title: Reduction of phase noise in nanowire spin orbit torque oscillators.
abstract: Spin torque oscillators (STOs) are compact, tunable sources of microwave radiation that serve as a test bed for studies of nonlinear magnetization dynamics at the nanometer length scale. The spin torque in an STO can be created by spin-orbit interaction, but low spectral purity of the microwave signals generated by spin orbit torque oscillators hinders practical applications of these magnetic nanodevices. Here we demonstrate a method for decreasing the phase noise of spin orbit torque oscillators based on Pt/Ni80Fe20 nanowires. We experimentally demonstrate that tapering of the nanowire, which serves as the STO active region, significantly decreases the spectral linewidth of the generated signal. We explain the observed linewidth narrowing in the framework of the Ginzburg-Landau auto-oscillator model. The model reveals that spatial non-uniformity of the spin current density in the tapered nanowire geometry hinders the excitation of higher order spin-wave modes, thus stabilizing the single-mode generation regime. This non-uniformity also generates a restoring force acting on the excited self-oscillatory mode, which reduces thermal fluctuations of the mode spatial position along the wire. Both these effects improve the STO spectral purity.
authors.affiliation: Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA.
authors.forename: Liu
authors.initials: L
authors.lastname: Yang
authors.affiliation: Institute of Magnetism, National Academy of Sciences of Ukraine, Kyiv 03142, Ukraine.
authors.forename: Roman
authors.initials: R
authors.lastname: Verba
authors.affiliation: Department of Physics, Oakland University, Rochester, MI 48309, USA.
authors.forename: Vasil
authors.initials: V
authors.lastname: Tiberkevich
authors.affiliation: Helmholtz-Zentrum Dresden - Rossendorf, Institute of Ion Beam Physics and Materials Research, Bautzner Landstraße 400, 01328 Dresden, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Schneider
authors.affiliation: Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA.
authors.forename: Andrew
authors.initials: A
authors.lastname: Smith
authors.affiliation: Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA.
authors.forename: Zheng
authors.initials: Z
authors.lastname: Duan
authors.affiliation: Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA.
authors.forename: Brian
authors.initials: B
authors.lastname: Youngblood
authors.affiliation: Helmholtz-Zentrum Dresden - Rossendorf, Institute of Ion Beam Physics and Materials Research, Bautzner Landstraße 400, 01328 Dresden, Germany.
authors.forename: Kilian
authors.initials: K
authors.lastname: Lenz
authors.affiliation: Helmholtz-Zentrum Dresden - Rossendorf, Institute of Ion Beam Physics and Materials Research, Bautzner Landstraße 400, 01328 Dresden, Germany.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Lindner
authors.affiliation: Department of Physics, Oakland University, Rochester, MI 48309, USA.
authors.forename: Andrei N
authors.initials: AN
authors.lastname: Slavin
authors.affiliation: Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA.
authors.forename: Ilya N
authors.initials: IN
authors.lastname: Krivorotov