Magnetic and electrical-thermal transport properties of Co<sub>3</sub>Sn<sub>2</sub>S<sub>2</sub> single crystal
Authors: Zhu Xin-Qiang , Wang Jian , Zhu Can , Luo Feng , Chen Shu-Quan , Xu Jia-Hui , Xu Feng , Wang Jia-Fu , Zhang Yan , Sun Zhi-Gang ,   State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China,   College of Science, Wuhan University of Technology, Wuhan 430070, China,   College of Material Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, China
Published: July 2023

Co<sub>3</sub>Sn<sub>2</sub>S<sub>2</sub> is a magnetic Weyl semimetal with special magnetic and electronic structure. Its unique band structure makes it have many interesting physical properties such as abnormal Hall effect, negative magnetoresistance effect, and abnormal Nernst effect. In this work, high quality Co<sub>3</sub>Sn<sub>2</sub>S<sub>2</sub> single crystal with a dimension of 8 mm×7 mm×0.5 mm is synthesized by self-flux method. We measure its electrical transport properties (magnetoresistance effect, Hall effect, etc.) and thermal transport properties (Seebeck effect) at low temperature. The free surface of the single crystal exhibits obvious layered growth characteristics, indicating that the Co<sub>3</sub>Sn<sub>2</sub>S<sub>2</sub> crystal grows along the <i>c</i>-axis direction. The value of the normalized resistivity <i>ρ</i><sub>3 K</sub>/<i>ρ</i><sub>300 K</sub> for the single crystal sample at 3 K is only 0.08, indicating that the crystal quality of the sample is at a relatively high level. The thermomagnetic (<i>M</i>-<i>T</i>) curves show that a special magnetic structure near 140 K (<i>T</i><sub>A</sub>) below the Curie temperature point (<i>T</i><sub>C</sub> = 178 K). This special magnetic structure is a magnetic transition state in which ferromagnetic state and antiferromagnetic state coexist, making them appear as a local minimum point in the <i>M</i>-<i>T</i> curve. The Co<sub>3</sub>Sn<sub>2</sub>S<sub>2</sub> shows a negative anomalous “convex” magnetoresistance in a large range of 100—160 K, and there exists a maximum critical magnetic field <i>B</i><sub>0</sub> (1.41 T), near <i>T</i><sub>A</sub>. The coercivity <i>H</i><sub>C</sub> drops to almost zero at <i>T</i><sub>A</sub> and the Hall resistivity <i>ρ</i><sub><i>yx</i></sub> also reaches a maximum value of about 20 μΩ·cm. This may be due to the competition between ferromagnetic state and antiferromagnetic state to form non-trivial spin texture, resulting in the unique electrical transport behavior near <i>T</i><sub>A</sub>. When the temperature rises to <i>T</i><sub>C</sub>, the Co<sub>3</sub>Sn<sub>2</sub>S<sub>2</sub> undergoes a ferromagnetic phase transition, with a saturation magnetization of <i>M</i><sub>S,</sub> anomalous Hall conductivity <inline-formula><tex-math id="M1">\begin{document}$ {\sigma }_{yx}^{\rm A} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20230621_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20230621_M1.png"/></alternatives></inline-formula>, and Hall resistivity <i>ρ</i><i><sub>yx</sub></i> sharply decreasing. Large anomalous Hall conductance <inline-formula><tex-math id="M2">\begin{document}$ {\sigma }_{yx}^{A} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20230621_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20230621_M2.png"/></alternatives></inline-formula> and anomalous Hall angle <inline-formula><tex-math id="M3">\begin{document}$ {\sigma }_{yx}^{\rm A}/\sigma $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20230621_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20230621_M3.png"/></alternatives></inline-formula> are also present in Co<sub>3</sub>Sn<sub>2</sub>S<sub>2</sub>, with these values reaching 1.4×10<sup>3</sup> Ω<sup>−1</sup>·cm<sup>−1</sup> and 18%, respectively. The magnetoresistance measurements reveal that the variation of the magnetoresistance with the magnetic field is due to the combination of linear and parabolic contributions. The change in magnetoresistance with the angle <i>θ</i> between the magnetic field <i>B</i> and the current <i>I</i> has a reversal symmetry with C<sub>2<i>x</i></sub> symmetry, and the change in <i>θ</i> does not affect the contribution of its magnetoresistance source. In addition, positive magnetoresistance and negative magnetoresistance are found to be shifted at about 60 K. the shift in positive magnetoresistance and negative magnetoresistance are mainly attributed to the competing positive contribution of the Lorentz force to the magnetoresistance and the negative contribution of the spin disorder. The scattering mechanism of Co<sub>3</sub>Sn<sub>2</sub>S<sub>2</sub> at low temperature is a combination of acoustic wave scattering and electron– phonon scattering. At 60–140 K, the enhancement of spin disorder causes enhanced electron–phonon scattering, resulting in a plateau characteristic of the Seebeck coefficient <i>S</i>. The research shows that the special magnetic structure and electron spin of Co<sub>3</sub>Sn<sub>2</sub>S<sub>2</sub> at low temperatures have an important influence on its electrothermal transport behavior.