Magneto-optical and magneto-transport studies of 2D CrBr3 and MnBi2Te4

• Main Author: Zhang, Zhaowei
• Other Authors: Gao Weibo
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Science::Physics::Electricity and magnetism
• Online Access: https://hdl.handle.net/10356/154931
• Institution: Nanyang Technological University

Two-dimensional ferromagnetic materials show different properties compared with their bulk counterparts. The fundamental origins are reduced dimensionality and crystal symmetry. Atomically thin van der Waals ferromagnets have shown their advantages in highly tunable magnetism, uniform surface and flexibility. The realization of two-dimensional ferromagnetism not only inspires the study of fundamental physics, but also leads to new applications in spintronics and optoelectronics. Utilizing polarization resolved magneto-photoluminescence, we studied ferromagnetism in two-dimensional insulating CrBr3. We attributed the strong photoluminescence in CrBr3 to the d-d transition. The Curie temperature of the monolayer is around 34 K, which is very close to that of the bulk. Ferromagnetic interlayer coupling is observed in layer-number dependent measurements. Our study on two-dimensional ferromagnetism in CrBr3 has potential applications in van der Waals spintronics. The magnetic proximity effect in two-dimensional graphene/CrBr3 heterostructures is observed. The Zeeman spin Hall effect probed by non-local measurements clearly indicates the strong magnetic proximity effect. The strength of the magnetic proximity effect is estimated by measuring Zeeman splitting field, which shows a significant magnetic proximity field even in a low magnetic field. More importantly, the anomalous longitudinal resistance at the Dirac point RXX;D is dependent on the magnetic field near filling factor of 0, which is ascribed to the magnetic proximity induced new ground state phases. The observation of the magnetic proximity effect in the graphene/CrBr3 heterostructures reveals strong magnetic coupling between van der Waals materials and thus provides guidance for designing next-generation 2D spin logic and memory devices. Nonreciprocal charge transport was observed in 5-SL MnBi2Te4. Though the crystal structure is inversion symmetric, the backscattering of charge carriers at the chiral edge is asymmetric, leading to directional magnetoresistance. We use the Al2O3 assisted method to fabricate MnBi2Te4 thin flakes. After estimating the thickness of obtained MnBi2Te4 thin flakes by optical contrast, the 5-SL sample is selected. The metal electrodes are then fabricated by a stencil mask. The quantum anomalous Hall effect at high magnetic field is demonstrated, which confirmed the chiral edge channel. The second-order magnetoresistance is used to study the nonreciprocal charge transport and dependence on the magnetic field shows clear hysteresis loops due to the finite net magnetic moment at zero magnetic field. The chirality of the hysteresis loop is tuned by applying the gate voltages, which results from the difference in scattering rate between electrons and holes. The study of nonreciprocal charge transport paves the way to develop new electronic devices using magnetic topological van der Waals materials.