The microelectronics industry faces relentless challenges to meet ever-growing demands for denser, faster, and more energy-efficient computing at lower cost. Born out of a Nobel winning physics discovery, magnetic random access memory (MRAM) uses electron spins, instead of charge, to store information. MRAM is inherently fast, enduring, and non-volatile, a combination unparalleled by existing charge-based memory technologies. Spin-orbit torque (SOT) is the leading technology to achieve low-energy magnetization switching for next-generation MRAM. In SOT, a charge current applied through a channel with strong spin-orbit coupling produces a transverse spin current that applies a torque to switch an adjacent ferromagnetic (FM) layer. Conventional 3D magnets, in which atoms are strongly bonded in all directions, have proven difficult to integrate at the ultrathin limit due to insufficiently clean interfaces and unreliable spin transparency. In contrast, 2D magnets have the potential to provide advantageous reductions in magnetic damping, total magnetic moment, and sample volume compared to 3D magnets.
Previous work highlights:
Discovery of room-temperature intrinsic monolayer ferromagnet in epitaxial Manganese Selenide film by molecular beam epitaxy (MBE)
We achieve the first experimental realization of large-area room-temperature monolayer intrinsic ferromagnetism in epitaxial Manganese Selenide films grown by MBE. Magnetic and structural characterizations show that in the monolayer limit, the ferromagnetism originates from a vdW manganese diselenide (MnSe2) monolayer, while for thicker films it originates from a combination of vdW MnSe2 and/or interfacial magnetism of α-MnSe(111). SQUID measurements of monolayer MnSex films on both GaSe and SnSe2 base layers show a ferromagnetic ordering with a large saturation magnetization of ∼4 Bohr magnetons per Mn.
Future directions:
High-efficiency SOT based on topological insulators and Weyl semimetals
Quantum materials, such as topological insulators (TI) and Weyl semimetals, provide unique opportunities to manipulate spin degrees of freedom, including the generation of strong spin currents and control of their polarization direction. TI has been shown to have the highest SOT efficiency to date2. In TI/FM systems, the SOTs have contributions from both the bulk spin Hall effect and the interfacial Rashba-Edelstein effect, where the latter is due to spin-momentum locking of the topological Dirac surface state. A strong surface state contribution is proposed for the enhanced spin Seebeck effect and enhanced Gilbert damping. These studies also motivate the use of Weyl semimetals for efficient SOT. We will plan to investigate key questions such as: What is the origin of the large spin Hall conductivity observed in topological insulators? Will the bulk Dirac cones with spin-momentum locking in Weyl semimetals generate even larger spin currents? How does the reduced symmetry of Weyl semimetals generate out-of-plane polarized spin currents?
High-resolution, accurate space-time SOT readout using magneto-optical techniques
Reliable and accurate measurements of SOT are necessary to address many of the questions above. Optical readout using the magneto-optical Kerr (MOKE) effect provides a unique advantage as it can avoid artifacts from transport techniques such as thermoelectric voltages, nonlinear-in-current Hall resistances, and modification of transport coefficients by magnons or heating. We will continue to advance the resolutions of magneto-optical microscopy in the Kerr sensitivity, temporal, and spatial domains by developing near-field MOKE microscopy and ultrafast Sagnac interferometry. These pioneering techniques will position our group uniquely to image the atomic-scale domains and disorders in low dimensional magnetic systems. In addition, ultrafast optics such as TRKR with THz resolution is a natural and powerful platform to investigate antiferromagnetic dynamics, where much faster magnetic resonance and switching rates are expected that are beyond the capacity of any transport measurement.
References: [1] L. Liu, C. -F. Pai, Y. Li, H. W. Tseng, D. C. Ralph, and R. A. Buhrman, “Spin-Torque Switching with the Giant Spin Hall Effect of Tantalum“, Science, 336, 555 (2022). [2] A. R. Mellnik, J. S. Lee, A. Richardella, J. L. Grab, P. J. Mintun, M. H. Fischer, A. Vaezi, A. Manchon, E.-A. Kim, N. Samarth, and D. C. Ralph, “Spin-transfer torque generated by a topological insulator“, Nature, 511, 449 (2014)