Tuneable Magneto-Resistance by Severe Plastic Deformation

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.

Microstructural Changes Influencing the Magnetoresistive Behavior of Bulk Nanocrystalline Materials

“…deformed Cu-Fe already shows an increasing bcc-Fe fraction when annealing at 150 °C [16] and substantial decomposition at 500 °C [16,22], whereas Cu-Co is stable regarding substantial decomposition up to at least 490 °C (this work, for higher Co-content [36]). For both Fe-containing specimens, GMR is higher in comparison to Cu85Fe15 [11] with a RT GMR in the as-deformed state of 0.2%. highest GMR for annealing at 300 °C and all other Co-materials also increase in GMR, this is not the case for Cu-Fe, where a smaller GMR was found after annealing at 300 °C.…”

“…Regarding the optimum composition for maximum GMR, the results of this study shall be discussed in combination with results taken from [11]. Therein, the RT GMR of both, Cu81Co19 and…”

“…Furthermore, the specimens were placed i) with the direction of applied current perpendicular to the magnetic field and ii) with the current parallel to the magnetic field. Specimens using the same production route and of varying Co-contents were already investigated under the same conditions and the results are presented in [11]. Therein, a gradual change from isotropic GMR for low ferromagnetic content to AMR with increasing Co-content is reported.…”

“…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 Cu85Fe15 showed the limitations of the existing setup, due to the specimen's increased conductivity and the small RT GMR-effect (~0.2% [11]). Details on investigated sample composition can be found in Table 1.…”

Magneto-Transport Properties of Co–Cu Thin Films Obtained by Co-Sputtering and Sputter Gas Aggregation

“…Co is practically immiscible in Cu at room temperature, but several preparation methods (as, e.g., sputtering, melt-quenching, electrodeposition, laser ablation, etc. [ 5 , 6 , 7 , 8 , 9 ]) allow to obtain a metastable solid solution of a small quantity of Co in Cu. In this situation, the (energetically more favorable) segregation of small precipitates of Co takes place (after annealing or, sometimes, even at preparation), giving rise to the so-called granular alloys.…”