NANOSCALE MAGNONIC DEVICES
The ever-increasing speed of communication demands high frequency (GHz upto sub THz) transmission and attenuation components. These frequencies were earlier in the 100kHz – 100MHz range. With many forms of digital wireless communications such as cellular phones and Wi-Fi devices, the most important RF bands are from 800MHz upto 210GHz. At these high frequencies, it is very difficult to fabricate efficient and high-gain microwave components. The existing components are very bulky and would take up too much space if fabricated on-chip. In addition, their response is highly frequency dependent. Thus, any improvement in the high-frequency behaviour and reduction in size of these components would have a direct commercial application/impact. Magnonic crystals have recently emerged as a powerful candidate for high frequency (GHz to sub THz) communication devices and components and it is even more interesting since these devices can fit to the nanoscale and hence are suitable for on-chip integration. Magnonic crystals are based upon the propagating and localized spin waves in magnetic medium and offer a large tunability in the frequency and size of the devices.
We plan to fabricate and characterize the magnetic nano devices to work in the GHz to sub-THz range. These include the microwave nano-oscillators as sources of microwave, and various nanoscale rf components including waveguides, broadband filters, attenuators and phase shifters and finally high frequency spin wave logic devices.
1. Microwave nano-oscillators: Microwave spin torque nano-oscillators (STNO) are currently the world’s smallest oscillator, based on a nanomagnet consisting of only about a million atoms. The magnetic oscillation is produced by the spin torque effect exerted on the nanomagnet by spin-polarized electrical current. The major shortcomings of STNO are large linewidth of the generated signals and small output power. Both of these issues can be solved in devices by synchronizing an array STNOs by locking their phases due to the interaction between the oscillators. The response of STNO to microwave fields exhibit a complex pattern of fractional synchronization to those fields (Devil’s staircase) that is linked to the symmetry of the system, and enables one to extract information about the properties of nanoscale oscillators which are too small for the standard characterization techniques. spin waves emitted by the STNO, which can by utilized as the mechanism for their synchronization [3]. We will fabricate arrays of the free-magnetic layers (arrays of elements with single domain or vortex states) of STNO and STVO (spin torque vortex oscillators) devices and investigate the coherence of microwave oscillation of the nanodevices. We will engineer the physical structure of the arrays to improve the coherence of oscillation, which will give narrow linewidth and enhanced peak power of the emitted microwave energy.
2. Nanoscale magnonic waveguides: We will fabricate and characterize nanoscale microwave waveguides based upon magnetic materials with high saturation magnetization and anisotropy. The magnetostatic surface and volume modes in such wave guides can be modified by careful structuring of the waveguide. A simple example of introducing 1-D chains of antidot lattice and the corresponding modifications of the band structures are shown in Fig. 1.
Fig. 1: (a) Dispersion of a nanostripe waveguide and (b) Dispersion of a nanostripe waveguide with 1-D antidot lattice (bandgaps open up)
We will introduce 2-D arrays of antidots with various shapes lattice constants and lattice geometries to engineer the bands over which the magnonic waveguide will have pass bands and stop bands. The pass and stop bands depend strongly upon the demagnetized regions around the antidots and hence application of a bias field along certain directions will also be used to manipulate the bandwidth of the magnonic waveguide.
3. Attenuators and Filters: The attenuators and broadband filters will be designed and fabricated based upon arrays of nanodots and composite structures. The interaction between the dots can affect the magnetic potential through which the spin wave will propagate. Hence, the magnetostatic interaction will be manipulated by engineering the shape of the elements and their positions in the array. This can have two effects, i) creation of stop bands if the interaction potential is too strong to extinct some of the excitation frequencies. ii) Attenuation of a broadband microwave signal, which can be continuously tuned by the physical structure as well as by application of a magnetic field.
A composite structure made of inclusion of a ferromagnetic material inside holes created into another ferromagnetic material with different magnetic parameters is a challenging but more effective way of creating stop and pass bands and attenuation.
4. Spin wave Interferometer and Phase shifters:
Fabrication:
For arrays of nano-oscillators, mask of bilayer PMMA resist by electron-beam lithography and deposit the material by magnetron sputtering/co-sputtering/e-beam evaporation.
The waveguides will be formed in two steps: In the first step a mask of bilayer PMMA resist will be prepared by electron-beam lithography and deposit the waveguide material by magnetron sputtering/co-sputtering/e-beam evaporation. The arrays of holes in the waveguide will then be prepared by focused ion beam lithography.
The attenuators and filters will be fabricated using the same technique as above.
For stimulation of the magnons by external microwave source we will fabricate the devices on a wave guide structure and use a high frequency probe to launch the microwave energy to the system.
Characterization:
We have developed the following measurement setup to measure the magnonic spectra, damping, magnonic band structure and for imaging the propagating and localized spin waves.
- Broadband Ferromagnetic Resonance Spectrometer
- Conventional and Microfocused BLS setup
- Time-resolved magneto-optical Kerr Effect Microscope