Spin Hall Magnetoresistance and Quadratic Magneto-Optic Studies of Antiferromagnets

Antiferromagnets with two sets of spins against each other pushes the operation of spintronics memory frequencies up to terahertz with a higher areal density, compared with the traditional ferromagnets due to the absence of the stray field and the strong coupling among spins. However, the net zero magnetic moment makes it difficult in probing and controlling the spin state in antiferromagnet. In this presentation, we discuss electrical and optical approaches to detect and manipulate spins, which pave the way for utilizing antiferromagnet in novel spintronic devices.<br/>First, we will discuss the use of magneto-transport or unidirectional spin Hall magnetoresistance (USMR) in Fe₂O₃ bilayer AFM bilayer. USMR has been widely reported in the heavy metal / ferromagnet (HM/FM) bilayer systems. We observed USMR in Pt/α-Fe₂O₃ bilayers where the α-Fe₂O₃ is an antiferromagnetic (AFM) insulator. Systematic field and temperature-dependent measurements confirm the magnonic origin of the USMR. The appearance of AFM-USMR is driven by the imbalance of creation and annihilation of AFM magnons by spin-orbit torque due to the thermal fluctuation field. Theoretical modeling reveals that the USMR in Pt/α-Fe₂O₃ is determined by the antiferromagnetic magnon number with a non-monotonic field dependence as compared with that of ferromagnetic materials. The magnonic origin of USMR is further confirmed by temperature-dependent measurements. This first evidence of USMR in other HM/AFM bilayers may be expanded to include the large family of AF insulators and paves the way for the highly sensitive detection of AF spin states in emerging the AF spintronics.<br/>Second, we will discuss the use of the quadratic magnetic-optical Kerr effect (QMOKE) to study dynamics of several AFMs. We will describe the experiments of e.g., α-MnTe; This antiferromagnet has a hexagonal close-packed structure with antiferromagnetic ordering along the c-axis. It is a collinear antiferromagnet with three in-plane easy axes, and its Néel temperature is approximately 310K [1,2], which is also known recently as an altermagnet, hosting anti-ferromagnetism in macroscopic scale and ferromagnetism in k space [3,4]. The hexagonal NiAs phase of MnTe (α-MnTe) is obtained by MBE on GaAs. We employ the time resolved pump-probe laser quadratic magnetic-optical Kerr effect (QMOKE) to study MnTe. The QMOKE of α-MnTe is more visible due to the spin splitting of electronic states in the k space. The pump process puts the Néel vector deviate from its equilibrium state. The pump results in the loss of the Néel order, referred to as the fast demagnetization, which endures for about 20 ps; the energy is redistributed between the electron, phonon and the magnon. This dynamics of the change of the Néel order is investigated through quadratic MOKE, reflecting the magnon dynamics. The dynamic study will also be used for other materials to compare the differences of the AFM dynamics.<br/>[1] Ferrer-Roca, C., Segura, A., Reig, C., & Munoz, V. (2000). Temperature and pressure dependence of the optical absorption in hexagonal MnTe. Physical Review B, 61(20), 13679.<br/>[2] Yin, G., Yu, J. X., Liu, Y., Lake, R. K., Zang, J., & Wang, K. L. (2019). Planar Hall effect in antiferromagnetic MnTe thin films. Physical review letters, 122(10), 106602.<br/>[3] Mazin, I. I. (2023). Altermagnetism in MnTe: Origin, predicted manifestations, and routes to detwinning. Physical Review B, 107(10), L100418.<br/>[4] Gonzalez Betancourt R.D. et al, Spontaneous Anomalous Hall Effect Arising from an Unconventional Compensated Magnetic Phase in a Semiconductor, Phys. Rev. Lett. 130, 036702(2023).