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A Two-dimensional Semiconducting GaN-based Ferromagnetic Monolayer

Ma, Yingqiao
http://orcid.org/0000-0001-8664-981X
http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1541513207541646

Abstract Details

2018, Doctor of Philosophy (PhD), Ohio University, Physics and Astronomy (Arts and Sciences).
Most of my PhD research is focused on a two-dimensional semiconducting GaN-based manganese gallium nitride surface monolayer structure referred as MnGaN-2D and its related magnetic heterostructure systems. All of these material systems are grown by a reflection high energy electron diffraction assisted molecular beam epitaxy system. The as-grown samples are then characterized by various cutting-edge techniques including spin-polarized scanning tunneling microscopy and spectroscopy at both room temperature and low temperature, Auger electron spectroscopy, and variable temperature superconducting quantum interference magnetometry, to explore their surfaces and interfaces atomic structure, electronic structure, and spin structure. In addition, density functional theory calculations are performed for model structure, spin-polarized electronic structure, spin-orbit coupling, and lattice strain effect to help to understand and to support the experimental discoveries. The scope of this research is to investigate the magnetic properties of the MnGaN-2D monolayer and its related exchange bias systems for future practical advanced spintronic applications such as novel nonvolatile magnetic storage devices with increased integration densities and high thermal stability, and practical quantum logic computation at room temperature with low energy cost. The MnGaN-2D monolayer is promising for such applications due to its remarkable properties discovered during my research including robust intrinsic ferromagnetism at room-temperature, large spin-polarization over 95% at specific energy level and large magnetic moment 3.94 µB per Mn, perpendicular magnetic anisotropy, high thermal stability up to 700 degree Celsius, nonvolatility after direct exposure to air, lattice strain tailoring ability of its electronic and magnetic properties, directly coupled to the technological important semiconductor GaN, and etc. Chapter 1 and 2 are an overall detailed introduction of the background of my research and techniques used in my research. Chapter 3 presents my SP-STM/STS study of the MnGaN-2D monolayer at both room temperature and low temperature. The dI/dV spin mappings reveal evident ferromagnetic domain structure at room temperature. The dI/dV point spectra also agreed well with the calculated density of states plots which indicate the MnGaN-2D surface states are highly spin-polarized and spin-split. The theoretical predicted spin-split nature between the MnGaN-2D filled and empty surface states are furthermore proved by dI/dV spin mappings with positive and negative bias voltages. Finally, the MnGaN-2D hysteretic behavior under an applied magnetic field is clearly mapped out. The beginning of Chapter 4 is stimulated by chapter 3 to further explore the MnGaN-2D magnetic properties. In this chapter, the SQUID measurements are performed for the MnGaN-2D monolayer, which agree well with previous SP-STM/STS results and reveal a robust magnetic hysteresis loop, a large magnetic moment of 3.94 µB per Mn, and a clear perpendicular magnetic anisotropy. New calculations which include the spin-orbit couplings further support the SQUID measurements and also indicate an out-of-plane magnetic easy axis for the MnGaN-2D monolayer. The second part of chapter 4 is about the local electronic structure tailoring of the MnGaN-2D atomic lattice, which is first indicated by a surprising phenomenon of giant DOS peak shift observed in a series of STM dI/dV point spectra on MnGaN-2D surface. Later, evidence of modifying the local MnGaN-2D atomic structure is found. Following theoretical structural calculations are performed by inducing an artificial isotropic lattice strain and by creating a point defect in the atomic lattice. For both the expanding lattice and the point defected lattice cases, the theoretical calculations reveal a new DOS plot highly consistent with the observed peak shift phenomenon. In chapter 5, a variety of brand new MnGaN-2D based antiferromagnetic thin film on ferromagnetic MnGaN-2D monolayer heterostructures are successfully developed to explore the exchange bias effect, including ~15 ML Mn thin film on MnGaN-2D and 20 ML Mn3N2 (001) thin film on MnGaN-2D. Then to investigate the origin of the exchange bias effect and the aFM/FM interface atomic structure and magnetic exchange couplings, a 2-D Mn/MnGaN-2D interface structure is synthesized and investigated directly using STM which reveals an interesting randomly located Mn adatom structure with partial local orderings on the MnGaN-2D surface. Based on the STM measurements, further DFT calculations for the new Mn adatom structures are performed, which indicate an interesting ferrimagentic nature of the new 2-D Mn/MnGaN-2D interface monolayer. The theoretical calculations further agree with the subsequent SQUID measurements. Finally, in Chapter 6, a summary of my work is made and a variety of future directions regarding to the MnGaN-2D material systems are discussed.
Arthur Smith (Advisor)
146 p.
Condensed Matter Physics; Materials Science; Physics
2-D magnetism; spintronics; spin-polarized STM; ferromagnetic semiconductor; room temperature

Recommended Citations

Citations

  • Ma, Y. (2018). A Two-dimensional Semiconducting GaN-based Ferromagnetic Monolayer [Doctoral dissertation, Ohio University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1541513207541646

    APA Style (7th edition)

  • Ma, Yingqiao. A Two-dimensional Semiconducting GaN-based Ferromagnetic Monolayer . 2018. Ohio University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1541513207541646.

    MLA Style (8th edition)

  • Ma, Yingqiao. "A Two-dimensional Semiconducting GaN-based Ferromagnetic Monolayer ." Doctoral dissertation, Ohio University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1541513207541646

    Chicago Manual of Style (17th edition)

ohiou1541513207541646
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This open access ETD is published by Ohio University and OhioLINK.