In most electronic devices we have only utilized the charge carried by electrons, while ignoring the other inherent property, the spin. In spintronics we explicitly make use of the spin degree of freedom of electrons to achieve new functionalities. A unique advantage of spintronics is its nonvolatility, which is particularly critical for future digital devices utilizing transistors smaller than 10nm. However, the Achilles heel for spintronics is the lowest switching energy through the well-known spin transfer torque effect is still three orders of magnitude higher than that of CMOS transistors, which severely limits the present applications of spintronic devices. After an introduction on fundamental spintronic effects based magnetic fields and electric currents. I will focus my talk on the exploration of new phenomena that can be controlled by electric fields, driven by the premise that voltage-controlled switching could be far more efficient therefore could eventually lead to ultra-low energy spintronics. I will describe the approaches to increase the magnetoresistance and thermal stability in perpendicular magnetic tunnel junctions (pMTJs) by employing new materials and new structures. Through the voltage controlled magnetic anisotropy (VCMA) effect where the energy barrier can be altered by redistribution of electron density amount different d orbitals of transitional ferromagnets, a large reduction in switching current density has been realized in pMTJs. Then I will present the voltage controlled magnetism (VCM) effect where both magnetic anisotropy field and saturation magnetization of Pt/Co/Gd2O3 trilayers can be manipulated by controlling the oxidation state of Co through external electric fields, reaching an unprecedentedly large control of 11 pJ/Vm and a large modulation ( up to 10 times) of spin Hall switching current density. Finally I will describe our recent effort to reduce the switching energy of pMTJ through effective in-plane magnetic fields generated by sub-ns voltage pulses.
Colloquium: Toward Ultra-low Energy Spintronics: Physics, Materials and Prototype Devices
Weigang Wang, University of Arizona
Friday, September 22, 2017 - 3:00pm