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Physics & Astronomy Colloquium – Fall 2018 -Majetich
October 31, 2018 @ 3:30 pm - 5:00 pm
Speaker: Sara Majetich (Carnegie Mellon University)
Title: Conductive Atomic Force Microscopy of Magnetic Tunnel Junctions
Abstract: Conductive atomic force microscopy (C-AFM) enables scanning magnetoresistance measurements of magnetic tunnel junction (MTJ) nanostructures. It can be used to image the tunnel current in an array of MTJs, which is beneficial both for studying size-dependent behavior and also for characterizing the distribution of switching properties among devices of the same size. The resistance of individual MTJs can be controlled either by a magnetic field, or electrically with a bias voltage or current.
Voltage-controlled magnetic anisotropy is a feature of MTJs with perpendicular magnetization due to thin CoFeB layers on either side of the MgO tunnel barrier. MTJs ranging from 18 to 500 nm were characterized as a function of magnetic field to determine the effective anisotropy of the free layer. When the free layer was metastable, the tunnel current showed random telegraph noise. Reversal occurs by nucleation and domain wall motion. Due to the magnetostatic field of the fixed layer, nucleation for an antiparallel to parallel switch occurs most often near the edge of the free layer.
A charge current passing through a heavy metal beneath the MTJ generates a spin orbit torque that can switch the adjacent magnetic layer. While magnetic random access memory (MRAM) devices based on this switching mechanism would ideally be sub-100 nm and would have a high thermal stability factor (60-80), measurements of spin orbit torque switching have focused on larger structures with lower thermal stability. Here we show how CAFM can detect reversal of a 20 nm nanomagnet with a thermal stability factor of 85. From our results, the charge current density needed for spin orbit torque switching is estimated to by only 15% of that needed for spin transfer torque reversal, and has an estimated write energy of 0.1 fJ. This approach is promising for low power MRAM.
A third type of tunnel junction uses thicker CoFeB layers and has in-plane magnetization. These MTJs are interesting for probabilistic computing because an applied bias voltage or current can stimulate superparamagnetic behavior with zero applied magnetic field. We show how a voltage can tune the time-averaged resistance of the MTJ, and how output from multiple MTJs can be used in logic gates and simple arithmetic.
Bio: Sara Majetich (Homepage) a Professor in physics department with a courtesy appointment in the department of electrical and computer engineering at Carnegie Mellon University in Pittsburgh, Pennsylvania, USA. She received her A.B. degree from Princeton University, a masters degree in from Columbia University, and a Ph.D. from the University of Georgia. Following postdoctoral work at Cornell University, she joined the faculty at Carnegie Mellon in 1990. Her research interests are in magnetic nanoparticles and nanostructures, including both materials preparation and characterization, for potential applications in data storage media, biomedicine, high frequency inductors, permanent magnets, and magnetic refrigeration. She has authored over 150 papers and has three patents. She has received a National Young Investigator Award from the US National Science Foundation for her work, and in 2007 was a Distinguished Lecturer of the IEEE Magnetics Society. She is also a Fellow of the American Physical Society and the IEEE.