Featured paper: Magnetic thin films for miniaturized devices.



Paper: Annealing effects on the microwave linewidth broadening of FeCuNbSiB ferromagnetic films. Alves, M.J.P.; Gonzalez-Chavez, D. E.; Bohn, F.; Sommer, R. L. Journal of Applied Physics. 117, 123913(2015) DOI: 10.1063/1.4915330.

Scheme of the broadband ferromagnetic resonance equipment used in the study. The Kepco source feeds the magnetizing circuit (Helmholtz coils) and the signal is measured with a Vector Network Analyzer (VNA). The sample is placed on a coplanar waveguide with micrometric dimensions.

A team of scientists from the Brazilian Center for Research in Physics (CBPF) and the Federal University of Rio Grande do Norte (UFRN) conducted a study on magnetic properties of thin films made of a nanocrystalline material (i.e, formed by nanometric grains) type FINEMET. The conclusions of this scientific research can help produce high quality magnetic materials suitable for use in small-sized devices, such as magnetic random access memories (MRAMs) or nano-oscillators. Results of the study were reported in a recent article in the Journal of Applied Physics.

FINEMET type materials are alloys based on iron (Fe), silicon (Si) and boron (B) with small additions of copper (Cu) and niobium (Nb). They have very good magnetic properties when they are produced by rapid cooling followed by annealing. However, there is not yet an established route that allows obtaining the material having these properties in the form of thin films, which are better suited for miniaturized applications.

In the work of the Brazilian team, magnetic thin films FeCuNbSiB were synthesized at CBPF by scientists from UFRN and CBPF. Samples of the film were analyzed using various techniques such as grazing incidence X-ray diffraction, magnetometry and, in particular, broadband ferromagnetic resonance (FMR). “We explored this technique to the limit,” said Rubem L. Sommer, one of authors of the Journal of Applied Physics paper. “It is powerful and has allowed the study of nanostructured materials with great efficiency,” added the CBPF researcher. Sommer and his team have been contributing to the development of broadband ferromagnetic resonance technique since 2011.

The ferromagnetic resonance technique is used to study the magnetization of the material, measuring the amount of electromagnetic radiation in the microwave range that a particular material absorbs. In the conventional version of this technique, explains Sommer, this absorption is measured at a fixed frequency, and the external magnetic field is varied to tune the equipment in resonance. In the microwave range, the frequency may be between 300 MHz and 300 GHz, and 1 Hz equals 1 oscillation per second. “In the case of ferromagnetic resonance broadband, we scan frequency and the external field, making a direct mapping of the material dispersion relation,” explains Sommer.

Typical results of broadband FMR: microwave absorption curve (color) depending on the field and frequency.

Based on the combination of the films analyses obtained by the various techniques, the team of scientists unveiled the mechanisms that are responsible for broadening the ferromagnetic resonance linewidht in the material. “The thinner the resonance line, the higher the quality of the material for applications”, said Sommer. The scientists could conclude that the residual stresses (those who remain in the materials after the elimination of their causes) were causing the enlargement of the resonance linewidth, and that annealing reduced these tensions.

The study reported in the article was funded by Brazilian agencies CNPq and CAPES and was developed mainly in the doctoral research of Marcos Alves held in CBPF and recently defended. The doctoral dissertation of Diego González-Chávez, defended in 2013, was also important for the article, as it allowed the successful development of the broadband FMR technique.

The authors of the article are part of a larger network of collaboration that includes, in addition to researchers from CBPF and UFRN, contributors from PUC-Rio and the federal universities of Santa Maria (UFSM) and Rio Grande do Sul (UFRGS), says Sommer. “I believe that this work, as well as the performance of a network spread over different institutions is a very positive aspect of the current Brazilian scientific and technological research reality”, he said. The network develops research on nanostructured materials and magnetic devices for use at high frequencies. “Our research has always a double bias: basic research to understand the phenomena and application development,” said Sommer.

Scheme of ferromagnetic resonance equipment used in broadband research. The Kepco source feeds the magnetizing circuit (Helmholtz coils) and the signal is measured with a vector Network Analyzer (VNA). The sample is placed on a coplanar waveguide with micrometric dimensions.

Results typical broadband of FMR: microwave absorption curve (color) depending on the field and frequency.


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