Microresonators and Microantennas—Tools to Explore Magnetization Dynamics in Single Nanostructures

Hamza Cansever,Jürgen Lindner

doi:10.3390/magnetochemistry7020028

Hamza Cansever, Jürgen Lindner

Open Access

https://doi.org/10.3390/magnetochemistry7020028

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Abstract

The phenomenon of magnetic resonance and its detection via microwave spectroscopy provide insight into the magnetization dynamics of bulk or thin film materials. This allows for direct access to fundamental properties, such as the effective magnetization, g-factor, magnetic anisotropy, and the various damping (relaxation) channels that govern the decay of magnetic excitations. Cavity-based and broadband ferromagnetic resonance techniques that detect the microwave absorption of spin systems require a minimum magnetic volume to obtain a sufficient signal-to-noise ratio (S/N). Therefore, conventional techniques typically do not offer the sensitivity to detect individual micro- or nanostructures. A solution to this sensitivity problem is the so-called planar microresonator, which is able to detect even the small absorption signals of magnetic nanostructures, including spin-wave or edge resonance modes. As an example, we describe the microresonator-based detection of spin-wave modes within microscopic strips of ferromagnetic A2 Fe60Al40 that are imprinted into a paramagnetic B2 Fe60Al40-matrix via focused ion-beam irradiation. While microresonators operate at a fixed microwave frequency, a reliable quantification of the key magnetic parameters like the g-factor or spin relaxation times requires investigations within a broad range of frequencies. Furthermore, we introduce and describe the step from microresonators towards a broadband microantenna approach. Broadband magnetic resonance experiments on single nanostructured magnetic objects in a frequency range of 2–18 GHz are demonstrated. The broadband approach has been employed to explore the influence of lateral structuring on the magnetization dynamics of a Permalloy (Ni80Fe20) microstrip.

Highlights

Determining dynamic properties of magnetic materials, i.e., their time-dependent behavior, often provides key insights into understanding of the magnetic response in microor nano-sized objects
ferromagnetic resonance (FMR) as a method is one of the main spectroscopic techniques, allowing for an investigation of static properties, such as the effective magnetization that provides the effective internal field acting on the spins, g-factor, and magnetic anisotropy, as well as dynamical properties such as magnetic relaxation mechanisms [5,6,7]
Understanding dynamical properties of nanomagnets plays a crucial role for future spin-based technologies

Summary

Introduction

Determining dynamic properties of magnetic materials, i.e., their time-dependent behavior, often provides key insights into understanding of the magnetic response in microor nano-sized objects. We review the use of a unique approach to study magnetization dynamics in magnetic samples with small volume via microresonator- and microantenna-based microwave spectroscopy. The main advantage of a using microresonator/microantenna approach relies on a tremendous increase of the filling factor upon scaling down the resonator volume to the micrometer size, comparable to the one of the nanostructure. This high filling factor allows the increase of the detection sensitivity by orders of magnitude as compared to conventional FMR setups, based on a measurement of the absorbed microwave power

Materials and Experimental Methods

Experimental

Experimental Results and Discussion

Snapshots

FMR taken

Conclusions