Magnetic Characterization of Exchange Coupled Ultrathin Magnetic Multilayers by Ferromagnetic Resonance Technique

Abstract

The magnetic–nonmagnetic multilayers have been widely used in various applications. As well known the important physical parameters depend on relevant applications. For giant magneto resistance (GMR), magnetic data storage, MRAM and spintronics applications, the most important magnetic parameters in multilayered structures are interlayer exchange coupling, magnetic anisotropy, saturation magnetization and spin relaxation time. All of these parameters strictly depend on the physical size of the elements which are continuously shrinking even down to nanometer scale for ultra high density data processes. However, as the dimensions (thickness) of the films continues to decrease the magnetic signal intensity gets so weak that its detection becomes one of the major issues. But still ferromagnetic resonance (FMR) can be powerful enough to study these multilayered structures.

Recently we have developed a theoretical model to analyse the FMR data to extract magnetic parameters. We have chosen the permalloy (Py) layers separated by very thin Cr for our study because Py is one of the softest magnetic materials and its bulk form is very well characterized. The FMR measurements were carried out by using an X-band ESR spectrometer at several temperatures. The experimental data was successfully simulated by proposed model. The saturation magnetization was observed close to the value that for bulk permalloy. However a significant perpendicular anisotropy induced for thin film case. The spectra strongly depend on the thickness of Cr layer. Even the relative positions of the strong and the weak modes are interchanged for particular thickness of Cr layer. It has been found that the exchange coupling between successive layers exhibits oscillatory behaviour with respect to Cr thickness, confirming usefulness of the developed FMR model.