# Microstructural Changes Influencing the Magnetoresistive Behavior of Bulk Nanocrystalline Materials
## 3 Results  
### In-Situ HEXRD Measurement  
In order to better understand microstructural changes of the severely deformed materials upon thermal treatments, one composition out of the Cu-Co-system, Cu\({}_{55}\)Co\({}_{45}\), was annealed in an in-situ HEXRD experiment. Figure 2 shows XRD patterns at well-defined points in time of the annealing treatment. Changes in peak shape and peak shift can easiest be seen for large diffraction angles 20, e.g., in the region between 11\({}^{\circ}\) and 12\({}^{\circ}\). There, the deconvolution of the peaks into a Cu-rich and a Co-rich phase using two pseudo-Voigt functions can be expected to be more accurate. The assumption of two prevailing phases was based on the appearance of broad initial [311]- and [222]-peaks (lowest curve in Figure 2). On the other hand, [PERSON] et al. [25; 26] found a single, supersaturated face centered cubic (fcc) phase, when investigating HPT-deformed material of a different chemical composition (Cu\({}_{24}\)Co\({}_{26}\)). Thus, the appearance of one or two phases in the as-deformed state can most likely be attributed to the difference in composition of the investigated materials. The Cu-Co fcc-phases stayed stable throughout the complete in-situ annealing experiment. No complete demixing or phase transformation of the fcc-Co-rich phase into hexagonal close packed (hcp) Co has been observed after holding the temperature for 1 h at 490 \({}^{\circ}\)C. Due to the passing of  
Figure 1: Schematic drawings of the giant magnetoresistance (GMR)—specimen position (green) within small (8 mm diameter, left) and large (30 mm diameter, right) HPT-discs. the fcc-hcp transition temperature of Co (422 \({}^{\circ}\)C [27]) upon cooling, a substantial increase in hcp-Co content could be expected. However, the amount of hcp-Co after thermal treatment was found to be negligible, see uppermost pattern in Figure 2.  
Comparison of the peak positions with calculated peak position for pure Cu (a = 0.3615 nm [28]) and pure Co\({}_{\text{fcc}}\) (a = 0.3537 nm [29]) shows that some Co is dissolved in Cu and vice versa. However, upon annealing both peaks tend towards the position for pure elements and the double peaks become more prominent. According to [PERSON]'s law [30] and taking the mean value of [311]- and [222]-peaks, the compositions Cu\({}_{94}\)Co\({}_{6}\) and Cu\({}_{51}\)Co\({}_{49}\) are calculated for the two fcc-phases in the as-deformed state. For the material annealed at 490 \({}^{\circ}\)C, they change to Cu\({}_{97}\)Co\({}_{3}\) and Cu\({}_{42}\)Co\({}_{58}\). Both phases, fcc-Cu-rich and fcc-Co-rich, are still supersaturated. The occurrence of supersaturated, metastable states after HPT deformation was already found for a variety of systems, such as Cu-Ag [31], Cu-Co [32], and Cu-Fe [16]. To give an even better idea of the processes taking place during the in-situ annealing, contour plots of the measured intensity are provided in Figure 3. This includes a detailed view for large 20-values (Figure 3b), where peak shifts and splitting in course of the experiment can easiest be followed. For doing so, pseudo-Voigt profiles are fitted in parts of each recorded pattern and the resulting center values for [222]-planes of Cu and Co are displayed in Figure 3b. The region of temperature increase, up to the time of \(\sim\)3000 s, is governed by a linear decrease in peak position due to thermal expansion. Within the region of constant temperature (time: \(\sim\)3000 s--5600 s) the [222]-peaks tend towards the positions of pure elements, due to ongoing demixing of Cu and Co. At maximum temperature, the Co-line shifts more strongly than the Cu-line. The final peak positions after cooling down, are slightly shifted towards the pure elemental lines. However, judging from the gap between  
Figure 2: Five high-energy X-ray diffraction (HEXRD) patterns taken out of the sequence of more than 1200 patterns, recorded during the in-situ annealing treatment of Cu\({}_{55}\)Co\({}_{45}\) at 490 \({}^{\circ}\)C. The time frame shown above covers the complete experiment.