Daniel Ralph

Overview

New nanofabrication techniques; spin transport and high-speed dynamics in magnetic devices; electron and spin states in magnets, superconductors, and 2D layered materials.

Research Focus

Our group's research focuses on the electronic, magnetic, and optical properties of nm-scale samples. The work in the group consists of making nanometer-size devices using equipment at the Cornell NanoScale Science Technology Facility (CNF), and then performing a wide variety of measurements in Clark Hall.  

In collaboration with the groups of Greg Fuchs, Katja Nowack, Farhan Rana, and several groups outside Cornell, we are investigating the “spin-transfer torque effect.”  This is a phenomenon by which the magnetic orientation of a small magnet can be manipulated by transferring angular momentum from a current of spin-polarized electrons, rather than by using magnetic fields.  We are using the spin torque as a tool for studying the fundamental physics of ferromagnetic and antiferromagnetic dynamics. This project is also progressing quickly toward applications for magnetic memory devices and high-speed signal processing.

Some of the most recent discoveries made by the group involve new and more efficient techniques for generating spin currents that can be used to exert spin transfer torques.  These include the spin Hall effect in certain heavy metals and the Rashba-Edelstein effect generated by topological insulators.  Several current experiments are aimed at better understanding and optimizing these phenomena.

The group is also in the process of starting new projects, including studies of spin and optical effects in devices containing graphene and other 2-dimensional atomic-layer materials.  Recent progress includes the development of Sagnac optical interferometry for studying magnetic systems and the demonstration of quantized anomalous Hall conductance in topological insulator/magnet samples at much higher temperatures than any other system.

Graduate Students
Maciej Olszewski, Bozo Vareskic, Daniel Pharis, Matthew Roddy, Orion Smedley

Postdocs
Xiaoxi Huang

Awards and Honors

• Lester B. Knight Director, Cornell NanoScale Science & Technology Facility (CNF) 2010-2016  
• Physics Chair 2021-2024
• Alfred P. Sloan Fellow, 1996-99
• David and Lucile Packard Foundation Fellow, 1997-2002
• William L. McMillan Award, 1997
• James C. McGroddy Prize for New Materials, 2022
• Member, Kavli Institute at Cornell

Professional Experience

• Postdoctoral Research Associate, Harvard University, 1993-96. 
• Assistant Professor, Physics, Cornell University, 1996-2000. 
• Associate Professor, Physics, Cornell University, 2000-2004. 
• Professor, Physics, Cornell University, 2004-2008
• Horace White Professor of Physics, Cornell University, 2008-2013. 
• F.R. Newman Professor of Physics, Cornell University, 2013-present. 

Publications

• Tilted spin current generated by the collinear antiferromagnet ruthenium oxide, Arnab Bose, Nathaniel J. Schreiber, Rakshit Jain, Ding-Fu Shao, Hari P. Nair, Jiaxin Sun, Xiyue S. Zhang, David A. Muller, Evgeny Y. Tsymbal, Darrell G. Schlom, and Daniel C. Ralph, Nature Electronics 5, 267-274 (2022).
• Controlling spin current polarization through non-collinear antiferromagnetism, Tianxiang Nan, Camilo X. Quintela, Julian Irwin, Gautam Gurung, Ding-Fu Shao, J. Gibbons, N. Campbell, Kuyngjun Song, S.-Y. Choi, Lu Guo, Roger D Johnson, Pascal Manuel, Rajesh V. Chopdekar, Ingrid Hallsteinsen, T. Tybell, Philip J. Ryan, J.-W. Kim, Yongseong Choi, Paolo G. Radaelli, Daniel C. Ralph, Evgeny Yu. Tsymbal, Mark S. Rzchowski, and Chang Beom Eom, Nature Communications 11, 4671 (2020).
• Probing and controlling magnetic states in 2D layered magnetic materials, Kin Fai Mak, Jie Shan, and D. C. Ralph, Nature Reviews Physics 1, 646-661 (2019).
• Control of spin-orbit torques through crystal symmetry in WTe₂/ferromagnet bilayers, D. MacNeill, G. M. Stiehl, M. H. D. Guimaraes, R. A. Buhrman, J. Park, and D. C. Ralph, Nature Physics 13, 300-305 (2017).
• Breaking of valley degeneracy breaking by magnetic field in monolayer MoSe₂, D. MacNeill, C. Heikes, K. F. Mak, Z. Anderson, A. Kormányos, Viktor Zólyomi, J. Park, and D. C. Ralph, Phys. Rev. Lett. 114, 037401 (2015).
• Spin-transfer torque generated by a topological insulator, 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, Nature 511, 449-451 (2014).
• Deterministic switching of ferromagnetism at room temperature using an electric field, J. T. Heron, J. L. Bosse, Q. He, Y. Gao, M. Trassin, J. D. Clarkson, C. Wang, J. Liu, S. Salahuddin, D. C. Ralph, D. G. Schlom, J. Íñiguez, B. D. Huey, and R. Ramesh, Nature 516, 370-373 (2014).
• Spin torque switching with the giant spin Hall effect of tantalum, Luqiao Liu, Chi-Feng Pai, Y. Li, H. W. Tseng, D. C. Ralph, and R. A. Buhrman, Science 336, 555-558 (2012).
• Spin Torque Ferromagnetic Resonance Induced by the Spin Hall Effect, Luqiao Liu, Takahiro Moriyama, D. C. Ralph, and R. A. Buhrman, Phys. Rev. Lett. 106, 036601 (2011).
• Mechanical Control of Spin States in Spin-1 Molecules and the Underscreened Kondo Effect, J. J. Parks, A. R. Champagne, T. A. Costi, W. W. Shum, A. N. Pasupathy, E. Neuscamman, S. Flores-Torres, P. S. Cornaglia, A. A. Aligia, C. A. Balseiro, G. K.-L. Chan, H. D. Abruña, and D. C. Ralph, Science 328, 1370-1373 (2010).
• Measurement of the Spin-Transfer-Torque Vector in Magnetic Tunnel Junctions, J. C. Sankey, Y.-T. Cui, J. Z. Sun, J. C. Slonczewski, R. A. Buhrman, and D. C. Ralph, Nature Physics 4, 67-71 (2008).
• Magnetic vortex oscillator driven by dc spin-polarized current, V. S. Pribiag, I. N. Krivorotov, G. D. Fuchs, P. M. Braganca, O. Ozatay, J. C. Sankey, D. C. Ralph, and R. A. Buhrman, Nature Physics 3, 498-503 (2007).
• The Kondo effect in the presence of ferromagnetism, A. N. Pasupathy, R. C. Bialczak, J. Martinek, J. E. Grose, L. A. K. Donev, P. L. McEuen, and D. C. Ralph, Science 306, 86-89 (2004).
• Microwave oscillations of a nanomagnet driven by a spin-polarized current, S. I. Kiselev, J. C. Sankey, I. N. Krivorotov, N. C. Emley, R. J. Schoelkopf, R. A. Buhrman, and D. C. Ralph, Nature 425, 380-383 (2003).
• Coulomb Blockade and the Kondo Effect in Single Atom Transistors, Jiwoong Park, A. N. Pasupathy, J. I. Goldsmith, C. Chang, Y. Yaish, J. R. Petta, M. Rinkoski, J. P. Sethna, H. D. Abruña, P. M. McEuen, and D. C. Ralph, Nature 417, 722-725 (2002).
• Current-Driven Magnetization Reversal and Spin-Wave Excitations in Co/Cu/Co Pillars, J. A. Katine, F. J. Albert, R. A. Buhrman, E. B. Myers, and D. C. Ralph, J. A. Katine, R. N. Louie, and R. A. Buhrman, Phys. Rev. Lett. 84, 3149 (2000).

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