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Register a new userReal-space imaging and flux noise spectroscopy of magnetic dynamics in Ho$_2$Ti$_2$O$_7$
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Holmium titanate (Ho$_2$Ti$_2$O$_7$) is a rare earth pyrochlore and a canonical example of a classical spin ice material. Despite the success of magnetic monopole models, a full understanding of the energetics and relaxation rates in this material has remained elusive, while recent studies have shown that defects play a central role in the magnetic dynamics. We used a scanning superconducting quantum interference device (SQUID) microscope to study the spatial and temporal magnetic fluctuations in three regions with different defect densities from a Ho$_2$Ti$_2$O$_7$ single crystal as a function of temperature. We found that the magnetic flux noise power spectra are not determined by simple thermally-activated behavior and observed evidence of magnetic screening that is qualitatively consistent with Debye-like screening due to a dilute gas of low-mobility magnetic monopoles. This work establishes magnetic flux spectroscopy as a powerful tool for studying materials with complex magnetic dynamics, including frustrated correlated spin systems.
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The elementary excitations of the spin-ice materials Ho$_2$Ti$_2$O$_7$ and Dy$_2$Ti$_2$O$_7$ in zero field can be described as independent magnetic monopoles. We investigate the influence of these exotic excitations on the heat transport by measuring the magnetic-field dependent thermal conductivity $kappa $. Additional measurements on the highly dilute reference compounds HoYTi$_2$O$_7$ and DyYTi$_2$O$_7$ enable us to separate $kappa $ into a sum of phononic ($kappa_{ph}$) and magnetic ($kappa_{mag}$) contributions. For both spin-ice materials, we derive significant zero-field contributions $kappa_{mag}$, which are rapidly suppressed in finite magnetic fields. Moreover, $kappa_{mag}$ sensitively depends on the scattering of phonons by magnetic excitations, which is rather different for the Ho- and the Dy-based materials and, as a further consequence, the respective magnetic-field dependent changes $kappa_{ph}(B)$ are even of opposite signs.
When degenerate states are separated by large energy barriers, the approach to thermal equilibrium can be slow enough that physical properties are defined by the thermalization process rather than the equilibrium. The exploration of thermalization pushes experimental boundaries and provides refreshing insights into atomic scale correlations and processes that impact steady state dynamics and prospects for realizing solid state quantum entanglement. We present a comprehensive study of magnetic relaxation in Ho$_2$Ti$_2$O$_7$ based on frequency-dependent susceptibility measurements and neutron diffraction studies of the real-time atomic-scale response to field quenches. Covering nearly ten decades in time scales, these experiments uncover two distinct relaxation processes that dominate in different temperature regimes. At low temperatures (0.6K<T<1K) magnetic relaxation is associated with monopole motion along the applied field direction through the spin-ice vacuum. The increase of the relaxation time upon cooling indicates reduced monopole conductivity driven by decreasing monopole concentration and mobility as in a semiconductor. At higher temperatures (1K<T<2K) magnetic relaxation is associated with the reorientation of monopolar bound states as the system approaches the single-spin tunneling regime. Spin fractionalization is thus directly exposed in the relaxation dynamics.
We present an extensive study on the effect of substrate orientation, strain, stoichiometry and defects on spin ice physics in Ho$_2$Ti$_2$O$_7$ thin films grown onto yttria-stabilized-zirconia substrates. We find that growth in different orientations produces different strain states in the films. All films exhibit similar c-axis lattice parameters for their relaxed portions, which are consistently larger than the bulk value of 10.10 AA. Transmission electron microscopy reveals anti-site disorder and growth defects to be present in the films, but stuffing is not observed. The amount of disorder depends on the growth orientation, with the (110) film showing the least. Magnetization measurements at 1.8 K show the expected magnetic anisotropy and saturation magnetization values associated with a spin ice for all orientations; shape anisotropy is apparent when comparing in and out-of-plane directions. Significantly, only the (110) oriented films display the hallmark spin ice plateau state in magnetization, albeit less well-defined compared to the plateau observed in a single crystal. Neutron scattering maps on the more disordered (111) oriented films show the Q=0 phase previously observed in bulk materials, but the Q=X phase giving the plateau state remains elusive. We conclude that the spin ice physics in thin films is modified by defects and strain, leading to a reduction in the temperature at which correlations drive the system into the spin ice state.
We report low temperature specific heat and muon spin relaxation/rotation ($mu$SR) measurements on both polycrystalline and single crystal samples of the pyrochlore magnet Yb$_2$Ti$_2$O$_7$. This system is believed to possess a spin Hamiltonian supporting a Quantum Spin Ice (QSI) ground state and to display sample variation in its low temperature heat capacity. Our two samples exhibit extremes of this sample variation, yet our $mu$SR measurements indicate a similar disordered low temperature state down to 16 mK in both. We report little temperature dependence to the spin relaxation and no evidence for ferromagnetic order, in contrast to recent reports by Chang emph{et al.} (Nat. Comm. {bf 3}, 992 (2012)). Transverse field (TF) $mu$SR measurements show changes in the temperature dependence of the muon Knight shift which coincide with heat capacity anomalies. We are therefore led to propose that Yb$_2$Ti$_2$O$_7$ enters a hidden order ground state below $T_csim265$ mK where the nature of the ordered state is unknown but distinct from simple long range order.
The single ion physics of Ho$_2$Ti$_2$O$_7$ is well-understood to produce strong Ising anisotropy, which is an essential ingredient to its low-temperature spin ice state. We present inelastic neutron scattering measurements on Ho$_2$Ti$_2$O$_7$ that reveal a clear inconsistency with its established single ion Hamiltonian. Specifically, we show that a crystal field doublet near 60~meV is split by approximately 3~meV. Furthermore, this crystal field splitting is not isolated to Ho$_2$Ti$_2$O$_7$ but can also be found in its chemical pressure analogs, Ho$_2$Ge$_2$O$_7$ and Ho$_2$Sn$_2$O$_7$. We demonstrate that the origin of this effect is a vibronic bound state, resulting from the entanglement of a phonon and crystal field excitation. We derive the microscopic Hamiltonian that describes the magneto-elastic coupling and provides a quantitative description of the inelastic neutron spectra.
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