BROAD BAND FERROMAGNETIC RESONANCE SPECTROMETER
Ferromagnetic resonance (FMR) experiments measure the collective spin excitations in a magnetic material. In conventional FMR the sample is subjected to a microwave field of fixed frequency in a resonant cavity with high Q-factor, and a bias magnetic field, the magnitude of which is varied. At each field value the reflected or absorbed power is measured, which may yield one of more peaks corresponding to the resonant modes of the system. The resonant fields depend on magnetic parameters including the gyromagnetic ratio, saturation magnetization, magnetic dipolar, exchange and anisotropy energies and interlayer couplings energies in case of magnetic multilayers. Further, the line-width of the resonance field gives information about the Gilbert damping coefficient of the material. In case of ordered arrays of ferromagnetic nanostructures a large frequency band may exist covering the MHz to high GHz regimes and usage of resonant cavity based FMR system is less efficient. In addition, conventional FMR does not offer broadband operation particularly below 1GHz, it is difficult to apply magnetic field along various orientations, which are all essential conditions for investigating and tailoring the magnonic band structures in magnonic crystals. Hence, a broad-band FMR spectrometer has been set up in our laboratory with all the above capabilities.
Fig: A photograph of the broadband (10 MHz to 20 GHz) ferromagnetic resonance spectrometer with a vector network analyzer and a custom built high frequency (40 GHz) probe station. The shorted coplanar waveguide structure with the picoprobe is shown on the monitor of the PC. The inset at the right top corner of the picture shows the FMR frequency of a permalloy microstructure as a function of the bias magnetic field, which is constructed by measuring the FMR spectra at 100 different values of the bias magnetic field.
The setup is based upon a vector network analyzer with 10 MHz to 20 GHz frequency bandwidth and a homemade high frequency probe station with an in-built electromagnet as shown in Fig. 1. The FMR spectrometer has been customized to study magnetic thin films and nanostructures fabricated on co-planar stripline or waveguide structures made of Au on Si or GaAs. The microwave devices are fabricated by a combination of optical and e-beam lithography. An electrically insulating layer (Al2O3 or SiO2) is deposited on top of the stripline structure and the patterned samples are fabricated on top of the microwave devices by using e-beam or focused ion beam lithography. In the simplest configuration a microwave signal is sent to the co-planar stripline or the waveguide and the S-parameters are measured in the transmission (S21) or reflection (S11) geometry. For the reflection geometry the waveguide/stripline is shorted at one end (Fig. 1) and the absorbed signal is doubled in the measured S11 parameter. However, in both cases impedance matching of the stripline or the waveguide structure is crucial to avoid spurious reflections. To avoid spurious reflections at the contacts and for measurements of a series of devices made on a single wafer, high frequency probes with micromanipulators and a probe station with high resolution stages and imaging system has been developed. In addition, phase sensitive detection using modulation of the microwave signal with a lower frequency rf signal and measurement by using lock-in amplifiers are used for increasing the sensitivity of measurement. An electromagnet fixed on a goniometer is used to apply bias magnetic fields at various directions w.r.t. the artificial magnonic lattices. Further, we also plan to perform local excitations of the magnons by using a microwave antenna and the propagation of the spin wave excitation along different directions of the artificial lattice. The wavevector and the frequency of the excitation may be selected by varying the width of the stripline structure. Using this method, we would be able to extract selective part of the magnonic band structures and investigate propagating and localized magnons.