| 1. |
- Bonanni, Valentina, et al.
(författare)
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First order reversal curve analysis of graded anisotropy FePtCu films
- 2010
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 97:20
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Tidskriftsartikel (refereegranskat)abstract
- The reversal mechanisms of graded anisotropy FePtCu films have been investigated by alternating gradient magnetometer (AGM) and magneto-optical Kerr effect (MOKE) measurements with first-order reversal curve (FORC) techniques. The AGM-FORC analysis, which clearly shows the presence of soft and hard components, is unable to resolve how these phases are distributed throughout the film thickness. MOKE-FORC measurements, which preferentially probe the surface of the film, reveal that the soft components are indeed located toward the top surface. Combining AGM-FORC with the inherent surface sensitivity of MOKE-FORC analysis allows for a comprehensive analysis of heterogeneous systems such as graded materials.
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| 2. |
- Bousquet, Jean, et al.
(författare)
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Allergic Rhinitis and its Impact on Asthma (ARIA) Phase 4 (2018) : Change management in allergic rhinitis and asthma multimorbidity using mobile technology
- 2019
-
Ingår i: Journal of Allergy and Clinical Immunology. - : Elsevier. - 0091-6749 .- 1097-6825. ; 143:3, s. 864-879
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Tidskriftsartikel (refereegranskat)abstract
- Allergic Rhinitis and its Impact on Asthma (ARIA) has evolved from a guideline by using the best approach to integrated care pathways using mobile technology in patients with allergic rhinitis (AR) and asthma multimorbidity. The proposed next phase of ARIA is change management, with the aim of providing an active and healthy life to patients with rhinitis and to those with asthma multimorbidity across the lifecycle irrespective of their sex or socioeconomic status to reduce health and social inequities incurred by the disease. ARIA has followed the 8-step model of Kotter to assess and implement the effect of rhinitis on asthma multimorbidity and to propose multimorbid guidelines. A second change management strategy is proposed by ARIA Phase 4 to increase self-medication and shared decision making in rhinitis and asthma multimorbidity. An innovation of ARIA has been the development and validation of information technology evidence-based tools (Mobile Airways Sentinel Network [MASK]) that can inform patient decisions on the basis of a self-care plan proposed by the health care professional.
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| 3. |
- Chen, Jianing, et al.
(författare)
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Plasmonic Nickel Nanoantennas
- 2011
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Ingår i: SMALL. - : Wiley. - 1613-6810 .- 1613-6829. ; 7:16, s. 2341-2347
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Tidskriftsartikel (refereegranskat)abstract
- The fundamental optical properties of pure nickel nanostructures are studied by far-field extinction spectroscopy and optical near-field microscopy, providing direct experimental evidence of the existence of particle plasmon resonances predicted by theory. Experimental and calculated near-field maps allow for unambiguous identification of dipolar plasmon modes. By comparing calculated near-field and far-field spectra, dramatic shifts are found between the near-field and far-field plasmon resonances, which are much stronger than in gold nanoantennas. Based on a simple damped harmonic oscillator model to describe plasmonic resonances, it is possible to explain these shifts as due to plasmon damping.
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| 4. |
- Dahlberg, D, et al.
(författare)
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Measurements of the ferromagnetic/antiferromagnetic interfacial exchange energy in CO/CoO and Fe/FeF2 layers
- 1998
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 83:11, s. 6893-6895
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Tidskriftsartikel (refereegranskat)abstract
- Two measurement techniques, both relying on reversible rotations of the magnetization, have been used to determine the magnitude of the interfacial exchange energy (IEE) between ferromagnetic and antiferromagnetic (F/AF) layers. One technique is to use the anisotropic magnetoresistance to determine rotations of the magnetization away from the unidirectional easy axis, where the rotation is accomplished by applying external magnetic fields less than the effective F/AF exchange field. The second technique uses measurements of the ac susceptibility as a function of the angle between the ac field and the unidirectional exchange field. Both of the reversible process techniques result in values of the IEE larger (by as much as a factor of 10 in Co/CoO bilayers) than the traditional irreversible technique of measuring a shift in the hysteresis loop. The ac susceptibility technique was also used to measure one Fe/FeF2 bilayer. For this sample, the IEE values obtained by reversible and irreversible methods are equivalent.
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| 5. |
- De Toro, José A., et al.
(författare)
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Remanence Plots as a Probe of Spin Disorder in Magnetic Nanoparticles
- 2017
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Ingår i: Chemistry of Materials. - Washington, D.C., USA : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 29:19, s. 8258-8268
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Tidskriftsartikel (refereegranskat)abstract
- Remanence magnetization plots (e.g., Henkel or delta M plots) have been extensively used as a straightforward way to determine the presence and intensity of dipolar and exchange interactions in assemblies of magnetic nanoparticles or single domain grains. Their evaluation is particularly important in functional materials whose performance is strongly affected by the intensity of interparticle interactions, such as patterned recording media and nanostructured permanent magnets, as well as in applications such as hyperthermia and magnetic resonance imaging. Here, we demonstrate that delta M plots may be misleading when the nanoparticles do not have a homogeneous internal magnetic configuration. Substantial dips in the delta M plots of gamma-Fe2O3 nanoparticles isolated by thick SiO2 shells indicate the presence of demagnetizing interactions, usually identified as dipolar interactions. results, however, demonstrate that it is the inhomogeneous spin structure of the nanoparticles, as most clearly evidenced by Mossbauer measurements, that has a pronounced effect on the delta M plots, leading to features remarkably similar to those produced by dipolar interactions. X-ray diffraction results combined with magnetic characterization indicate that this inhomogeneity is due to the presence of surface structural (and spin) disorder. Monte Carlo simulations unambiguously corroborate the critical role of the internal magnetic structure in the delta M plots. Our findings constitute a cautionary tale on the widespread use of remanence plots to assess interparticle interactions as well as offer new perspectives in the use of Henkel and delta M plots to quantify the rather elusive inhomogeneous magnetization states in nanoparticles.
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| 6. |
- Dumas, Randy K., et al.
(författare)
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Graded Anisotropy FePtCu Films
- 2011
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Ingår i: IEEE transactions on magnetics. - 0018-9464 .- 1941-0069. ; 47:6, s. 1580-1586
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Tidskriftsartikel (refereegranskat)abstract
- The fabrication and subsequent analysis of continuously graded anisotropy films are discussed. During deposition, a compositional gradient is first achieved by varying the Cu concentration from Cu-rich (Fe53Pt47)(70)Cu-30 to Cu-free Fe53Pt47. The anisotropy gradient is then realized after thermal post-annealing, and by utilizing the strong composition dependence of the low-anisotropy (A1) to high-anisotropy (L1(0)) ordering temperature. The magnetic properties are investigated by surface sensitive magneto-optical Kerr effect and alternating gradient magnetometer (AGM) measurements. AGM first-order reversal curve (FORC) measurements are employed in order to provide a detailed analysis of the reversal mechanisms, and therefore the induced anisotropy gradient. At low annealing temperatures, the FORC measurements clearly indicate the highly coupled reversal of soft and hard phases. However, significant interdiffusion results in virtually uniform films at elevated annealing temperatures. Additionally, the A1 to L1(0) ordering process is found to depend on the film thickness.
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| 7. |
- Dumas, Randy K., et al.
(författare)
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Probing vertically graded anisotropy in FePtCu films
- 2011
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Ingår i: Physical Review B. - 1098-0121 .- 1550-235X. ; 84, s. 054434-
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Tidskriftsartikel (refereegranskat)abstract
- Field-dependent polarized neutron reflectivity (PNR) and magnetometry are employed to study the magnetic properties of compositionally uniform and graded FePtCu films as a function of annealing temperature (TA). The PNR results are able to directly probe the compositional and anisotropy variations through the film thickness. Further details about how the reversal mechanisms evolve are then elucidated by using a first-order reversal curve technique. The reversal of the graded sample annealed at 300°C occurs by an initial rapid switching of the dominant soft A1 phase toward the surface of the film, followed by the gradual reversal of the residual hard phase components toward the bottom. This indicates that the anisotropy gradient is not well established at this low TA. A fundamentally different mechanism is found after annealing at 400°C, where the rapid switching of the entire film is preceded by a gradual reversal of the soft layers. This suggests that the anisotropy gradient has become better established through the film thickness. The field-dependent PNR measurements confirm the existence of an anisotropy gradient, where the lower (higher) anisotropy portions are now toward the bottom (top) of the film because of the Cu compositional gradient. However, after annealing at 500°C, a single rapid reversal is found, indicating the formation of a uniform hard film. In this case, PNR demonstrates a more uniform magnetic depth profile that is consistent with a uniform reference sample, suggesting significant interdiffusion of the Cu is degrading the compositional and induced anisotropy gradient at this elevated TA.
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| 8. |
- Estrader, Marta, et al.
(författare)
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Origin of the large dispersion of magnetic properties in nanostructured oxides : FexO/Fe3O4 nanoparticles as a case study
- 2015
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3364 .- 2040-3372. ; 7:7, s. 3002-3015
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Tidskriftsartikel (refereegranskat)abstract
- The intimate relationship between stoichiometry and physicochemical properties in transition-metal oxides makes them appealing as tunable materials. These features become exacerbated when dealing with nanostructures. However, due to the complexity of nanoscale materials, establishing a distinct relationship between structure-morphology and functionalities is often complicated. In this regard, in the FexO/Fe3O4 system a largely unexplained broad dispersion of magnetic properties has been observed. Here we show, thanks to a comprehensive multi-technique approach, a clear correlation between the magneto-structural properties in large (45 nm) and small (9 nm) FexO/Fe3O4 core/shell nanoparticles that can explain the spread of magnetic behaviors. The results reveal that while the FexO core in the large nanoparticles is antiferromagnetic and has bulk-like stoichiometry and unit-cell parameters, the FexO core in the small particles is highly non-stoichiometric and strained, displaying no significant antiferromagnetism. These results highlight the importance of ample characterization to fully understand the properties of nanostructured metal oxides.
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| 9. |
- Fornara, Andrea, 1980-
(författare)
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Magnetic nanostructured materials for advanced bio-applications
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the recent years, nanostructured magnetic materials and their use in biomedical and biotechnological applications have received a lot of attention. In this thesis, we developed tailored magnetic nanoparticles for advanced bio-applications, such as direct detection of antibodies in biological samples and stimuli responsive drug delivery system. For sensitive and selective detection of biomolecules, thermally blocked iron oxide nanoparticles with specific magnetic properties are synthesized by thermal hydrolysis to achieve a narrow size distribution just above the superparamagnetic limit. The prepared nanoparticles were characterized and functionalized with biomolecules for use in a successful biosensor system. We have demonstrated the applicability of this type of nanoparticles for the detection of Brucella antibodies as model compound in serum samples and very low detection limits were achieved (0.05 mg/mL). The second part is concerning an in-depth investigation of the evolution of the thermally blocked magnetic nanoparticles. In this study, the formation of the nanoparticles at different stages during the synthesis was investigated by high resolution electron microscopy and correlated to their magnetic properties. At early stage of the high temperature synthesis, small nuclei of 3.5 nm are formed and the particles size increases successively until they reach a size of 17-20 nm. The small particles first exhibit superparamagnetic behavior at the early stage of synthesis and then transform to thermally blocked behavior as their size increases and passes the superparamagnetic limit. The last section of the Thesis is related to the development of novel drug delivery system based on magnetically controlled release rate. The system consists of hydrogel of Pluronic FP127 incorporating superparamagnetic iron oxide nanoparticles to form a ferrogel. The sol to gel formation of the hydrogel could be tailored to be solid at body temperature and thus have the ability to be injected inside living biological tissues. In order to evaluate the drug loading and release, the hydrophobic drug indomethacin was selected as a model compound. The drug could be loaded in the ferrogel owning to the oil in water micellar structure. We have studied the release rate from the ferrogel in the absence and presence of magnetic field. We have demonstrated that the drug release rate can be significantly enhanced by use of external magnetic field decreasing the half time of the release to more than 50% (from 3200 to 1500 min) upon the application of the external magnetic field. This makes the developed ferrogel a very promising drug delivery system that does not require surgical implant procedure for medical treatment and gives the possibility of enhancing the rate of release by external magnetic field.
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| 10. |
- Guell-Grau, Pau, et al.
(författare)
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Elastic Plasmonic-Enhanced Fabry-Perot Cavities with Ultrasensitive Stretching Tunability
- 2022
-
Ingår i: Advanced Materials. - : WILEY-V C H VERLAG GMBH. - 0935-9648 .- 1521-4095. ; 34:7
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Tidskriftsartikel (refereegranskat)abstract
- The emerging stretchable photonics field faces challenges, like the robust integration of optical elements into elastic matrices or the generation of large optomechanical effects. Here, the first stretchable plasmonic-enhanced and wrinkled Fabry-Perot (FP) cavities are demonstrated, which are composed of self-embedded arrays of Au nanostructures at controlled depths into elastomer films. The novel self-embedding process is triggered by the Au nanostructures catalytic activity, which locally increases the polymer curing rate, thereby inducing a mechanical stress that simultaneously pulls the Au nanostructures into the polymer and forms a wrinkled skin layer. This geometry yields unprecedented optomechanical effects produced by the coupling of the broad plasmonic modes of the Au nanostructures and the FP modes, which are modulated by the wrinkled optical cavity. As a result, film stretching induces drastic changes in both the spectral position and intensity of the plasmonic-enhanced FP resonances due to the simultaneous cavity thickness reduction and cavity wrinkle flattening, thus increasing the cavity finesse. These optomechanical effects are exploited to demonstrate new strain-sensing approaches, achieving a strain detection limit of 0.006%, i.e., 16-fold lower than current optical strain-detection schemes.
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| 11. |
- Krycka, Kathryn L., et al.
(författare)
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Resolving Material-Specific Structures within Fe3O4 vertical bar gamma-Mn2O3 Core vertical bar Shell Nanoparticles Using Anomalous Small-Angle X-ray Scattering
- 2013
-
Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 7:2, s. 921-931
-
Tidskriftsartikel (refereegranskat)abstract
- Here it is demonstrated that multiple-energy, anomalous small-angle X-ray scattering (ASAXS) provides significant enhancement in sensitivity to internal material boundaries of layered nanoparticles compared with the traditional modeling of a single scattering energy, even for cases in which high scattering contrast naturally exists. Specifically, the material-specific structure of monodispersed Fe3O4 vertical bar gamma-Mn2O3 core vertical bar shell nanoparticles is determined, and the contribution of each component to the total scattering profile is identified with unprecedented clarity. We show that Fe3O4 vertical bar gamma-Mn2O3 core vertical bar shell nanoparticles with a diameter of 8.2 +/- 0.2 nm consist of a core with a composition near Fe3O4 surrounded by a (MnxFe1-x)(3)O-4 shell with a graded composition, ranging from x approximate to 0.40 at the Inner shell toward x approximate to 0.46 at the surface. Evaluation of the scattering contribution arising from the interference between material-specific layers additionally reveals the presence of Fe3O4 cores without a coating shell. Finally, it is found that the material-specific scattering profile shapes and chemical compositions extracted by this method are independent of the original input chemical compositions used in the analysis, revealing multiple-energy ASAXS as a powerful tool for determining internal nanostructured morphology even if the exact composition of the individual layers is not known a priori.
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| 12. |
- Lopez-Ortega, Alberto, et al.
(författare)
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Applications of exchange coupled bi-magnetic hard/soft and soft/hard magnetic core/shell nanoparticles
- 2015
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Ingår i: Physics reports. - : Elsevier BV. - 0370-1573 .- 1873-6270. ; 553, s. 1-32
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Forskningsöversikt (refereegranskat)abstract
- The applications of exchange coupled bi-magnetic hard/soft and soft/hard ferromagnetic core/shell nanoparticles are reviewed. After a brief description of the main synthesis approaches and the core/shell structural morphological characterization, the basic static and dynamic magnetic properties are presented. Five different types of prospective applications, based on diverse patents and research articles, are described: permanent magnets, recording media, microwave absorption, biomedical applications and other applications. Both the advantages of the core/shell morphology and some of the remaining challenges are discussed.
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| 13. |
- Lopez-Ortega, Alberto, et al.
(författare)
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Size-Dependent Passivation Shell and Magnetic Properties in Antiferromagnetic/Ferrimagnetic Core/Shell MnO Nanoparticles
- 2010
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 132:27, s. 9398-9407
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Tidskriftsartikel (refereegranskat)abstract
- The magnetic properties of bimagnetic core/shell nanoparticles consisting of an antiferromagnetic MnO core and a ferrimagnetic passivation shell have been investigated. It is found that the phase of the passivation shell (gamma-Mn2O3 or Mn3O4) depends on the size of the nanoparticles. Structural and magnetic characterizations concur that while the smallest nanoparticles have a predominantly gamma-Mn2O3 shell, larger ones have increasing amounts of Mn3O4. A considerable enhancement of the Neel temperature, T-N, and the magnetic anisotropy of the MnO core for decreasing core sizes has been observed. The size reduction also leads to other phenomena such as persistent magnetic moment in MnO up to high temperatures and an unusual temperature behavior of the magnetic domains.
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| 14. |
- Maccaferri, N., et al.
(författare)
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Polarizability and magnetoplasmonic properties of magnetic general nanoellipsoids
- 2013
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Ingår i: Optics Express. - 1094-4087 .- 1094-4087. ; 21:8, s. 9875-9889
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Tidskriftsartikel (refereegranskat)abstract
- An approach to compute the polarizability tensor of magnetic nanoparticles having general ellipsoidal shape is presented. We find a surprisingly excellent quantitative agreement between calculated and experimental magneto-optical spectra measured in the polar Kerr configuration from nickel nanodisks of large size (exceeding 100 nm) with circular and elliptical shape. In spite of its approximations and simplicity, the formalism presented here captures the essential physics of the interplay between magneto-optical activity and the plasmonic resonance of the individual particle. The results highlight the key role of the dynamic depolarization effects to account for the magneto-optical properties of plasmonic nanostructures.
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| 15. |
- Maccaferri, Nicolò, et al.
(författare)
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Polarizability and magnetoplasmonic properties of magnetic general nanoellipsoids
- 2013
-
Ingår i: Optics Express. - : Optica Publishing Group. - 1094-4087. ; 21:8, s. 9875-9889
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Tidskriftsartikel (refereegranskat)abstract
- An approach to compute the polarizability tensor of magnetic nanoparticles having general ellipsoidal shape is presented. We find a surprisingly excellent quantitative agreement between calculated and experimental magneto-optical spectra measured in the polar Kerr configuration from nickel nanodisks of large size (exceeding 100 nm) with circular and elliptical shape. In spite of its approximations and simplicity, the formalism presented here captures the essential physics of the interplay between magneto-optical activity and the plasmonic resonance of the individual particle. The results highlight the key role of the dynamic depolarization effects to account for the magneto-optical properties of plasmonic nanostructures.
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| 16. |
- Maccaferri, Nicolò, et al.
(författare)
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Tuning the Magneto-Optical Response of Nanosize Ferromagnetic Ni Disks Using the Phase of Localized Plasmons
- 2013
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 111:16
-
Tidskriftsartikel (refereegranskat)abstract
- We explore the influence of the phase of localized plasmon resonances on the magneto-optical activity of nanoferromagnets. We demonstrate that these systems can be described as two orthogonal damped oscillators coupled by the spin-orbit interaction. We prove that only the spin-orbit induced transverse plasmon plays an active role on the magneto-optical properties by controlling the relative amplitude and phase lag between the two oscillators. Our theoretical predictions are fully confirmed by magneto-optical Kerr effect and optical extinction measurements in nanostructures of different size and shape.
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| 17. |
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| 18. |
- Menditto, Enrica, et al.
(författare)
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Adherence to treatment in allergic rhinitis using mobile technology : The MASK Study
- 2019
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Ingår i: Clinical and Experimental Allergy. - : WILEY. - 0954-7894 .- 1365-2222. ; 49:4, s. 442-460
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Tidskriftsartikel (refereegranskat)abstract
- Background: Mobile technology may help to better understand the adherence to treatment. MASK-rhinitis (Mobile Airways Sentinel NetworK for allergic rhinitis) is a patient-centred ICT system. A mobile phone app (the Allergy Diary) central to MASK is available in 22 countries. Objectives: To assess the adherence to treatment in allergic rhinitis patients using the Allergy Diary App. Methods: An observational cross-sectional study was carried out on all users who filled in the Allergy Diary from 1 January 2016 to 1 August 2017. Secondary adherence was assessed by using the modified Medication Possession Ratio (MPR) and the Proportion of days covered (PDC) approach. Results: A total of 12143 users were registered. A total of 6949 users reported at least one VAS data recording. Among them, 1887 users reported >= 7 VAS data. About 1195 subjects were included in the analysis of adherence. One hundred and thirty-six (11.28%) users were adherent (MPR >= 70% and PDC <= 1.25), 51 (4.23%) were partly adherent (MPR >= 70% and PDC = 1.50) and 176 (14.60%) were switchers. On the other hand, 832 (69.05%) users were non-adherent to medications (MPR <70%). Of those, the largest group was non-adherent to medications and the time interval was increased in 442 (36.68%) users. Conclusion and clinical relevance: Adherence to treatment is low. The relative efficacy of continuous vs on-demand treatment for allergic rhinitis symptoms is still a matter of debate. This study shows an approach for measuring retrospective adherence based on a mobile app. This also represents a novel approach for analysing medication-taking behaviour in a real-world setting.
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| 19. |
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| 20. |
- Qin, Jian, 1980-, et al.
(författare)
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Differences in the Magnetic Properties of Co, Fe and Ni 250-300 nm Wide Nanowires Electrodeposited in Amorphous Anodized Alumina Templates
- 2005
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 17:7, s. 1829-1834
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Tidskriftsartikel (refereegranskat)abstract
- Anodized alumina membranes (AAMs) were synthesized by a three-step electrochemical anodization of aluminum. The anodization results in a hexagonally pseudo-ordered 2D array of nanochannels. The AAMs were used as templates to grow Ni, Co, Fe nanowires, with diameters in the range of 250-300 nm, by electrodeposition. The AAM appears to be amorphous, while the metal nanowires are polycrystalline. The angular dependence of the coercivity, HC, of the Ni nanowires presents a smooth variation from a moHC = 5.5 mT when the field is applied perpendicular to the wires to moHC = 53 mT when the field is applied parallel to them. However, the Co and Fe nanowires exhibit a peak in the angular dependence of HC for fields applied close to the AAM plane (i.e. perpendicular to the wires). The competition between shape anisotropy and dipolar interaction between the nanowires seems to be responsible for the difference in magnetic behavior between the different metals.
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| 21. |
- Roldan, Manuel A., et al.
(författare)
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Probing the meta-stability of oxide core/shell nanoparticle systems at atomic resolution
- 2021
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Ingår i: Chemical Engineering Journal. - : Elsevier BV. - 1385-8947 .- 1873-3212. ; 405
-
Tidskriftsartikel (refereegranskat)abstract
- Hybrid nanoparticles allow exploiting the interplay of confinement, proximity between different materials and interfacial effects. However, to harness their properties an in-depth understanding of their (meta)stability and interfacial characteristics is crucial. This is especially the case of nanosystems based on functional oxides working under reducing conditions, which may severely impact their properties. In this work, the in-situ electron-induced selective reduction of Mn3O4 to MnO is studied in magnetic Fe3O4/Mn3O4 and Mn3O4/Fe3O4 core/shell nanoparticles by means of high-resolution scanning transmission electron microscopy combined with electron energy-loss spectroscopy. Such in-situ transformation allows mimicking the actual processes in operando environments. A multi-stage image analysis using geometric phase analysis combined with particle image velocity enables direct monitoring of the relationship between structure, chemical composition and strain relaxation during the Mn3O4 reduction. In the case of Fe3O4/Mn3O4 core/shell the transformation occurs smoothly without the formation of defects. However, for the inverse Mn3O4/Fe3O4 core/shell configuration the electron beam-induced transformation occurs in different stages that include redox reactions and void formation followed by strain field relaxation via formation of defects. This study highlights the relevance of understanding the local dynamics responsible for changes in the particle composition in order to control stability and, ultimately, macroscopic functionality.
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| 22. |
- Salazar-Alvarez, German, et al.
(författare)
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Enhanced coercivity in Co-rich near-stoichiometric CoFe3-xO4+delta nanoparticles prepared in large batches
- 2007
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 19:20, s. 4957-4963
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Tidskriftsartikel (refereegranskat)abstract
- The relationship between the structure and composition with the magnetic properties of near stoichiometric cobalt ferrite nanoparticles CoxFe3-xO4+delta (0.85 < x < 1.1) prepared in large batches with average sizes in the range 60-210 nm has been investigated. Chemical analysis and Rietveld refinement of the X-ray diffraction data in conjunction with Mossbauer spectroscopy allowed us to identify an interplay between particle size, microstructure (concentration of interstitial ions, microstrain, cation arrangement in octahedral and tetrahedral sites), and composition, which sensitively controls the magnetic properties such as coercivity and saturation magnetization. In all cases, cobalt-rich compositions resulted in a higher coercivity, whereas lower degrees of inversion and higher iron contents led to slightly higher saturation magnetization values.
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| 23. |
- Salazar-Alvarez, German, et al.
(författare)
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Reversible post-synthesis tuning of the superparamagnetic blocking temperature of gamma-Fe2O3 nanoparticles by adsorption and desorption of Co(II) ions
- 2007
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Ingår i: Journal of Materials Chemistry. - : Royal Society of Chemistry (RSC). - 0959-9428 .- 1364-5501. ; 17:4, s. 322-328
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Tidskriftsartikel (refereegranskat)abstract
- The influence of the post-synthesis adsorption of Co(II) ions on the structural and magnetic properties of maghemite (gamma-Fe2O3) nanoparticles with a mean particle size of about 10 nm has been investigated. It is shown that the step-wise adsorption of Co( II) can controllably increase the blocking temperature, T-B, of the system up to 60 K with respect to that of untreated particles, while neither the particle size nor the particle size distribution are significantly modified. This is accompanied by a four-fold increase in the coercivity, H-C, at low temperatures. Using a selective leaching of the previously adsorbed Co(II) ions the T-B and H-C values of the pristine gamma-Fe2O3 nanoparticles are recovered. Hence, a reversible and controllable tailoring of the magnetic properties (e.g., T-B and H-C) of the gamma-Fe2O3 nanoparticles can be achieved by a simple adsorption and desorption process of Co( II) ions after their synthesis.
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| 24. |
- Salazar-Alvarez, German, et al.
(författare)
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Tunable High-Field Magnetization in Strongly Exchange-Coupled Freestanding Co/CoO Core/Shell Coaxial Nanowires
- 2016
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 8:34, s. 22477-22483
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Tidskriftsartikel (refereegranskat)abstract
- The exchange bias properties of Co/CoO coaxial core/shell nanowires were investigated with cooling and applied fields perpendicular to the wire axis. This configuration leads to unexpected exchange-bias effects. First, the magnetization value at high fields is found to depend on the field-cooling conditions. This effect arises from the competition between the magnetic anisotropy and the Zeeman energies for cooling fields perpendicular to the wire axis. This allows imprinting predefined magnetization states to the antiferromagnetic (AFM) shell, as corroborated by micromagnetic simulations. Second, the system exhibits a high-field magnetic irreversibility, leading to open hysteresis loops attributed to the AFM easy axis reorientation during the reversal (effect similar to athermal training). A distinct way to manipulate the high-field magnetization in exchange-biased systems, beyond the archetypical effects, was thus experimentally and theoretically demonstrated.
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| 25. |
- Salazar-Alvarez, German, et al.
(författare)
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Two-, Three-, and Four-Component Magnetic Multilayer Onion Nanoparticles Based on Iron Oxides and Manganese Oxides
- 2011
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 133:42, s. 16738-16741
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic multilayered, onion-like, hetero-structured nanoparticles are interesting model systems for studying magnetic exchange coupling phenomena. In this work, we synthesized heterostructured magnetic nanopartides composed of two, three, or four components using iron oxide seeds for the subsequent deposition of manganese oxide. The MnO layer was allowed either to passivate fully in air to form an outer layer of Mn(3)O(4) or to oxidize partially to form MnO vertical bar Mn(3)O(4) double layers. Through control of the degree of passivation of the seeds, particles with up to four different magnetic layers can be obtained (i.e., FeO vertical bar Fe(3)O(4)vertical bar MnO vertical bar Mn(3)O(4)). Magnetic characterization of the samples confirmed the presence of the different magnetic layers.
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| 26. |
- Sánchez, Elena H., et al.
(författare)
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Crossover From Individual to Collective Magnetism in Dense Nanoparticle Systems: Local Anisotropy Versus Dipolar Interactions
- 2022
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Ingår i: Small. - : Wiley. - 1613-6810 .- 1613-6829. ; 18:28
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Tidskriftsartikel (refereegranskat)abstract
- Dense systems of magnetic nanoparticles may exhibit dipolar collective behavior. However, two fundamental questions remain unsolved: i) whether the transition temperature may be affected by the particle anisotropy or it is essentially determined by the intensity of the interparticle dipolar interactions, and ii) what is the minimum ratio of dipole–dipole interaction (Edd) to nanoparticle anisotropy (KefV, anisotropy⋅volume) energies necessary to crossover from individual to collective behavior. A series of particle assemblies with similarly intense dipolar interactions but widely varying anisotropy is studied. The Kef is tuned through different degrees of cobalt-doping in maghemite nanoparticles, resulting in a variation of nearly an order of magnitude. All the bare particle compacts display collective behavior, except the one made with the highest anisotropy particles, which presents “marginal” features. Thus, a threshold of KefV/Edd ≈ 130 to suppress collective behavior is derived, in good agreement with Monte Carlo simulations. This translates into a crossover value of ≈1.7 for the easily accessible parameter TMAX(interacting)/TMAX(non-interacting) (ratio of the peak temperatures of the zero-field-cooled magnetization curves of interacting and dilute particle systems), which is successfully tested against the literature to predict the individual-like/collective behavior of any given interacting particle assembly comprising relatively uniform particles.
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| 27. |
- Sanchez, Elena H., et al.
(författare)
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Simultaneous Individual and Dipolar Collective Properties in Binary Assemblies of Magnetic Nanoparticles
- 2020
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 32:3, s. 969-981
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Tidskriftsartikel (refereegranskat)abstract
- Applications based on aggregates of magnetic nanoparticles are becoming increasingly widespread, ranging from hyperthermia to magnetic recording. However, although some uses require collective behavior, others need a more individual-like response, the conditions leading to either of these behaviors are still poorly understood. Here, we use nanoscale-uniform binary random dense mixtures with different proportions of oxide magnetic nanoparticles with low/high anisotropy as a valuable tool to explore the crossover from individual to collective behavior. Two different anisotropy scenarios have been studied in two series of binary compacts: M1, comprising maghemite (gamma-Fe2O3) nanoparticles of different sizes (9.0 nm/11.5 nm) with barely a factor of 2 between their anisotropy energies, and M2, mixing equally sized pure maghemite (low-anisotropy) and Co-doped maghemite (high-anisotropy) nanoparticles with a large difference in anisotropy energy (ratio > 8). Interestingly, while the M1 series exhibits collective behavior typical of strongly coupled dipolar systems, the M2 series presents a more complex scenario where different magnetic properties resemble either "individual-like" or "collective", crucially emphasizing that the collective character must be ascribed to specific properties and not to the system as a whole. The strong differences between the two series offer new insight (systematically ratified by simulations) into the subtle interplay between dipolar interactions, local anisotropy and sample heterogeneity to determine the behavior of dense assemblies of magnetic nanoparticles.
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| 28. |
- Torruella, Pau, et al.
(författare)
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3D Visualization of the Iron Oxidation State in FeO/Fe3O4 Core-Shell Nanocubes from Electron Energy Loss Tomography
- 2016
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Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 16:8, s. 5068-5073
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Tidskriftsartikel (refereegranskat)abstract
- The physicochemical properties used in numerous advanced nanostructured devices are directly controlled by the oxidation states of their constituents. In this work we combine electron energy-loss spectroscopy, blind source separation, and computed tomography to reconstruct in three dimensions the distribution of Fe2+ and Fe3+ ions in a FeO/Fe3O4 core/shell cube-shaped nanoparticle with nanometric resolution. The results highlight the sharpness of the interface between both oxides and provide an average shell thickness, core volume, and average cube edge length measurements in agreement with the magnetic characterization of the sample.
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| 29. |
- Ulusoy, Seda, et al.
(författare)
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Elucidating the Lithiation Process in Fe3−δO4 Nanoparticles by Correlating Magnetic and Structural Properties
- 2024
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 16:12, s. 14799-14808
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Tidskriftsartikel (refereegranskat)abstract
- Due to their high potential energy storage, magnetite (Fe3O4) nanoparticles have become appealing as anode materials in lithium-ion batteries. However, the details of the lithiation process are still not completely understood. Here, we investigate chemical lithiation in 70 nm cubic-shaped magnetite nanoparticles with varying degrees of lithiation, x = 0, 0.5, 1, and 1.5. The induced changes in the structural and magnetic properties were investigated using X-ray techniques along with electron microscopy and magnetic measurements. The results indicate that a structural transformation from spinel to rock salt phase occurs above a critical limit for the lithium concentration (xc), which is determined to be between 0.5< xc ≤ 1 for Fe3−δO4. Diffraction and magnetization measurements clearly show the formation of the antiferromagnetic LiFeO2 phase. Upon lithiation, magnetization measurements reveal an exchange bias in the hysteresis loops with an asymmetry, which can be attributed to the formation of mosaic-like LiFeO2 subdomains. The combined characterization techniques enabled us to unambiguously identify the phases and their distributions involved in the lithiation process. Correlating magnetic and structural properties opens the path to increasing the understanding of the processes involved in a variety of nonmagnetic applications of magnetic materials.
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| 30. |
- Zha, Chaolin, et al.
(författare)
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Continuously graded anisotropy in single (Fe53Pt47)(100-x)Cu-x films
- 2010
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 97:18, s. 182504-
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Tidskriftsartikel (refereegranskat)abstract
- We report on continuously graded anisotropy. During deposition, a compositional gradient is achieved by varying the Cu concentration from Cu-rich (Fe53Pt47)(70)Cu-30 to Cu-free Fe53Pt47. The anisotropy gradient is then realized after annealing using the composition dependence of the low-anisotropy (A1) to high-anisotropy (L1(0)) ordering temperature. The critical role of the annealing temperature on the resultant anisotropy gradient is investigated. Magnetic measurements support the creation of an anisotropy gradient in properly annealed films which exhibit both a reduced coercivity and moderate thermal stability. These results demonstrate that an anisotropy gradient can be realized, and tailored, in single continuous films without the need for multilayers.
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| 31. |
- Zha, Chaolin, et al.
(författare)
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Exchange Bias in L1(0) (111)-Oriented FePt-Based Pseudo Spin Valves
- 2009
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Ingår i: IEEE transactions on magnetics. - 0018-9464 .- 1941-0069. ; 45:10, s. 3881-3884
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Tidskriftsartikel (refereegranskat)abstract
- The coupling between hard FePt and soft NiFe layers through a Cu interlayer has been studied in L1(0) (111) FePt(20)/CoFe(1.5)/Cu(3.5 or 4.5)/CoFe(2)/NiFe(3) (in nanometers) pseudo spin valves. The soft-layer hysteresis loops exhibit clear shifts in the field axis (exchange-bias-like) and a marked coercivity enhancement. It is found that several parameters such as the thickness of the Cu layer or the interface roughness influence the exchange bias properties of the systems, while others (e. g., temperature) are less important. This interlayer coupling arises from the competition between dipolar and Ruderman-Kittel-Kasuya-Yoshida (RKKY) interactions between the layers.
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| 32. |
- Zha, Chaolin, et al.
(författare)
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Exchange-bias-like effects in L1_0 (111)-oriented FePt based pseudo spin valves
- 2010
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Ingår i: Journal of Physics: Conference Series. - BRISTOL : IOP Publishing. - 1742-6596. ; 200, s. UNSP 072110-
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Tidskriftsartikel (refereegranskat)abstract
- The coupling between hard FePt/CoFe and soft CoFe/NiFe layers through a Cu spacer has been studied in L10 (111) FePt(20)/CoFe(1.5)/Cu(3.5 or 4.5)/CoFe(2)/NiFe(3) (in nm) pseudo spin valves. The soft layer hysteresis loops exhibit clear shifts in the ¯eld axis (exchange bias-like) and a marked coercivity enhancement. The thickness of the Cu layer or the interface roughness in°uence the exchange bias properties of the systems. This interlayer coupling arises from the competition between dipolar and RKKY interactions between the layers.
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| 33. |
- Zha, Chaolin, et al.
(författare)
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Improved magnetoresistance through spacer thickness optimization in tilted pseudo spin valves based on L10 (111)-oriented FePtCu fixed layers
- 2009
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 106:5
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Tidskriftsartikel (refereegranskat)abstract
- Through optimization of the Cu spacer thickness, we demonstrate magnetoresistance (MR) up to 5% in FePtCu/CoFe/Cu/CoFe/NiFe pseudo spin valves based on L10 (111) FePtCu fixed layers with a tilted magnetization. We find an optimum spacer thickness of about 2.4 nm which correlates with a clear onset of strong interlayer exchange coupling below 2.4 nm and spin-independent current shunting in the spacer above 2.4 nm. We argue that yet higher MR should be possible through further reduction in the interlayer exchange coupling.
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| 34. |
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| 35. |
- Zha, Chaolin, et al.
(författare)
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Nanostructured MnGa films on Si/SiO(2) with 20.5 kOe room temperature coercivity
- 2011
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 110:9, s. 093902-
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Tidskriftsartikel (refereegranskat)abstract
- Nanostructured Mn(67)Ga(33) films exhibiting high room temperature coercivity (H(C) = 20.5 kOe) have been prepared by sputtering onto thermally oxidized Si substrates. Both the morphology and the coercivity of the films can be tuned by varying the growth parameters. The low deposition rate film, sputtered at a reduced power and working pressure, demonstrates a discontinuous island-like growth and the highest H(C). The large H(C) is linked to the presence of the high anisotropy DO(22) Mn(3)Ga phase and the single domain character of the exchange isolated, dipolar interacting, single crystal islands.
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| 36. |
- Zha, Chaolin, et al.
(författare)
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Pseudo spin valves using a (112)-Textured D022 Mn2.3-2.4Ga fixed layer
- 2010
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Ingår i: IEEE Magnetics Letters. - 1949-307X .- 1949-3088. ; 1, s. 2500104-
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate pseudo spin valves (PSVs) with a (112)-textured D022 Mn2.3-2.4Ga (MnGa) tilted magnetization fixed layer and an in-plane CoFe free layer. Single D022 MnGa films exhibit a small magnetoresistance (MR) typically observed in metals. In MnGa/Cu/CoFe PSVs, a transition from a negative (-0.08%) to positive (3.88%) MR is realized by introducing a thin spin polarizing CoFe insertion layer at the MnGa/Cu interface and tailoring the MnGa thickness. Finally, the exchange coupling between the MnGa and CoFe insertion layer is studied using a first-order reversal curve technique.
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