In-Plane Thermal Conductivity of ${\mathrm{N}\mathrm{d}}_2{\mathrm{C}\mathrm{u}\mathrm{O}}_4$: Evidence for Magnon Heat Transport

We report the temperature and magnetic field dependence of the in-plane thermal conductivity (${\kappa }_{ab}$) of high-quality monocrystalline ${\mathrm{N}\mathrm{d}}_2{\mathrm{C}\mathrm{u}\mathrm{O}}_4$. Isothermal measurements of the field dependence of ${\kappa }_{ab}$ at low temperatures ($2\mathrm{K}\le T\le 5\mathrm{K}$) show no change in ${\kappa }_{ab}$ below a critical magnetic field $H_c$ ($H_c\approx 4.5\mathrm{T}$ for $H\parallel [100]$) and $H_c\approx 2.5\mathrm{T}$ for $H\approx \parallel [110]$). Above $H_c$, ${\kappa }_{ab}$ more than doubles as $H$ is increased to 9 T. At $H_c$, there is a transition from a noncollinear to a collinear arrangement of the Nd and Cu spins and a collapse of the gap, $\Delta$, in an acoustic magnon branch at $\mathbf{k}=0$. Closure of this gap appears to allow the conduction of substantial amounts of heat by acoustic magnons.

Figure 1

(a) Magnetic structure of ${\mathrm{N}\mathrm{d}}_2{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ below 30 K. The brown dots denote the Cu ions and the blue ones Nd ions. Red arrows point the rotation direction of spins when $H$ is applied along the [100] direction. (b) Temperature dependence of the in-plane magnetic susceptibility (${\chi }_{ab}$) of ${\mathrm{N}\mathrm{d}}_2{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ measured by applying $H=1000\mathrm{O}\mathrm{e}$ along the $ab$ plane. The inset is the plot of $d{\chi }_{ab}/dT$ vs $T$ between 250 and 350 K, where the Néel temperature is indicated.

Figure 2

Linear plot of temperature dependence of ${\kappa }_{ab}$ of ${\mathrm{N}\mathrm{d}}_2{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ at $H=0$ (open circles) and $H=9\mathrm{T}$ (solid circles) with $H\parallel ab$. The inset shows the temperature dependence of magnetothermal conductivity ${\kappa }_{ab}(H)\mathrm{\text{−}}{\kappa }_{ab}(0)/{\kappa }_{ab}(0)$ at $H=9\mathrm{T}$. Note that the application of magnetic field induces a peak near 5 K and a sign change of ${\kappa }_{ab}(H)\mathrm{\text{−}}{\kappa }_{ab}(0)/{\kappa }_{ab}(0)$ at 10 K.

Figure 3

Magnetic field dependence of ${\kappa }_{ab}$ of ${\mathrm{N}\mathrm{d}}_2{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ at $T=2$ (solid circles), 5 (diamonds), 10 (triangles), and 18 K (squares) for (a) $H\parallel [100]$ and (b) $H\approx \parallel [110]$, respectively. The dashed lines are an estimate of thermal conductivity change due to phonon-magnon scattering.

Figure 4

(a) Comparison of magnon-related thermal conductivity ${\kappa }_{ab}^m$ (open circles) with the magnon population ($P_m$) residing in the lower branch of the magnon spectrum at $H=9\mathrm{T}$ (solid line); (b) magnetic field dependence of $P_m$ (solid line) and ${\kappa }_{ab}$ (open circles) at 2 K. As the experimental data are for $H\parallel [100]$, $H_c=4.5\mathrm{T}$ are used for calculating $P_m$.