kagome-Like S = 1/2 Antiferromagnet Volborthite Cu~3V~2O~7(OH)~2 · 2H~2O

The volborthite compound is one of the very few realizations of S=1/2 quantum spins on a highly frustrated kagomé-like lattice. Low-T SQUID measurements reveal a broad magnetic transition below 2 K which is further confirmed by a peak in the 51V nuclear spin relaxation rate (1/T1) at 1.4 K +/- 0.2 K. Through 51V NMR, the ground state (GS) appears to be a mixture of different spin configurations, among which 20% corresponds to a well defined short-range order, possibly of the sqrt(3) x sqrt(3) type. While the freezing involves all the Cu2+ spins, only 40% of the copper moment is actually frozen which suggests that quantum fluctuations strongly renormalize the GS.

Cascade of Bulk Magnetic Phase Transitions in NaxCoO2 as Studied by Muon Spin Rotation

Using muon spin rotation, well-defined bulk ~ 100% magnetic phases in NaxCoO2 are revealed. A novel magnetic phase is detected for x = 0.85 with the highest transition temperature ever observed for x>= 0.75. This stresses the diversity of x>=0.75 magnetic phases and the link between magnetic and structural degrees of freedom. For the charge-ordered x = 0.50 compound, a cascade of transitions is observed below 85 K. From a detailed analysis of our data, we conclude that the ordered moment varies continuously with temperature and suggest that the two secondary transitions at 48 K and 29 K correspond to a moderate reorientation of antiferromagnetically coupled moments.

Intrinsic susceptibility and bond defects in the novel two dimensional frustrated antiferromagnet Ba2Sn2ZnCr7pGa10-7pO22.

We present microscopic and macroscopic magnetic properties of the highly frustrated antiferromagnet Ba(2)Sn(2)ZnCr(7p)Ga(10-7p)O22, respectively, probed with NMR and SQUID experiments. The T variation of the intrinsic susceptibility of the Cr3+ frustrated Kagomé bilayer, chi(Kag), displays a maximum around 45 K. The dilution of the magnetic lattice has been studied in detail for 0.29</=p</=0.97. Novel dilution independent defects, likely related with magnetic bond disorder, are evidenced and discussed. We compare our results to SrCr(9p)Ga(12-9p)O19. Both bond defects and spin vacancies do not affect the average susceptibility of the Kagomé bilayers.

Dilution in volborthite S = 1/2 frustrated magnet: a μSR and NMR study

Volborthite, Cu3V2O7(OH)22H2O, is a natural frustrated antiferromagnet ( K) with S = 1/2 spins (Cu2+) sitting at the vertices of a Kagome-like lattice built on isosceles triangles. We report on the static (SQUID, 51V 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 Kagome bilayer compounds, namely a dynamical state as and a low temperature maximum in the local susceptibility, we found some surprising specificities. The 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.

Correlations, spin dynamics, defects: the highly frustrated kagomé bilayer

The compounds SrCr9pGa12−9pO19 and Ba2Sn2ZnGa10−7pCr7pO22 are two highly frustrated magnets possessing a quasi-two-dimensional kagome bilayer of spin-3/2 chromium ions with antiferromagnetic interactions. Their magnetic susceptibility was measured by local nuclear magnetic resonance and nonlocal (SQUID) techniques, and their low-temperature spin dynamics by muon spin resonance. Consistent with the theoretical picture drawn for geometrically frustrated systems, the kagome bilayer is shown here to exhibit: (i) short range spin-spin correlations down to a temperature much lower than the Curie–Weiss temperature, no conventional long-range transition occurring; (ii) a Curie contribution to the susceptibility from paramagnetic defects generated by spin vacancies; (iii) low-temperature spin fluctuations, at least down to 30 mK, which are a trademark of a dynamical ground state. These properties point to a spin-liquid ground state, possibly built on resonating valence bonds with unconfined spinons as the magnetic...

Oxygen doped S = 1/2 Cu delafossites: a muon spin rotation/relaxation study

We present a μSR study of oxygen-doped delafossites RCuO 2+δ (δ = 0.5, 0.66; R 3+ = La 3+ , Y 3+ ) which present triangular based lattices of Cu 2+ , S = 1/2 spins. A slowing down of the spin dynamics without onset of a magnetic static freezing down to 1.6 K is found for YCuO 2.5 , which is believed to be an example of a Δ-chain. In contrast, YCuO 2.66 and LaCuO 2.66 clearly order, probably into an antiferromagnetic state, which was rather unexpected.

Low-temperature relaxation in kagome bilayer antiferromagnets

The pyrochlore slab (kagome bilayer) compounds SrCr9x Ga12−9x O19 (SCGO; x < 1) and Ba2Sn2ZnCr7x Ga10−7x O22 (BSZCGO; x < 1), are frustrated systems with quite similar magnetic properties of the spin S = 3/ 2C r 3+ ions. Neutron scattering studies have shown that the two compounds have a completely dynamic magnetic response in a broad temperature range. In both systems the development of short-ranged dynamic correlations leads to a low-T state that can be understood as local clusters with antiferromagnetic character. At liquid He temperatures a partial freezing of the magnetic fluctuations is observed as an increase of the elastic resolved response. A large majority of the magnetic moments remain fluctuating and one also observes a low-energy (long-time) relaxation in the vicinity of the macroscopic freezing. Time and temperature dependence of this relaxation appear system dependent without critical behaviour, and we conclude that the freezing is a consequence of the establishment of a coherent quantum state. (Some figures in this article are in colour only in the electronic version)