Anisotropic magnetic properties and anomalous thermal conductivity in the bc plane of the quasi-two-dimensional spin system Cu₃B ₂O₆: Relation between the thermal conductivity and the spin state in magnetic fields

We have measured the magnetic susceptibility in a magnetic field of 1 T (rotating the field direction) in the bc plane, the magnetization curve in magnetic fields up to 30 T and the thermal conductivity in magnetic fields up to 14 T for the quasi-two-dimensional spin system Cu₃B ₂O₆. Two-fold symmetry of the susceptibility in the bc plane has been found to develop at low temperatures below the Néel temperature T_N = 10 K. This is understood on the basis of the low symmetry of the Cu-O-Cu network. The susceptibility does not appear to vanish as T → 0 K even in the direction where the magnetization exhibits the minimum, indicating that the spin arrangement in the bc plane at T < T _N is noncollinear. The magnetization curve exhibits a bend and/or a jump associated with either spin-flop or metamagnetic transition at T < T_N, depending on the field direction. The thermal conductivity along the direction where the magnetization exhibits the minimum in the bc plane has been found to exhibit marked enhancement and a peak at T < T_N, which is understood to be caused by the increase of the thermal conductivity due to magnons. The peak is suppressed with increasing magnetic-field below 12 T and tends to recover in high field above 12 T. It has been found that the thermal conductivity strongly correlates with the spin state in Cu ₃B₂O₆.