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 Nd 3+ magnons can act as either heat carriers or phonon scatterers, which strongly depends on the long-range antiferromagnetic transition and the fieldinduced transitions of spin structure. In particular, the Nd 3+ magnons can effectively transport heat in the spin-flopped state of the Nd 3+ sublattice. However, both the magnon transport and the magnetic scattering are quenched at very high fields. The spin re-orientations under the in-plane field can be conjectured from the detailed field dependence of κ.

The very-low-temperature thermal conductivity (κ) is studied for BaCo2V2O8, a quasi-onedimensional Ising-like antiferromagnet exhibiting an unusual magnetic-field-induced order-todisorder transition. The nearly isotropic transport in the longitudinal field indicates that the magnetic excitations scatter phonons rather than conduct heat. The field dependence of κ 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 κ(H) in the transverse field shows a very deep valley-like 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.

We report a study of the low-temperature heat transport in the quasi-one-dimensional S = 1/2 alternating antiferromagnetic-ferromagnetic chain compound (CH 3 ) 2 NH 2 CuCl 3 . 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.

We study the low-temperature heat transport of Nd 3 Ga 5 SiO 14 , which is a spin-liquid candidate, to probe the nature of the ground state and the effect of the magnetic field on the magnetic properties. The thermal conductivity (κ) shows a purely phononic transport in zero field. The external magnetic field along the c axis induces a diplike behavior of κ(H ), which can be attributed to a simple paramagnetic scattering on phonons. However, the magnetic field along the ab plane induces another steplike decrease of κ. This kind of κ(H ) behavior is discussed to be related to a field-induced partial order, which yields low-energy magnetic excitations that significantly scatter phonons. These results point to a paramagnetic ground state that partial magnetic order can be induced by magnetic field along the ab plane, which is also signified by the low-T specific heat data.