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Topological phenomena in magnetic thin films

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TitleInfo
Title
Topological phenomena in magnetic thin films
Name (type = personal)
NamePart (type = family)
Wang
NamePart (type = given)
Wenbo
NamePart (type = date)
1989-
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Wenbo Wang
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author
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NamePart (type = family)
Wu
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Weida
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Weida Wu
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Advisory Committee
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chair
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Zimmerman
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Frank M.
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Frank M. Zimmerman
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Advisory Committee
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Gilman
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Ronald
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Ronald Gilman
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Advisory Committee
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internal member
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Haule
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Kristjan
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Kristjan Haule
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Advisory Committee
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internal member
Name (type = personal)
NamePart (type = family)
Chang
NamePart (type = given)
Cui-zu
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Cui-zu Chang
Affiliation
Advisory Committee
Role
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outside member
Name (type = corporate)
NamePart
Rutgers University
Role
RoleTerm (authority = RULIB)
degree grantor
Name (type = corporate)
NamePart
School of Graduate Studies
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school
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Text
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theses
OriginInfo
DateCreated (qualifier = exact)
2018
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2018-10
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2018
Place
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xx
Language
LanguageTerm (authority = ISO639-2b); (type = code)
eng
Abstract (type = abstract)
This thesis covers the study of various topological phenomena in magnetic thin films by utilizing magnetic force microscopy (MFM) at cryogenic temperature. In the first part, we will focus on the magnetic topological insulators (TI) which exhibit quantum anomalous Hall effect (QAHE). The QAHE is of great fundamental interest and potential application because of their dissipationless conduction without the need of external magnetic field. Up to now, this fascinating effect has been achieved in magnetic topological insulators, e.g. Cr- and V-doped (Bi,Sb)2Te3 thin films, at extremely low temperature (T<50 mK). The magnetic inhomogeneity has been proposed as one of the main reasons that limit the temperature for realization of QAHE. In the first part, we will present direct visualization of long-range ferromagnetism in the Cr,V co-doped BST films, by using the MFM and textit{in-situ} transport measurement. The magnetization reversal process reveals typical ferromagnetic domain behavior, i.e. domain nucleation and domain wall propagation. The ferromagnetic behavior at charge neutral point is consistent with the Van Vleck exchange mechanism. Furthermore, this ferromagnetism is robust against significant change in bulk charge carriers, though exchange interaction is enhanced by hole doping. This indicates a significant contribution from the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange coupling. The direct evidence of long-range ferromagnetic order eases the concern of the fragility of QAHE due to magnetic inhomogeneity, alleviating the need for ultra-low temperature to achieve full quantization. Our results encourage further exploration of QAHE and related phenomena in magnetically doped topological materials for disspationless conduction at elevated temperature.

In the second part, we will discuss topological Hall effect (THE), which is a real space Berry phase phenomenon originated from scalar spin chirality. The THE has been observed in static spin texture with integer Berry phase (skyrmions)in chiral magnets. Recently, this effect has been also observed in some ultra-thin film systems. The existence of static skyrmions in these systems is still unknown. While thermally driven spin chirality fluctuation in 2 dimensional chiral magnets has been predicted to exist, the resultant THE have not been observed. In this thesis, we report a surprising observation of substantial THE around the ferromagnetic transition Tc of ultra-thin films of SrRuO3, a uniaxial ferromagnet with very weak chiral interaction. The temperature, magnetic field, and thickness dependence of THE are in good agreement with our Monte-Carlo simulations, which unambiguously confirms the emergence of net topological charges driven by spin chirality fluctuation. Our discovery opens a new paradigm of exploring the chirality fluctuations with topological Hall transport in 2D ferromagnets. In addition to the chiral fluctuation driven THE (fTHE) in SRO films around Tc, the THE is also observed at low temperature in the ultra-thin SRO films, as they approach the critical thickness (3 u.c.). MC simulations indicate the magnetic disorder plays an important role in the low-temperature THE. This disorder-driven THE (dTHE) will be briefly discussed at the end of this thesis.
Subject (authority = RUETD)
Topic
Physics and Astronomy
Subject (authority = ETD-LCSH)
Topic
Magnetic force microscopy
Subject (authority = ETD-LCSH)
Topic
Thin Films
RelatedItem (type = host)
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Title
Rutgers University Electronic Theses and Dissertations
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ETD
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ETD_9220
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electronic resource
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application/pdf
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Extent
1 online resource (120 pages : illustrations)
Note (type = degree)
Ph.D.
Note (type = bibliography)
Includes bibliographical references
Note (type = statement of responsibility)
by Wenbo Wang
RelatedItem (type = host)
TitleInfo
Title
School of Graduate Studies Electronic Theses and Dissertations
Identifier (type = local)
rucore10001600001
Location
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NjNbRU
Identifier (type = doi)
doi:10.7282/t3-n2w8-9898
Genre (authority = ExL-Esploro)
ETD doctoral
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The author owns the copyright to this work.
RightsHolder (type = personal)
Name
FamilyName
Wang
GivenName
Wenbo
Role
Copyright Holder
RightsEvent
Type
Permission or license
DateTime (encoding = w3cdtf); (qualifier = exact); (point = start)
2018-09-21 16:09:40
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Name
Wenbo Wang
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Affiliation
Rutgers University. School of Graduate Studies
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I hereby grant to the Rutgers University Libraries and to my school the non-exclusive right to archive, reproduce and distribute my thesis or dissertation, in whole or in part, and/or my abstract, in whole or in part, in and from an electronic format, subject to the release date subsequently stipulated in this submittal form and approved by my school. I represent and stipulate that the thesis or dissertation and its abstract are my original work, that they do not infringe or violate any rights of others, and that I make these grants as the sole owner of the rights to my thesis or dissertation and its abstract. I represent that I have obtained written permissions, when necessary, from the owner(s) of each third party copyrighted matter to be included in my thesis or dissertation and will supply copies of such upon request by my school. I acknowledge that RU ETD and my school will not distribute my thesis or dissertation or its abstract if, in their reasonable judgment, they believe all such rights have not been secured. I acknowledge that I retain ownership rights to the copyright of my work. I also retain the right to use all or part of this thesis or dissertation in future works, such as articles or books.
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DateTime (encoding = w3cdtf); (qualifier = exact); (point = start)
2018-10-31
DateTime (encoding = w3cdtf); (qualifier = exact); (point = end)
2019-10-31
Detail
Access to this PDF has been restricted at the author's request. It will be publicly available after October 31st, 2019.
Copyright
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Copyright protected
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Status
Open
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Permission or license
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