| 1. |
- Bulbucan, Claudiu
(author)
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Characterization of Magnetic Nanoscale Systems: From Molecules to Particles and Self-Assembled Chains
- 2021
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Doctoral thesis (other academic/artistic)abstract
- This thesis characterizes magnetic nanoscale systems with increasing size and aspect ratio using synchrotron radiation-based spectroscopy and microscopy techniques, combined with conventional magnetometry. The systems studied are molecular magnets, bimagnetic nanoparticles and self-assembled magnetic nanochains.Single-molecule magnets are compounds that exhibit magnetic bistability and hysteresis at low temperatures. To investigate how their properties are affected when in contact with different surfaces, sub-monolayers of endofullereneDynSc3−nN@C80 (n = 1, 2) were prepared on conducting and insulating substrates by thermal evaporation and chemical deposition. In the latter case, the SMMs were functionalized with surface-anchoring thioether groups, which substantially affected the magnetic properties. However, magnetic hysteresis was observed on a surface of Au(111). The magnetic bistability was better preserved when sublimating the di-dysprosium compound (n = 2) onto Au(111), Ag(100), and MgO|Ag(100), exhibiting a wide substrate-independent hysteresis. While the magnetic bistability was unaffected, the orientation of the magnetic clusters was highly influenced by the choice of substrate.Bimagnetic nanoparticles composed of two different magnetically ordered phases were synthesized by means of a spark ablation aerosol method. Cr substituted Fe-spinel oxide particles with embedded FeO subdomains with sizes of 10, 20 and 40 nm were investigated in detail. The unique structure was attributed to the presence of Cr and its influence on the oxidation of Fe. The 40 nm NPs exhibited a large exchange bias and an increase in coercivity when cooled in an applied field across the Néel temperature of FeO. The influence of particle size on the formation of the FeO subdomains, as well as the emergence of exchange bias were investigated and the results point to a decrease in the exchange field as the particle size decreases, with virtually no exchange bias present for the 10 nm system.Nanochains were generated in a bottom-up approach by field-assisted self-assembly of aerosolized Co nanoparticles. Local magnetic properties were investigated by means of scanning transmission x-ray microscopy, complimented by micromagnetic simulations. While uniform chains have a single domain due to significant shape anisotropy, the inclusion of large multi-domain particles can facilitate domain formation.
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| 2. |
- Bulbucan, Claudiu, et al.
(author)
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Cr-substituted Fe3O4 nanoparticles : The role of particle size in the formation of FexO sub-domains and the emergence of exchange bias
- 2023
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In: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853. ; 570
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Journal article (peer-reviewed)abstract
- Here we study the influence of particle size on the formation of antiferromagnetic FexO subdomains and the emergence of exchange bias in novel ferrimagnetic Cr-substituted Fe3O4 nanoparticles generated using an aerosol technique based on spark ablation. Cooling in an applied magnetic field across the Néel temperature of the FexO subdomains results in significant shifts in the hysteresis loops, which decrease in magnitude with the particle diameter for the two large systems (D≈20nm&D≈40nm), whereas only minor exchange fields are observed for the smallest system (D≈10nm). The reduced exchange bias is attributed to the metastable FexO's lower volume fraction and size due to a facile transformation into the thermodynamically stable (Fe,Cr)3O4 phase with decreasing particle diameter.
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| 3. |
- Bulbucan, Claudiu, et al.
(author)
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Large exchange bias in Cr substituted Fe3O4nanoparticles with FeO subdomains
- 2021
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In: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3364 .- 2040-3372. ; 13:37, s. 15844-15852
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Journal article (peer-reviewed)abstract
- Tuning the anisotropy through exchange bias in bimagnetic nanoparticles is an active research strategy for enhancing and tailoring the magnetic properties for a wide range of applications. Here we present a structural and magnetic characterization of unique FeCr-oxide nanoparticles generated from seed material with a Fe : Cr ratio of 4.71 : 1 using a physical aerosol method based on spark ablation. The nanoparticles have a novel bimagnetic structure composed of a 40 nm ferrimagnetic Cr-substituted Fe3O4 structure with 4 nm antiferromagnetic FexO subdomains. Cooling in an applied magnetic field across the Néel temperature of the FexO subdomains results in a significant shift in the hysteresis, demonstrating the presence of a large exchange bias. The observed shift of μ0HE = 460 mT is among the largest values reported for FexO-Fe3O4-based nanoparticles and is attributed to the large antiferromagnetic-ferrimagnetic interface area provided by the subdomains.
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| 4. |
- Koutsouflakis, Emmanouil, et al.
(author)
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Metamagnetic transition and a loss of magnetic hysteresis caused by electron trapping in monolayers of single-molecule magnet Tb2@C79N
- 2022
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In: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3364 .- 2040-3372. ; 14:27, s. 9877-9892
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Journal article (peer-reviewed)abstract
- Realization of stable spin states in surface-supported magnetic molecules is crucial for their applications in molecular spintronics, memory storage or quantum information processing. In this work, we studied the surface magnetism of dimetallo-azafullerene Tb2@C79N, showing a broad magnetic hysteresis in a bulk form. Surprisingly, monolayers of Tb2@C79N exhibited a completely different behavior, with the prevalence of a ground state with antiferromagnetic coupling at low magnetic field and a metamagnetic transition in the magnetic field of 2.5-4 T. Monolayers of Tb2@C79N were deposited onto Cu(111) and Au(111) by evaporation in ultra-high vacuum conditions, and their topography and electronic structure were characterized by scanning tunneling microscopy and spectroscopy (STM/STS). X-ray photoelectron spectroscopy (XPS), in combination with DFT studies, revealed that the nitrogen atom of the azafullerene cage tends to avoid metallic surfaces. Magnetic properties of the (sub)monolayers were then studied by X-ray magnetic circular dichroism (XMCD) at the Tb-M4,5 absorption edge. While in bulk powder samples Tb2@C79N behaves as a single-molecule magnet with ferromagnetically coupled magnetic moments and blocking of magnetization at 28 K, its monolayers exhibited a different ground state with antiferromagnetic coupling of Tb magnetic moments. To understand if this unexpected behavior is caused by a strong hybridization of fullerenes with metallic substrates, XMCD measurements were also performed for Tb2@C79N adsorbed on h-BN|Rh(111) and MgO|Ag(100). The co-existence of two forms of Tb2@C79N was found on these substrates as well, but magnetization curves showed narrow magnetic hysteresis detectable up to 25 K. The non-magnetic state of Tb2@C79N in monolayers is assigned to anionic Tb2@C79N− species with doubly-occupied Tb-Tb bonding orbital and antiferromagnetic coupling of the Tb moments. A charge transfer from the substrate or trapping of secondary electrons are discussed as a plausible origin of these species.
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| 5. |
- Krylov, Denis S., et al.
(author)
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Substrate-Independent Magnetic Bistability in Monolayers of the Single-Molecule Magnet Dy2ScN@C80 on Metals and Insulators
- 2020
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In: Angewandte Chemie - International Edition. - : Wiley. - 1433-7851 .- 1521-3773. ; 59:14, s. 5756-5764
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Journal article (peer-reviewed)abstract
- Magnetic hysteresis is demonstrated for monolayers of the single-molecule magnet (SMM) Dy2ScN@C80 deposited on Au(111), Ag(100), and MgO|Ag(100) surfaces by vacuum sublimation. The topography and electronic structure of Dy2ScN@C80 adsorbed on Au(111) were studied by STM. X-ray magnetic CD studies show that the Dy2ScN@C80 monolayers exhibit similarly broad magnetic hysteresis independent on the substrate used, but the orientation of the Dy2ScN cluster depends strongly on the surface. DFT calculations show that the extent of the electronic interaction of the fullerene molecules with the surface is increasing dramatically from MgO to Au(111) and Ag(100). However, the charge redistribution at the fullerene-surface interface is fully absorbed by the carbon cage, leaving the state of the endohedral cluster intact. This Faraday cage effect of the fullerene preserves the magnetic bistability of fullerene-SMMs on conducting substrates and facilitates their application in molecular spintronics.
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| 6. |
- Messing, Maria, et al.
(author)
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Iron-based magnetic nanoparticles by spark ablation
- 2019
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Conference paper (peer-reviewed)abstract
- Magnetic nanoparticles have shown great potential for use in drug delivery and bioimaging applications and are prospective building blocks in future high-performing permanent magnets. Today, magnetic nanoparticles are most often produced in batches by chemical methods resulting in the risk of chemical impurities and production of vast amounts of chemical waste. Aerosol generation methods on the other hand, particularly spark ablation, are promising for future generation of magnetic nanoparticles since they are simple, fast, continuous, scalable, provide good control of size and composition, and offer the possibility to form alloys of material combinations not miscible on the macroscopic scale. High controllability is of utmost importance when generating magnetic nanoparticles since small deviations in size can significantly alter the magnetic coercivity. Also, the composition of the particles is of high significance since the magnetic properties can be completely transformed by changes in elemental composition or the oxidation state of the particle.In this work, we present the successful generation of monodisperse bimetallic FeCr and FeMn nanoparticles by spark ablation, and the results from the thorough characterization of individual particles with aerosol instruments, transmission electron microscopy, and synchrotron-based X-ray photoelectron spectroscopy. We demonstrate how the carrier gas can be used to dictate the oxidation and how to alternate between self-passivated and completely oxidized nanoparticles. We also show how the produced particles can be deposited to yield a low surface concentration which is critical for minimizing interparticle interactions during magnetic measurements. Finally, as a proof of concept, measurements using a magnetometer equipped with a SQUID on samples with different particle coverages are presented.
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| 7. |
- Preger, Calle, et al.
(author)
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Controlled Oxidation and Self-Passivation of Bimetallic Magnetic FeCr and FeMn Aerosol Nanoparticles
- 2019
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In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 123:26, s. 16083-16090
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Journal article (peer-reviewed)abstract
- Nanoparticle generation by aerosol methods, particularly spark ablation, has high potential for creating new material combinations with tailored magnetic properties. By combining elements into complex alloyed nanoparticles and controlling their size and structure, different magnetic properties can be obtained. In combination with controlled deposition, to ensure nanoparticle separation, it is possible to minimize interparticle interactions and measure the intrinsic magnetic property of the nanoparticles. Most magnetic materials are highly sensitive to oxygen, and it is therefore crucial to both understand and control the oxidation of magnetic nanoparticles. In this study, we have successfully generated oxidized, bimetallic FeCr and FeMn nanoparticles by spark ablation in combination with a compaction step and thoroughly characterized individual particles with aerosol instruments, transmission electron microscopy and synchrotron-based X-ray photoelectron spectroscopy. The generated nanoparticles had an almost identical transition-metal ratio to the electrodes used as seed materials. Further, we demonstrate how the carrier gas can be used to dictate the oxidation and how to alternate between self-passivated and entirely oxidized nanoparticles. We also discuss the complexity of compacting alloyed nanoparticles consisting of elements with different vapor pressures and how this will affect the composition. This knowledge will further the understanding of design and generation of complex alloyed nanoparticles based on transition metals using aerosol methods, especially for the size regime where a compaction step is needed. As a proof of concept, measurements using a magnetometer equipped with a superconducting quantum interference device were performed on samples with different particle coverages. These measurements show that the magnetic properties could be explored for both high and low surface coverages, which open a way for studies where interparticle interactions can be systematically controlled.
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| 8. |
- Sedrpooshan, Mehran, et al.
(author)
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Single-step generation of 1D FeCo nanostructures
- 2024
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In: Nano Express. ; 5:2
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Journal article (peer-reviewed)abstract
- Magnetic one-dimensional structures are attractive nanomaterials due to the variety of potential applications they can provide. The fabrication of bimetallic 1D structures further expands the capabilities of such structures by tailoring the magnetic properties. Here, a single-step template-free method is presented for the fabrication of 1D FeCo alloy nanochains. In this approach, charged single-crystalline FeCo nanoparticles are first generated by the co-ablation of pure Fe and Co electrodes under a carrier gas at ambient pressures and attracted to a substrate using an electric field. When reaching the surface, the particles are self-assembled into parallel nanochains along the direction of an applied magnetic field. The approach allows for monitoring the self-assembly particle by particle as they are arranged into linear 1D chains with an average length controlled by the deposited particle concentration. Magnetometry measurements revealed that arranging nanoparticles into nanochains results in a 100% increase in the remanent magnetization, indicating significant shape anisotropy. Furthermore, by combining x-ray microscopy and micromagnetic simulations, we have studied the local magnetization configuration along the nanochains. Our findings show that variations in magnetocrystalline anisotropy along the structure play a crucial role in the formation of magnetic domains.
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| 9. |
- Sedrpooshan, Mehran, et al.
(author)
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Template-free generation and integration of functional 1D magnetic nanostructures
- 2023
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In: Nanoscale. - : Royal Society of Chemistry. - 2040-3372 .- 2040-3364.
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Journal article (peer-reviewed)abstract
- The direct integration of 1D magnetic nanostructures into electronic circuits is crucial for realizing their great potential as components in magnetic storage, logical devices, and spintronic applications. Here, we present a novel template-free technique for producing magnetic nanochains and nanowires using directed self-assembly of gas-phase-generated metallic nanoparticles. The 1D nanostructures can be self-assembled along most substrate surfaces and can be freely suspended over micrometer distances, allowing for direct incorporation into different device architectures. The latter is demonstrated by a one-step integration of nanochains onto a pre-patterned Si chip and the fabrication of devices exhibiting magnetoresistance. Moreover, fusing the nanochains into nanowires by post-annealing significantly enhances the magnetic properties, with a 35% increase in the coercivity. Using magnetometry, X-ray microscopy, and micromagnetic simulations, we demonstrate how variations in the orientation of the magnetocrystalline anisotropy and the presence of larger multi-domain particles along the nanochains play a key role in the domain formation and magnetization reversal. Furthermore, it is shown that the increased coercivity in the nanowires can be attributed to the formation of a uniform magnetocrystalline anisotropy along the wires and the onset of exchange interactions.
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