M. Zawodzki¹, L. Weissitsch¹, R. Pippan¹, P. Knoll², H. Krenn², S. Wurster¹ and A. Bachmaier¹

¹Erich Schmid Institute of Materials Science of the Austrian Academy of Sciences, Jahnstrasse 12, 8700 Leoben, Austria

²Institute of Physics, University of Graz, Universitätsplatz 5, 8010 Graz, Austria

High pressure torsion (HPT), a severe plastic deformation (SPD) technique, has already been used over two decades to synthesise bulk nanocrystalline materials to study its mechanical and physical properties. Through the large variation of applied strain and hydrostatic pressure HPT enables, to a certain degree, a control of microstructure and therefore a tailoring of material properties.

The focus in this study is the processing of ferromagnetic (FM) and antiferromagnetic (AFM) materials obtaining a composite material with nanostructured phase sizes. Furthermore, to demonstrate a correlation between microstructure and exchange bias (H_eb). This H_eb is a permanent coupling of FM-spins over the phase interface to the AFM-spins causing a shift of the hysteresis loop. H_eb was studied in thin films techniques to investigate influences of thickness, AFM-material, crystallographic orientation at the interface. Still detailed reports on bulk material are seldom, here HPT offers a unique opportunity to gain an insight in the dependence of Heb on microstructure.

The investigated bulk nanocomposites were processed from initial powders. Powder compositions consist of (Fe)Ni as FM- and NiO as AFM-phase. For powder blends with a high fracture of NiO prior ball milling to HPT processing was performed to enhance phase intermixing. Processed bulk material was analysed by scanning electron microscopy (SEM), X-ray diffraction (XRD), Vickers microhardness testing and superconducting quantum interference device (SQUID). Microhardness of deformed nanocomposites was unusually high and SEM measurements showed a refinement of phase size over applied strain. Further wide angle X-ray scattering (WAXS) investigations conducted at Deutsches Elektronen-Synchrotron (DESY) revealed a coherent scattering domain size of a few tenths of nanometres.

Microstructural characterisations were correlated to hysteresis loop measurements. By SQUID measurements, an H_eb of the hysteresis loops was detected. Furthermore, an influence of applied strain was demonstrated on magnetic parameters like H_eb and magnetic saturation. Comparing powders blends with a lower and higher fraction of NiO display the importance of powder composition on the refinement process during HPT synthesis. Additionally, the influence of phase interface morphology on magnetic parameters could be demonstrated.

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).

We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at (PETRA III) and we would like to thank (N. Schell and E. Maawad) for assistance in using (P07B- High Energy Materials Science).