Exploiting the Topological Electronic Properties of Weyl Semimetal Heusler Alloys for Spintronics Applications

Abstract

This thesis explores the physical properties, such as structure, magnetic, and electrical transport of 3 distinct families of Heusler thin-film materials. The primary goal is to understand how small adjustments in their chemical formation and growth conditions can impact important electronic phenomena such as Berry curvature effects, half-metallicity, and topological electronic states. Within the Co2MnGe1-xGax series, systematic substitution of Ga with Ge leads to a gradual and well-controlled evolution of the material’s electronic properties. This compositional tuning preserves high quality epitaxial L21 crystal order across the entire series on MgO (001) substrates, accompanied by smooth changes in lattice parameters and a reduction in X ray diffraction peak widths at intermediate compositions (x = 0.5). These structural trends confirm a continuous compositional evolution. There is a transition from Weyl semimetallic behavior in Co2MnGa to half metallic behavior in Co2MnGe identified through electronic transport measurements, particularly the magnitude of the anomalous Hall effect, rather than structural characteristics alone. Magnetic measurements reveal a consistent increase in saturation magnetization from approximately 700 to 900 emu/cc with increasing Ge content. Electrical resistance also evolves from non linear behavior in Ga rich films to low resistance metallic conduction in Ge rich compositions. An adapted resistivity model, based on established frameworks, successfully describes both the low temperature resistivity upturn and thermally activated magnon scattering. Crucially, the anomalous Hall effect is suppressed by nearly three orders of magnitude, consistent with the downward shift of Weyl nodes from approximately 20 meV below the Fermi level in Co2MnGa to about 0.5 eV below the Fermi level in Co2MnGe. This demonstrates a highly controlled modulation of Berry curvature through chemical substitution. The Mn-doped Co2MnGa system further extends this tunability by enabling systematic control of excess Mn, for x = -0.21 to +0.44. All prepared films exhibit highly ordered epitaxial L21 ordering. The saturation magnetization reaches a maximum value of 928 emu/cc under optimized growth conditions. The electrical resistivity maintains the characteristic Co2MnGa like behaviour, including a low temperature upturn and a bulge around 150 K, while its overall magnitude increases steadily with Mn content. This rise in resistivity reflects enhanced electron scattering associated with modified magnon and electron-magnon scatterings, rather than a degradation of long-range crystalline order, which instead improves with increasing Mn content as confirmed by X ray diffraction. Temperature dependent transport measurements over 5-300 K reveal that both the magnon activation energy and the degree of structural order are maximized near stoichiometric compositions. In addition, Mn rich films display a larger anomalous Hall effect, highlighting strong Berry curvature driven transport arising from Mn incorporation. Together, these results emphasize the interaction between tuning of electronic structure and scattering processes in stabilizing half metallic and topological transport properties. Lastly, the inverse-Heusler material Ti2MnAl was examined as a potential spin-gapless semiconductor or a compensated Weyl semimetal. This investigation was prompted by theoretical predictions of Weyl nodes very close to the Fermi level and inherent anomalous Hall conductivity, even without any overall magnetic moment. Growing thin films of this material proved extremely challenging due to its high sensitivity to precise atomic ratios, requiring a multi-target co-sputtering method to manage significant imbalances in Ti, Mn, and Al. Optimized films grown on MgO (001) substrates exhibited nearly ideal compositions, smooth interfaces, and good epitaxial alignment, though some crystalline imperfections remained. Magnetic measurements confirmed the expected near-zero magnetic behavior. Electrical transport indicated a mixture of semiconducting and metallic characteristics, along with a minor but visible anomalous Hall effect. While the observed effect was considerably smaller than theoretical predictions, these findings suggest that the material's topological electronic structure is partially preserved. It also highlights that disorder is the main barrier to fully achieving the ideal spin-gapless semiconductor state. In summary, this thesis establishes that key properties of Heusler alloys, including Berry curvature, half-metallicity, anomalous Hall behavior, and magnon-scattering dynamics, can be tailored through adjustments in composition, choice of substrate, and optimized growth techniques. The seamless transition from Weyl-semimetallic to half-metallic characteristics in the Co-based materials (such as Co2MnGe1-xGex and Mn-doped Co2MnGa, coupled with the Ti2MnAl, positions these materials as potent foundations for future spintronic applications.