Anisotropic thermal transport in robust crystalline yttrium aluminium garnet tunable by long oriented amorphous nano-tracks

Developing robust functional materials with tunable nano-engineered heat flow directionality in micro-scale devices for operation in high demanding applications presents a major challenge for conventional nano-fabrication techniques. In this work, we explore the control of emergent anisotropic heat propagation in single-crystalline yttrium aluminium garnet (YAG) Y₃Al₅O₁₂ nano-patterned by long amorphous cylindrical tracks induced by 167 MeV Xe ion irradiation. The resulting nano-composite layer consists of high aspect-ratio, nanoscale-thick and microscale-long amorphous tracks embedded within a YAG matrix. The formation of amorphous phase is finely tuned by varying the ion fluence, spanning from isolated tracks to the regime of their partial overlap. High-resolution scanning transmission electron microscopy (HR-STEM) reveals a uniform and directionally consistent amorphous structure across the irradiated depth. Thermal anisotropy of the resulting crystalline-amorphous nano-composite is confirmed by spatial domain thermoreflectance measurements and aligned nano-channel model based on phonon scattering from oriented tracks with the radial and axial thermal resistances. Heat is conducted primarily along the ion beam direction due to the elongated crystalline domains, while the radial transport is inhibited by increased phonon scattering on multiple crystalline-amorphous boundaries. The Klemens model considering ion track scattering terms is shown to be an effective tool to estimate the ion track cross-section area, which is confirmed by HR-STEM measurements and molecular dynamics simulations. Amorphous extended nano-patterning by swift heavy ion irradiation of robust crystalline insulators and semiconductors provides a viable path to control orientation-dependent heat flow.