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Ferromagnetic Thin and Ultra-Thin Film Alloys of Manganese and Iron with Gallium and Their Structural, Electronic, and Magnetic Properties

Mandru, Andrada Oana
http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1458572741

Abstract Details

2016, Doctor of Philosophy (PhD), Ohio University, Physics and Astronomy (Arts and Sciences).
The behavior of ferromagnetic alloys of manganese and iron with gallium when coupled with different magnetic and/or non-magnetic systems is investigated. The studies explore how the structural and electronic/magnetic properties vary with thickness and composition, probing systems in the sub-monolayer, ultra-thin, and thin film regimes. Molecular beam epitaxy is used to prepare clean sample surfaces that are subsequently investigated in-situ down to atomic level using scanning tunneling microscopy and Auger electron spectroscopy. A variety of ex-situ methods are also utilized to obtain information about the overall system properties, with additional theoretical calculations accompanying the experimental findings for two of the investigated systems. The first study refers to L10-structured ferromagnetic MnGa(111) ultra-thin films grown on semiconducting GaN(0001) substrates under lightly Mn-rich conditions. Room temperature scanning tunneling microscopy investigations reveal smooth and reconstructed terraces, with the surface structure consisting primarily of a hexagonal-like 2 x 2 reconstruction. Theoretical calculations are carried out using density functional theory, revealing that a Mn-rich 2 x 2 surface structure gives the best agreement with the observed experimental images and Auger electron spectroscopy surface composition investigations. It is found that under such growth conditions, the Mn atoms incorporate at di fferent rates: surfaces become highly Mn-rich, while the bulk remains stoichiometric, making the MnGa system very sensitive to the ratio of elements in its structure. Such behavior reveals a potential recipe for tuning, for example, magnetic properties by carefully controlling the surface reconstruction during growth. With the aim of understanding how the properties change as the growth conditions are varied, we also investigate the structure, surface, and magnetism of ferromagnetic Ga-rich MnGa thin and ultra-thin films grown again on GaN(0001). For this study, the Mn:Ga composition ratio is varied from ≈ 1 (stoichiometric) to ≈ 0.42 (very Ga-rich) for different samples. We find that the L10 MnGa phase is preserved down to a Mn:Ga ratio of ≈ 0.81. As the Ga concentration increases, we observe the coexistence of more Ga-rich phases, namely Mn3Ga5 and Mn2Ga5. Room temperature scanning tunneling microscopy imaging reveals highly epitaxial films, with atomically smooth and highly reconstructed surfaces. Magnetic characterizations show how the magnetic properties evolve with changing composition and that giant perpendicular magnetic anisotropy is induced by reducing the film thickness. Next, the initial stages of interface formation for a real-world ferromagnet/antiferromagnet bi-layer system, iron/manganese nitride, is investigated down to the atomic scale using a combination of experiment and first-principles theoretical calculations. Sub-monolayer deposition of iron onto manganese nitride surfaces results in an unexpected yet well-ordered structural and magnetic arrangement. It is shown that although the island structures seen in scanning tunneling microscopy images are of single monolayer height, their chemical composition, based on Auger electron spectroscopy, conductance map imaging, and theoretical models, does not consist of iron. It is found theoretically that models that consider iron on the surface of manganese nitride are highly unfavorable. Instead, models with iron atoms incorporated into specific subsurface layers are most stable, in excellent agreement with Auger spectroscopy measurements. Calculations also reveal the magnetic alignment of iron with the manganese nitride layers. The last study combines iron with gallium in the form of highly magnetostrictive FeGa alloys and investigates their structural and magnetic properties when deposited onto non-magnetic MgO and antiferromagnetic manganese nitride substrates. Films with compositions ranging between ≈ 8 at.% Ga and ≈ 24 at.% Ga are investigated. From X-ray diffraction measurements it is found that the FeGa films are single crystalline. Scanning tunneling microscopy imaging reveals that the surface morphologies are dictated by the growth temperature, composition, and substrate. The magnetic properties can be tailored by the substrate, as found by magnetic force microscopy imaging and vibrating sample magnetometry measurements. Depositing FeGa onto manganese nitride substrates leads to the formation of stripe-like magnetic domain patterns and to the appearance of perpendicular magnetic anisotropy.
Arthur Smith (Advisor)
140 p.
Condensed Matter Physics; Physics
ferromagnets; antiferromagnets; manganese gallium; molecular beam epitaxy; manganese nitride; iron gallium; scanning tunneling microscopy

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Citations

  • Mandru, A. O. (2016). Ferromagnetic Thin and Ultra-Thin Film Alloys of Manganese and Iron with Gallium and Their Structural, Electronic, and Magnetic Properties [Doctoral dissertation, Ohio University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1458572741

    APA Style (7th edition)

  • Mandru, Andrada Oana. Ferromagnetic Thin and Ultra-Thin Film Alloys of Manganese and Iron with Gallium and Their Structural, Electronic, and Magnetic Properties. 2016. Ohio University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1458572741.

    MLA Style (8th edition)

  • Mandru, Andrada Oana. "Ferromagnetic Thin and Ultra-Thin Film Alloys of Manganese and Iron with Gallium and Their Structural, Electronic, and Magnetic Properties." Doctoral dissertation, Ohio University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1458572741

    Chicago Manual of Style (17th edition)

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This open access ETD is published by Ohio University and OhioLINK.