*3.2. Magnetic Properties—Magneto-Resistivity*

To investigate the change in magneto-resistive behavior for all produced samples, four-point resistance measurements were performed at room temperature in magnetic fields up to 22.5 kOe. The results of the drop in resistance with applied magnetic field, displayed according to Equation (1), are summarized in Figures 9 and 10. Two different testing directions were taken into account. First, applied current and applied magnetic field are parallel; second, current and magnetic field are perpendicular to each other. Probe current flows within the shear-tangential HPT-plane, except for Cu55Co45. Depending on the composition and processing parameters, different types of resistive behavior within magnetic fields can be identified.

**Figure 9.** Drop in room-temperature resistance as a function of magnetic fields for Co-based HPT-deformed samples. (**a**) Cu81Co19. (**b**) Cu64Co36. (**c**) black: Cu52Co48 deformed at room temperature; red: Cu49Co51 deformed at 200◦C; blue: Cu55Co45 deformed at elevated temperature with the large HPT-tool. (**d**) Cu43Co57. (**e**) Cu22Co78, deformed at 300 ◦C. (**f**) Pure Co. The continuous lines (parallel alignment) and dashed lines (perpendicular alignment) are derived from polynomial fits and serve only as a guide for the eye.

**Figure 10.** Drop in room-temperature resistance as a function of applied magnetic fields for HPT-deformed Cu85Fe15. The continuous line (parallel alignment) and dashed line (perpendicular alignment) are derived from polynomial fits and serve only as a guide for the eye.

For small Co-content, an almost isotropic dependence of resistance with magnetic field can be found (Figure 9a,b). With increasing ferromagnetic content, there is a gradual development of an anisotropic component, which is due to larger, percolating ferromagnetic regions. Thus, for medium and high ferromagnetic content, the resistance curve can be seen as a superposition of GMR and AMR. Pure Co (Figure 9f) is the best example for AMR with the resistance being higher for parallel current and magnetic field. For pure Co, the difference in resistivity between parallel and perpendicular orientation increases quickly for small fields; for higher fields, the curves are about parallel.

In case for replacing Co with Fe, the effect for low ferromagnetic content is identical for Fe and Co (Figure 10). The decrease in resistance with applied field is immediate at low fields. However, the effect is much smaller for Fe by around a factor of ten for fields as high as 22.5 kOe. A detailed discussion of the results of Figures 9 and 10 is provided in Section 4.2.

**Figure 11.** Hysteresis loop (specific magnetic moment versus applied field) of Cu55Co45, measured at 300 K, with the inset showing a detailed view for lower applied fields. For comparison, the magnetic moment of fcc-Co (164.8 emu/g [53]), is indicated as a dashed line.

The large sample deformed at elevated temperature (Cu55Co45) shows the largest reduction in resistance (~3.5% in perpendicular orientation) including a saturation of the decrease in resistance for high fields. For smaller Co content, an even larger GMR effect is expected—e.g., according to reference [17], the largest effect can be expected for a Co-concentration of 16%. Thin films, which were electrodeposited and annealed, were used for this work. However, as HPT-samples of these compositions saturate at very high fields due to paramagnetic contributions [40], the applied magnetic field is not large enough to saturate them. A sample that shows a satisfactory GMR as well as saturation in GMR at lower fields is Cu55Co45. It was chosen to be thoroughly investigated using SQUID. Figure 11 shows the result of hysteresis measurements at 300 K. The hysteresis was measured with the specimen's axial direction being parallel to the applied field. The coercivity was determined by linear interpolating between fields of +/− 1000 Oe and was found to be 316 Oe. For high applied fields, the saturation magnetization of bulk fcc-Co (164.8 emu/g [53]) is almost reached.
