Th. Orth

Thermal wave imaging by photothermally modulated ferromagnetic resonance

Thermal wave modulated ferromagnetic resonance has been used to investigate local magnetic properties of ferromagnetic materials. The sample inside the microwave cavity is exposed to a modulated laser beam and the reflected microwave power is measured as a function of beam position and modulation frequency. First results are obtained from magnetic recording tapes.

Experimental and theoretical studies of transverse susceptibility in recording media

Introduction An experimental study of the transverse susceptibility xt has been undertaken. Measurements have been performed on various types of samples. In parallel with the expenmental work a model has been set up with a system of Stoner Woblfarth (S-W) particles. Good agreement is found between the experimental and theoretical characteristics of the k t response. Theory Aharoni eta1 developed a theoretical expression for x t for a random system. which we have generalised for some arbitrary texture function f(&, q6k) representing the distribution of easy axis directions. Thus;

Photothermal Imaging Using Microwave Detection

Local thermal modulation of magnetic resonance is used to obtain images of magnetic structures in recording tapes, amorphous metallic alloys and metallic films. The theoretical and experimental concepts of the photothermally modulated magnetic resonance are discussed and the figures of merit of this novel method are compared with other photothermal detection techniques in microwave experiments.

Characterisation of the anisotropy behaviour of different cobalt modified γ-Fe2O3 tapes

Abstract We have investigated Co-surface adsorbed iron oxide particles with constant cobalt content and varying penetration depth of the Co 2+ ions by means of ferromagnetic resonance, transverse susceptibility and VSM measurements. For all surface modified particles we have found that beside shape anisotropy, uniaxial as well as multiaxial anisotropy contributions play an important role in the magnetic behaviour, depending on the distribution width of the adsorption layer. The multiaxial contribution increases with increasing distribution width, while the uniaxial contribution decreases. In the scope of our theoretical models, we have calculated the uniaxial anisotropy constants at room temperature to be K u = 3.5 × 10 5 J/m 3 for a distribution width of 2.5 A and K u = 1.1 × 10 5 J/m 3 for a distribution width of 6 A.

Deconvolution of anisotropy field distributions from transverse susceptibility measurements

Abstract The reversible transverse susceptibility is a measurement often used to determine the anisotropy field of magnetic materials. A maximum entropy approach issued to determine the dispersion of H k values for a system of randomly oriented CrO 2 particles.

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.

Temperature-dependent conventional and photothermally modulated FMR measurements on CrO2 particulate tapes

Abstract We have used conventional and photothermally modulated (PM-) FMR to determine the temperature dependence of the magnetisation M ( T ), and the uniaxial magnetocrystalline anisotropy parameters, K 1 ( T ) and K 2 ( T ), of CrO 2 tapes. The distribution of the particle easy axes and magnetostatic interactions are considered in the theoretical analysis of the resonance field, the line width and shape of the absorption curve.

Imaging of magnetic materials by photothermally modulated ferromagnetic resonance

Abstract Thermal modulation of magnetic properties produced by focused laser illumination has been used to measure locally the temperature derivative of ferromagnetic resonance microwave absorption. Images have been obtained from stored information in magnetic recording tapes, strain-induced structures in amorphous metallic alloys and inhomogeneities in thin evaporated permalloy films.

Photothermally modulated readback from a magnetic tape

The photothermally modulated (PM) readback technique is based on the thermal modulation of a recorded magnetic pattern. The PM-readback signal that is detected by a conventional tape recorder head comprises lateral and depth resolved information of the recorded signal. The PM-readback has been studied from homogeneously magnetized tapes and from a sinusoidal track recorded on a CrO/sub 2/ tape. From the signals measured as a function of laser spot position and of modulation frequency the depth and lateral variation of the magnetization has been determined.

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].