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 the 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.
Schematic of a vortex nano-oscillator and a FFT image of the azimuthal spin wave mode.
Electrical synchronisation of two nano-oscillators, displaying large output powers (p > 10 µW).
Schematic representation of the experimental setup used for the RF emission characterization.
DC and RF electrical characterization. TMR versus R×A 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 color scale of the points represents the maximum P out of the RF emission and the size of the points the linewidth for the oscillation with the highest Q. The inset shows a schematic representation of the deposited MTJ stack. More details at Costa et al..
Range of operation of STNOs. Critical current density for STT-induced oscillations J STT (blue triangles), breakdown current density J break (red circles) and current for which the highest Q is achieved (white diamonds). The lines are splines fitted to the data separating the region without STT effects (dark grey), the STNO region (light grey) and the breakdown region (white). More details at Costa et al..
Spin Hall Nano-Oscillators
Spin Hall nano-oscillators are believe to be the potential replacement of spin-torque nano-oscillators with increase reliability, output and high device endurance. The three goals of this projects 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 demonstrate a high performance 3-terminal STNOs based on MTJs with a concurrent spin injection from a spin-polarized tunneling current and a pure spin Hall current. A combination of both current efficiently excites the free layer into dynamic regimes which is six times larger than the oscillation amplitudes achieved by either of the individual mechanisms.
Optical microscope image of the final three-terminal device geometry and schematic of a three-terminal measurement setup. The results of this measurements were published by Tarequzzaman et al..
This color map shows the integrated output power due to the combined excitation by a spin Hall current and a tunneling current. The black dashed lines represent the contour lines of equal power. The right graph integrated output powerand linewidth as a function of the spin Hall current density at an tunneling current of -22.3 × 109 A/m2 and fixed magnetic field of -150 Oe. The results of this measurements were published 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 scheme using an MTJ as a field sensor.
SEM image of test samples with micromagnetic simulations of the spin waves along the waveguide.
Communications Physics, 2 (1), pp. 20, 2019, ISSN: 2399-3650.
Applied Physics Letters, 112 (25), pp. 252401, 2018, (arXiv: 1804.04104).
IEEE Transactions on Magnetics, pp. 1–4, 2018.
Scientific Reports, 7 (1), 2017.
IEEE Transactions on Magnetics , 51 (11), 2015.