# Microstructural Changes Influencing the Magnetoresistive Behavior of Bulk Nanocrystalline Materials
## 5 Conclusions  
Using SEM and synchrotron diffraction in combination with resistivity measurements in magnetic fields, it was possible to establish a link between the microstructure and functional properties of severe plastically deformed materials. Magnetoresistance was found to be improvable by adequate thermal treatments of as-deformed samples.  
Different binary compositions containing Cu and Fe or Co were severe plastically deformed and investigated in the as-deformed and different annealed states. For Cu\({}_{55}\)Co\({}_{45}\), an annealing treatment for 1 h at 300 \({}^{\circ}\)C was found to give the highest GMR at fields of 22.5 kOe. An increase in GMR was also found for lower Co-content using the same thermal treatment. Cu-Fe shows smaller drops in resistivity in fields of 22.5 kOe compared to Cu-Co. Furthermore, annealing Cu-Fe at 300 \({}^{\circ}\)C was found to be too high for improving GMR.  
In general, with the technique of SPD using HPT, it is possible to produce bulk nanocrystalline materials showing GMR behavior at RT. The GMR-ratio is smaller in comparison to well-established multilayer systems, already in application, however comparable to literature values of materials showing _granular_ GMR. The usage of SPD by HPT offers several advantages: First, the sample size processed in this paper reaches 5 cm\({}^{3}\), with a possibility to achieve sample volumes up to 34 cm\({}^{3}\)[38]. Second, for a particle size aspect ratio close to 1 of a dilute ferromagnetic phase within the large HPT sample (diameter >30 mm), an isotropic behavior independent of GMR-probe extraction direction can be expected. Third, the temperature stability of the investigated Cu-Co system is astonishingly high with the GMR-ratio remaining stable for annealing treatments for 1 h at temperatures up to 400 \({}^{\circ}\)C.  
Conceptualization, S.W. and A.B.; methodology, S.W., M.S., L.W., T.M., and A.B.; software, S.W. and T.M.; validation, S.W.; formal analysis, S.W.; investigation, S.W., M.S., L.W., T.M., and A.B.; resources, T.M. and A.B.; data curation, S.W.; writing--original draft preparation, S.W.; writing--review and editing, S.W., M.S., L.W., T.M., and A.B.; visualization, S.W.; supervision, A.B.; project administration, A.B.; funding acquisition, A.B. All authors have read and agreed to the published version of the manuscript. 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).  
S.W. deeply appreciates the help of [PERSON] and [PERSON] for performing the GMR specimen preparation and assistance in resistance measurements. 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 beamline P21.2 at PETRA III under proposal I-20190577.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.