# Microstructural Changes Influencing the Magnetoresistive Behavior of Bulk Nanocrystalline Materials
## 3 Results
### Improving the Magnetoresistive Effect  
After screening the influence of annealing temperature onto the GMR-behavior of a material of medium ferromagnetic content, the same treatment is performed on materials of lower ferromagnetic content. As it is known from literature [13], there is an optimum ferromagnetic content of approximately from 15 to 25%. The HPT-processability for binary Cu-Fe and Cu-Co alloys was investigated in [21], where it was stated that samples of low Co content feature large remaining Co-particles, while the deformation of samples of increasing Fe-content (\(>\) -25%) is accompanied with shear band formation. Furthermore, preliminary experiments on annealed Cu\({}_{55}\)Fe\({}_{15}\) showed the limitations of the existing setup, due to the specimen's increased conductivity and the small RT GMR-effect (\(\sim\)0.2% [11]). Details on investigated sample composition can be found in Table 1.  
The HEXRD results, shown in Figure 8 demonstrate the supersaturation of Cu with Co or Fe, except for the specimen of Cu\({}_{69}\)Fe\({}_{31}\) still showing a substantial amount of the body centered cubic (bcc) phase.  
GMR was determined for the specimens in the as-deformed and in one annealed state. An annealing temperature of 300 \({}^{\circ}\)C yielded best results for Cu\({}_{55}\)Co\({}_{45}\). Thus, this temperature was chosen to be used for annealing of MR-specimens of low ferromagnetic content and the results are summarized in Figure 9. Therein, only the perpendicular specimen orientation is shown, due to the almost perfect isotropic behavior. GMR increases for the annealed Cu-Co, while the 300 \({}^{\circ}\)C-annealed Cu-Fe already decreases in GMR. The occurrence of the maximum GMR already in the as-deformed (or as-sputtered state, as it was the case for [PERSON] and [PERSON] [5]) state cannot be ruled out. Decreasing GMR-effect for annealed Cu-Fe can be explained by formation and growth of Fe grains at low temperatures. HPT-deformed  
Figure 7: Drop in resistance as a function of applied magnetic field for Cu\({}_{55}\)Co\({}_{45}\) in as-deformed and different annealed states. (**a**) Current applied perpendicular to the magnetic field, (**b**) current applied parallel to the magnetic field.  
Cu-Fe already shows an increasing bcc-Fe fraction when annealing at 150 \({}^{\circ}\)C [16] and substantial decomposition at 500 \({}^{\circ}\)C [16; 22], whereas Cu-Co is stable regarding substantial decomposition up to at least 490 \({}^{\circ}\)C (this work, for higher Co-content [36]). For both Fe-containing specimens, GMR is higher in comparison to Cu\({}_{85}\)Fe\({}_{15}\)[11] with a RT GMR in the as-deformed state of 0.2%.  
A summary of all GMR values in magnetic fields of 22.5 kOe can be found in Figure 10.  
Figure 8: HEXRD pattern of Cu–Co and Cu–Fe samples of low ferromagnetic content, in as-deformed states. In addition, the lines of pure elements are shown, data for bcc-Fe was taken from [35].  
Figure 9: GMR as a function of applied magnetic field for specimens of low ferromagnetic content in the as-deformed state and after annealing for 1 h at 300 \({}^{\circ}\)C. Current flow was perpendicular to the applied magnetic field.