F. B. Zhang

Magnetic phase transitions and magnetoelectric coupling of GdFeO3 single crystals probed by low-temperature heat transport

The low-temperature thermal conductivity (�) of GdFeO3 single crystals is found to be strongly dependent on magnetic field. The low-field �(H) curves show two “dips” for H k a and only one “dip” for H k c, with the characteristic fields having good correspondence with the spin-flop and the spin-polarization transitions. A remarkable phenomenon is that the subKelvin thermal conductivity shows hysteretic behaviors on the history of applying magnetic field, that is, the �(H) isotherms measured with field increasing are larger than those with field decreasing. 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 �(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.

Single crystal growth of the pyrochlores R2Ti2O7 (R=rare earth) by the optical floating-zone method

Abstract We report a systematic study on the crystal growth of the rare-earth titanates R2Ti2O7 (R=Gd, Tb, Dy, Ho, Y, Er, Yb and Lu) and Y-doped Tb2−xYxTi2O7 (x=0.2 and 1) using an optical floating-zone method. High-quality single crystals were successfully obtained and the growth conditions were carefully optimized. The oxygen pressure was found to be the most important parameter and the appropriate ones are 0.1–0.4xa0MPa, depending on the radius of rare-earth ions. The growth rate is another parameter and was found to be 2.5–4xa0mm/h for different rare-earth ions. X-ray diffraction data demonstrated the good crystallinity of these crystals. The basic physical properties of these crystals were characterized by the magnetic susceptibility and specific heat measurements.

Ground state and magnetic phase transitions of orthoferriteDyFeO3

Low-temperature thermal conductivity (kappa), as well as magnetization (M) and electric polarization (P), of multiferroic orthoferrite DyFeO_3 single crystals are studied with H parallel c. When the crystal is cooled in zero field, M, P, and kappa all consistently exhibit irreversible magnetic-field dependencies. In particular, with 500 mK < T le 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-low-T (T < 500 mK) kappa(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 Fe^{3+} spins. In particular, the ground state, obtained with cooling to subKelvin temperatures in zero field, is found to be an unexplored phase.

Single crystal growth of the hexagonal manganites R MnO3 (R=rare earth) by the optical floating-zone method

Abstract We report a study on the crystal growth of the hexagonal manganites R MnO 3 ( R =Y, Lu, Ho, Er, and Tm) by using an optical floating-zone method. It was found that high-quality single crystals of R =Y, Lu, and Ho could be easily grown with essentially the same conditions as those reported in the literature, that is, with an atmosphere of normal pressure Ar and oxygen mixture and a growth rate of 2–4xa0mm/h. However, these conditions were not feasible for growing good crystals of R =Er and Tm. The chemical analysis indicated that it was due to an off-stoichiometric phenomenon in the formed single crystals. We used an effective and simple way to resolve this problem by adjusting the nominal compositions of the polycrystal feed rods to be 1–2% rare-earth excess. The structures and physical properties were characterized by X-ray diffraction, magnetic susceptibility, specific heat, resistivity, and dielectric constant measurements.

Irreversible magnetic-field dependence of low-temperature heat transport of spin-ice compound Dy2Ti2O7 in a [111] field

We study the low-temperature thermal conductivity (kappa) of Dy_2Ti_2O_7 along and perpendicular to the (111) plane and under the magnetic field along the [111] direction. Besides the step-like decreases of kappa at the field-induced transitions from the spin-ice state to the kagome-ice state and then to the polarized state, an abnormal phenomenon is that the kappa(H) isotherms show a clear irreversibility at very low temperatures upon sweeping 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 spin ice compound by the weak disorders is proposed.

Phonon-glass-like behavior of magnetic origin in single-crystal Tb 2 Ti 2 O 7

Q. J. Li, Z. Y. Zhao, C. Fan, F. B. Zhang, H. D. Zhou, 3 X. Zhao, ∗ and X. F. Sun † Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996-1200, USA National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32306-4005, USA School of Physical Sciences, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China (Dated: May 30, 2013)

Low-temperature heat transport, specific heat, and magnetic properties of the hexagonal TmMnO3single crystals

We study the low-temperature heat transport, as well as the magnetization and the specific heat, of TmMnO_3 single crystals to probe the transitions of magnetic structure induced by magnetic field. It is found that the low-T thermal conductivity (kappa) shows strong magnetic-field dependence and the overall behaviors can be understood in the scenario of magnetic scattering on phonons. In addition, a strong dip-like feature shows up in kappa(H) isotherms at 3.5--4 T for H parallel c, which is related to a known spin re-orientation of Mn^{3+} moments. The absence of this phenomenon for H parallel a indicates that the magnetic-structure transition of TmMnO_3 cannot be driven by the in-plane field. In comparison, the magnetothermal conductivity of TmMnO_3 is much larger than that of YMnO_3 but smaller than that of HoMnO_3, indicating that the magnetisms of rare-earth ions are playing the key role in the spin-phonon coupling of the hexagonal manganites.

Thermal conductivity of IPA-CuCl3: Evidence for ballistic magnon transport and the limited applicability of the Bose-Einstein condensation model

The heat transport of the spin-gapped material (CH_3)_2CHNH_3CuCl_3 (IPA-CuCl_3), a candidate quantum magnet with Bose-Einstein condensation (BEC), is studied at ultra-low temperatures and in high magnetic fields. Due to the presence of the spin gap, the zero-field thermal conductivity (kappa) is purely phononic and shows a ballistic behavior at T H_{c1}) and demonstrates limited applicability of the BEC model to IPA-CuCl_3.

Low-temperature heat transport in the geometrically frustrated antiferromagnets R2Ti2O7 (R = Gd and Er)

We report a systematic study on the low-temperature thermal conductivity (kappa) of R_2Ti_2O_7 (R = Gd and Er) single crystals with different directions of magnetic field and heat current. It is found that the magnetic excitations mainly act as phonon scatterers rather than heat carriers, although these two materials have long-range magnetic orders at low temperatures. The low-T kappa(H) isotherms of both compounds show rather complicated behaviors and have good correspondences with the magnetic transitions, where the kappa(H) curves show drastic dip- or step-like changes. In comparison, the field dependencies of kappa are more complicated in Gd_2Ti_2O_7, due to the complexity of its low-T phase diagram and field-induced magnetic transitions. These results demonstrate the significant coupling between spins and phonons in these materials and the ability of heat-transport properties probing the magnetic transitions.

Low-temperature heat transport ofNd2−xCexCuO4single crystals

We report a study of the Ce doping effect on the thermal conductivity (kappa) of Nd_{2-x}Ce_{x}CuO_{4} (NCCO) at low temperatures down to 0.3 K and in magnetic fields up to 14 T. It is found that with Ce doping, the electronic thermal conductivity increases; at the same time, the a-axis field induced changes in kappa(H), associated with the spin flop and spin polarization of Nd^{3+} sublattice, and the spin flop of Cu^{2+} sublattice, gradually disappear. These are clearly due to the electron doping and the destruction of the antiferromagnetic orders. In the superconducting NCCO with x = 0.14 and 0.18, although the electronic thermal conductivity shows sizable field dependencies with H // c, the paramagnetic scattering of phonons is still playing the dominant role in the heat transport, which is different from many other cuprates. In the lightly doped samples (x = 0.03 and 0.06), the low-T kappa(H) isotherms with H // c show a step-like anomaly and is likely related to the spin/charge stripes.