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Engineering Magnetic and Topological Properties in Epitaxial Heusler Compounds
Commercially viable spintronic devices require magnetic contacts with high electrical conductivity, high spin polarization, low Gilbert damping, and perpendicular magnetic anisotropy. The contact must also be amenable to thin film growth techniques to allow device scalability. Until now, this combination of properties had yet to be obtained in a single material. The exquisite control over crystal growth conditions and elemental composition imparted by molecular beam epitaxy can be leveraged to tune magnetic and electronic material properties closer to the ideal set desired by device researchers.Ferromagnetic metals composed of elements with low atomic weight are commonly used for spintronics, but the industry standard CoFeB does not possess high spin polarization, and its perpendicular magnetic anisotropy depends on film thickness, limiting its versatility. On the other hand, Heusler compounds are a class of over 1000 ternary intermetallic materials with highly variable magnetic and electronic properties. The Heusler compound Co2MnSi is well known as a half-metal with 100% spin polarization at the Fermi level, making it an ideal source of spin-polarized current. However, Co2MnSi does not possess perpendicular magnetic anisotropy. In this work, the magnetic anisotropy of Heusler compounds is engineered by breaking their cubic crystal symmetry. This can be accomplished by growing tetragonal crystal structures with the unique axis aligned out-of-plane, or by engineering superlattices composed of alternating layers of dissimilar Heusler compounds. In both cases, the resulting perpendicular magnetic anisotropy does not depend on film thickness, making the materials attractive for a broad range of spintronic device applications. Additionally, the Heusler compound superlattices studied here are composed of Co2MnAl and Fe2MnAl, which combine their electronic structures to produce 95% spin polarization as measured by spin-resolved photoemission spectroscopy. This combines two important magnetic properties never before seen in a single material system. The growth, structural, electronic and magnetic properties of the engineered films will be presented.Finally, Co2TiGe is explored as a candidate of an exotic class of topological materials known as Weyl semimetals. These systems possess a unique band structure that arises due to broken time-reversal symmetry resulting from the internal magnetization. Electrons with energy and momentum near so-called Weyl points have zero effective mass and a discrete chiral charge, making them analogous to the elusive Weyl fermion. The signatures of Weyl semimetallicity in Co2TiGe are probed using magnetotransport and synchrotron-based angle-resolved photoemission spectroscopy
Weak antilocalization in quasi-two-dimensional electronic states of epitaxial LuSb thin films
Observation of large non-saturating magnetoresistance in rare-earth
monopnictides has raised enormous interest in understanding the role of its
electronic structure. Here, by a combination of molecular-beam epitaxy,
low-temperature transport, angle-resolved photoemssion spectroscopy, and hybrid
density functional theory we have unveiled the bandstructure of LuSb, where
electron-hole compensation is identified as a mechanism responsible for large
magnetoresistance in this topologically trivial compound. In contrast to bulk
single crystal analogues, quasi-two-dimensional behavior is observed in our
thin films for both electron and holelike carriers, indicative of dimensional
confinement of the electronic states. Introduction of defects through growth
parameter tuning results in the appearance of quantum interference effects at
low temperatures, which has allowed us to identify the dominant inelastic
scattering processes and elucidate the role of spin-orbit coupling. Our
findings open up new possibilities of band structure engineering and control of
transport properties in rare-earth monopnictides via epitaxial synthesis.Comment: 20 pages, 12 figures; includes supplementary informatio
Revealing quantum Hall states in epitaxial topological half-Heusler semimetal
Prediction of topological surface states (TSS) in half-Heusler compounds
raises exciting possibilities to realize exotic electronic states and novel
devices by exploiting their multifunctional nature. However, an important
prerequisite is identification of macroscopic physical observables of the TSS,
which has been difficult in these semi-metallic systems due to prohibitively
large number of bulk carriers. Here, we introduce compensation alloying in
epitaxial thin films as an effective route to tune the chemical potential and
simultaneously reduce the bulk carrier concentration by more than two orders of
magnitude compared to the parent compound. Linear magnetoresistance is shown to
appear as a precursor phase that transmutes into a TSS induced quantum Hall
phase on further reduction of the coupling between the surface states and the
bulk carriers. Our approach paves the way to reveal and manipulate exotic
properties of topological phases in Heusler compounds.Comment: 8 pages, 4 figures. Supplementary Infromation contains 7 sections and
17 figure