O. von Geisau

Conventional and photothermally modulated ferromagnetic resonance investigations of anisotropy fields in an epitaxial Fe(001) film

Conventional angle‐dependent ferromagnetic resonance (FMR) measurements on an epitaxial (001) Fe film grown on a GaAs substrate were performed as a function of the orientation of the external magnetic field Bext for three configurations: One with Bext in the film plane and two with Bext out of plane starting in plane in different crystallographic orientations. From these measurements the magnetization M, the crystalline anisotropy constants K1 and K2, the surface anisotropy Ks and an additional uniaxial anisotropy Ku were deduced self‐consistently. The temperature dependence of K1 and M was investigated for the in‐plane [110] orientation in the range from 100 to 570 K. In addition, locally resolved photothermally modulated FMR measurements were carried out to study the homogeneity of the magnetization and of the crystalline anisotropy across the film. Disturbances of both magnetic parameters as a function of position were observed which are related to local inhomogeneities in the semiconducting substrate.

Photothermal investigation of magnetostatic modes in yttrium iron garnet at high microwave power levels

Laterally resolved magnetostatic mode spectra in an yttrium-iron-garnet single-crystal (0.98 mm*8.40 mm*4.40 mm) were obtained by microwave spectroscopy using the photothermal laser beam deflection technique. For a microwave frequency of 9.33 GHz and a power of 2.2 W, both magnetostatic surface and magnetostatic backward volume wave modes were observed. The lateral variation of these modes can be explained by a modified WKB approximation for the magnetostatic potential and by taking into account the presence of an inhomogeneous demagnetization field inside the sample. At high incident microwave power, a foldover effect in the range of the surface wave modes and one distinctly larger volume wave mode peak occur. The photothermal signal phase provides information on the penetration depth of the surface wave modes. >

Investigation of the photothermally modulated ferromagnetic resonance signal from magnetostatic modes in yttrium iron garnet films

The conventional and photothermally modulated (PM) ferromagnetic resonance (FMR) of magnetostatic modes (MSM) in yttrium iron garnet (YIG) films have been investigated as a function of temperature. Approaching the ferrimagnetic transition at Tc=560 K a strong enhancement of the PM-FMR signal amplitude is observed which is accompanied by a change of the signal shape. The observations are discussed in the framework of a model that takes into account the temperature derivatives of those quantities that contribute to the high-frequency susceptibility. At temperatures still below Tc a paramagnetic line emerges. The MSM disappear in a state of finite magnetization which is explained on the basis of damping of the MSM being important in the vicinity of the magnetic phase transition. Additionally, frequency and power dependent measurements are presented and the imaging ability of PM-FMR is demonstrated.

LOCALLY RESOLVED PHOTOTHERMALLY MODULATED FERROMAGNETIC RESONANCE INVESTIGATION OF MAGNETIC ANISOTROPIES IN A (001) FE WEDGE FILM

Photothermally modulated ferromagnetic resonance (PM‐FMR) was used to investigate the lateral dependence of the magnetic parameters in an epitaxially grown (001) Fe wedge. The wedge thickness varied from 2.2 to 12.8 nm on a length of 10 mm. The PM‐FMR measurements, which were carried out with a lateral resolution of 50 μm, permitted to determine the surface anisotropy with a high accuracy. In addition, a small uniaxial in‐plane anisotropy was detected which shows an increase with the Fe‐film thickness.

Ferromagnetic resonance investigations of anisotropy fields of Fe(001) epitaxial layers

Abstract Epitaxially grown Fe(001) thin films have been investigated by ferromagnetic resonance. The angular dependence of the resonance field position was measured in different planes with respect to the crystallographic axes of the sample and analysed by a theoretical model. Values of the anisotropy constants are obtained self-consistently from the measurements.

3D-imaging of magnetostatic modes using photothermally modulated FMR-technique

Abstract The spatial distribution of magnetostatic modes in rectangular YIG samples is investigated with the photothermally modulated FMR-technique for varying microwave power and modulation frequency. Surface and volume waves are identified in the images obtained at fixed modulation frequency and are distinguished by the frequency dependence of the photothermal signal.

Temperature-dependent FMR investigations on epitaxial Fe (001) films with different thicknesses

Abstract The temperature dependence of ferromagnetic resonance (FMR) spectra of single epitaxial (001) Fe films of different thicknesses ( d = 1.0, 2.0, 4.0, 20.0 nm) were examined in the range from 4 to 570 K. Two resonance lines were observed. The change in line position is essentially governed by the strong temperature dependence of the crystalline anisotropy constant K 1 which was found to be close to the value of bulk Fe. The linewidth of the observed resonances decreases roughly with the inverse power of temperature. The narrowing is attributed to the decrease of the crystalline anisotropy and of the magnetostriction.

Ferromagnetic resonance investigations of particulate magnetic recording tapes

Abstract The FMR spectra of particulate magnetic tapes have been studied to determine their dependence on the type of material used, on the orientation of the tape with respect to the external magnetic field, on the angular distribution and packing density of the particles, and on the temperature. It is demonstrated that FMR is a very sensitive tool to investigate the influence of these parameters.

Imaging of Magnetic Properties Using Photothermal Techniques in Microwave Resonance

The interaction of microwaves with magnetic materials in determined by the high frequency magnetic susceptibility tensor. To achieve a spatially resolved measurement of this property two different photothermal approaches have been developed and applied [1]. One technique makes use fo the heat dissipated in the material in the course of the absorption of microwaves. Modulating the microwave input power thermal waves are generated which can be detected by the photoacoustic effect or by laser beam deflection. The photoacoustic mehtod was successfully applied to image the depth variation of the magnetization in layered magnetic tapes [2]. The laser beam deflection technique, in addition, provides a lateral resolution which has been used to visualize collective magnetic excitations in soft ferrites and in yttrium iron garnet slab [3,4]. In the latter material also the spatial evolution of magnetostatic modes in the linear and in the non-linear regime could be studied. To improve the spatial resolution and the sensitivity, a second technique has been developed that relies on the local modulation of the high frequency susceptibility by a thermal wave generated with an intensity modulated laser beam incident on the sample [5,6]. This technique has been used, for the first time, to obtain lateral resolved images of spherical magnetostatic modes excited at a microwave frequency of 9.2 GHz in an yttrium iron garnet sphere [7]. A further benefit of the photothermally modulated ferromagnetic resonance is the ability to visualize magnetic signals. The technique has been applied for the imaging of the magnetization distribution due to a magnetic signal recorded by a tape recorder on a particulate tape. As compared to the photoacoustic method and to the laser beam deflection technique, the photothermally modulated magnetic resonance provides a true three-dimensional tomographic method for magnetic properties with a superior sensitivity and spatial resolution. The signal strenth and shape, on the other hand, depends beside the magnitude of the high frequency susceptibility also on its temperature derivative, which complicates the quantitative description of the signal generation process. In most systems investigated untili now, the photothermally modulated ferromagnetic resonance signal is governed by the temperature coefficient of the spontaneous magnetization. Here the signal can be modelled straight forward on the basis of the magnetization behaviour. A dramatic intensity enhancement accompanied by a change of the signal shape is predicted on the basis of this mechanism when the magnetic transition temperature is approached. The predicted behaviour could be confirmed experimentally in the case of a yttrium iron garnet sample [8].

Photothermally modulated ferromagnetic resonance investigations of epitaxially grown thin films (abstract)

Photothermally modulated (PM) ferromagnetic resonance (FMR) is applied, for the first time, to locally investigate the magnetic anisotropy of thin epitaxially grown Fe and Co films. The PM‐FMR measurements were performed at 9.2 GHz in an optical cavity. The microwave resonance absorption was thermally modulated by an intensity modulated laser beam and the change of the microwave power reflected at the cavity was synchronously detected by a lock‐in amplifier. By focusing the laser beam a lateral resolution of 20 μm could be achieved. The PM‐FMR signal was studied as a function of the position of the laser spot on the film, of the modulation frequency (10 Hz–10 MHz) and of the temperature. The PM‐FMR measurements were complemented by temperature (5–700 K) and angle dependent conventional FMR studies. The temperature variation of the PM‐FMR and its comparison with the behavior of the conventional signal yields evidence for a signal generation process of the PM‐FMR that is governed by anisotropy effects. Ther...