Large magnetothermal conductivity of ${\text{HoMnO}}_3$ single crystals and its relation to the magnetic-field-induced transitions of magnetic structure

We study the low-temperature heat transport of ${\text{HoMnO}}_3$ single crystals to probe the magnetic structures and their transitions induced by magnetic field. It is found that the low-$T$ thermal conductivity $\left(\kappa \right)$ shows very strong magnetic-field dependence, with the strongest suppression of nearly 90% and the biggest increase of 20 times of $\kappa$ compared to its zero-field value. In particular, some “dip”-like features show up in $\kappa \left(H\right)$ isotherms for field along both the $ab$ plane and the $c$ axis. These behaviors are found to shed light on the complex $H\text{−}T$ phase diagram and the field-induced reorientations of ${\text{Mn}}^{3+}$ and ${\text{Ho}}^{3+}$ spin structures. The results also demonstrate a significant spin-phonon coupling in this multiferroic compound.

Figure 1

(Color online) Temperature dependences of the $ab$-plane and the $c$-axis thermal conductivities of ${\text{HoMnO}}_3$ single crystals. The heat-transport data of ${\text{YMnO}}_3$ single crystals grown by the floating-zone method are also shown for comparison.

Figure 2

(Color online) Temperature dependences of thermal conductivities of ${\text{HoMnO}}_3$ single crystals in both the zero field and several different magnetic fields up to 14 T. The heat current are applied along the $ab$ plane or the $c$ axis while the magnetic field applied either in the $ab$ plane (along the $a$ axis or [210] directions) or along the $c$ axis.

Figure 3

(Color online) Magnetic-field dependences of thermal conductivities of ${\text{HoMnO}}_3$ single crystals at low temperatures.

Figure 4

(Color online) Magnetic structure of ${\text{HoMnO}}_3$ at low temperatures and those in applied magnetic field. Panel (a) shows the zero-field $\left(P{6}_{3}cm\right)$ and high-field $\left(P{6}_3{c}^{\prime }{m}^{\prime }\right)$ magnetic structures of ${\text{Mn}}^{3+}$ moments for $H\parallel ab$. Panels (b)–(d) show the proposed two transitions of magnetic structure when the magnetic field $\left(H\parallel c\right)$ is increasing. The two sublattices of AF ${\text{Ho}}^{3+}$ moments are polarized separately at two critical fields. Correspondingly, the ${\text{Mn}}^{3+}$ moments make $90°$ rotations twice.