Nondestructive Testing of Nanocrystalline Materials by Barkhausen-Noise, Ultrasound and Magneto-optic Kerr-Effect

Project coordinator: Univ.Prof. Dr. H. Krenn (University of Graz)

The microphysical properties like internal strain, chemical composition and disorder of novel nano-phase materials (= NPM) which are prepared either by severe plastic deformation (SPD) or by the crystallization/compaction route should be investigated by nondestructive testing methods. Contactless measurements will be performed to study the propagation and damping of laser-pulse-induced bulk and surface acoustic waves for nonmagnetic as well as magnetic samples. The spectral evaluation of the Barkhausen-noise takes into account dynamical aspects of the magnetization reversal which is expected to differ for SPD-materials, and the magneto-optic Kerr effect is more convenient than magnetometric measurements using superconducting quantum interference devices (SQUIDs), if magnetic properties are modulated by external influences apart from a magnetic field (e.g. by electric charging and stress-modulation). The interpretation of such "engineering"-like investigations on NPM's necessitates a strong feedback to the other project partners  expertise in sample processing, X-ray and neutron scattering, scanning (magnetic) force and electron microscopy, electric transport measurements and micromagnetic modelling. The relation between microstructure/micromagnetism and ultrasound propagation and damping, between coercivity and Barkhausen noise, between magnetization and Kerr rotation should be explored since research on SPD- and compacted functionalized NPM's in these fields is rather scarce. This could (i) augment the basic understanding of the physics behind in such complex, magnetostrictive and magnetic-shape-memory functional materials, but could also lead (ii) to useful specifications important for engineering applications. Of specific technical interest is the exploration of the structural homogeneity (porosity, texture, single- or multi-phase status, nano-size effects) of non-magnetic samples using acoustic waves. Hysteresis loop and magnetic relaxation measurements usually performed by SQUID-magnetometry shall be related to Barkhausen noise studies. Another interesting phenomenon connected with the Barkhausen jumps during magnetization reversal is the excitation of acoustic waves, which is called magnetoacoustic effect (MAE). The magnetic properties of compacted porous magnetic materials which are modulated by interfacial charging shall be monitored by magnetooptic Kerr rotation. It is intended to apply these techniques to bulk samples of various sizes and shapes, which are fabricated and pre-characterized by the partners. Micromagnetic modelling conducted by a project partner will substantiate the nondestructive testing results.