MSGID: 1:317/3 6274a48e   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Mechanism 'splits' electron spins in magnetic material    
      
     Date:   
         May 5, 2022   
     Source:   
         Cornell University   
     Summary:   
         Holding the right material at the right angle, researchers have   
         discovered a strategy to switch the magnetization in thin layers   
         of a ferromagnet -- a technique that could eventually lead to the   
         development of more energy-efficient magnetic memory devices.   
      
      
      
   FULL STORY   
   ==========================================================================   
   Holding the right material at the right angle, Cornell researchers have   
   discovered a strategy to switch the magnetization in thin layers of a   
   ferromagnet -- a technique that could eventually lead to the development   
   of more energy-efficient magnetic memory devices.   
      
      
   ==========================================================================   
   The team's paper, "Tilted Spin Current Generated by the Collinear   
   Antiferromagnet Ruthenium Dioxide," published May 5 in Nature   
   Electronics. The paper's co-lead authors are postdoctoral researcher   
   Arnab Bose and doctoral students Nathaniel Schreiber and Rakshit Jain.   
      
   For decades, physicists have tried to change the orientation of electron   
   spins in magnetic materials by manipulating them with magnetic fields. But   
   researchers including Dan Ralph, the F.R. Newman Professor of Physics   
   in the College of Arts and Sciences and the paper's senior author, have   
   instead looked to using spin currents carried by electrons, which exist   
   when electrons have spins generally oriented in one direction.   
      
   When these spin currents interact with a thin magnetic layer, they   
   transfer their angular momentum and generate enough torque to switch   
   the magnetization 180 degrees. (The process of switching this magnetic   
   orientation is how one writes information in magnetic memory devices.)   
   Ralph's group has focused on finding ways to control the direction of   
   the spin in spin currents by generating them with antiferromagnetic   
   materials. In antiferromagnets, every other electron spin points in the   
   opposite direction, hence there is no net magnetization.   
      
   "Essentially, the antiferromagnetic order can lower the symmetries of the   
   samples enough to allow unconventional orientations of spin current to   
   exist," Ralph said. "The mechanism of antiferromagnets seems to give a way   
   of actually getting fairly strong spin currents, too."  The team had been   
   experimenting with the antiferromagnet ruthenium dioxide and measuring   
   the ways its spin currents tilted the magnetization in a thin layer of a   
   nickel-iron magnetic alloy called Permalloy, which is a soft ferromagnet.   
      
   In order to map out the different components of the torque, they measured   
   its effects at a variety of magnetic field angles.   
      
      
      
   ==========================================================================   
   "We didn't know what we were seeing at first. It was completely different   
   from what we saw before, and it took us a lot of time to figure out what   
   it is," Jain said. "Also, these materials are tricky to integrate into   
   memory devices, and our hope is to find other materials that will show   
   similar behavior which can be integrated easily."  The researchers   
   eventually identified a mechanism called "momentum-dependent spin   
   splitting" that is unique to ruthenium oxide and other antiferromagnets   
   in the same class.   
      
   "For a long time, people assumed that in antiferromagnets spin up and   
   spin down electrons always behave the same. This class of materials is   
   really something new," Ralph said. "The spin up and spin down electronic   
   states essentially have different dependencies. Once you start applying   
   electric fields, that immediately gives you a way of making strong spin   
   currents because the spin up and spin down electrons react differently. So   
   you can accelerate one of them more than the other and get a strong spin   
   current that way."  This mechanism had been hypothesized but never before   
   documented. When the crystal structure in the antiferromagnet is oriented   
   appropriately within devices, the mechanism allows the spin current to   
   be tilted at an angle that can enable more efficient magnetic switching   
   than other spin-orbit interactions.   
      
   Now, Ralph's team is hoping to find ways to make antiferromagnets in   
   which they can control the domain structure -- i.e., the regions where   
   the electrons' magnetic moments align in the same direction -- and study   
   each domain individually, which is challenging because the domains are   
   normally mixed.   
      
      
      
   ==========================================================================   
   Eventually, the researchers' approach could lead to advances in   
   technologies that incorporate magnetic random-access memory.   
      
   "The hope would be to make very efficient, very dense and nonvolatile   
   magnetic memory devices that would improve upon the existing silicon   
   memory devices," Ralph said. "That would allow a real change in the   
   way that memory is done in computers because you'd have something with   
   essentially infinite endurance, very dense, very fast, and the information   
   stays even if the power is turned off. There's no memory that does that   
   these days."  Co-authors include former postdoctoral researcher Ding-Fu   
   Shao; Hari Nair, assistant research professor of materials science and   
   engineering; doctoral students Jiaxin Sun and Xiyue Zhang; David Muller,   
   the Samuel B. Eckert Professor of Engineering; Evgeny Tsymbal of the   
   University of Nebraska; and Darrell Schlom, the Herbert Fisk Johnson   
   Professor of Industrial Chemistry.   
      
   The research was supported by the U.S. Department of Energy, the Cornell   
   Center for Materials Research (CCMR), with funding from the National   
   Science Foundation's Materials Research Science and Engineering Center   
   program, the NSF-supported Platform for the Accelerated Realization,   
   Analysis and Discovery of Interface Materials (PARADIM), the Gordon and   
   Betty Moore Foundation's EPiQS Initiative, and the NSF's Major Instrument   
   Research program.   
      
   The devices were fabricated using the shared facilities of the Cornell   
   NanoScale Science and Technology Facility and CCMR.   
      
      
   ==========================================================================   
   Story Source: Materials provided by Cornell_University. Original written   
   by David Nutt, courtesy of the Cornell Chronicle. Note: Content may be   
   edited for style and length.   
      
      
   ==========================================================================   
   Journal Reference:   
      1. 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, Daniel C. Ralph. Tilted spin current   
         generated by the collinear antiferromagnet ruthenium dioxide. Nature   
         Electronics, 2022; DOI: 10.1038/s41928-022-00744-8   
   ==========================================================================   
      
   Link to news story:   
   https://www.sciencedaily.com/releases/2022/05/220505143823.htm   
      
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