Muon spin relaxation investigation of the spin dynamics of geometrically frustrated antiferromagnetsY2Mo2

Abstract

The spin dynamics of geometrically frustrated pyrochlore antiferromagnets Y2Mo2O7 and Tb2Mo2O7 have been investigated using muon spin relaxation. A dramatic slowing down of the moment fluctuations occurs as one approaches the spin freezing temperatures (TF =22 K and 25 K respectively) from above. Below TF there is a disordered magnetic state similar to that found in a spin glass but with a residual muon spin relaxation rate at low temperatures. These results show that there is a large density of states for magnetic excitations in these systems near zero energy. * Address after September 01, 1996: Dept. of Physics, University of Waterloo, Waterloo, Ontario, N2L-3G1, CANADA Antiferromagnets which are frustrated or diluted can exhibit novel electronic and magnetic behaviour. Recently, there has been considerable interest in the behaviour of systems where the natural antiferromagnetic coupling between ions is frustrated by the geometry of the lattice. In two dimensions, Heisenberg spins on triangular and corner sharing triangular (kagomé) lattices are simple examples of geometric frustration, while in three dimensions, the most well studied systems have a pyrochlore structure, in which the magnetic ions occupy a lattice of corner sharing tetrahedra. A system of Heisenberg spins interacting via nearest-neighbor antiferromagnetic couplings on the pyrochlore lattice displays a classical ground state with macroscopic degeneracy, since the lowest energy spin configuration requires only that Σ 4 i=1 S i = 0 for each tetrahedron. This feature led Villain to argue that these systems remain in a cooperative paramagnetic state with only short range spin-spin correlations for all T>0 [1] and this has been confirmed by Monte Carlo simulations [2]. Possibly, the most interesting feature of the ground state of pyrochlore [3] and kagomé [4,5] lattice antiferromagnets is the prediction of a dispersionless spin-wave branch ("zero modes"). These zero modes manifestly affect the thermodynamics of these classical systems as demonstrated by Monte Carlo simulations, where the low temperature specific heat, C v , falls below the classical value k B expected from equipartition of energy [2,5,6]. Also, again because of these zero modes, the kagomé and pyrochlore antiferromagnets display large spin fluctuations down to T= s.In this letter we report an investigation of the low temperature magnetic properties of pyrochlores Y 2 Mo 2 O 7 and Tb 2 Mo 2 O 7 using the technique of muon spin rotation/relaxation (µSR ), which is sensitive to spin fluctuation rates in the range 10 4 − 10 11 s −1 [13], below that detectable with neutron scattering. We find that, despite its nominally disorder free structure, the magnetic behaviour in Y 2 Mo 2 O 7 is close to that observed in conventional random spin glasses. Specifically, a large static internal magnetic field with a very broad distribution develops below T F , such that no coherent muon spin precession is observed. At the same time, the muon spin relaxation rate 1/T 1 decreases according to a power l...

First-Order Transition in the Spin Dynamics of Geometrically FrustratedYb2Ti2O7Hodges1, Bonville2, Forget3 et al. 2002Phys. Rev. Lett.182252119View full textAdd to dashboardCite

“…3). A similar misfit was also observed for Y 2 Mo 2 O 7 near 0.03 ms at 2.5 K [13]. A better fit is provided by the Gaussian broadened Gaussian (GBG) model [25], where the single Gaussian distribution of the KT model is replaced by a collection of distributions (solid lines at 0.200 K in Fig.…”

“…First, under the influence of the frustration the system does not experience a magnetic phase transition and remains in a collective paramagnetic state with the spin fluctuations persisting as T ! 0 [3,4,[6][7][8][9][10][11]13]. Second, a long range ordered state is reached through a phase transition which may be first order [5,14,15].…”

A comparison of the local magnetic susceptibility in rare earth pyrochlores

“…As shown, there is a difference of B5-10% in the inverse fractional frequency shift between spectra taken on cooling or warming. The total frequency shift of B50% observed at 0:5 K corresponds with a magnetic field of 5 kOe and hence the history dependence implies there is a static component to the internal field of B0:025-0:05 kOe: It is speculated that the sample is magnetized on cooling, inducing a small ferromagnetic component which persists on warming to 1 K: Note that such a small static field would not have been picked up by previous T À1 1 measurements since they could easily be masked by the signal from the dynamic internal fields, which are known to be of the order of 10 kOe from studies of the isostructural compound Tb 2 Mo 2 O 7 [25], where the behavior of the Tb 3þ moments in the paramagnetic regime is measured to be roughly the same. In Tb 2 Mo 2 O 7 one can measure the magnitude of the internal field directly since spin freezing is observed.…”

Evidence for Gapped Spin-Wave Excitations in the FrustratedGd2Sn2O7Pyrochlore Antiferromagnet from Low-Temperature Specific Heat Measurements

“…Examples of observed phenomenology include spin ice [1], spin glass [2,3], and spin liquid [4] behaviors as well as long-range magnetic order [5,6,7,8,9,10,11] and perhaps some novel type of hidden order [12]. A common trend observed in all of these systems is an apparent persistence of spin dynamics down to the lowest temperatures as revealed by muon spin relaxation (µSR) [3,4,7,8,9,11,12], Mössbauer spectroscopy [13] and neutron spin echo [1] experiments. The origin of persistent spin dynamics (PSDs) is a major open question in the study of highly frustrated insulating magnetic oxide materials [5].…”

“…Examples of observed phenomenology include spin ice [1], spin glass [2,3], and spin liquid [4] behaviors as well as long-range magnetic order [5,6,7,8,9,10,11] and perhaps some novel type of hidden order [12]. A common trend observed in all of these systems is an apparent persistence of spin dynamics down to the lowest temperatures as revealed by muon spin relaxation (µSR) [3,4,7,8,9,11,12] Perhaps what is most perplexing, is that PSDs have been found in pyrochlores, such as Gd 2 Ti 2 O 7 (GTO) [9,11] and Gd 2 Sn 2 O 7 (GSO) [7,8,13], which, according to neutron scattering experiments [6,10], display long-range magnetic order below a critical temperature T c = 0.74 K for GTO [6] and T c = 1.0 K for GSO [10]. This is highly unusual since conventional wisdom suggests that collective magnon-like excitations, hence spin dynamics, should freeze out in the limit of zero temperature.…”