# Microstructural Changes Influencing the Magnetoresistive Behavior of Bulk Nanocrystalline Materials
## 2 Materials and Methods  
Binary powder blends of Cu (Alfa Aesar, Ward Hill, MA, USA, 99.9%, \(-\)170 +400 mesh) with either Co (Goodfellow, Hamburg, Germany, 99.9%, 50-150 \(\upmu\)m) or Fe (MaTeck, Julich, Germany, 99.9%, \(-\)100 +200 mesh) were used for sample production. Powder blends were stored in a glove box, compressed in argon atmosphere or air and deformed to very high strains at RT using an HPT device. The applied strain linearly increases with the radius of the HPT-disc. Samples of varying low ferromagnetic content, where according to literature highest GMR can be expected [13], were produced. On the other hand, materials of medium ferromagnetic content show saturation of magnetization in smaller fields [21; 22] and consequently, saturation of GMR is achieved easier. Thus, one sample of medium ferromagnetic content was produced. While specimens of low ferromagnetic content were deformed with small HPT-tools (diameter \(\sim\)8 mm, resulting sample thickness \(\sim\)0.5 mm), the specimen of medium Co content was deformed with HPT-tools of 30 mm diameter (thickness \(\sim\)7 mm), demonstrating the ability to upscale the HPT-process. Table 1 gives an overview of the produced samples, including their exact composition determined with energy dispersive X-ray (EDX) spectroscopy. Long and thin specimens for measuring the resistance at RT via the 4-point-probe method were prepared by careful cutting and grinding. For GMR-specimens made out of the smaller HPT-discs, the distance of GMR-specimen to the center of the disc was at least 1.5 mm, for the large HPT-disc at least 9 mm. The GMR-specimens were prepared along a secant line of the small HPT-discs, while the GMR-specimen is in axial direction for the large disc (see Figure 1 for positioning the GMR-specimen within the HPT-sample). The thickness of GMR-specimens was \(\sim\)200 \(\upmu\)m, the width \(\sim\)1 mm, the length (current flow direction) is several millimeters. For measuring the GMR, the specimens were placed in the air gap of an electromagnet. RT resistivity was determined in fields up to 22.5 kOe, taking into account two different current flow orientations (parallel and perpendicular) in respect to the magnetic field. Please note, the maximum achievable magnetic field is smaller than the saturation field of most investigated materials. Consequently, the reported GMR values are not saturation values, but values measured at the maximum field of 22.5 kOe. \begin{table}
\begin{tabular}{c c c} \hline
**Composition [at\%]** & **HPT—Tool and Sample Diameter [mm]** & **No. of Rotations** \\ \hline Cu\({}_{82}\)Co\({}_{18}\) & 8 & 100 \\ Cu\({}_{70}\)Co\({}_{30}\) & 8 & 100 \\ Cu\({}_{55}\)Co\({}_{45}\) & 30 & 250 \\ Cu\({}_{76}\)Fe\({}_{24}\) & 8 & 100 \\ Cu\({}_{69}\)Fe\({}_{31}\) & 8 & 100 \\ \hline \end{tabular}
\end{table} 
Table 1: High-pressure torsion (HPT)-processed samples, with their composition in at% and processing parameters such as HPT-tool diameter and number of rotations. Microstructural and chemical investigations of the as-deformed, annealed HPT-samples and GMR-specimens were performed using a scanning electron microscope (SEM) equipped with EDX (SEM: LEO1525, Zeiss, Oberkochen Germany, EDX: e'Flash, Bruker, Berlin, Germany). All compositions within this publication were determined by EDX and are given in atomic percentage if not stated otherwise. High-energy X-ray diffraction (HEXRD) experiments were performed at beamline P21.2 [23] at the synchrotron PETRA III (DESY, Hamburg, Germany) using a photon energy of 60 keV and a beam size of about \(0.2\times 0.2\) mm on the sample. The diffraction patterns were recorded with a Varex XRD 4343 flat panel detector and azimuthally integrated using the pyFAI software package [24]. Besides ex-situ measurements of the material in as-deformed and annealed states, the complete process of annealing was captured for one specimen by in-situ HEXRD.