Tuning spin wave modes in yttrium iron garnet films with stray fields.

The nanopatterning of Yttrium Iron Garnets (YIGs) has proven to be a non-trivial problem even with advances in modern lithography techniques due to non-compatibility with a conventional complementary metal oxide semiconductor platform. In an attempt to circumvent this problem, we demonstrate a simple and reliable method to indirectly pattern YIG films on a Gadolinium Gallium Garnet (GGG) substrate. We fabricated exchange-coupled arrays of Py dots onto the underlying YIG films using nanostencil lithography. The stray fields generated from the Py dots were used to transfer patterned magnetic information to the underlying YIG films. The static and dynamic properties of the fabricated hybrid YIG/Py dot structure and reference YIG film were characterized using the focused magneto-optic Kerr effect and by broadband ferromagnetic resonance spectroscopy. For the reference YIG film, as expected, a single field-dependent resonance mode with a narrow linewidth was observed in contrast to the splitting into three distinct resonance modes for the YIG/Py dot structure as predicted by micromagnetic simulations. We have thus shown that it is possible to utilize stray field effects from easily patternable magnetic materials for the development of future YIG-based magnonic devices.

Magnetoimpedance of Epitaxial Y3Fe5O12 (001) Thin Film in Low-Frequency Regime.

The atomically flat interface of the Y3Fe5O12 (YIG) thin film and the Gd3Ga5O12 (GGG) substrate plays a vital role in obtaining the magnetization dynamics of YIG below and above the anisotropy field. Here, magnetoimpedance (MI) is used to investigate the magnetization dynamics in fully epitaxial 45 nm YIG thin films grown on the GGG (001) substrates using a copper strip coil in the MHz-GHz frequency region. The resistance (R) and reactance (X), which are components of impedance (Z), allow us to probe the absorptive and dispersive components of the dynamic permeability, whereas a conventional spectrometer only measures the field derivative of the power absorbed. The distinct excitation modes arising from the resonance in the uniform and dragged magnetization states of YIG are respectively observed above and below the anisotropy field. The magnetodynamics clearly shows the visible dichotomy between two resonant fields below and above the anisotropy field and its motion as a function of the direction of the applied magnetic field. A low value of a damping factor of ∼4.7 - 6.1 × 10-4 is estimated for uniform excitation mode with an anisotropy field of 65 ± 2 Oe. Investigation of below and above anisotropy field-dependent magnetodynamics in the low-frequency mode can be useful in designing the YIG-based resonators, oscillators, filters, and magnonic devices.

Broadband Electron Spin Resonance Study in a Sr2FeMoO6 Double Perovskite.

We report broadband magnetic resonance in polycrystalline Sr2FeMoO6 measured over the wide temperature (T = 10-370 K) and frequency (f = 2-18 GHz) ranges. Sr2FeMoO6 was synthesized by the sol-gel method and found to be ferromagnetic below T C = 325 K. A coplanar waveguide-based broadband spectrometer was used to record the broadband electron spin resonance (ESR) both in frequency sweep and field sweep modes. From the frequency sweep mode at fixed dc magnetic fields, we obtain the spectroscopic splitting factor g ∼ 2.02 for T ≥ T C K, which confirms the 3+ ionic state of Fe in the material. The effective g value was found to decrease monotonically with decreasing temperature in the ferromagnetic regime. Resonance frequency decreases and the line width of the spectra increases as the temperature decreases below T C. At room temperature (RT) and above, the line width (ΔH) of the ESR signal increases linearly with frequency, giving Gilbert damping constant α ∼0.032 ± 0.005 at RT. However, at lower temperatures, a minimum emerges in the ΔH vs frequency curve, and the minimum shifts to a higher frequency with decreasing temperature, confining the linear frequency regime to a narrow-frequency regime. Additional inhomogeneous broadening and low-field-loss terms are needed to describe the line width in the entire frequency range.

Detection of L-band electron paramagnetic resonance in the DPPH molecule using impedance measurements.

Detection of electron paramagnetic resonance (EPR) using a microwave cavity resonating at a fixed frequency (between 9 and 10 GHz) remains the most popular method to date. Here, we report a cavity-less technique which makes use of only an impedance analyzer and a copper strip coil to detect L-band EPR (f = 1-3 GHz) in the standard EPR marker 2,2-diphenyl-1-picrylhydrazyl (DPPH). Our method relies on measuring the magnetoimpedance (MI) response of DPPH through a copper strip coil that encloses DPPH. In contrast to commercial EPR which measures only the field derivative of power absorption, our method enables us to deduce both absorption and dispersion. Changes in resistance (R) and reactance (X) of the copper strip while sweeping an external dc magnetic field, were measured for different frequencies (f = 0.9 to 2.5 GHz) of radio frequency current in the coil. R exhibits a sharp peak at a critical value of the dc magnetic field, which is identified as the resonance field and X shows a dispersion at the same frequency. The data were analyzed to obtain line width and resonance field parameters. The resonance field increased linearly with frequency and the obtained Landé g factor of 1.999 ± 0.0197 is close to the accepted value of 2.0036, measured in the X-band. The simplicity of this technique can be exploited to study paramagnetic centers in catalysis and other materials.

Direct mapping of spin wave modes of individual Ni80Fe20 nanorings.

Ferromagnetic nanorings exhibit tunable magnetic states with unique magnetization reversal processes and dynamic behavior that can be exploited in data storage and magnonic devices. Traditionally, probing the magnetization dynamics of individual ferromagnetic nanorings and mapping the resonance modes has proved challenging. In this study, micro-focused Brillouin light scattering spectroscopy is used to directly map the spin wave modes and their intensities in nanorings as a function of ring width and applied magnetic field. Micromagnetic simulations provide further insights into the experimental observations and are in good agreement with the experimental results. These results can help in improving the understanding of spin wave confinement in single elements for magnonic devices and waveguides.