Claas Thede

Magnetic moment investigation by frequency mixing techniques

Gas turbines and other large industrial equipment are subjected to high-temperature oxidation and corrosion. Research and development of efficient protective coatings is the main task in the field. Also, knowledge about the depletion state of the coating during the operation time is important. To date, practical nondestructive methods for the measurement of the depletion state do not exist. By integrating magnetic phases into the coating, the condition of the coating can be determined by measuring its magnetic properties. In this paper, a new technique using frequency mixing is proposed to investigate the thickness of the coatings based on their magnetic properties. A sensor system is designed and tested on specific magnetic coatings. New approaches are proposed to overcome the dependency of the measurement on the distance between coil and sample that all noncontact techniques face. The novelty is a low cost sensor with high sensibility and selectivity which can provide very high signal-to-noise ratios. Prospects and limitations are discussed for future use of the sensor in industrial applications.

Thermal Stability of the Ferromagnetic In-Plane Uniaxial Anisotropy of Fe-Co-Hf-N/Ti-N Multilayer Films for High-Frequency Sensor Applications

In this study, the thermal stability of the in-plane uniaxial anisotropy field of Fe₃₂Co₄₄Hf₁₂N₁₂/Ti₅₀N₅₀ multilayer films was investigated from room temperature up to 500 °C with regard to contactless high-frequency sensor applications. The uniaxial anisotropy field of about μ_oH_u ≈ 5 mT was thermally induced by annealing the films in a static magnetic field of 50 mT in vacuum for one hour either at an annealing temperature of T_α = 400 °C or T_α = 600 °C. The films which were annealed at T_α = 600 °C show a clear distinction between the easy and hard axis of polarization up to temperatures of 500 °C. Hence, the uniaxial anisotropy, thermally induced at T_α = 600 °C, is stable at working temperatures up to 500 °C. In contrast, films with a uniaxial anisotropy induced at T_α = 400 °C show a different behavior. Here, hysteresis loop measurements indicate that above a working temperature of 200 °C the uniaxial anisotropy field turns away from its direction at room temperature. In conclusion, only the Fe₃₂Co₄₄Hf₁₂N₁₂/Ti₅₀N₅₀ multilayer films with a uniaxial anisotropy field induced at T_α = 600 °C promise a great potential for contactless high-frequency sensor applications up to 500 °C.

Reconstruction of Magnetization Curve Using Magnetic Spectroscopy

A new measurement principle based on the frequency mixing technique for investigating the shape of the magnetization curve of soft non-hysteretic magnetic materials is introduced. Based on Taylor expansion of the magnetization curve and spectral investigation of an inductively detected signal, a mathematical model for the reconstruction of M(H) is proposed, [7]. Here, the model is experimentally verified using a nanocrystalline soft magnetic material with defined properties. It is shown that the magnetization curve can be reconstructed very accurately and the influence of an additional parameter, i.e. strain, can be investigated in detail as well.