{"id":{"repo_id":"aachen","oai_identifier":"oai:publications.rwth-aachen.de:61755"},"canonical_url":"https://search.dev.ndltd.org/etd/aachen/oai:publications.rwth-aachen.de:61755","repository":{"repo_id":"aachen","name":"RWTH Aachen University","base_url":"https://publications.rwth-aachen.de/oai2d"},"display":{"title":"Theory of magnetic transition metal nanoclusters on surfaces","abstract":"The question how magnetism behaves when the dimension of materials is reduced to increasingly smaller sizes has attracted much research and led to the development of the field of magnetic nanostructures. This research has been fueled by the technological potential of these systems for the field of high-density magnetic storage media and has been accelerated by the many novel experimental methods and techniques developed exhibiting atomic resolution. This thesis is motivated by the quest for the understanding and the exploration of complex magnetism provided by atomic scale magnetic clusters deposited on surfaces or embedded in the bulk. The nature of magnetism in these systems can be very rich, in that the properties depend on the atomic species, the cluster size, shape and symmetry or choice of the substrate. Small variations of the cluster parameter may change the properties dramatically. Particularly rich and particularly challenging for experiment and theory is the behavior of clusters with competing magnetic interactions either between the cluster atoms or between the cluster and the substrate. In both cases magnetic frustration can lead to non-collinear magnetic structures for which the magnetic quantization axis changes from atom to atom. This thesis sheds light onto these systems from a theoretical perspective. Use is made of the density functional theory (DFT), the most successful material specific theory for describing electronic and derived properties from first-principles. Acting within this framework, we have developed and implemented the treatment of non-collinear magnetism into the Juelich version of the full-potential Korringa-Kohn-Rostoker Green Function (KKR-GF) method. The KKR-GF method provides several advantages compared to other first-principles methods. Based on solving the Dyson equation it allows an elegant treatment of non-periodic systems such as impurities and clusters in bulk or on surfaces. Electronic, magnetic properties and the observables provided by experimental techniques such as x-ray, scanning tunneling microscopy and spectroscopy can be accessed with the KKR-GF method. Firstly, the method was applied to 3d transition-metal clusters on different ferromagnetic surfaces. Different types of magnetic clusters were selected. Clusters of Fe, Co, Ni atoms are ferromagnetic and thus magnetically collinear. In order to investigate magnetic frustration due to competing interactions within the ad-cluster we considered a (001) oriented surface of fcc metals, a topology which usually does not lead to non-collinear magnetism. We tuned the strength of the magnetic coupling between the ad-clusters and the ferromagnetic surface by varying the substrate from the case of Ni(001) with a rather weak hybridization of the Ni d-states with the adatom d-states to the case of Fe(3ML)/Cu(001) with a much stronger hybridization due to the larger extend of the Fe wavefunctions. Cr and Mn clusters of different sizes and shapes have been found to exhibit non-collinear magnetic structures. The second type of frustration was investigated employing a Ni(111) surface, a surface with a triangular lattice of atoms, were both kind of competing magnetic interactions occur: intra-cluster magnetic frustration and cluster-substrate magnetic interaction. Parity of number of adatoms in finite antiferromagnetic nanowires is shown to be crucial in predicting whether the magnetic ground state is non-collinear or collinear. We show that nanochains with an even number of adatoms are always magnetically non-collinear while an odd number of adatoms leads under given conditions to a collinear ferrimagnetic ground state. In the second part of the thesis we applied the KKR-GF method to reveal experimental issues related to scanning tunneling microscopy (STM). Hence, we investigated the scattering of a two-dimensional surface state at adatoms. Here it is shown theoretically for some special cases as well as experimentally (STM) that any attractive potential should lead to a localized state. With a systematic study, including sp as well as d adatoms on or in the surface, we demonstrate that the cited statement, that any attractive potential should lead to a localized state, is not correct. Indeed, we derive a better criteria based on the scattering length of the adatom. At last, we show that Fermi surfaces can be imaged through STM. The Fermi surface is one of the most important properties in metals as it determines many transport properties as well as long-ranged interactions. The new effect is explained by means of scattering of electrons at subsurface impurities. Since the electrons propagate away from the impurities with velocity vectors perpendicular to the Fermi surface, flat regions on this surface focus the electrons in special space-angle directions, leading in real space to highly symmetric STM spots around the impurity.","abstract_html":"The question how magnetism behaves when the dimension of materials is reduced to increasingly smaller sizes has attracted much research and led to the development of the field of magnetic nanostructures. This research has been fueled by the technological potential of these systems for the field of high-density magnetic storage media and has been accelerated by the many novel experimental methods and techniques developed exhibiting atomic resolution. This thesis is motivated by the quest for the understanding and the exploration of complex magnetism provided by atomic scale magnetic clusters deposited on surfaces or embedded in the bulk. The nature of magnetism in these systems can be very rich, in that the properties depend on the atomic species, the cluster size, shape and symmetry or choice of the substrate. Small variations of the cluster parameter may change the properties dramatically. Particularly rich and particularly challenging for experiment and theory is the behavior of clusters with competing magnetic interactions either between the cluster atoms or between the cluster and the substrate. In both cases magnetic frustration can lead to non-collinear magnetic structures for which the magnetic quantization axis changes from atom to atom. This thesis sheds light onto these systems from a theoretical perspective. Use is made of the density functional theory (DFT), the most successful material specific theory for describing electronic and derived properties from first-principles. Acting within this framework, we have developed and implemented the treatment of non-collinear magnetism into the Juelich version of the full-potential Korringa-Kohn-Rostoker Green Function (KKR-GF) method. The KKR-GF method provides several advantages compared to other first-principles methods. Based on solving the Dyson equation it allows an elegant treatment of non-periodic systems such as impurities and clusters in bulk or on surfaces. Electronic, magnetic properties and the observables provided by experimental techniques such as x-ray, scanning tunneling microscopy and spectroscopy can be accessed with the KKR-GF method. Firstly, the method was applied to 3d transition-metal clusters on different ferromagnetic surfaces. Different types of magnetic clusters were selected. Clusters of Fe, Co, Ni atoms are ferromagnetic and thus magnetically collinear. In order to investigate magnetic frustration due to competing interactions within the ad-cluster we considered a (001) oriented surface of fcc metals, a topology which usually does not lead to non-collinear magnetism. We tuned the strength of the magnetic coupling between the ad-clusters and the ferromagnetic surface by varying the substrate from the case of Ni(001) with a rather weak hybridization of the Ni d-states with the adatom d-states to the case of Fe(3ML)/Cu(001) with a much stronger hybridization due to the larger extend of the Fe wavefunctions. Cr and Mn clusters of different sizes and shapes have been found to exhibit non-collinear magnetic structures. The second type of frustration was investigated employing a Ni(111) surface, a surface with a triangular lattice of atoms, were both kind of competing magnetic interactions occur: intra-cluster magnetic frustration and cluster-substrate magnetic interaction. Parity of number of adatoms in finite antiferromagnetic nanowires is shown to be crucial in predicting whether the magnetic ground state is non-collinear or collinear. We show that nanochains with an even number of adatoms are always magnetically non-collinear while an odd number of adatoms leads under given conditions to a collinear ferrimagnetic ground state. In the second part of the thesis we applied the KKR-GF method to reveal experimental issues related to scanning tunneling microscopy (STM). Hence, we investigated the scattering of a two-dimensional surface state at adatoms. Here it is shown theoretically for some special cases as well as experimentally (STM) that any attractive potential should lead to a localized state. With a systematic study, including sp as well as d adatoms on or in the surface, we demonstrate that the cited statement, that any attractive potential should lead to a localized state, is not correct. Indeed, we derive a better criteria based on the scattering length of the adatom. At last, we show that Fermi surfaces can be imaged through STM. The Fermi surface is one of the most important properties in metals as it determines many transport properties as well as long-ranged interactions. The new effect is explained by means of scattering of electrons at subsurface impurities. Since the electrons propagate away from the impurities with velocity vectors perpendicular to the Fermi surface, flat regions on this surface focus the electrons in special space-angle directions, leading in real space to highly symmetric STM spots around the impurity.","abstract_has_math":false,"creators":["Lounis, Samir"],"institution":"Publikationsserver der RWTH Aachen University","degree_name":null,"degree_level":null,"degree_discipline":null,"degree_department":null,"school":null,"contributors":["Blügel, Stefan"],"advisors":[],"committee_chairs":[],"committee_members":[],"year":2007,"date_issued":"2007","date_published":"2007","updated_at":"2026-07-30T19:43:10Z","subjects":["info:eu-repo/classification/ddc/530","Magnetismus","Nanopartikel","Elektronenstruktur","Elektronische Eigenschaft","Green-Funktion","Physik","Noncollinear magnetism","Density functional theory","Nanocluster","transition metal","KKR"],"languages":["eng"],"rights":["info:eu-repo/semantics/openAccess"],"rights_urls":[],"identifier_entries":[{"key":"dc:identifier","label":"Identifier","values":["https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-123387%22"],"render_values":[{"text":"https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-123387%22","href":"https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-123387%22","code":true}]}]},"links":{"outbound_url":"https://publications.rwth-aachen.de/record/61755","outbound_label":"Repository record","outbound_source":"dc:identifier"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor","label":"Contributor","values":["Blügel, Stefan"]},{"key":"dc:creator","label":"Author","values":["Lounis, Samir"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:coverage","label":"Dc Coverage","values":["DE"]},{"key":"dc:date","label":"Dc Date","values":["2007"]},{"key":"dc:publisher","label":"Institution","values":["Publikationsserver der RWTH Aachen University"]},{"key":"dc:relation","label":"Dc Relation","values":["info:eu-repo/semantics/altIdentifier/urn/urn:nbn:de:hbz:82-opus-18839"]},{"key":"dc:type","label":"Dc Type","values":["info:eu-repo/semantics/doctoralThesis","info:eu-repo/semantics/publishedVersion"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["info:eu-repo/classification/ddc/530","Magnetismus","Nanopartikel","Elektronenstruktur","Elektronische Eigenschaft","Green-Funktion","Physik","Noncollinear magnetism","Density functional theory","Nanocluster","transition metal","KKR"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language","label":"Dc Language","values":["eng"]},{"key":"dc:rights","label":"Dc Rights","values":["info:eu-repo/semantics/openAccess"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier","label":"Identifier","values":["https://publications.rwth-aachen.de/record/61755","https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-123387%22"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description","label":"Description","values":["The question how magnetism behaves when the dimension of materials is reduced to increasingly smaller sizes has attracted much research and led to the development of the field of magnetic nanostructures. This research has been fueled by the technological potential of these systems for the field of high-density magnetic storage media and has been accelerated by the many novel experimental methods and techniques developed exhibiting atomic resolution. This thesis is motivated by the quest for the understanding and the exploration of complex magnetism provided by atomic scale magnetic clusters deposited on surfaces or embedded in the bulk. The nature of magnetism in these systems can be very rich, in that the properties depend on the atomic species, the cluster size, shape and symmetry or choice of the substrate. Small variations of the cluster parameter may change the properties dramatically. Particularly rich and particularly challenging for experiment and theory is the behavior of clusters with competing magnetic interactions either between the cluster atoms or between the cluster and the substrate. In both cases magnetic frustration can lead to non-collinear magnetic structures for which the magnetic quantization axis changes from atom to atom. This thesis sheds light onto these systems from a theoretical perspective. Use is made of the density functional theory (DFT), the most successful material specific theory for describing electronic and derived properties from first-principles. Acting within this framework, we have developed and implemented the treatment of non-collinear magnetism into the Juelich version of the full-potential Korringa-Kohn-Rostoker Green Function (KKR-GF) method. The KKR-GF method provides several advantages compared to other first-principles methods. Based on solving the Dyson equation it allows an elegant treatment of non-periodic systems such as impurities and clusters in bulk or on surfaces. Electronic, magnetic properties and the observables provided by experimental techniques such as x-ray, scanning tunneling microscopy and spectroscopy can be accessed with the KKR-GF method. Firstly, the method was applied to 3d transition-metal clusters on different ferromagnetic surfaces. Different types of magnetic clusters were selected. Clusters of Fe, Co, Ni atoms are ferromagnetic and thus magnetically collinear. In order to investigate magnetic frustration due to competing interactions within the ad-cluster we considered a (001) oriented surface of fcc metals, a topology which usually does not lead to non-collinear magnetism. We tuned the strength of the magnetic coupling between the ad-clusters and the ferromagnetic surface by varying the substrate from the case of Ni(001) with a rather weak hybridization of the Ni d-states with the adatom d-states to the case of Fe(3ML)/Cu(001) with a much stronger hybridization due to the larger extend of the Fe wavefunctions. Cr and Mn clusters of different sizes and shapes have been found to exhibit non-collinear magnetic structures. The second type of frustration was investigated employing a Ni(111) surface, a surface with a triangular lattice of atoms, were both kind of competing magnetic interactions occur: intra-cluster magnetic frustration and cluster-substrate magnetic interaction. Parity of number of adatoms in finite antiferromagnetic nanowires is shown to be crucial in predicting whether the magnetic ground state is non-collinear or collinear. We show that nanochains with an even number of adatoms are always magnetically non-collinear while an odd number of adatoms leads under given conditions to a collinear ferrimagnetic ground state. In the second part of the thesis we applied the KKR-GF method to reveal experimental issues related to scanning tunneling microscopy (STM). Hence, we investigated the scattering of a two-dimensional surface state at adatoms. Here it is shown theoretically for some special cases as well as experimentally (STM) that any attractive potential should lead to a localized state. With a systematic study, including sp as well as d adatoms on or in the surface, we demonstrate that the cited statement, that any attractive potential should lead to a localized state, is not correct. Indeed, we derive a better criteria based on the scattering length of the adatom. At last, we show that Fermi surfaces can be imaged through STM. The Fermi surface is one of the most important properties in metals as it determines many transport properties as well as long-ranged interactions. The new effect is explained by means of scattering of electrons at subsurface impurities. Since the electrons propagate away from the impurities with velocity vectors perpendicular to the Fermi surface, flat regions on this surface focus the electrons in special space-angle directions, leading in real space to highly symmetric STM spots around the impurity."]},{"key":"dc:source","label":"Dc Source","values":["Aachen : Publikationsserver der RWTH Aachen University XII, 189 S. : Ill., graph. Darst. (2007). = Aachen, Techn. Hochsch., Diss., 2007"]},{"key":"dc:title","label":"Title","values":["Theory of magnetic transition metal nanoclusters on surfaces"]}]}],"canonical_facts":{"dc:contributor":["Blügel, Stefan"],"dc:coverage":["DE"],"dc:creator":["Lounis, Samir"],"dc:date":["2007"],"dc:description":["The question how magnetism behaves when the dimension of materials is reduced to increasingly smaller sizes has attracted much research and led to the development of the field of magnetic nanostructures. This research has been fueled by the technological potential of these systems for the field of high-density magnetic storage media and has been accelerated by the many novel experimental methods and techniques developed exhibiting atomic resolution. This thesis is motivated by the quest for the understanding and the exploration of complex magnetism provided by atomic scale magnetic clusters deposited on surfaces or embedded in the bulk. The nature of magnetism in these systems can be very rich, in that the properties depend on the atomic species, the cluster size, shape and symmetry or choice of the substrate. Small variations of the cluster parameter may change the properties dramatically. Particularly rich and particularly challenging for experiment and theory is the behavior of clusters with competing magnetic interactions either between the cluster atoms or between the cluster and the substrate. In both cases magnetic frustration can lead to non-collinear magnetic structures for which the magnetic quantization axis changes from atom to atom. This thesis sheds light onto these systems from a theoretical perspective. Use is made of the density functional theory (DFT), the most successful material specific theory for describing electronic and derived properties from first-principles. Acting within this framework, we have developed and implemented the treatment of non-collinear magnetism into the Juelich version of the full-potential Korringa-Kohn-Rostoker Green Function (KKR-GF) method. The KKR-GF method provides several advantages compared to other first-principles methods. Based on solving the Dyson equation it allows an elegant treatment of non-periodic systems such as impurities and clusters in bulk or on surfaces. Electronic, magnetic properties and the observables provided by experimental techniques such as x-ray, scanning tunneling microscopy and spectroscopy can be accessed with the KKR-GF method. Firstly, the method was applied to 3d transition-metal clusters on different ferromagnetic surfaces. Different types of magnetic clusters were selected. Clusters of Fe, Co, Ni atoms are ferromagnetic and thus magnetically collinear. In order to investigate magnetic frustration due to competing interactions within the ad-cluster we considered a (001) oriented surface of fcc metals, a topology which usually does not lead to non-collinear magnetism. We tuned the strength of the magnetic coupling between the ad-clusters and the ferromagnetic surface by varying the substrate from the case of Ni(001) with a rather weak hybridization of the Ni d-states with the adatom d-states to the case of Fe(3ML)/Cu(001) with a much stronger hybridization due to the larger extend of the Fe wavefunctions. Cr and Mn clusters of different sizes and shapes have been found to exhibit non-collinear magnetic structures. The second type of frustration was investigated employing a Ni(111) surface, a surface with a triangular lattice of atoms, were both kind of competing magnetic interactions occur: intra-cluster magnetic frustration and cluster-substrate magnetic interaction. Parity of number of adatoms in finite antiferromagnetic nanowires is shown to be crucial in predicting whether the magnetic ground state is non-collinear or collinear. We show that nanochains with an even number of adatoms are always magnetically non-collinear while an odd number of adatoms leads under given conditions to a collinear ferrimagnetic ground state. In the second part of the thesis we applied the KKR-GF method to reveal experimental issues related to scanning tunneling microscopy (STM). Hence, we investigated the scattering of a two-dimensional surface state at adatoms. Here it is shown theoretically for some special cases as well as experimentally (STM) that any attractive potential should lead to a localized state. With a systematic study, including sp as well as d adatoms on or in the surface, we demonstrate that the cited statement, that any attractive potential should lead to a localized state, is not correct. Indeed, we derive a better criteria based on the scattering length of the adatom. At last, we show that Fermi surfaces can be imaged through STM. The Fermi surface is one of the most important properties in metals as it determines many transport properties as well as long-ranged interactions. The new effect is explained by means of scattering of electrons at subsurface impurities. Since the electrons propagate away from the impurities with velocity vectors perpendicular to the Fermi surface, flat regions on this surface focus the electrons in special space-angle directions, leading in real space to highly symmetric STM spots around the impurity."],"dc:identifier":["https://publications.rwth-aachen.de/record/61755","https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-123387%22"],"dc:language":["eng"],"dc:publisher":["Publikationsserver der RWTH Aachen University"],"dc:relation":["info:eu-repo/semantics/altIdentifier/urn/urn:nbn:de:hbz:82-opus-18839"],"dc:rights":["info:eu-repo/semantics/openAccess"],"dc:source":["Aachen : Publikationsserver der RWTH Aachen University XII, 189 S. : Ill., graph. Darst. (2007). = Aachen, Techn. 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