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We study the low-temperature thermal conductivity (κ) of Dy2Ti2O7 along and perpendicular to the (111) plane and under the magnetic field along the [111] direction. Besides the steplike decreases of κ at the field-induced transitions from the spin-ice state to the kagomé-ice state and then to the polarized state, an abnormal phenomenon is that the κ(H) isotherms show a clear irreversibility at very low temperatures upon sweeping the magnetic field up and down. This phenomenon surprisingly has no correspondence with the well-known magnetization hysteresis. Possible origins for this irreversibility are discussed; in particular, a pinning effect of magnetic monopoles in a spin-ice compound by the weak disorders is proposed. [ABSTRACT FROM AUTHOR]

We report a study on the thermal conductivity (κ) of Tb2Ti2O7 single crystals at low temperatures. It is found that in zero field this material has an extremely low phonon thermal conductivity in a broad temperature range. The mean free path of phonons is even smaller than that of amorphous materials and is 3-4 orders of magnitude smaller than the sample size at 0.3 K. The strong spin fluctuation of the spin-liquid state is discussed to be the reason of the strong phonon scattering. The magnetic-field dependence of κ and comparison with Y2Ti2O7 and TbYTi2O7 confirm the magnetic origin of this phonon-glass-like behavior. [ABSTRACT FROM AUTHOR]

We study the low-temperature heat transport, as well as the magnetization and the specific heat, of TmMnO3 single crystals to probe the transitions of magnetic structure induced by magnetic field. It is found that the low-T thermal conductivity (k) shows strong magnetic-field dependence and the overall behaviors can be understood in the scenario of magnetic scattering on phonons. In addition, a strong "diplike" feature shows up in k(H) isotherms at 3.5--4 T for H ||c, which is related to a known spin reorientation of Mn3+ moments. The absence of this phenomenon for H||a indicates that the magnetic-structure transition of TmMnO3 cannot be driven by the in-plane field. In comparison, the magnetothermal conductivity of TmMnO3 is much larger than that of YMnO3 but smaller than that of HoMn03, indicating that the magnetisms of rare-earth ions are playing the key role in the spin-phonon coupling of the hexagonal manganites. [ABSTRACT FROM AUTHOR]

The low-temperature thermal conductivity (κ) of GdFeO3 single crystals is found to be strongly dependent on magnetic field. The low-field κ(H) curves show two "dips" for H ∥ a and only one dip for H ∥ c, with the characteristic fields having good correspondence with the spin-flop and the spin-polarization transitions. A remarkable phenomenon is that the sub-Kelvin thermal conductivity shows hysteretic behaviors on the history of applying the magnetic field; that is, the κ(H) isotherms measured with increasing field are larger than those with decreasing field. Intriguingly, the broad region of magnetic field (~0-3 T) showing the irreversibility of heat transport coincides with that presenting the ferroelectricity. It is discussed that the irreversible κ(H) behaviors are due to the phonon scattering by ferroelectric domain walls. This result shows an experimental feature that points to the capability of controlling the ferroelectric domain structures by magnetic field in multiferroic materials. [ABSTRACT FROM AUTHOR]

We study the heat transport of S=1/2 chain compound CuCl2 · 2((CH3)2SO) along the b axis (vertical to the chain direction) at very low temperatures. The zero-field thermal conductivity (j) shows a distinct kink at about 0.9 K, which is related to the long-range antiferromagnetic (AF) transition. With applying magnetic field along the c axis, j(H) curves also show distinct changes at the phase boundaries between the AF and the high-field disordered states. These results indicate a strong spin-phonon interaction and the magnetic excitations play a role in the b-axis heat transport as phonon scatterers. [ABSTRACT FROM AUTHOR]

We study the low-temperature thermal conductivity (κ) of Dy2Ti2O7 and Yb2Ti2O7 single crystals in magnetic fields up to 14 T along the [111], [100], and [110] directions. For Dy2Ti2O7 we find experimentally that (i) the low-Tκ(H) isotherms exhibit not only the steplike decreases at the low-field (<2T) magnetic transitions but also obvious field dependencies in high fields (>7T); (ii) at T≤0.5 K, the κ(H) curves show anisotropic irreversibility in low fields, that is, the κ(H) hysteresis is located at the first-order transition with H|| [100] and [110], while it is located between two successive transitions with H|| [111]; (iii) the κ in the hysteresis loops for H|| [100] and [110] show an extremely slow relaxation with the time constant of ~1000 min. For Yb2Ti2O7 we find that (i) the zero-field κ(T) show a kinklike decrease at the first-order transition (~200mK) with decreasing temperature, (ii) the low-Tκ(H) isotherms show a decrease in low field and a large enhancement in high fields, and (iii) the low-Tκ(H) curves show a sharp minimum at 0.5 T for H|| [110] and [111]. The roles of monopole excitations, field-induced transitions, spin fluctuations, and magnetoelastic coupling are discussed. [ABSTRACT FROM AUTHOR]

Low-temperature thermal conductivity (κ), as well as magnetization (M) and electric polarization (P), of multiferroic orthoferrite DyFeO3 single crystals are studied with H ∥ c. When the crystal is cooled in zero field, M, P, and k all consistently exhibit irreversible magnetic-field dependencies. In particular, with 500 mK < T ⩽ 2 K, all these properties show two transitions at the first run of increasing field but only the higher-field transition is present in the subsequent field sweepings. Moreover, the ultra-Iow-T (T < 500 mK) κ(H) shows a different irreversibility and there is only one transition when the field is swept both up and down. All the results indicate a complex low-T H-T phase diagram involving successive magnetic phase transitions of the Fe3+ spins. In particular, the ground state, obtained with cooling to sub-Kelvin temperatures in zero field, is found to be an unexplored phase. [ABSTRACT FROM AUTHOR]

The very-low-temperature thermal conductivity (κ) is studied for BaCo2V2O8, a quasi-one-dimensional Ising-like antiferromagnet exhibiting an unusual magnetic-field-induced order-to-disorder transition. The nearly isotropic transport in the longitudinal field indicates that the magnetic excitations scatter phonons rather than conduct heat. The field dependence of K shows a sudden drop at ∼4 T, where the system undergoes the transition from the Néel order to the incommensurate state. Another dip at lower field of ∼3 T indicates an unknown magnetic transition, which is likely due to the spin-flop transition. Moreover, the K{H) in the transverse field shows a very deep valleylike feature, which moves slightly to higher field and becomes sharper upon lowering the temperature. This indicates a magnetic transition induced by the transverse field, which however is not predicted by the present theories for this low-dimensional spin system. [ABSTRACT FROM AUTHOR]

We report a study of the low-temperature heat transport in the quasi-one-dimensional S = 1/2 alternating antiferromagnetic-ferromagnetic chain compound (CH3)2NH2CuCl3. Both the temperature and magnetic-field dependencies of thermal conductivity are very complicated, pointing to the important role of spin excitations. It is found that magnetic excitations act mainly as the phonon scatterers in a broad temperature region from 0.3 to 30 K. In magnetic fields, the thermal conductivity shows drastic changes, particularly at the field-induced transitions from the low-field Néel state to the spin-gapped state, the field-induced magnetic ordered state, and the spin polarized state. In high fields, the phonon conductivity is significantly enhanced because of the weakening of spin fluctuations. [ABSTRACT FROM AUTHOR]

Abstract: The bulk single crystals of S=1 chain compound Ni(C3H10N2)2NO2ClO4 are grown by using a slow evaporation method at a constant temperature and a slow cooling method. It is found that the optimum condition of growing large crystals is via slow evaporation at 25°C using 0.015mol , 0.015mol NaNO2, and 0.03mol 1,3-propanediamine liquid dissolved into 30ml aqueous solvent. High-quality crystals with size up to mm3 are obtained. The single crystals are characterized by measurements of X-ray diffraction, magnetic susceptibility, specific heat and thermal conductivity. The susceptibilities along three crystallographic axes are found to exhibit broad peaks at , and then decrease abruptly to zero at lower temperatures, which is a characteristic of a Haldane chain system. The specific heat and the thermal conductivity along the c-axis can be attributed to the simple phononic contribution and are analyzed using the Debye approximation. [Copyright &y& Elsevier]

We report the magnetic-field dependence of thermal conductivity (κ) of an insulating cuprate Nd2CuO4 at very low temperatures down to 0.3 K. It is found that, apart from the paramagnetic moments scattering on phonons, the Nd3+ magnons can act as either heat carriers or phonon scatterers, which strongly depends on the long-range antiferromagnetic transition and the field-induced transitions of spin structure. In particular, the Nd3+ magnons can effectively transport heat in the spin-flopped state of the Nd3+ sublattice. However, both the magnon transport and the magnetic scattering are quenched at very high fields. The spin reorientations under the in-plane field can be conjectured from the detailed field dependence of ?. [ABSTRACT FROM AUTHOR]