(17일) Application of ferromagnetic resonance techniques to the studies of magnetic thin films and nanostructures
(18일) Probing magnetic microdots and their arrays by ferromagnetic resonance
연사: Dr. Gleb Kakazei (Department of Physics, Oviedo University, Oviedo, Spain)
일시: 2008년 1월 17일 (목) 오전 10시 30분 / 1월 18일 (금) 오후 2시
장소: 서울대학교 33동 307호
(17일) Ferromagnetic resonance (FMR) has proven to be a powerful technique in the investigation of the magnetic parameters of continuous thin films and multilayers, to determine exchange interactions and different types of magnetic anisotropy fields. In the case of narrow resonance linewidth, FMR experiments have the capability of obtaining the resonance positions with an extremely high degree of accuracy (± few Oersted). It is very important that the measurements of resonance field angular dependence, both polar and azimuthal, and temperature dependence can be easily done. FMR was also successfully used to study standing spin waves in continuous thin magnetic films, both single layered and multilayers. And finally, FMR is a judicious technique to investigate the dynamics of internal spin interactions in ferromagnets that determine the relaxation processes.
The most conventional and frequently used FMR setups is so called continuous wave spectrometer (cw – FMR), where the frequency is fixed and the sample is placed inside cavity. Usually commercial electron spin resonance spectrometers that operate at frequencies 10 - 36 GHz are used for these purposes. This method provides the highest possible sensitivity. One of advantage of the high frequency FMR (10 GHz and above) is that the resonance field in most cases is larger than the saturation field of the sample, thus the influence of domain structures is avoided.
Another type of FMR setups is broadband spectrometer, where frequency can be changed in the broad range (0 - 40 GHz). This can be reached by placing the sample directly over stripline or coplanar waveguide. In this case it is possible to investigate the frequency dependence of resonance position and linewidth as well as to study samples in unsaturated state. The drawback of this technique is a lower homogeneity of radiofrequency (rf) field and lower sensitivity. Recently the sensitivity of this method was improved by using Vector Network Analyzer as a source/detector of rf radiation.
To study individual micron-size elements by FMR a new experimental technique, ferromagnetic resonance force microscopy (FMRFM) was recently developed. FMRFM is a variation of magnetic resonance force microscopy (MRFM) that was first proposed by J.A. Sidles in 1991 as a noninductive method to detect nuclear magnetic resonance in microscopic samples. Since then the technique has evolved into a powerful microscopy technique for electron spin resonance, nuclear magnetic resonance and ferromagnetic resonance. In 2004 D. Rugar et al. reported the detection of a single unpaired electron spin in silicon dioxide, achieving a spatial resolution of 25 nm.
(18일) Continuous wave ferromagnetic resonance (cw – FMR) at 10 GHz was used to characterize static and dynamic properties of rectangular and square arrays of circular nickel and permalloy microdots. In the case of a rectangular lattice, as interdot distances in one direction decrease, the in-plane uniaxial anisotropy field increases, in good agreement with a simple theory of magnetostatically interacting uniformly magnetized dots. In the case of a square lattice a four-fold anisotropy of the in-plane FMR field Hr is found when the interdot distance a gets comparable to the dot diameter D. This anisotropy, not expected in the case of uniformly magnetized dots, is explained by a non-uniform magnetization m(r) in a dot in response to dipolar forces in the patterned magnetic structure. It is well described by an iterative solution of a continuous variation procedure. In the case of perpendicular magnetization multiple sharp resonance peaks are observed below the main FMR peak in all the samples, and the relative positions of these peaks are independent of the interdot separations. Quantitative descript-xion of the observed multiresonance FMR spectra is given using the dipole-exchange spin wave dispersion equation for a perpendicularly magnetized film where in-plane wave vector is quantized due to the finite dot radius, and the inhomogenetiy of the intradot static demagnetization field in the nonellipsoidal dot is taken into account. It is also found that with an increase of θ (angle between magnetic field and sample normal), higher-order peaks approach the main one, cross it (each one at its own θ) and continue their shift to higher (in comparison with the main peak) fields. At θ > 15o the spectra have a reversed order – the main peak has the lowest resonance field value, and the intensities of higher-order peaks decrease with increasing peak number. Such structure of spin wave spectra is preserved until θ = 90o.
In addition, the same arrays of circular permalloy microdots were characterized by Vector Network Analyzer (VNA) technique. Vibrating sample magnetometry measurements demonstrate a typical magnetization curve for the vortex ground state. High frequency vortex resonance (two modes) with positive dispersion law was observed below the nucleation field. The broad uniform procession resonance peak occurs above nucleation field and reaches the similar to cw – FMR values of linewidth above annihilation peak. Its Hr in the whole frequency range (4 – 10 GHz) was perfectly described by Kittel formula that takes into account the demagnetizing factor of individual magnetic dot. An additional field independent low frequency peak was observed in a high density sample.
It was also demonstrated that ferromagnetic resonance force microscopy (FMRFM) can be used to determine both local and global properties of patterned submicron ferromagnetic samples. Local spectroscopy together with the possibility to vary the tip-sample spacing enables the separation of those two contributions to a FMRFM spectrum.
문 의 : 김상국 교수 (880-5854)