Default Normal Template

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:

A.Keren, J.S.Gardner, G.Ehlers, A.Fukaya, E.Segal and Y.J.Uemura, Dynamic Properties of a Diluted Pyrochlore Cooperative Paramagnet ( Tb_pY_(1-p))₂ Ti₂O₇, Phys. Rev. Lett. 92, (2004) 107204.

Abstract:

Investigations of the spin dynamics of the geometrically frustrated pyrochlore (Tb_p Y_{1- 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 p_c 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.

The paper in PDF format(325Kb).

A. Keren and J. S. Gardner, Frustration driven lattice distortion; an NMR investigation of Y₂Mo₂O₇, Phys. Rev. Lett. 22-Oct, (2001).

Abstract:

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.

The paper in PDF format (205kB).

A. Keren, Y. J. Uemura, G. Luke, P. Mendels, M. Mekata and T. Asano, Magnetic dilution in the geometrically frustrated SrCr_9pGa_12-3pO₁₉, and the role of local dynamics: a mSR study, Phys. Rev. Lett. 84, (2000) 3450-3453.

Abstract:

We investigate the spin dynamics of SrCr_9pGa_12-3pO₁₉ for p below and above the percolation threshold p_c 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 lim_p->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 p_c, indicating that they are controlled by local excitation.

The paper in PDF format (80kB).

A. Keren, P. Mendels, M. Horvatic, F. Ferrer, Y. J. Uemura, M. Mekata and T. Asano, ^69,71Ga NMR in the kagome lattice compound SrCr₈Ga₄O₁₉, Phys. Rev. B 57, (1997).

Abstract:

We report the NMR site assignment of ^69,71 Ga in the kagome lattice compound SrCr_9-xGa_3+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.

The paper in PDF format (222 kB).

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.

Abstract:

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 T_g ~ 2K where the history dependence in the magnetic susceptibility appears.

The paper in PDF format (95 kB).

A. Keren, Dynamical Simulation of Spins on Kagome and Square Lattices, Phys. Rev. Lett. 72, (1994) 3254-3257.

Abstract:

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.

The paper in PDF format (405 kB).

A. Keren, Simulation of spin dynamics on kagome and square lattices, JMMM 140-144 (1995) 1493-1494.

Abstract:
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.

The paper in PDF format(86Kb).

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.

Abstract:

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 T_g=3.5(2) K. The slowing down of Cr spin fluctuations is found to occur over a very wide temperature range T_g<T<30T_g. As T/T_g ->0 these fluctuations remain without static polarization (order parameter). Such strong fluctuations below T_g have not been observed before in a conventional SG system.

The paper in PDF format(250Kb).