This research project is funded by the United States Israel Binational Science Foundation. The project name is Degenerate Magnets: A Study by Local Probes, and it duration is 1998-2001. The principal investigators are: A. Keren (Technion), Y. J. Uemura and G. M. Luke (Columbia Univ.).<\/p>\n\n\n\n
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We report magnetization, electron-spin resonance (ESR), and muon-spin relaxation (\u03bcSR) measurements on single crystals of the S = 1\/2 (Cu+2<\/sup>) kagome compound Cu(1,3-benzendicarboxylate). The \u03bcSR is carried to temperatures as low as 45 mK. The spin-Hamiltonian parameters are determined from the analysis of the magnetization and ESR data.We find that this compound has anisotropic ferromagnetic interactions.Nevertheless, no spin freezing is observed even at temperatures two orders of magnitude lower than the coupling constants. In light of this finding, the relation between persistent spin dynamics and spin liquids on kagome lattices is reexamined.<\/p>\n\n\n\n\n
We present transverse field muon spin rotation\/relaxation measurements on single crystals of the spin-1\/2 kagome antiferromagnet Herbertsmithite. We find that the spins are more easily polarized when the field is perpendicular to the kagome plane. We demonstrate that the difference in magnetization between the different directions cannot be accounted for by Dzyaloshinskii-Moriya-type interactions alone and that anisotropic axial interaction is present.<\/p>\n\n\n\n\n
We investigated the nature of the spin-glass like phase transition in the geometrically frustrated pyrochlore lattices Y2<\/sub>Mo2<\/sub>O7<\/sub> using the local probes nuclear and muon magnetic resonances, and the field-dependent long-range probes x-ray and neutron scatterings. The long-range probes indicated that Y2<\/sub>Mo2<\/sub>O7<\/sub> does not undergo any global symmetry changes, even in a field of 6 T. In contrast, the local signal indicates a lattice distortion close to the critical temperature. The nuclei show at least two inequivalent Y sites, and the muons show sublinear line broadening as a function of moment size, over a wide temperature range. The conclusion from all the measurements is that even in high field, the distortion of Y2<\/sub>Mo2<\/sub>O7<\/sub> takes place within the unit cell while its global cubic symmetry is preserved. Moreover, the muon result clearly indicates the presence of magnetoelastic coupling.<\/p>\n\n\n\n\n
We present magnetization measurements on oriented powder of ZnCu3<\/sub>(OH)6<\/sub>Cl2<\/sub> along and perpendicular to the orienting field. We find a dramatic difference in the magnetization between the two directions. It is biggest at low measurement fields H<\/em> or high temperatures. We show that the difference at high temperatures must emerge from the Ising-type exchange anisotropy. This allows us to explain muon spin rotation data at T<\/em> → 0 in terms of an exotic ferromagnetic ground state.<\/p>\n\n\n\n\n
Using muon-spin resonance, we examine the organometallic hybrid compound Cu(1,3- benzenedicarboxylate) [Cu(1,3-bdc)], which has structurally perfect spin- 1 2 copper kagome planes separated by pure organic linkers. This compound has antiferromagnetic interactions with Curie-Weiss temperature of -33 K. We found slowing down of spin fluctuations starting at T=1.8 K and that the state at T->0 is quasistatic with no long-range order and extremely slow spin fluctuations at a rate of 3.6 ms-1. This indicates that Cu(1,3-bdc) behaves as expected from a kagome magnet and could serve as a model kagome compound.<\/p>\n\n\n\n
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We investigated the possibility of temperature-dependent lattice distortions in the pyrochlore compound Tb2<\/sub>Ti2<\/sub>O7<\/sub> by measuring the internal magnetic field distribution, using muon spin resonance, and comparing it to the susceptibility. The measurements are done at temperatures as low as 70 mK and external fields up to 6 kG. We find that the evolution of the width of the field distribution can be explained by spin susceptibility only, thus ruling out a temperaturedependent hyperfine coupling. We conclude that lattice deformations are absent in Tb2<\/sub>Ti2<\/sub>O7<\/sub>.<\/p>\n\n
We investigated the nature of the freezing in the geometrically frustrated Heisenberg spin glass Y2<\/sub>Mo2<\/sub>O7<\/sub> by measuring the temperature dependence of the static internal magnetic field distribution above the spin-glass temperature, Tg, using the muon spin relaxation technique. The evolution of the field distribution cannot be explained by changes in the spin susceptibility alone and suggests a lattice deformation. This possibility is addressed by numerical simulations of the Heisenberg Hamiltonian with magnetoelastic coupling at T >0.<\/p>\n\n
Investigations of the spin dynamics of the geometrically frustrated pyrochlore (Tbp<\/sub> Y1- p<\/sub>)2<\/sub>Ti2<\/sub>O7<\/sub> , using muon spin relaxation and neutron spin echo,as a function of magnetic coverage p ,have been carried out.Our major finding is that paramagnetic fluctuations prevail as T ->0 for all values of p, and that they are sensitive to dilution, indicating a cooperative spin motion. However, the percolation threshold pc<\/sub> is not a critical point for the fluctuations.We also find that the low temperature spectral density has a 1\/f behavior, and that dilution slows down the spin fluctuations.<\/p>\n<\/div>\n\n
We have investigated the 89<\/sup>Y NMR spectrum and spin lattice relaxation, T1<\/sub>, in the magnetically frustrated pyrochlore Y2<\/sub>Mo2<\/sub>O7<\/sub>. We find that upon cooling the spectrum shifts, and broadens asymmetrically. A detailed examination of the low T spectrum reveals that it is constructed from multiple peaks, each shifted by a different amount. We argue that this spectrum is due to discrete lattice distortions, and speculate that these distortions relieve the frustration and reduce the system’s energy.<\/p>\n<\/div>\n\n
We investigate the spin dynamics of SrCr9p<\/sub>Ga12-3p<\/sub>O19<\/sub> for p<\/i> below and above the percolation threshold pc<\/sub><\/i> using muon spin relaxation. Our major findings are: (I) At T->0<\/i> the relaxation rate is T<\/i> independent and propotional to p3<\/sup><\/i>, (II) the slowing down of spin fluctuation is activated with an energy U<\/i> which is also a linear function of p3<\/sup><\/i> and limp->0<\/sub>U=8<\/i> K; this energy scale could stem only from a single ion anisotropy, and (III) the p<\/i> ependence of the dynamical properties is identical below and above pc<\/sub><\/i>, indicating that they are controlled by local excitation.<\/p>\n<\/div>\n\n
We report the NMR site assignment of 69,71<\/sup> Ga in the kagome lattice compound SrCr9-x<\/sub>Ga3+x<\/sub>O19<\/sub>, and discuss the couplings of the different Ga nuclei with Cr moments. We find that Ga(4e) is dominantly coupled to the triangular plane, and Ga(4f) is coupled to both the kagome and triangular planes. The low temperature behavior of the NMR signal in SrCr8<\/sub>Ga4<\/sub>O19<\/sub> suggests that slowing down of some spin degrees of freedom occurs first on the kagome plane.<\/p>\n<\/div>\n\n
We have examined the magnetic properties of the frustrated kagome lattice spin systems KFe3<\/sub>(OH)6<\/sub> (SO4<\/sub>)2<\/sub>(Fe-jarosite) and KCr3<\/sub>(OH)6<\/sub>(SO4<\/sub>)2 <\/sub>(Cr-jarosite) using the\u00a0 mSR technique. In Fe-jarosite, a clear muon precession signal in zero field was found below 55K, corresponding to the long-range order of Fe moments.\u00a0 In Cr-jarosite spin fluctuations persist without clear signature of spin freezing even at T = 25 mK, far below Tg<\/sub> ~ 2K where the history dependence in the magnetic susceptibility appears.<\/p>\n<\/div>\n