Magnetoresistive materials by severe plastic deformation
Severe plastic deformation (SPD) by high pressure torsion (HPT) was used to consolidate and deform different powder blends of diamagnetic and ferromagnetic powders. The resulting bulk materials were investigated regarding their resistive behaviour in magnetic fields. For low and medium ferromagnetic contents, an almost isotropic drop in resistivity with increasing magnetic field was found. This behaviour is well-known as giant magnetoresistance (GMR). It originates from spin-dependent scattering of conducting electrons at small ferromagnetic clusters and is mainly studied in layered thin films. HPT offers the opportunity to study this effect in bulk granular systems. Well-investigated model systems (Cu-Co, Cu-Fe) were used to demonstrate the possibility to produce these GMR granular systems by HPT; results are compared with literature, where materials were prepared with other techniques e.g. magnetron sputtering or ball milling.
In addition, HPT-deformed samples are subjected to thermal treatments to demonstrate whether it is possible to further increase the GMR-effect, as Fe or Co segregate out of a supersaturated fcc-phase and form ferromagnetic clusters. It could be shown for the CuCo system that GMR can be enhanced due to thermal treatments at intermediate temperatures. If the annealing temperature is chosen too high, Co-clusters grow too large and the GMR effect diminishes. To connect the results of resistivity measurements with the prevailing microstructure, this study is completed by electron microscopy and synchrotron measurements. For investigating the evolution of phases upon thermal treatments, in-situ synchrotron heating experiments were performed.
This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant No. 757333).