Despite a well-ordered pyrochlore crystal structure and strong magnetic interactions between the Dy 3+ or Ho 3+ ions, no long range magnetic order has been detected in the pyrochlore titanates Ho2Ti2O7 and Dy2Ti2O7. To explore the actual magnetic phase formed by cooling these materials, we measure their magnetization dynamics using toroidal, boundary-free magnetization transport techniques. We find that the dynamical magnetic susceptibility of both compounds has the same distinctive phenomenology, that is indistinguishable in form from that of the dielectric permittivity of dipolar glass-forming liquids. Moreover, Ho2Ti2O7 and Dy2Ti2O7 both exhibit microscopic magnetic relaxation times that increase along the super-Arrhenius trajectories analogous to those observed in glass-forming dipolar liquids. Thus, upon cooling below about 2K, Dy2Ti2O7 and Ho2Ti2O7 both appear to enter the same magnetic state exhibiting the characteristics of a glass-forming spin-liquid. PACS numbers: 75.50.Lk, 75.47.LxA recent proposal 29 that the magnetic state of arXiv:1707.09014v3 [cond-mat.str-el]

A "supercooled" liquid develops when a fluid does not crystallize upon cooling below its ordering temperature. Instead, the microscopic relaxation times diverge so rapidly that, upon further cooling, equilibration eventually becomes impossible and glass formation occurs. Classic supercooled liquids exhibit specific identifiers including microscopic relaxation times diverging on a Vogel-TammannFulcher (VTF) trajectory, a Havriliak-Negami (HN) form for the dielectric function e(ω, T ), and a general Kohlrausch-Williams-Watts (KWW) form for time-domain relaxation. Recently, the pyrochlore Dy 2 Ti 2 O 7 has become of interest because its frustrated magnetic interactions may, in theory, lead to highly exotic magnetic fluids. However, its true magnetic state at low temperatures has proven very difficult to identify unambiguously. Here, we introduce highprecision, boundary-free magnetization transport techniques based upon toroidal geometries and gain an improved understanding of the time-and frequency-dependent magnetization dynamics of Dy 2 Ti 2 O 7 . We demonstrate a virtually universal HN form for the magnetic susceptibility χ (ω, T ), a general KWW form for the realtime magnetic relaxation, and a divergence of the microscopic magnetic relaxation rates with the VTF trajectory. Low-temperature Dy 2 Ti 2 O 7 therefore exhibits the characteristics of a supercooled magnetic liquid. One implication is that this translationally invariant lattice of strongly correlated spins may be evolving toward an unprecedented magnetic glass state, perhaps due to many-body localization of spin.spin liquid | supercooled liquids | magnetic dynamics | periodic boundaries C ooling a pure liquid usually results in crystallization via a first-order phase transition. However, in glass-forming liquids when the cooling is sufficiently rapid, a metastable "supercooled" state is achieved instead (1-3). Here, the microscopic relaxation times diverge until equilibration of the system is no longer possible at a given cooling rate. At this juncture there is generally a broad peak in the specific heat preceding the glass transition, at which no symmetry-breaking phase transition occurs (Fig. 1A). The antecedent fluid exhibits a set of phenomena characteristic of the supercooled liquid state (1-3). For example, the divergence of microscopic relaxation times τ 0 ðTÞ typically shows substantial departures from Arrhenius behavior [τ 0 ðTÞ = AexpðΔ=kTÞ and, instead, is described characteristically using the Vogel-Tammann-Fulcher (VTF) form (4)Here, T 0 is a temperature at which the relaxation time diverges to ∞ while D characterizes the extent of the super-Arrhenius behavior (Fig. 1B). One way to establish τ 0 ðTÞ is by measuring the characteristic frequency ω 0 ðTÞ = 1=τ 0 ðTÞ of peaks in the dissipative (imaginary) component of the dielectric function «ðω, TÞ.For classic supercooled liquids, «ðω, TÞ generally exhibits the Havriliak-Negami (HN) form (5, 6) «ðω,[2]Here, the exponents α and γ describe, respectively, the broadening and asymmetry of the rela...