Staff directory
Josep Nogués Sanmiquel
ICREA Research Professor and Group Leader
josep.nogues(ELIMINAR)@icn2.cat
Magnetic Nanostructures
- ORCID: 0000-0003-4616-1371
Publications
2017
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Lateral Magnetically Modulated Multilayers by Combining Ion Implantation and Lithography
Menéndez E., Modarresi H., Petermann C., Nogués J., Domingo N., Liu H., Kirby B.J., Mohd A.S., Salhi Z., Babcock E., Mattauch S., Van Haesendonck C., Vantomme A., Temst K. Small; 13 (11, 1603465) 2017. 10.1002/smll.201603465. IF: 8.643
The combination of lithography and ion implantation is demonstrated to be a suitable method to prepare lateral multilayers. A laterally, compositionally, and magnetically modulated microscale pattern consisting of alternating Co (1.6 µm wide) and Co-CoO (2.4 µm wide) lines has been obtained by oxygen ion implantation into a lithographically masked Au-sandwiched Co thin film. Magnetoresistance along the lines (i.e., current and applied magnetic field are parallel to the lines) reveals an effective positive giant magnetoresistance (GMR) behavior at room temperature. Conversely, anisotropic magnetoresistance and GMR contributions are distinguished at low temperature (i.e., 10 K) since the O-implanted areas become exchange coupled. This planar GMR is principally ascribed to the spatial modulation of coercivity in a spring-magnet-type configuration, which results in 180° Néel extrinsic domain walls at the Co/Co-CoO interfaces. The versatility, in terms of pattern size, morphology, and composition adjustment, of this method offers a unique route to fabricate planar systems for, among others, spintronic research and applications. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Magnetically-actuated mesoporous nanowires for enhanced heterogeneous catalysis
Serrà A., Grau S., Gimbert-Suriñach C., Sort J., Nogués J., Vallés E. Applied Catalysis B: Environmental; 217: 81 - 91. 2017. 10.1016/j.apcatb.2017.05.071. IF: 9.446
We study the optimization of the catalytic properties of entirely magnetic Co–Pt compact and mesoporous nanowires of different diameters (25–200 nm) by using magnetic actuation. The nanowires are a single-entity, robust, magnetic-catalyst with a huge catalytically-active surface area. We show that apart from conventional parameters, like the size and morphology of the nanowires, other factors can be optimized to enhance their catalytic activity. In particular, given the magnetic character of the nanowires, rotating magnetic fields are a very powerful approach to boost the performance of the catalyst. In particular, the magnetic field induces them to act as nano-stirrers, improving the local flow of material towards the active sites of the catalyst. We demonstrate the versatility of the procedure by optimizing (i) the degradation of different types of pollutants (4-nitrophenol and methylene blue) and (ii) hydrogen production. For example, by using as little as 0.1 mg mL−1 of 25 nm wide Co–Pt mesoporous nanowires (with ∼3 nm pore size) as catalysts, kinetic normalized constants knor as high as 20,667 and 21,750 s−1 g−1 for 4-nitrophenol and methylene blue reduction, respectively, are obtained. In addition, activity values for hydrogen production from borohydride are as high as 25.0 L H2 g−1 min−1, even at room temperature. These values outperform any current state-of-the-art proposed catalysis strategies for water remediation reactions by at least 10-times and are superior to most advanced approaches to generate hydrogen from borohydride. The recyclability of the nanowires together with the simplicity of the synthetic method makes this approach (using not only Co–Pt, but also other mesoporous magnetic catalysts) very appealing for very diverse types of catalytic applications. © 2017 Elsevier B.V.
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Maximizing Exchange Bias in Co/CoO Core/Shell Nanoparticles by Lattice Matching between the Shell and the Embedding Matrix
González J.A., Andrés J.P., López Antón R., De Toro J.A., Normile P.S., Muniz P., Riveiro J.M., Nogués J. Chemistry of Materials; 29 (12): 5200 - 5206. 2017. 10.1021/acs.chemmater.7b00868. IF: 9.466
The exchange bias properties of 5 nm Co/CoO ferromagnetic/antiferromagnetic core/shell nanoparticles, highly dispersed in a CuxO matrix, have been optimized by matching the lattice parameter of the matrix with that of the CoO shell. Exchange bias and coercivity fields as large as HE = 7780 Oe and HC = 6950 Oe are linked to the presence of a Cu2O matrix (0.3% lattice mismatch with respect to the shells). The small mismatch between Cu2O and CoO plays a dual role: it (i) structurally stabilizes the CoO and (ii) favors the existence of a large amount of uncompensated moments in the shell that enhance the exchange bias effects. The results indicate that lattice matching may be a very efficient way to improve the exchange bias properties of core/shell nanoparticles, paving the way to novel approaches to tune their magnetic properties. © 2017 American Chemical Society.
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Novel Ba-hexaferrite structural variations stabilized on the nanoscale as building blocks for epitaxial bi-magnetic hard/soft sandwiched maghemite/hexaferrite/maghemite nanoplatelets with out-of-plane easy axis and enhanced magnetization
Belec B., Dražić G., Gyergyek S., Podmiljšak B., Goršak T., Komelj M., Nogués J., Makovec D. Nanoscale; 9 (44): 17551 - 17560. 2017. 10.1039/c7nr05894b. IF: 7.367
Atomic-resolution scanning-transmission electron microscopy showed that barium hexaferrite (BHF) nanoplatelets display a distinct structure, which represents a novel structural variation of hexaferrites stabilized on the nanoscale. The structure can be presented in terms of two alternating structural blocks stacked across the nanoplatelet: a hexagonal (BaFe6O11)2- R block and a cubic (Fe6O8)2+ spinel S block. The structure of the BHF nanoplatelets comprises only two, or rarely three, R blocks and always terminates at the basal surfaces with the full S blocks. The structure of a vast majority of the nanoplatelets can be described with a SR∗S∗RS stacking order, corresponding to a BaFe15O23 composition. The nanoplatelets display a large, uniaxial magnetic anisotropy with the easy axis perpendicular to the platelet, which is a crucial property enabling different novel applications based on aligning the nanoplatelets with applied magnetic fields. However, the BHF nanoplatelets exhibit a modest saturation magnetization, MS, of just over 30 emu g-1. Given the cubic S block termination of the platelets, layers of maghemite, γ-Fe2O3, (M), with a cubic spinel structure, can be easily grown epitaxially on the surfaces of the platelets, forming a sandwiched M/BHF/M platelet structure. The exchange-coupled composite nanoplatelets exhibit a remarkably uniform structure, with an enhanced MS of more than 50 emu g-1 while essentially maintaining the out-of-plane easy axis. The enhanced MS could pave the way for their use in diverse platelet-based magnetic applications. © 2017 The Royal Society of Chemistry.
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Remanence plots as a probe of spin disorder in magnetic nanoparticles
De Toro J.A., Vasilakaki M., Lee S.S., Andersson M.S., Normile P.S., Yaacoub N., Murray P., Sánchez E.H., Muñiz P., Peddis D., Mathieu R., Liu K., Geshev J., Trohidou K.N., Nogués J. Chemistry of Materials; 29 (19): 8258 - 8268. 2017. 10.1021/acs.chemmater.7b02522. IF: 9.466
Remanence magnetization plots (e.g., Henkel or δM 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 δM plots may be misleading when the nanoparticles do not have a homogeneous internal magnetic configuration. Substantial dips in the M plots of γ-Fe2O3 nanoparticles isolated by thick SiO2 shells indicate the presence of demagnetizing interactions, usually identified as dipolar interactions. Our results, however, demonstrate that it is the inhomogeneous spin structure of the nanoparticles, as most clearly evidenced by Mössbauer measurements, that has a pronounced effect on the δM 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 δM 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 δM plots to quantify the rather elusive inhomogeneous magnetization states in nanoparticles. © 2017 American Chemical Society.
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Seeded Growth Synthesis of Au-Fe3O4 Heterostructured Nanocrystals: Rational Design and Mechanistic Insights
Fantechi E., Roca A.G., Sepúlveda B., Torruella P., Estradé S., Peiró F., Coy E., Jurga S., Bastús N.G., Nogués J., Puntes V. Chemistry of Materials; 29 (9): 4022 - 4035. 2017. 10.1021/acs.chemmater.7b00608. IF: 9.466
Multifunctional hybrid nanoparticles comprising two or more entities with different functional properties are gaining ample significance in industry and research. Due to its combination of properties, a particularly appealing example is Au-Fe3O4 composite nanoparticles. Here we present an in-depth study of the synthesis of Au-Fe3O4 heterostructured nanocrystals (HNCs) by thermal decomposition of iron precursors in the presence of preformed 10 nm Au seeds. The role of diverse reaction parameters on the HNCs formation was investigated using two different precursors: iron pentacarbonyl (Fe(CO)5) and iron acetylacetonate (Fe(acac)3). The reaction conditions promoting the heterogeneous nucleation of Fe3O4 onto Au seeds were found to significantly differ depending on the precursor chosen, where Fe(acac)3 is considerably more sensitive to the variation of the parameters than Fe(CO)5 and more subject to homogeneous nucleation processes with the consequent formation of isolated iron oxide nanocrystals (NCs). The role of the surfactants was also crucial in the formation of well-defined and monodisperse HNCs by regulating the access to the Au surface. Similarly, the variations of the [Fe]/[Au] ratio, temperature, and employed solvent were found to act on the mean size and the morphology of the obtained products. Importantly, while the optical properties are rather sensitive to the final morphology, the magnetic ones are rather similar for the different types of obtained HNCs. The surface functionalization of dimer-like HNCs with silica allows their dispersion in aqueous media, opening the path to their use in biomedical applications. © 2017 American Chemical Society.
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Tri-segmented magnetic nanowires with antiparallel alignment: Suitable platforms for biomedical applications with minimized agglomeration?
Sort J., Zhang J., Agramunt S., Del-Valle N., Navau C., Estradé S., Peiró F., Pané S., Sánchez A., Pellicer E., Nogues J. 2017 IEEE International Magnetics Conference, INTERMAG 2017; (8007843) 2017. 10.1109/INTMAG.2017.8007843.
Materials structured in the form of one-dimensional nanoarchitectures, such as nanorods and nanowires (NWs), have found widespread applications in several technological areas, such as optoelectronics, magnetism, catalysis, piezo- and thermo-electricity, biosensing, or micro-/nanoelectromechanical systems (MEMS/NEMS), among others. © 2017 IEEE.
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Unveiling a New High-Temperature Ordered Magnetic Phase in ϵ-Fe2O3
García-Muñoz J.L., Romaguera A., Fauth F., Nogués J., Gich M. Chemistry of Materials; 29 (22): 9705 - 9713. 2017. 10.1021/acs.chemmater.7b03417. IF: 9.466
Iron oxides are among the most abundant materials on Earth, and yet there are some of their basic properties which are still not well-established. Here, we present temperature-dependent magnetic, X-ray, and neutron diffraction measurements refuting the current belief that the magnetic ordering temperature of ϵ-Fe2O3 is ∼500 K, i.e., well below that of other iron oxides such as hematite, magnetite, or maghemite. Upon heating from room temperature, the ϵ-Fe2O3 nanoparticles' saturation magnetization undergoes a monotonic decrease while the coercivity and remanence sharply drop, virtually vanishing around ∼500 K. However, above that temperature the hysteresis loops present a nonlinear response with finite coercivity, making evident signs of ferrimagnetic order up to temperatures as high as 850 K (TN1). The neutron diffraction study confirms the presence of ferrimagnetic order well above 500 K with Pna'21' magnetic symmetry, but only involving two of the four Fe3+ sublattices which are ordered below TN2 ≈ 480 K, and with a reduced net ferromagnetic component, that vanishes at above 850 K. The results unambiguously show the presence of a high-temperature magnetic phase in ϵ-Fe2O3 with a critical temperature of TN1 ∼ 850 K. Importantly, this temperature is similar to the Curie point in other iron oxides, indicating comparable magnetic coupling strengths. The presence of diverse magnetic phases is further supported by the nonmonotonic evolution of the thermal expansion. The existence of a high-temperature ferrimagnetic phase in ϵ-Fe2O3 may open the door to further expand the working range of this multifunctional iron oxide. © 2017 American Chemical Society.
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Voltage-Induced Coercivity Reduction in Nanoporous Alloy Films: A Boost toward Energy-Efficient Magnetic Actuation
Quintana A., Zhang J., Isarain-Chávez E., Menéndez E., Cuadrado R., Robles R., Baró M.D., Guerrero M., Pané S., Nelson B.J., Müller C.M., Ordejón P., Nogués J., Pellicer E., Sort J. Advanced Functional Materials; 27 (32, 1701904) 2017. 10.1002/adfm.201701904. IF: 12.124
Magnetic data storage and magnetically actuated devices are conventionally controlled by magnetic fields generated using electric currents. This involves significant power dissipation by Joule heating effect. To optimize energy efficiency, manipulation of magnetic information with lower magnetic fields (i.e., lower electric currents) is desirable. This can be accomplished by reducing the coercivity of the actuated material. Here, a drastic reduction of coercivity is observed at room temperature in thick (≈600 nm), nanoporous, electrodeposited Cu–Ni films by simply subjecting them to the action of an electric field. The effect is due to voltage-induced changes in the magnetic anisotropy. The large surface-area-to-volume ratio and the ultranarrow pore walls of the system allow the whole film, and not only the topmost surface, to effectively contribute to the observed magnetoelectric effect. This waives the stringent “ultrathin-film requirement” from previous studies, where small voltage-driven coercivity variations were reported. This observation expands the already wide range of applications of nanoporous materials (hitherto in areas like energy storage or catalysis) and it opens new paradigms in the fields of spintronics, computation, and magnetic actuation in general. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
2016
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3D Visualization of the Iron Oxidation State in FeO/Fe3O4 Core-Shell Nanocubes from Electron Energy Loss Tomography
Torruella P., Arenal R., De La Peña F., Saghi Z., Yedra L., Eljarrat A., López-Conesa L., Estrader M., López-Ortega A., Salazar-Alvarez G., Nogués J., Ducati C., Midgley P.A., Peiró F., Estradé S. Nano Letters; 16 (8): 5068 - 5073. 2016. 10.1021/acs.nanolett.6b01922. IF: 13.779
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. © 2016 American Chemical Society.
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Effective ionic-liquid microemulsion based electrodeposition of mesoporous Co-Pt films for methanol oxidation catalysis in alkaline media
Serrà A., Gómez E., Golosovsky I.V., Nogués J., Vallés E. Journal of Materials Chemistry A; 4 (20): 7805 - 7814. 2016. 10.1039/c6ta02035f. IF: 8.262
Pt-based direct methanol fuel cells are attracting increasing interest as environmentally friendly alternative energy sources. However, the high price of Pt and the difficulty to prepare favourable morphologies for catalysis (e.g., mesoporous materials) are hampering their development into feasible products. Here, we demonstrate a novel approach to efficiently grow mesoporous films of Pt-poor alloys (Co3Pt and CoPt3), based on electrodeposition in ionic liquid-in-water (IL/W) microemulsions. The high proportion of the electrolytic aqueous solution in the IL/W microemulsion favors a significant deposition rate, while the presence of IL drops induces the formation of highly mesoporous films. The mesoporous alloys, with pores in the 8-11 nm range, exhibit excellent durability in acidic and alkaline aggressive media, maintaining their peculiar morphology. The structures are very efficient for the catalysis of methanol electro-oxidation in alkaline media, with minimal poisoning of the catalysts. These results pave the way to develop simple, versatile environmentally friendly fuel cell catalysts to commercialize new viable ecological alternative energy sources. © 2016 The Royal Society of Chemistry.
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Electrochemically synthesized amorphous and crystalline nanowires: Dissimilar nanomechanical behavior in comparison with homologous flat films
Zeeshan M.A., Esqué-De Los Ojos D., Castro-Hartmann P., Guerrero M., Nogués J., Suriñach S., Baró M.D., Nelson B.J., Pané S., Pellicer E., Sort J. Nanoscale; 8 (3): 1344 - 1351. 2016. 10.1039/c5nr04398k. IF: 7.760
The effects of constrained sample dimensions on the mechanical behavior of crystalline materials have been extensively investigated. However, there is no clear understanding of these effects in nano-sized amorphous samples. Herein, nanoindentation together with finite element simulations are used to compare the properties of crystalline and glassy CoNi(Re)P electrodeposited nanowires (φ ≈ 100 nm) with films (3 μm thick) of analogous composition and structure. The results reveal that amorphous nanowires exhibit a larger hardness, lower Young's modulus and higher plasticity index than glassy films. Conversely, the very large hardness and higher Young's modulus of crystalline nanowires are accompanied by a decrease in plasticity with respect to the homologous crystalline films. Remarkably, proper interpretation of the mechanical properties of the nanowires requires taking the curved geometry of the indented surface and sink-in effects into account. These findings are of high relevance for optimizing the performance of new, mechanically-robust, nanoscale materials for increasingly complex miniaturized devices. © 2016 The Royal Society of Chemistry.
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Galvanic Replacement onto Complex Metal-Oxide Nanoparticles: Impact of Water or Other Oxidizers in the Formation of either Fully Dense Onion-like or Multicomponent Hollow MnOx/FeOx Structures
López-Ortega A., Roca A.G., Torruella P., Petrecca M., Estradé S., Peiró F., Puntes V., Nogués J. Chemistry of Materials; 28 (21): 8025 - 8031. 2016. 10.1021/acs.chemmater.6b03765. IF: 9.407
Multicomponent metal-oxide nanoparticles are appealing structures from applied and fundamental viewpoints. The control on the synthetic parameters in colloidal chemistry allows the growth of complex nanostructures with novel morphologies. In particular, the synthesis of biphase metal-oxide hollow nanoparticles has been reported based on galvanic replacement using an organic-based seeded-growth approach, but with the presence of H2O. Here we report a novel route to synthesize either fully dense or hollow core/shell metal-oxide nanoparticles (MnOx/FeOx) by simply adding or not oxidants in the reaction. We demonstrate that the presence of oxidants (e.g., O2 carried by the not properly degassed H2O or (CH3)3NO) allows the formation of hollow structures by a galvanic reaction between the MnOx and FeOx phases. In particular, the use of (CH3)3NO as oxidant allows for the first time a very reliable all-organic synthesis of hollow MnOx/FeOx nanoparticles without the need of water (with a somewhat unreliable oxidation role). Oxidants permit the formation of MnOx/FeOx hollow nanoparticles by an intermediate step where the MnO/Mn3O4 seeds are oxidized into Mn3O4, thus allowing the Mn3+ → Mn2+ reduction by the Fe2+ ions. The lack of proper oxidative conditions leads to full-dense onion-like core/shell MnO/Mn3O4/Fe3O4 particles. Thus, we show that the critical step for galvanic replacement is the proper seed oxidation states so that their chemical reduction by the shell ions is thermodynamically favored. © 2016 American Chemical Society.
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Highly efficient electrochemical and chemical hydrogenation of 4-nitrophenol using recyclable narrow mesoporous magnetic CoPt nanowires
Serrà A., Alcobé X., Sort J., Nogués J., Vallés E. Journal of Materials Chemistry A; 4 (40): 15676 - 15687. 2016. 10.1039/c6ta07149j. IF: 8.262
Toxic nitro-compounds, such as 4-nitrophenol, are one of the most common wastewater industrial pollutants. Thus, efficient ways to neutralize them are actively pursued. Here novel procedures to degrade these types of compounds based on the use of mesoporous magnetic nanowires are demonstrated. Fully-mesoporous magnetic narrow (25 nm) CoPt nanowires with an extraordinary effective area are grown using ionic liquid-in-water microemulsions in alumina templates. These mesoporous nanowires are shown to be efficient catalysts for the hydrogenation of 4-nitrophenol by electrocatalysis. Additionally, these nanowires also present exceptional conventional catalytic activity when used in conjunction with NaBH4, particularly when magnetic stirring is utilized. In fact, magnetically actuated mesoporous CoPt nanowires drastically outperform all state-of-the-art 4-nitrophenol catalysts. Additionally, given their magnetic character, these nanowires can be easily recycled and reused. Thus, the outstanding catalytic performance of mesoporous CoPt nanowires makes them excellent candidates for wastewater treatment agents. © 2016 The Royal Society of Chemistry.
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Modeling the collective magnetic behavior of highly-packed arrays of multi-segmented nanowires
Agramunt-Puig S., Del-Valle N., Pellicer E., Zhang J., Nogués J., Navau C., Sanchez A., Sort J. New Journal of Physics; 18 (1, 013026) 2016. 10.1088/1367-2630/18/1/013026. IF: 3.570
A powerful model to evaluate the collective magnetic response of large arrays of segmented nanowires comprising two magnetic segments of dissimilar coercivity separated by a non-magnetic spacer is introduced. The model captures the essential aspects of the underlying physics in these systems while being at the same time computationally tractable for relatively large arrays. The minimum lateral and vertical distances rendering densely packed weakly-interacting nanowires and segments are calculated for optimizing their performance in applications like magnetic sensors or recording media. The obtained results are appealing for the design of multifunctional miniaturized devices actuated by external magnetic fields, whose successful implementation relies on achieving a delicate balance between two opposing technological demands: the need for an ultra-high density of nanowires per unit area and the minimization of inter-wire and inter-segment dipolar interactions. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
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Spontaneous formation of spiral-like patterns with distinct periodic physical properties by confined electrodeposition of Co-In disks
Golvano-Escobal I., Gonzalez-Rosillo J.C., Domingo N., Illa X., López-Barberá J.F., Fornell J., Solsona P., Aballe L., Foerster M., Surinãch S., Baró M.D., Puig T., Pané S., Nogués J., Pellicer E., Sort J. Scientific Reports; 6 ( 30398) 2016. 10.1038/srep30398. IF: 5.228
Spatio-temporal patterns are ubiquitous in different areas of materials science and biological systems. However, typically the motifs in these types of systems present a random distribution with many possible different structures. Herein, we demonstrate that controlled spatio-temporal patterns, with reproducible spiral-like shapes, can be obtained by electrodeposition of Co-In alloys inside a confined circular geometry (i.e., in disks that are commensurate with the typical size of the spatio-temporal features). These patterns are mainly of compositional nature, i.e., with virtually no topographic features. Interestingly, the local changes in composition lead to a periodic modulation of the physical (electric, magnetic and mechanical) properties. Namely, the Co-rich areas show higher saturation magnetization and electrical conductivity and are mechanically harder than the In-rich ones. Thus, this work reveals that confined electrodeposition of this binary system constitutes an effective procedure to attain template-free magnetic, electric and mechanical surface patterning with specific and reproducible shapes.
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Tailoring Staircase-like Hysteresis Loops in Electrodeposited Trisegmented Magnetic Nanowires: A Strategy toward Minimization of Interwire Interactions
Zhang J., Agramunt-Puig S., Del-Valle N., Navau C., Baró M.D., Estradé S., Peiró F., Pané S., Nelson B.J., Sanchez A., Nogués J., Pellicer E., Sort J. ACS Applied Materials and Interfaces; 8 (6): 4109 - 4117. 2016. 10.1021/acsami.5b11747. IF: 7.145
A new strategy to minimize magnetic interactions between nanowires (NWs) dispersed in a fluid is proposed. Such a strategy consists of preparing trisegmented NWs containing two antiparallel ferromagnetic segments with dissimilar coercivity separated by a nonmagnetic spacer. The trisegmented NWs exhibit a staircase-like hysteresis loop with tunable shape that depends on the relative length of the soft- and hard-magnetic segments and the respective values of saturation magnetization. Such NWs are prepared by electrodepositing CoPt/Cu/Ni in a polycarbonate (PC) membrane. The antiparallel alignment is set by applying suitable magnetic fields while the NWs are still embedded in the PC membrane. Analytic calculations are used to demonstrate that the interaction magnetic energy from fully compensated trisegmented NWs with antiparallel alignment is reduced compared to a single-component NW with the same length or the trisegmented NWs with the two ferromagnetic counterparts parallel to each other. The proposed approach is appealing for the use of magnetic NWs in certain biological or catalytic applications where the aggregation of NWs is detrimental for optimized performance. © 2016 American Chemical Society.
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Tunable High-Field Magnetization in Strongly Exchange-Coupled Freestanding Co/CoO Core/Shell Coaxial Nanowires
Salazar-Alvarez G., Geshev J., Agramunt-Puig S., Navau C., Sanchez A., Sort J., Nogués J. ACS Applied Materials and Interfaces; 8 (34): 22477 - 22483. 2016. 10.1021/acsami.6b05588. IF: 7.145
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. © 2016 American Chemical Society.
2015
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A new reversal mode in exchange coupled antiariomagnetic/ferromagnetic disks: Distorted viscous vortex
Gilbert D.A., Ye L., Varea A., Agramunt-Puig S., Del Valle N., Navau C., Lopez-Barbera J.F., Buchanan K.S., Hoffmann A., Sanchez A., Sort J., Liu K., Nogues J. Nanoscale; 7 (21): 9878 - 9885. 2015. 10.1039/c5nr01856k. IF: 7.394
Magnetic vortices have generated intense interest in recent years due to their unique reversal mechanisms, fascinating topological properties, and exciting potential applications. In addition, the exchange coupling of magnetic vortices to antiferromagnets has also been shown to lead to a range of novel phenomena and functionalities. Here we report a new magnetization reversal mode of magnetic vortices in exchange coupled Ir
20 Mn80 /Fe20 Ni80 microdots: distorted viscous vortex reversal. In contrast to the previously known or proposed reversal modes, the vortex is distorted close to the interface and viscously dragged due to the uncompensated spins of a thin antiferromagnet, which leads to unexpected asymmetries in the annihilation and nucleation fields. These results provide a deeper understanding of the physics of exchange coupled vortices and may also have important implications for applications involving exchange coupled nanostructures. This journal is © The Royal Society of Chemistry.
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Applications of exchange coupled bi-magnetic hard/soft and soft/hard magnetic core/shell nanoparticles
López-Ortega A., Estrader M., Salazar-Alvarez G., Roca A.G., Nogués J. Physics Reports; 553: 1 - 32. 2015. 10.1016/j.physrep.2014.09.007. IF: 20.033
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. © 2014 Elsevier B.V.
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Enhanced magnetic properties in antiferromagnetic-core/ferrimagnetic-shell nanoparticles
Vasilakaki M., Trohidou K.N., Nogués J. Scientific Reports; 5 ( 9609) 2015. 10.1038/srep09609. IF: 5.578
Bi-magnetic core/shell nanoparticles are gaining increasing interest due to their foreseen applications. Inverse antiferromagnetic(AFM)/ferrimagnetic(FiM) core/shell nanoparticles are particularly appealing since they may overcome some of the limitations of conventional FiM/AFM systems. However, virtually no simulations exist on this type of morphology. Here we present systematic Metropolis Monte Carlo simulations of the exchange bias properties of such nanoparticles. The coercivity, H C, and loop shift, H ex, present a non-monotonic dependence with the core diameter and the shell thickness, in excellent agreement with the available experimental data. Additionally, we demonstrate novel unconventional behavior in FiM/AFM particles. Namely, while H C and H ex decrease upon increasing FiM thickness for small AFM cores (as expected), they show the opposite trend for large cores. This presents a counterintuitive FiM size dependence for large AFM cores that is attributed to the competition between core and shell contributions, which expands over a wider range of core diameters leading to non-vanishing H ex even for very large cores. Moreover, the results also hint different possible ways to enhance the experimental performance of inverse core/shell nanoparticles for diverse applications.
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High Temperature Magnetic Stabilization of Cobalt Nanoparticles by an Antiferromagnetic Proximity Effect
De Toro J.A., Marques D.P., Muñiz P., Skumryev V., Sort J., Givord D., Nogués J. Physical Review Letters; 115 (5, 057201) 2015. 10.1103/PhysRevLett.115.057201. IF: 7.512
Thermal activation tends to destroy the magnetic stability of small magnetic nanoparticles, with crucial implications for ultrahigh density recording among other applications. Here we demonstrate that low-blocking-temperature ferromagnetic (FM) Co nanoparticles (TB<70K) become magnetically stable above 400 K when embedded in a high-Néel-temperature antiferromagnetic (AFM) NiO matrix. The origin of this remarkable TB enhancement is due to a magnetic proximity effect between a thin CoO shell (with low Néel temperature, TN, and high anisotropy, KAFM) surrounding the Co nanoparticles and the NiO matrix (with high TN but low KAFM). This proximity effect yields an effective antiferromagnet with an apparent TN beyond that of bulk CoO, and an enhanced anisotropy compared to NiO. In turn, the Co core FM moment is stabilized against thermal fluctuations via core-shell exchange-bias coupling, leading to the observed TB increase. Mean-field calculations provide a semiquantitative understanding of this magnetic-proximity stabilization mechanism. © 2015 American Physical Society.
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Origin of the large dispersion of magnetic properties in nanostructured oxides: FexO/Fe3O4 nanoparticles as a case study
Estrader M., Lopez-Ortega A., Golosovsky I.V., Estrade S., Roca A.G., Salazar-Alvarez G., Lopez-Conesa L., Tobia D., Winkler E., Ardisson J.D., Macedo W.A.A., Morphis A., Vasilakaki M., Trohidou K.N., Gukasov A., Mirebeau I., Makarova O.L., Zysler R.D., Peiro F., Baro M.D., Bergstrom L., Nogues J. Nanoscale; 7 (7): 3002 - 3015. 2015. 10.1039/c4nr06351a. IF: 7.394
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. © 2015 The Royal Society of Chemistry.
2014
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Direct evidence for an interdiffused intermediate layer in bi-magnetic core-shell nanoparticles
Juhin A.; Lopez-Ortega A.; Sikora M.; Carvallo C.; Estrader M.; Estrade S.; Peiro F.; Baro M.D.; Sainctavit P.; Glatzel P.; Nogues J. Nanoscale; 6 (20): 11911 - 11920. 2014. . IF: 6.739
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Interdependence between training and magnetization reversal in granular Co-CoO exchange bias systems
Menendez E.; Dias T.; Geshev J.; Lopez-Barbera J.F.; Nogues J.; Steitz R.; Kirby B.J.; Borchers J.A.; Pereira L.M.C.; Vantomme A.; Temst K. Physical Review B - Condensed Matter and Materials Physics; 2014. 10.1103/PhysRevB.89.144407. IF: 3.664
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Oxide wizard: An EELS application to characterize the white lines of transition metal edges
Yedra L.; Xuriguera E.; Estrader M.; Lopez-Ortega A.; Baro M.D.; Nogues J.; Roldan M.; Varela M.; Estrade S.; Peiro F. Microscopy and Microanalysis; 20 (3): 698 - 705. 2014. 10.1017/S1431927614000440. IF: 2.161