EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

High pressure torsion(HPT), a method of severe plastic deformation(SPD) was initially used by Bridgman to get deeper insights into geological and mineralogical processes. HPT has unique process conditions and enables scientists to study bulk specimens of normally rather difficult to deform materials like oxides, ceramics, glasses, etc. For oxides in particular, HPT is capable of influencing properties as e.g. grain size, oxygen vacancies, dislocation activity and phase transformation.
In this study, HPT is used to produce bulk nanostructured materials from initial powders. The aim is a synthesis of nanocomposite, consisting of an antiferromagnetic(NiO) and a ferromagnetic-phase(FeNi). Such a nanocomposite can exhibit a coupling of spins over phase boundaries, the so-called exchange-bias.
Interestingly, during HPT processing NiO is extremely sensitive to composition of FeNi and deformation temperature. When initial powders are processed at low temperatures hardly any NiO is observed to be deformed in the desired nanostructure. By optimizing processing parameters, it is possible to obtain and control a nanostructured NiO or even be able to produce a lamella nanostructure of FeNi-NiO. Combination of NiO with Fe or Ni alone, show severe difficulties provoking synthesis of a nanocomposite by HPT. Just with an additional ball milling preparation step prior to HPT processing, a lamella nanostructure of Ni-NiO is obtained.
Squid magnetometric measurements on FeNi-NiO reveal the presence of the exchange-bias effect, being a direct evidence to successful preparation of nanostructured antiferromagnetic-ferromagnetic phases by HPT. These results are comparable to other processing methods like thin film deposition. This study is largely devoted to demonstrate the possibility of tailoring magnetic materials with SPD techniques.
This project has received funding from the European Research Council(ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No:757333)