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Engineering Magnetic and Topological Properties in Epitaxial Heusler Compounds

  • Author(s): Brown-Heft, Tobias Levi
  • Advisor(s): Palmstrøm, Chris J
  • et al.
Abstract

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.

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