Spatially resolved Ferromagnetic Resonance using 2-dimensional microresonators – Investing Spindynamics at the Nanoscale

Funded by DFG: LI 1567/3-1

Collaboration:

Dr. Ryszard Narkowicz

Dr. Ralf Meckenstock (Universität Duisburg-Essen)

Summary

Active area (‚loop‘) of a microresonator prepared by e-beam litho­gra­phy.

Fabrication and Electron micros­copy image by M. Buhl (FWIO)

Within processes in which the magnetization state of a system is changed are governed by magnetostatics as well as magnetodynamics. Thus, properties such as magnetic anisotropy and magnetic relaxation play an important role in this context. A well-established method to study both fields is Ferromagnetic Resonance (FMR). This method is, however, not sensititve enough to (in its conventional form) investigate single nanostructures. Nevertheless, with ongoing demand to reduce the structural size of magnetic systems surface/interface and egde properties become important issues. The main goal of the project is therefore to investigate spin wave excitation in single nanostructures and to extract the above-mentioned parameters in quatitative way by means of an improved FMR approach. The basis of the sensitivity enhancement are so-called microresonators that are prepared by means of electron-beam lithography. Their sensitivity allows for detecting the resonance signal of 10⁴ - 10⁵ Spins (in ferromagnetic specimens) [1,2].

Experimental Methods:

Fig. 2: Schematic representation of a microresonator shown (a) as cross section showing the bottom and top metallization separated by the dielectric interlayer, (b) topview showing the upper metal layer. (c) and (d) show the calculated E-field- and H-field distribution, respectively (for better visibility a ‘loop’-diameter of 200µm was chosen).

(a) und (b) show schematic representations of the microresonator structure used within the project (reproduced from [3,4]). The resonant structure is based on microstrip layout and works in reflexion geometry. The microstrip layout comprises a dielectric material that is fully metallized on one side (acting as ground line) and a second metallic layer on the other side being laterally structured by means of e-beam lithography (signal line, see also cross section shown in (a) and topview in (b)). For the purpose of conductiong a microwave signal the structured layer would just be given by a single stripline. In case of the microresonators, however, the shape of the metalization layer is chosen to be such that a standing electro­magnetic wave of given frequency will develop, thus providing a resonant structure at that frequency. The magnetic induction generated by the microwave signal is concentratedby a small ‘loop’ of diameter D_i. In order to achieve impedance matching to the input line (having an impedance of 50Ω), a ‘shunt-stub’ of length L_s, is patterned on the side of the main line, while the resonant frequency of the structure is furthermore determined by a ‘radial stub’ of radius R_S. In (c) a snapshot of the calculated spatial distribution of the electric field generated by the microresonator structure is vizualized for a resonator with ‚loop‘-diameter of 200µm). The standing electromagnetic wave with maxima of the electric field component at the edges of the ‘radial stub’ and the ‘shunt-stub’ is clearly visible. The associated magnetic H-field at the same time is shown in (d). At microwave powers of 1W magnetic inductions of several 10mT are built up inside the ‚loop‘. Via the ‘loop’ the resonator becomes scalable, whereas a reduction of the „loop“-diameter requires an adjustment of the other resonator components (‘shunt-stub’ and ‘radial stub’) bedingt. The sensititvity of a microresonator approximately is proportional to 1/D_i.

Proposed research

1. Magnetic anisotropy and spin wave excitations in naostructures:
   

   In contrast to bulk samples in case of nanostructure the influence of the surface becomes important. The crystalline as well as shape magnetic anisotropy therefore have a strong impact on the overall magnetic anisotropy energy.

   1. By investigating the Ferromagnetic Resonance response of individual nanostructures magnetic anisotropy as well as spin wave excitations within laterally confined structures are studied.
   2. In apllications ensembles of nanostructures are often used. The interaction via e.g. dipolar forces between the entities thus is of fundamental interest. In some cases those interactions are undesired, while in other cases they may be used to stabilize magnetization states. We thus investigate such interactions and their impact on the spin wave sprectrum as well as on the magnetic anisotropy.
      
      
2. Magnetic damping in nanostructures:
   In addition to magnetic anisotropy the relaxation of magnetic excitations plays a major role within applications of magnetic nanostructures, since it determines the behavior of the system upon magnetization reversal.
   
   
   1. The magnetic damping parameter of individual nanostructures is determined by analyzing the linewidth of the Ferromagnetic Resonance signal.
   2. Since the planar structure of the microresonators enables one to place a second nanostructure within the ‚loop‘-area after investigating a first, the influence of magnetic dipole-dipole interaction between individual nanostructures is investigated.
      
      
3. Tailoring of magnetic properties in nanostructures:
   The possibility of manipulating magnetic parameters of nanostructures is highly desireable. Due to the planar geometry of the microresonators samples may be manipulated after investigation in a controled manner. Int the project key parameters such as magntic anisotropy, the spin wave spectrum and the magnetic damping parameter is influenced by structural as well as chemical surface modifications of the nanostructure.

Project Members

Jürgen Lindner
Kilian Lenz
Sven Stienen

Review Articles and Publications

[1] Frequency-domain Magnetic Resonance – Alternative Detection Schemes for Samples at the Nanoscale, M. Möller, K. Lenz, J. Lindner
Journal of Surfaces and Interfaces of Materials, in press.

[2] A. Banholzer, R Narkowicz, C. Hassel, R. Meckenstock, S. Stienen, O. Posth, D. Suter, M. Farle, J. Lindner., Visualization of spin dynamics in single nanosized magnetic elements, Nanotechnology 22, 295713 (2011).

[3] R. Narkowicz, Suter, I. Niemeyer, Rev. Sci. Instrum. 79, 084702 (2008).

[4] R. Narkowicz, D. Suter, R. Stonies, J. Magn. Reson. 175, 275 (2003).