2018 Physics/Theoretical Colloquium
 
Thursday, April 19th, 2018
3:45 – 4:45 p.m.
Rosen Auditorium (TA-53, Bldg. 1)
Refreshments at 3:15pm
 
Speaker: Dr. Olle Heinonen
  
Argonne National Laboratory and the Northwestern-Argonne Institute for Science and Engineering, Northwestern University
 
 
 
“Nanomagnets”
 
Abstract: 
 
My research is in the areas of computational materials science and nanoscale magnetic systems. I will start with a brief overview of magnetism, and then spend most of the time talking about nanoscale magnetic systems. These have been the subject of intense research over the past decade. This has partly been driven by developments in magnetic hard disc drives, where the readback sensor is now smaller than 50 nm x 50 nm(!), but partly also driven by new emergent physics in nanoscale magnetic systems. One such example is the spin transfer torque effect, by which a dc current can induce magnetization dynamics in the GHz range. In nanoscale magnetic systems, interactions can be also be made to compete, giving rise to complex energy surfaces with interesting (and sometimes unexpected) quasi-static and dynamical behavior. One example is given by effects that arise in the presence of the Dzyaloshinskii-Moriya interaction. This interaction may appear inversion symmetry and gives rise to structures such as skyrmions.
 
In this presentation, I will give some background on materials and interactions in these systems, and a brief overview of the spin transfer torque effect. I will then give some examples of specific systems, such as artificial spin ice lattices and spin torque oscillators, and the complex and interesting behavior they exhibit, as well as an overview of skyrmions in thin film systems.
 
Frequency versus time of a spin torque oscillator: The top figure shows that frequency is stable over a long time. The bottom figure shows that the frequency is hopping with time (mode hopping). The phenomena depends on the orientation of the magnetic layers as shown in the right.