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Evidence for unidimensional low-energy excitations as the origin of persistent spin dynamics in geometrically frustrated magnets

A. Yaouanc, P. Dalmas de Réotier, A. Bertin, C. Marin, E. Lhotel, A. Amato, and C. Baines
Phys. Rev. B 91, 104427 – Published 27 March 2015
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    Abstract

    We report specific heat, magnetic, and muon spin relaxation measurements performed on a polycrystalline sample of the normal spinel CdHo2S4. The rare-earth ions sit on a lattice of corner-sharing regular tetrahedra as in pyrochlore compounds. Magnetic ordering is detected at Tc≃0.87 K. From spin-lattice relaxation rate measurements on both sides of Tc we uncover similar magnetic excitation modes driving the so-called persistent spin dynamics at T<Tc. Unidimensional excitations are argued to be at its origin. Often observed spin loop structures are suggested to support these excitations. The possibility of a generic mechanism for their existence is discussed.

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    • Received 20 November 2013
    • Revised 23 December 2014

    DOI:https://doi.org/10.1103/PhysRevB.91.104427

    ©2015 American Physical Society

    Authors & Affiliations

    A. Yaouanc1,2,3, P. Dalmas de Réotier1,2, A. Bertin1,2, C. Marin1,2, E. Lhotel4, A. Amato3, and C. Baines3

    • 1Université Grenoble Alpes, INAC-SPSMS, F-38000 Grenoble, France
    • 2CEA, INAC-SPSMS, F-38000 Grenoble, France
    • 3Laboratory for Muon-Spin Spectroscopy, Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland
    • 4Institut Néel, CNRS and Université Joseph Fourier, BP 166, F-38042 Grenoble Cedex 9, France

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    Vol. 91, Iss. 10 — 1 March 2015

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    • Figure 1
      Figure 1

      (a) Rare-earth ions lattice in the pyrochlore R2M2O7 and normal spinel CdR2X4 compounds. The thicker light blue (thinner dark blue) bold line represents a 6 (10)-site loop. (b) Low-temperature heat capacity of CdHo2S4.

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    • Figure 2
      Figure 2

      Inverse of the magnetic susceptibility versus temperature. The data have been measured in a magnetic field of 1.1 mT. The solid line results from a fit of the Curie-Weiss law to the data measured above 150 K. The susceptibility is dimensionless since we use SI units.

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    • Figure 3
      Figure 3

      Magnetic measurements for a CdHo2S4 powder sample. The data for the magnetic susceptibility χ versus temperature were recorded with external fields Bext ranging from 0.5 to 50 mT and were found to match one another. The field was applied in the plane of the sample pellet so that the demagnetization field is small. The solid line at low temperature together with the dotted line extension above Tc represents a fit of the function Cfs/(T−θCW,fs)+a+bT to the data recorded below Tc. The former term represents the contribution of weakly interacting Ho3+ spins, while the remaining terms describe the majority spin state; see main text. The quantity Kexp, defined in the main text and measured for different Bext as indicated in the figure, is shown for comparison.

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    • Figure 4
      Figure 4

      μSR spectra recorded in a transverse of 50 mT at 0.2 and 1.2 K. The full lines are the results of fits to the data. The model consists of a sum of two exponentially damped cosine functions, the former and latter accounting for muons stopped in the sample and its surroundings, respectively. The amplitudes extracted from the fits are temperature independent for the two components. The precession frequency νμ of the former provides a measure of the mean field 〈Bμ〉 = 2πνμ/γμ at the muon site in the sample; see main text for details. γμ=851.615Mrads−1T−1 is the muon gyromagnetic ratio.

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    • Figure 5
      Figure 5

      Magnetic moment per holmium ion of a CdHo2S4 powder versus field measured for different temperatures as indicated in the figure.

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    • Figure 6
      Figure 6

      External field derivative of the Ho3+ magnetic moment of a CdHo2S4 powder for different temperatures as indicated in the figure.

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    • Figure 7
      Figure 7

      Four μSR spectra recorded for a CdHo2S4 powder sample, two zero-field spectra taken on both sides of the magnetic phase transition temperature Tc, and two longitudinal field spectra recorded at T=0.12 K, i.e., T≪Tc. The early time details are shown in the insert. The solid lines result from fits as explained in the main text.

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    • Figure 8
      Figure 8

      Spin-lattice relaxation rate λZ versus longitudinal field intensity Bext and temperature T for a CdHo2S4 powder. In the main frame is displayed λZ(Bext) for two temperatures below and above Tc. The λZ(T=0.12K) maximum occurs at 20 mT. The solid lines are explained in the main text. The dashed line at small Bext and T=0.12 K is a guide to the eyes. In the insert is displayed λZ(T) measured from 0.019 to 55 K under zero field or Bext=5 mT. The temperature Tc at which the compound exhibits a magnetic phase transition is specified by an arrow.

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