Revealing the anisotropic thermal conductivity using time-resolved magneto-optical Kerr effect

Ultrafast thermo-magneto-optical effect upon femtosecond laser excitation, has enabled new capabilities in the thermal characterization of nanomaterials. In this work, we demonstrate time-resolved magneto-optical Kerr effect (TR-MOKE) as an advanced thermal characterization technique for studying the anisotropic thermal properties of materials. The original factors of the MOKE signal are revealed for four magnetic transducers, including TbFe, GdFeCo, Co/Pd, and CoFe/Pt. A figure of merit is proposed to evaluate the performance of magnetic transducers by examining the improvement of the signal-to-noise ratio (SNR) in TR-MOKE measurements. Excellent agreement between experimental TR-MOKE signals and computed figure of merit is achieved for all four magnetic transducers. We observe the best SNR for TR-MOKE measurements with rare-earth transition metal (RE-TM)-based TbFe and GdFeCo transducers. As an example, we conduct TR-MOKE measurements on a model system of single crystalline black phosphorus (BP) flakes using the optimal TbFe transducer. The measured anisotropic thermal conductivity of BP along three primary crystalline directions well corroborates first-principles predictions. Further, we demonstrate that TR-MOKE, compared with the standard time-domain thermoreflectance, offers an enhanced measurement sensitivity to the in-plane thermal conductivity of materials, and meanwhile is immune to other uncertainties in the transducer properties. Thus, the overall uncertainties of TR-MOKE for in-plane thermal measurements can be significantly improved.