TY - THES

T1 - Magnetoresistive behavior of nanocrystalline materials processed by high-pressure torsion

AU - Kasalo, Manoel

N1 - no embargo

PY - 2022

Y1 - 2022

N2 - Severe plastic deformation by high-pressure torsion was used to fabricate nanocrystalline materials in a bulk form showing the granular giant magnetoresistance effect. Immiscible binary and ternary alloys, consisting of nonmagnetic (Ag, Cu, and Cr) and ferromagnetic (Co, Fe, and Ni) elements, were investigated. The ferromagnetic content was chosen to be 20 vol% since a peak in magnetoresistance is expected to occur, and additionally to be 40 vol% for Cu- and Cr-based compositions. Microstructural changes due to deformation and annealing significantly influence the magnetoresistive properties of the material. These changes were studied using light microscopy, scanning electron microscopy, microhardness, and X-ray diffraction measurements. A single-phase structure was observed for all as-deformed Cu-based and Cr-based alloys, indicating the formation of a supersaturated solid solution. However, Cr-based alloys showed higher brittleness, whereby Ag-based compositions exhibited partially insufficient co-deformation of the ferromagnetic phase. Granular giant magnetoresistance was measured in almost all as-deformed materials and tuned in a further step with appropriate thermal treatments. Primarily, annealing had a positive effect on the magnitude of magnetoresistance. The highest drop in resistivity of 2.45 % was found in Cu60Fe20Ni20 after annealing for 1h at 400°C, whereby the effect decreases for higher annealing temperatures. Severe plastically deformed materials showed remarkable high-temperature stability of phases and microstructure.

AB - Severe plastic deformation by high-pressure torsion was used to fabricate nanocrystalline materials in a bulk form showing the granular giant magnetoresistance effect. Immiscible binary and ternary alloys, consisting of nonmagnetic (Ag, Cu, and Cr) and ferromagnetic (Co, Fe, and Ni) elements, were investigated. The ferromagnetic content was chosen to be 20 vol% since a peak in magnetoresistance is expected to occur, and additionally to be 40 vol% for Cu- and Cr-based compositions. Microstructural changes due to deformation and annealing significantly influence the magnetoresistive properties of the material. These changes were studied using light microscopy, scanning electron microscopy, microhardness, and X-ray diffraction measurements. A single-phase structure was observed for all as-deformed Cu-based and Cr-based alloys, indicating the formation of a supersaturated solid solution. However, Cr-based alloys showed higher brittleness, whereby Ag-based compositions exhibited partially insufficient co-deformation of the ferromagnetic phase. Granular giant magnetoresistance was measured in almost all as-deformed materials and tuned in a further step with appropriate thermal treatments. Primarily, annealing had a positive effect on the magnitude of magnetoresistance. The highest drop in resistivity of 2.45 % was found in Cu60Fe20Ni20 after annealing for 1h at 400°C, whereby the effect decreases for higher annealing temperatures. Severe plastically deformed materials showed remarkable high-temperature stability of phases and microstructure.

KW - severe plastic deformation

KW - high-pressure torsion

KW - nanocrystalline materials

KW - magnetic properties

KW - magnetoresistance

KW - microstructural characterization

KW - Hochverformung

KW - Hochdruck-Torsionsverformung

KW - Nanokristalline Werkstoffe

KW - Magnetische Eigenschaften

KW - Magnetowiderstand

KW - Gefügecharakterisierung

M3 - Master's Thesis

ER -