*Article* **Tuneable Magneto-Resistance by Severe Plastic Deformation**

**Stefan Wurster 1,\*, Lukas Weissitsch 1, Martin Stückler 1, Peter Knoll 2, Heinz Krenn 2, Reinhard Pippan <sup>1</sup> and Andrea Bachmaier <sup>1</sup>**


Received: 9 October 2019; Accepted: 31 October 2019; Published: 5 November 2019

**Abstract:** Bulk metallic samples were synthesized from different binary powder mixtures consisting of elemental Cu, Co, and Fe using severe plastic deformation. Small particles of the ferromagnetic phase originate in the conductive Cu phase, either by incomplete dissolution or by segregation phenomena during the deformation process. These small particles are known to give rise to granular giant magneto-resistance. Taking advantage of the simple production process, it is possible to perform a systematic study on the influence of processing parameters and material compositions on the magneto-resistance. Furthermore, it is feasible to tune the magneto-resistive behavior as a function of the specimens' chemical composition. It was found that specimens of low ferromagnetic content show an almost isotropic drop in resistance in a magnetic field. With increasing ferromagnetic content, percolating ferromagnetic phases cause an anisotropy of the magneto-resistance. By changing the parameters of the high pressure torsion process, i.e., sample size, deformation temperature, and strain rate, it is possible to tailor the magnitude of giant magneto-resistance. A decrease in room temperature resistivity of ~3.5% was found for a bulk specimen containing an approximately equiatomic fraction of Co and Cu.

**Keywords:** severe plastic deformation; high pressure torsion; microstructural characterization; magnetic properties; hysteresis; magneto-resistance
