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Author
Date
2021Type
- Doctoral Thesis
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Abstract
Antiferromagnetic materials are receiving widespread attention because they promise
to enable smaller, faster, and more resilient data processing units compared to state-of-
the-art ferromagnets. The possibility to control the orientation of the magnetic domains
in antiferromagnets by either electrical or optical means is revolutionizing the
field of magnetism and opening a wealth of applications in spintronics. Outstanding
open questions bear on both fundamental problems (switching mechanism) as well
as on applied aspects (amplitude of the readout signal). One of the great unknowns
in the field of antiferromagnetism is the structure of domain walls, which is key to
both of these aspects. In particular, chiral walls are required for current-controlled
magnetisation switching. The limited knowledge about antiferromagnetic domain
walls as well as the switching mechanism is mainly due to the difficulty of imaging
antiferromagnets.
The main focus of this thesis is to address the aforementioned problems by imaging
antiferromagnets using nanoscale scanning diamond magnetometry. We will
elaborate throughout this thesis on the question of what information about the spin
structure can be inferred when measuring the magnetic stray field on the surface
of an antiferromagnet. Related to that, we discuss current-induced effects in antiferromagnets
and to what extent those can be detected using nanoscale diamond
magnetometry.
We first calibrate our technique by determining the internal structure of a domain
wall in a ferrimagnetic insulator Tm3Fe5O12 and estimate its magnetisation.
Ferrimagnets, being in a broader sense between ferro- and antiferromagnets, are detectable
by common magnetic probes, which we use to confirm our methodology.
We reveal that the domain walls in Tm3Fe5O12 have a left Néel character indicating
the presence of interfacial Dzyaloshinskii-Moriya interaction. These results open the
possibility to stabilize chiral spin textures in centrosymmetric magnetic insulators
important for spintronic applications.
We then extend these insights and methods to the archetypical antiferromagnet
Cr2O3. Here we present, to the best of our knowledge, the first experimental observation
of a domain wall in a pure, intrinsic antiferromagnet. We reveal that the
intrinsic domain wall structure is Bloch-like and can become Néel-like if sufficient
in-plane magnetic anisotropy is present. Our experimental observation is significant
because the theory does not predict any preference for Bloch or Néel domain wall
structure.
Finally, we combine nanoscale scanning diamond magnetometry with electrical
pulsing and resistance measurements to shed light on the switching mechanism and
readout signal in the metallic antiferromagnet CuMnAs. We find that, besides the
known reorientation of the Néel vector, the domain pattern fragments upon injection
of current pulses. This provides an explanation for the recently-observed unipolar
high-resistive switching signals in CuMnAs, and their relaxation and demonstrates a
novel memristive effect. Besides these observations, we show that nanoscale scanning
diamond magnetometry can be successfully applied to map the domain structure
of in-plane antiferromagnets.
Our results point out directions for future research in the field of antiferromagnetic
spintronics and provide a new methodology and concepts relevant for the quantum
sensing and imaging communities. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000488794Publication status
publishedExternal links
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Contributors
Examiner: Gambardella, Pietro
Examiner: Degen, Christian L.
Examiner: Spaldin, Nicola
Examiner: Makarov, Denys
Publisher
ETH ZurichOrganisational unit
03986 - Gambardella, Pietro / Gambardella, Pietro
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