Ferromagnetism in atomically thin van der Waals materials
<p>The characterization of two-dimensional materials, which have different properties compared to those of their bulk counterparts, has intensified in the last two decades. Moreover, the ability to stack atomically thin sheets into heterostructures is enabled by the van der Waals interaction,...
| Main Author: | |
|---|---|
| Other Authors: | |
| Format: | Thesis |
| Language: | English |
| Published: |
2023
|
| _version_ | 1811139785776955392 |
|---|---|
| author | Fujita, RH |
| author2 | Hesjedal, T |
| author_facet | Hesjedal, T Fujita, RH |
| author_sort | Fujita, RH |
| collection | OXFORD |
| description | <p>The characterization of two-dimensional materials, which have different properties compared to those of their bulk counterparts, has intensified in the last two decades. Moreover, the ability to stack atomically thin sheets into heterostructures is enabled by the van der Waals interaction, which weakly binds atomic layers together within a broader class of van der Waals materials. Fascinating two-dimensional physics, in addition to the relative ease of fabricating atomically thin van der Waals heterostructures of high structural quality, has arguably made two-dimensional materials the most studied area of condensed matter physics in recent years. Despite the rich physics, technical challenges will likely limit the implementation of two-dimensional materials to niche purposes, and will more generally obfuscate their incorporation in practical devices. Nevertheless, the integration of two-dimensional materials into nascent technologies such as quantum computing strongly motivate further characterization and subsequent device optimization.</p>
<br>
<p>The two-dimensional magnet is a relatively recent addition to the broader van der Waals class, consisting of two-dimensional semiconductors, ferroelectrics and superconductors. In particular, two-dimensional ferromagnetism is generally stabilized by a large magnetic anisotropy. This thesis examines if such an anisotropic magnetic state can be tuned in the atomically thin limit, and if other magnetic states exist in two dimensions.</p>
<br>
<p>Firstly, it is shown that ferromagnetism is fundamentally distinct in two dimensions compared to in the bulk. In detail, the magnetic orbital moment 𝑚<sub>𝑙</sub> and magnetic spin moment 𝑚<sub>𝑠</sub> are estimated for CrSiTe<sub>3</sub>, a 2D Ising ferromagnet for which the magnetic exchange interaction is mostly limited to within the 𝑎𝑏 plane. An enhancement of the 𝑚<sub>𝑙</sub>/𝑚<sub>𝑠</sub> ratio in atomically thin CrSiTe<sub>3</sub> demonstrates that even for a 2D Ising ferromagnet with limited interlayer interactions, dimensionality effects are non-negligible.</p>
<br>
<p>Secondly, a novel method of tuning the magnetic state in Fe<sub>3</sub>GeTe<sub>2</sub>, a van der Waals ferromagnet with a large unaxial anisotropy, is demonstrated. In particular, the inclusion of α-In<sub>2</sub>Se<sub>3</sub>, a van der Waals piezoelectric, is shown to strain Fe<sub>3</sub>GeTe<sub>2</sub>, resulting in a reduced critical temperature and domain wall energy. The straining of other van der Waals materials with α-In<sub>2</sub>Se<sub>3</sub>, including graphite, is also demonstrated.</p>
<br>
<p>Thirdly, a search for two-dimensional magnetic states with weak anisotropy is thoroughly discussed. In particular, magnetic domains are characterized in few-layer Fe<sub>5</sub>GeTe<sub>2</sub>, a van der Waals ferromagnet with a small unaxial anisotropy compared to its counterpart Fe<sub>3</sub>GeTe<sub>2</sub>. The observation of such domains in two-dimensional Fe<sub>5</sub>GeTe<sub>2</sub> is counterintuitive, as the magnetic anisotropy energy is usually too large to accomodate domain walls. These domains, as well as deviations from the easy axis, are discussed in terms of different energetic quantities and domain wall pinning. It is concluded that for Fe<sub>5</sub>GeTe<sub>2</sub>, the magnetism is characterized by intercompetition among the magnetic anisotropy and stray field energy, meaning that it should be possible to stabilize magnetic domains with varying properties in the atomically thin limit.</p>
<br>
<p>Lastly, easy-plane ferromagnetism is considered for Fe<sub>5</sub>GeTe<sub>2</sub>. In detail, a temperature-induced magnetostructural transition is utilized to induce easy-plane magnetic anisotropy in Fe<sub>5</sub>GeTe<sub>2</sub>. In the atomically thin limit, magnetic domains with an in-plane orientation, as well as magnetic vortices and antivortices are observed. This is a significant finding which demonstrates the formation of spin textures in an atomically thin 2D material, potentially for the first time. Future work includes the characterization of the topological properties of these spin textures, as well as incorporating them in ferromagnet-superconductor van der Waals heterostructures to test proposals regarding the Majorana bound state, which holds relevance for quantum computing.</p> |
| first_indexed | 2024-09-25T04:11:37Z |
| format | Thesis |
| id | oxford-uuid:141fd44c-7fff-4871-af18-bbc25283588d |
| institution | University of Oxford |
| language | English |
| last_indexed | 2024-09-25T04:11:37Z |
| publishDate | 2023 |
| record_format | dspace |
| spelling | oxford-uuid:141fd44c-7fff-4871-af18-bbc25283588d2024-07-03T14:03:49ZFerromagnetism in atomically thin van der Waals materialsThesishttp://purl.org/coar/resource_type/c_db06uuid:141fd44c-7fff-4871-af18-bbc25283588dEnglishHyrax Deposit2023Fujita, RHHesjedal, TJohnston, MSantos, E<p>The characterization of two-dimensional materials, which have different properties compared to those of their bulk counterparts, has intensified in the last two decades. Moreover, the ability to stack atomically thin sheets into heterostructures is enabled by the van der Waals interaction, which weakly binds atomic layers together within a broader class of van der Waals materials. Fascinating two-dimensional physics, in addition to the relative ease of fabricating atomically thin van der Waals heterostructures of high structural quality, has arguably made two-dimensional materials the most studied area of condensed matter physics in recent years. Despite the rich physics, technical challenges will likely limit the implementation of two-dimensional materials to niche purposes, and will more generally obfuscate their incorporation in practical devices. Nevertheless, the integration of two-dimensional materials into nascent technologies such as quantum computing strongly motivate further characterization and subsequent device optimization.</p> <br> <p>The two-dimensional magnet is a relatively recent addition to the broader van der Waals class, consisting of two-dimensional semiconductors, ferroelectrics and superconductors. In particular, two-dimensional ferromagnetism is generally stabilized by a large magnetic anisotropy. This thesis examines if such an anisotropic magnetic state can be tuned in the atomically thin limit, and if other magnetic states exist in two dimensions.</p> <br> <p>Firstly, it is shown that ferromagnetism is fundamentally distinct in two dimensions compared to in the bulk. In detail, the magnetic orbital moment 𝑚<sub>𝑙</sub> and magnetic spin moment 𝑚<sub>𝑠</sub> are estimated for CrSiTe<sub>3</sub>, a 2D Ising ferromagnet for which the magnetic exchange interaction is mostly limited to within the 𝑎𝑏 plane. An enhancement of the 𝑚<sub>𝑙</sub>/𝑚<sub>𝑠</sub> ratio in atomically thin CrSiTe<sub>3</sub> demonstrates that even for a 2D Ising ferromagnet with limited interlayer interactions, dimensionality effects are non-negligible.</p> <br> <p>Secondly, a novel method of tuning the magnetic state in Fe<sub>3</sub>GeTe<sub>2</sub>, a van der Waals ferromagnet with a large unaxial anisotropy, is demonstrated. In particular, the inclusion of α-In<sub>2</sub>Se<sub>3</sub>, a van der Waals piezoelectric, is shown to strain Fe<sub>3</sub>GeTe<sub>2</sub>, resulting in a reduced critical temperature and domain wall energy. The straining of other van der Waals materials with α-In<sub>2</sub>Se<sub>3</sub>, including graphite, is also demonstrated.</p> <br> <p>Thirdly, a search for two-dimensional magnetic states with weak anisotropy is thoroughly discussed. In particular, magnetic domains are characterized in few-layer Fe<sub>5</sub>GeTe<sub>2</sub>, a van der Waals ferromagnet with a small unaxial anisotropy compared to its counterpart Fe<sub>3</sub>GeTe<sub>2</sub>. The observation of such domains in two-dimensional Fe<sub>5</sub>GeTe<sub>2</sub> is counterintuitive, as the magnetic anisotropy energy is usually too large to accomodate domain walls. These domains, as well as deviations from the easy axis, are discussed in terms of different energetic quantities and domain wall pinning. It is concluded that for Fe<sub>5</sub>GeTe<sub>2</sub>, the magnetism is characterized by intercompetition among the magnetic anisotropy and stray field energy, meaning that it should be possible to stabilize magnetic domains with varying properties in the atomically thin limit.</p> <br> <p>Lastly, easy-plane ferromagnetism is considered for Fe<sub>5</sub>GeTe<sub>2</sub>. In detail, a temperature-induced magnetostructural transition is utilized to induce easy-plane magnetic anisotropy in Fe<sub>5</sub>GeTe<sub>2</sub>. In the atomically thin limit, magnetic domains with an in-plane orientation, as well as magnetic vortices and antivortices are observed. This is a significant finding which demonstrates the formation of spin textures in an atomically thin 2D material, potentially for the first time. Future work includes the characterization of the topological properties of these spin textures, as well as incorporating them in ferromagnet-superconductor van der Waals heterostructures to test proposals regarding the Majorana bound state, which holds relevance for quantum computing.</p> |
| spellingShingle | Fujita, RH Ferromagnetism in atomically thin van der Waals materials |
| title | Ferromagnetism in atomically thin van der Waals materials |
| title_full | Ferromagnetism in atomically thin van der Waals materials |
| title_fullStr | Ferromagnetism in atomically thin van der Waals materials |
| title_full_unstemmed | Ferromagnetism in atomically thin van der Waals materials |
| title_short | Ferromagnetism in atomically thin van der Waals materials |
| title_sort | ferromagnetism in atomically thin van der waals materials |
| work_keys_str_mv | AT fujitarh ferromagnetisminatomicallythinvanderwaalsmaterials |