Iron-and Cobalt-based Heusler Alloy Nanostructures and Their Device Applications
Heusler alloys are compounds with chemical formula X2YZ, where X and Y are transition metals or lanthanides (rare-earth metals) and Z is a main group element. The properties of this remarkable class of materials have been of interest since 1903 when Heusler discovered the ferromagnetic behavior from non-ferromagnetic constituents Cu, Mn, and Al. The spectrum of their possible applications ranges from magnetic and magneto-mechanical materials over semiconductors and thermoelectrics to superconductors and topological insulators. An important feature of the Heusler compounds is the possibility of controlling the valence electron concentration by partial substitution of elements. On the other hand, the properties also depend on the degree of ordering in the crystal structure. They make one of the leading candidate material classes for achieving high spin polarization. A number of Heusler alloys are predicted to be half-metallic ferromagnets that would theoretically provide high spin polarization at the Fermi energy. Furthermore, magnetic Heusler compounds can exhibit topological semimetal (TSM) behavior. TSM are electronic strong spin -orbit metals/semimetals whose fermi surface arise from crossing between valence band and conduction band, which cannot be avoided due to non-trivial topology. This new state has recently attracted worldwide interest because they may realize the particles that remain elusive in high energy physics. The possible existence of massless chiral fermions known as Weyl fermion exist in a class of TSM known as Weyl semimetals (WSM). Although Weyl fermion systems proposed in condensed matter physics share many similarities with the massless fermionic particle propagating in the vacuum, they have some unusual properties. WSM have unclosed fermi arc which connect two Weyl fermions with opposite chirality. This can lead to show anomalous Hall effect associated with Berry phase. Despite interest, room temperature ferromagnetic semimetals, regardless of its topological trivial/non-trivial nature, are rare in nature. So, Heusler alloy are the most suitable member of room- temperature topological Weyl semimetal. Magnetic Heusler alloys have several advantages over other compounds where Weyl fermions have been proposed and detected. They usually have considerable electron-electron interaction which helps to study the interplay between the topological semimetal and electronic correlation. The excitement surrounding magnetic Heusler alloys is not only due to the possibility of studying new quantum phenomena, or to the realization of new exotic phases in matter. It also springs from the promise of developing quantum devices working at relatively at high temperature, as the topological protection makes their quantum properties robust against perturbations.In this thesis, we will explore the properties of iron and cobalt based Heusler alloy. So far, no epitaxial thin films study has been reported. We have grown cobalt and iron based Heusler alloy thin films using ultra-high vacuum electron-beam evaporation. We have used magnesium oxide as a buffer layer to get well ordered thin film. We have fabricated Hall bar devices using metal contact mask method. Our Co2TiGe(CTG) Heusler alloy display semimetallic behavior with band gap of 24 meV. Negative magnetoresistance (MR) has been observed in CTG films. At low magnetic field, weak-localization dominates on the MR behavior. At intermediate magnetic field, chiral anomaly contributes to the magnetoconductivity of thin films. Anomalous Hall conductivity of 25 S/cm have been observed due to Berry curvature. We have theoretically calculated the band structure of CTG using Quantum espresso. The calculated band structure exactly match the reported band structure in the literature. In general, Heusler compounds crystallize in the cubic crystal structure with a space group Fm3 ̅m, but in many cases, certain types of disorder are observed. In this thesis, it is also shown that iron -based Heusler alloy exhibit disorder in the crystal lattice. This lattice disorder can lead to different nearest neighbors to Fe, which can result in remarkable magnetic interactions. Our Fe-based alloy, Fe¬2CrAl(FCA) exhibit high Curie temperature and large magnetization due to disordering in lattice sites. Semimetallic behavior of FCA is destroyed by this disorder.
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