Magnetotransport studies of two-dimensional magnetism in van der Waals materials

• Main Author: Ye, Chen
• Other Authors: Wang Xiao, Renshaw
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Science::Physics::Electricity and magnetism; Engineering::Materials::Magnetic materials
• Online Access: https://hdl.handle.net/10356/160958
• Institution: Nanyang Technological University

From ancient compass to modern spintronics, magnetism has had a pivotal role in fundamental research and technological advance for millennia. To fulfill the exploded information introduced by the rapid development of artificial intelligence, extensive efforts have been paid to reduce the dimensionality of magnetic materials. The discovery of two-dimensional (2D) magnetism in van der Waals (vdW) materials, where each atomic layer vertically stacks and experiences vdW interaction, enables the long-range magnetic ordering to emerge at the monolayer limit. Exotic magnetotransport phenomena with great sensitivity toward varying parameters in the magnetic vdW materials provide unique insights into 2D magnetism. The universal compatibility to dissimilar materials without lattice-matching constraints also enables the interplay between 2D magnetism with other physical properties. This can be leveraged to the highly demanded electronic and spintronic devices with miniaturized circuits and ultimate performance. Hence, my research focuses on magnetotransport studies of 2D magnetic vdW materials to reveal the novel phenomena in reduced dimensions and the underlying mechanism. The investigation is composed of three parts, namely (i) searching for new candidates to enrich the 2D magnetic family, (ii) revealing the magnetic order governed by fundamental physics, and (iii) investigating the interplay of magnetism and topology. First, I studied the magnetism in 2D magnetic crystals FeTe, whose structural phase can be tuned during the chemical vapour deposition. By altering the growth temperature, the antiferromagnetic FeTe in the tetragonal phase and ferromagnetic FeTe in the hexagonal phase were fabricated and the magnetic order can be retained down to 3.6 and 2.8 nm, respectively. I experimentally characterized the layer-dependent magnetic properties in FeTe flakes with different phases through magnetotransport measurements. For the antiferromagnetic phase, the Néel temperature experiences a decrease from 70 to 45 K when the thickness declines from 32 to 5 nm. For the ferromagnetic phase, the Curie temperature also decreases from 220 to 170 K when the thickness decreases from 30 to 4 nm. Second, I demonstrated the layer-dependent interlayer antiferromagnetic spin reorientation in an air-stable semiconductor CrSBr. The semiconducting antiferromagnetic order survives down to a bilayer thickness with the Néel temperature of 140 K. Using magnetotransport measurements and the antiferromagnetic linear-chain model, I revealed an unprecedented odd-even layer effect in the spin-flop transition, which stems from the competition of internal energy components composed of interlayer exchange coupling, magnetic anisotropy and Zeeman energy. Moreover, I quantitatively construct the diagram of layer-dependent antiferromagnetic textures and their responses to the magnetic field. The symmetric and asymmetric textures during the spin reorientation observed in the even- and odd-layer CrSBr were introduced by the energy competition. Last, I studied the nonreciprocal response of a bilayer composed of intrinsic magnetic topological insulator MnBi2Te4 and nonmagnetic Pt. I found that the nonreciprocal resistance is responsive to both magnetic field and electrical current, and the nonreciprocal resistance only emerges when the temperature is below the Néel temperature of MnBi2Te4, confirming a correlation between magnetism and nonreciprocity. Furthermore, the angular dependence of nonreciprocal transport proves the existence of asymmetric scattering of electrons at the surface of MnBi2Te4 mediated by magnon. I also achieved the electrically switching of uncompensated magnetization at the surface with a current density of 9.26 x 10^11 A m^-2.