Özhan Özatay, Cornell University
on Manipulation of Extrinsic Magnetic Damping and Spin Transfer Switching Currents in Nanomagnets by Nonuniform Spin Injection and Controlled Sidewall Oxidation

The idea of employing electron spin for information technologies has unique advantages over the conventional charge-based electronics because it potentially enables nonvolatile data storage, improved data processing speed, more efficient power consumption as well as high integration density. In metallic spintronics the interaction of the spin of a current-carrying electron and a localized moment of a ferromagnet has two important outcomes: the spin dependent scattering of the current carrying electrons leading to magnetoresistance effects and the ability to manipulate the local moment through a mutual spin transfer torque. In this talk I will present a series of experiments that address issues regarding the latter, spin-transfer phenomenon and that addresses some of the challenges for advancing this phenomenon toward technological applications. . In the first part of the talk I will discuss the nucleation and subsequent depinning of a domain wall driven by a nonuniform spin polarized current injection into a nanomagnet. This is accomplished by defining a 20-30nm diameter aperture inside a 3.5nm thick AlOx in between the Cu spacer and permalloy(Py) free layer of a Py/Cu/Py nanopillar spin valve. The resulting concentrated spin polarized current injection into the free layer nucleates and applies pressure to a domain wall at the contact region. The magnetic reversal is via the propagation of the domain wall driven by spin torque. This mechanism reduces the absolute level of spin transfer switching currents required to achieve magnetization reversal by two orders of magnitude. In the second part of my talk I will focus on the adverse effects of sidewall oxides in a Py nanomagnet on both field and current driven switching characteristics as a function of temperature. Analytical electron microscopy and surface sensitive x-ray photoemission spectroscopy measurements reveal that the Py surface has NiO, FeO and Fe2O3 native antiferromagnetic oxides. These adventitious oxides can have a major impact on the efficiency of spin torque switching as well as field driven switching by enhancing magnetic damping as well as causing unstable switching fields due to a rotatable anisotropy. I will show that the passivation of such an oxide layer results in minimal temperature dependence of spin transfer switching currents as well as in stabilizing the switching fields.
References: 1. Emley NC, Krivorotov IN, Ozatay O, Garcia AGF, Sankey JC, Ralph DC, Buhrman RA, Phys. Rev. Lett. 96, 247204 (2006) 2. Ozatay O, Emley NC, Braganca PM, Garcia AGF, Fuchs GD, Krivorotov IN, Buhrman RA, Ralph DC, Appl. Phys. Lett. 88, 202502 (2006) 3. Ozatay O, Chalsani P, Emley NC, Krivorotov IN, Buhrman RA, J. Appl. Phys. 95, 7315 (2004)