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.).
We have published the following papers on the subject:
- O.Ofer, L.Marcipar, V.R.Chandra, S.Gazit, D.Podolsky, D.P.Arovas, and A.Keren, Dynamic spin fluctuations at T → 0 in a spin-1/2 ferromagnetic kagome lattice, Phys. Rev B, 89, (2014) 205116.
We report magnetization, electron-spin resonance (ESR), and muon-spin relaxation (μSR) measurements on single crystals of the S = 1/2 (Cu+2) kagome compound Cu(1,3-benzendicarboxylate). The μSR 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.
- O. Ofer, A.Keren, J.H.Brewer, T.H.Han and Y.S.Lee, The_Herbertsmithite_Hamiltonian_μSR_measurements_on_single_crystals, J. Phys.: Condens. Matter, 23, (2011) 164207.
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.
- O. Ofer, A. Keren, J.S. Gardner, Y. Ren, and W.A. MacFarlane, Origin of magnetic freezing in pyrochlore Y2Mo2O7, Phys. Rev B, 82, (2010) 092403.
We investigated the nature of the spin-glass like phase transition in the geometrically frustrated pyrochlore lattices Y2Mo2O7 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 Y2Mo2O7 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 Y2Mo2O7 takes place within the unit cell while its global cubic symmetry is preserved. Moreover, the muon result clearly indicates the presence of magnetoelastic coupling.
- O. Ofer and A. Keren, Symmetry of the spin Hamiltonian for herbertsmithite: A spin -1/2 kagome lattice, Phys. Rev B, 79, (2009) 134424.
We present magnetization measurements on oriented powder of ZnCu3(OH)6Cl2 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 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 → 0 in terms of an exotic ferromagnetic ground state.
- L.Marcipar, O.Ofer, A.Keren and Ch.Baines, Muon-spin spectroscopy of the organometallic spinspin-1/2 kagome-lattice compound Cu(1,3-benzenedicarboxylate),Phys. Rev B, 80, (2009) 132402.
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.
- O.Ofer, A.Keren and Ch.Baines, Lack of static lattice distortion in Tb2Ti2O7, J. Phys.: Condens. Matter, 19, (2007) 145270.
We investigated the possibility of temperature-dependent lattice distortions in the pyrochlore compound Tb2Ti2O7 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 Tb2Ti2O7.
- E.Sagi, O.Ofer, A.Keren, J. S. Gardner, Quest for Frustration Driven Distortion in Y2Mo2O7, Phys. Rev. Lett., 94, 237202 (2005).
We investigated the nature of the freezing in the geometrically frustrated Heisenberg spin glass Y2Mo2O7 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.
- A.Keren, J.S.Gardner, G.Ehlers, A.Fukaya, E.Segal and Y.J.Uemura, Dynamic Properties of a Diluted Pyrochlore Cooperative Paramagnet ( TbpY(1-p))2 Ti2O7, Phys. Rev. Lett. 92, (2004) 107204.
Investigations of the spin dynamics of the geometrically frustrated pyrochlore (Tbp Y1- p)2Ti2O7 , 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 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.
- A. Keren and J. S. Gardner, Frustration driven lattice distortion: an NMR investigation of Y2Mo2O7, Phys. Rev. Lett. 87, (2001) 117201.
We have investigated the 89Y NMR spectrum and spin lattice relaxation, T1, in the magnetically frustrated pyrochlore Y2Mo2O7. 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.
- A. Keren, Y. J. Uemura, G. Luke, P. Mendels, M. Mekata and T. Asano, Magnetic dilution in the geometrically frustrated SrCr9pGa12-3pO19, and the role of local dynamics: a mSR study, Phys. Rev. Lett. 84, (2000) 3450-3453.
We investigate the spin dynamics of SrCr9pGa12-3pO19 for p below and above the percolation threshold pc using muon spin relaxation. Our major findings are: (I) At T->0 the relaxation rate is T independent and propotional to p3, (II) the slowing down of spin fluctuation is activated with an energy U which is also a linear function of p3 and limp->0U=8 K; this energy scale could stem only from a single ion anisotropy, and (III) the p ependence of the dynamical properties is identical below and above pc, indicating that they are controlled by local excitation.
- A. Keren, P. Mendels, M. Horvatic, F. Ferrer, Y. J. Uemura, M. Mekata and T. Asano, 69,71Ga NMR in the kagome lattice compound SrCr9-xGa3+xO19, Phys. Rev. B 57, (1997).
We report the NMR site assignment of 69,71 Ga in the kagome lattice compound SrCr9-xGa3+xO19, 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 SrCr8Ga4O19 suggests that slowing down of some spin degrees of freedom occurs first on the kagome plane.
- A. Keren, K. Kojima, L. P. Le, G. L. Luke, W. D. Wu, Y. J. Uemura, M. Takano, H. Dabkowska, and M. J. P. Gingras, mSR Measurements in the Kagome Lattice Systems: Cr-Jarosite and Fe-Jarosite, Phys. Rev. B 53, (1996) 6451-6454.
We have examined the magnetic properties of the frustrated kagome lattice spin systems KFe3(OH)6 (SO4)2(Fe-jarosite) and KCr3(OH)6(SO4)2 (Cr-jarosite) using the 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. In Cr-jarosite spin fluctuations persist without clear signature of spin freezing even at T = 25 mK, far below Tg ~ 2K where the history dependence in the magnetic susceptibility appears.
- A. Keren, Dynamical Simulation of Spins on Kagome and Square Lattices, Phys. Rev. Lett. 72, (1994) 3254-3257.
The time evolution of classical Heisenberg spins on kagome and square lattices is numerically evaluated. A clear difference is found between the two cases in the time dependence of an individual spin S(t). The stability of possible ground states is discussed in light of the sample-averaged correlation function [S(t)S(0)]. The spectral density j(w) as a function of temperature is also computed. In the square lattice two phases are observed, whereas in the kagome lattice only motional narrowing is seen. The application of these simulations to in SrCr8 Ga4O19 is discussed.
- A. Keren, Simulation of spin dynamics on kagome and square lattices, JMMM 140-144 (1995) 1493-1494.
The spin-spin correlation function <S(t)S(0)> of classical Heisenberg spins on the kagome lattice is numerically evaluated for varying concentrations of non-magnetic impurities. This function is also used to obtain the correlation time t as a function of temperature for the kagome and square lattices.
- A. Keren, L. P. Le, G. L. Luke, W. D. Wu, Y. J. Uemura, Y. Ajiro, T. Asano, H. Kuriyama, M. Mekata and H. Kiruchi, Muon Spin Relaxation Measurements in Kagome Lattice System SrCr8Ga4O19, Hyp. Int. 85, (1994) 181-186.
Dynamical spin fluctuations in SrCr8Ga4O19, a frustrated spin system on a kagome lattice, is examined by the longitudinal field muon spin relaxation technique. This system shows a spin-glass (SG) like cusp in the susceptibility at Tg=3.5(2) K. The slowing down of Cr spin fluctuations is found to occur over a very wide temperature range Tg<T<30Tg. As T/Tg ->0 these fluctuations remain without static polarization (order parameter). Such strong fluctuations below Tg have not been observed before in a conventional SG system.