Spin dynamics and Spin Transfer Torque Nano-Oscillators

The dynamical behaviour of thin film magnetic multilayers is a rich and diverse area of research and allows a unique opportunity to design and explore the high frequency properties of magnetic materials. Due to the high quality magnetic tunnel junctions (MTJs) produced at our state-of-the-art facilities at INL, we are able to design systems which not only allow us to explore the fundamental dynamics of such systems, but also to target future and emerging technologies, including 5G wireless communications, bio-inspired neuromorphic computing architectures and wireless energy harvesting. By carefully controlling the geometry and material properties of the magnetic and adjacent non-magnetic layers, novel spin textures, such as magnetic vortices, bubbles and skyrmions, can be created, and their dynamical behaviour explored. The dynamic modes of these magnetic materials can be excited via spin transfer torque, local magnetic fields or Spin Hall induced spin currents.

Here at INL we focus on three key areas of high-frequency dynamics utilising magnetic tunnel junctions:

  • Spin Torque Nano-Oscillators – direct excitation of confined MTJ nanopillar via spin transfer torque or integrated magnetic fields;
  • Spin Hall Nano-Oscillators – indirect excitation of a MTJ nanopillar via the spin current induced in an adjacent spin Hall line;
  • Spin wave detection – The detection of spin waves in magnetic waveguides via an adjacent MTJ

Spin Torque Nano-Oscillators

In the Spintronics group at INL we specialize in the deposition, characterization and exploitation of Spin torque nano-oscillators (STNOs) for future and emerging technologies. We use a range of magnetic alloys (CoFe, FeB, CoFeB, NiFe) and explore their magnetic dynamical response when excited by electrical currents and magnetic fields. We investigate a range of magnetic textures, from in-plane and out-of-plane uniform magnetised free layers to more novel magnetic textures, i.e. magnetic vortices, bubbles and Skyrmions. By harnessing these dynamic magnetic systems, we target an exciting range of applications and are actively pursuing the demonstrator level prototypes and integration of these STNOs in CMOS technologies in order to prove their industrial viability for future products.

During 2013 to 2016, INL was able to demonstrate the ability to produce STNOs with power values up to 200 nW in homogeneous free-layers, reasonable linewidths and monochromatic spectrums using MTJs using thicker insulating barriers for junctions than typically used. This research focused on this role of the tunnel barrier in the dynamics.

Vortex-oscillator both – “A schematic of a vortex nano-oscillator and an FFT image of the azimuthal spin wave mode”

Vortex-oscillator result – “Electrical synchronisation of two nano-oscillators, displaying large output powers (p>10 µW)

STNO setup – “Schematic representation of the experimental setup used for the RF emission characterisation”

Spintronics Description – “DC and RF electrical characterisation. TMR versus RxA for all the studied STNOs (circles with black border correspond to a free layer thickness of 2.0 nm and the circles with dashed red border to 1.4 nm). The colour scale of the points represents the maximum P out of the RF emission and the size of the points the line width for the oscillation with the highest Q. The inset shows a schematic representation of the deposited MTJ stack. More details at Costa et.al.”

Spin Hall Nano-oscillators

Spin Hall nano-oscillators are believed to be the potential replacement of spin-torque nano-oscillators with increased reliability, output and high device endurance. The three goals of this project were the following:

  • Generation of pure spin current in Ta Hall bar.
  • Nanofabrication of 3-terminal device with high quality magnetic tunnel junctions.
  • Detection of high frequency oscillations in 3-terminal spin Hall nano-oscillators

Results: We experimentally demonstrated a high-performance 3-terminal STNOs based on MTJs with a concurrent spin injection from a spin-polarised tunnelling current and a pure spin Hall current. A combination of both currents efficiently excites the free layer into dynamic regimes, providing a result six times larger than the oscillation amplitudes achieved by either of the individual mechanisms.

SHNO setup “Optical microscope image of the final 3-terminal device geometry and schematic of a 3-terminal measurement setup. The results for these measurements were published by Tarequzzaman et. al.”

SHNO power map – “The colour map shows the integrated output power due to the combined excitation by a spin Hall current and a tunnelling current. The black dashed lines represent the contour lines of equal power. The right graph integrated output power and linewidth as a function of the spin Hall current density at a tunnelling current of -22.3 x 10^9 A/m2 and fixed magnetic field of 150 Oe. The results of this measurement were reported by Tarequzzaman et al.”

Spin Wave Detection

Integration of MTJs on waveguide structures to allow efficient detection of spin waves for 5G technologies, magnonic logic and coupled arrays.

Spin wave detection “Spin wave detection scheme using an MTJ as a field sensor” Spin wave detection2 “SEM image of test samples with micromagnetic simulations of the spin waves along the wave guide.

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Research Groups