Colloquium
Department of Physics, NCU
Spintronics: from magnetoresistance to spin transfer torque
Speaker
Prof. Hiroyoshi Itoh
Department of Pure and Applied Physics, Kansai University, Japan
Date 2019.05.21(Tue)
Time 14:00-16:00
Place S4-625
Spintronics, also known as spin-electronics or magneto-electronics, is an emerging field of basic and applied research in electronics. It utilizes the spin of electrons in addition to the charge and has provided new physics (magnetoresistance, spin Hall effect, spin transfer torque, etc.) and new functional devices (read head of HDD, magnetic sensor, memory, etc.).
The development of spintronics is triggered by the discovery of the giant magnetoresistance (GMR) in the 1980s, in which the electrical resistance of nano-scaled magnetic multilayers, e.g. Fe/Cr system, changes a lot as the magnetization alignment of the adjacent magnetic layers separated by a thin nonmagnetic spacer varies by applying a magnetic field. GMR revealed that the magnetic structure affects significantly the electrical current or transport properties of the system. In the 2000s, the current-induced domain wall motion in magnetic nano-wires and the current-induced magnetization reversal in GMR systems showed that the electrical current also affects significantly the magnetic structure via spin-transfer torque. Now, magnetic random access memory (MRAM) comprised of magnetic tunnel junction (MTJ), which is expected to replace DRAM and eFlash memory, is being developed by semiconductor companies such as TSMC (Taiwan), Samsung (Korea), Globalfoundries (USA), Toshiba Memory (Japan), and so on.
After an introductory talk about spintronics, I will show you some of our recent works on magnetoresistance and spin-transfer torque. As for the tunnel magnetoresistance (TMR), we consider MTJ of FM/ NM/ FI/ NM where FM, NM, and FI are ferromagnetic metal, nonmagnetic metal, and ferromagnetic insulator, respectively. By using a free electron model and the transfer matrix method, the conductance and TMR are calculated. It is shown that the important factors to obtain large TMR effect are the combination of the materials of the FM electrode and the NM spacer and also the optimization of the NM thickness. As for spintransfer torque, we examine magnetization reversals in magnetic thin films when pure spin currents are injected into the film. By performing micromagnetic simulations based on the Landau-Lifshitz-Gilbert (LLG) equation including a spin-transfer torque term, the magnetization dynamics of the films is clarified. It is shown how the critical current required for the magnetization reversal depends on the size of the film.