Exchange bias of nanostructured materials processed by severe plastic deformation

In the course of this thesis, different metallic powder blends, consisting of ferromagnetic and antiferromagnetic phases, were consolidated and deformed using high-pressure torsion. The aim was to detect a magnetic phenomenon called exchange bias in the resulting nanocomposites. The exchange bias has been predominantly detected in bilayer or multilayer thin-film samples and in nanoparticles. Over three decades, the exchange bias has been used for magnetic read heads in hard disc drives. There, the exchange bias stabilises the ferromagnetic reference layer by magnetic coupling it to an antiferromagnetic material. The introduction of antiferromagnetic materials enabled the use of the giant magnetoresistance phenomenon and allowed the miniaturisation of hard disk drives to progress. A detailed description of the exchange bias and the giant magnetoresistance is provided in the first chapter in the present thesis. During this thesis, it was possible to overcome the so far existing two-dimensional limitation using the high-pressure torsion method. These novel synthesised, three-dimensional, nanostructured composites were systematically analysed regarding the microstructure and magnetic parameters. In addition, important insights into the deformation behaviour of metallic oxides in combination with a metallic phase, were gained through the application of high-pressure torsion deformation. The most extensively investigated powder blends were Fe50NiO50, Fe10Ni40NiO50, Ni50NiO50, Ni40Cr60, and Fe50FeS50, as well as the powder blends Ni05NiO95, Ni17NiO83, and Ni30NiO70, which were ball milled before deformation. The methodical investigation of the Fe50NiO50 composition demonstrated the significant influence of the deformation temperature on microstructure evolution using scanning electron microscopy and high-energy X-ray diffraction. For the nanocomposite based on the Fe10Ni40NiO50 powder blend, a positive correlation between the applied strain and the exchange bias was demonstrated. The exchange bias was determined by measuring a complete hysteresis loop with a superconducting quantum interference device. Furthermore, the exchange bias was also stable at room temperature, and the energy product of the nanocomposite was significantly increased compared to the ferromagnetic phase alone. With the improvement of the magnetic parameters, a decrease in crystallite size in the ferromagnetic phase was detected by high-energy X-ray diffraction and a refinement of the microstructure was observed by scanning electron microscopy. The Vickers microhardness was extremely high for the Fe10Ni40NiO50 nanocomposite. In contrast, no significant change in crystallite size or exchange bias was detected with the Ni50NiO50 nanocomposites. The magnetic parameters were relatively constant for different amounts of applied strain. By directional field cooling of the nanocomposites within an external magnetic field in the radial, axial, and tangential directions in regard to the shear plane, it was demonstrated that the coercivity and consequently the exchange bias were approximately the same in all directions, although a slight texture of the ferromagnetic phase was detected in high-energy X-ray diffraction experiments. This result is new and has not been found before in other ferromagnetic-antiferromagnetic material systems. Using transmission Kikuchi diffraction, it was possible to estimate the grain size distribution in the nanocomposites. The influence of this distribution on the magnetic parameters was shown by setting different target temperatures during the field cooling procedure. Furthermore, an exchange bias was detected that was perpendicular to the applied magnetic field during field cooling. This detection is a further novelty and was attributed to the complex interfacial structure of the ferromagnetic and antiferromagnetic phases. An annealing treatment of the Fe10Ni40NiO50 and Ni50NiO50 nanocomposites showed for both compositions a clear phase growth and change of the interface morphology in scanning electron microscopy investigations, a crystal growth detected by X-ray diffraction and a reduction of the exchange bias as well as the remanence. Using NixNiO1-x nanocomposites, it was demonstrated that the Ni content is crucial for good deformation of both phases during processing. The microstructure and magnetic parameters improved drastically by increasing the Ni content from 5 to 50 at.%. Increasing the deformation temperature from 300 to 450 °C showed a significant improvement in the microstructure and the associated magnetic properties. The improved deformation was attributed to the activation of additional slip systems in NiO due to the higher deformation temperature. No exchange bias was detected for the previously mentioned powder blends Ni40Cr60 and Fe50FeS50 , but deformation by high-pressure torsion was possible for both powder blends.