Published articles
Magnetic super-structure and active surface role in the onset of magnetic excitons revealed in TbCu2 nanoparticles
Elizabeth M. Jefremovas*, María de la Fuente Rodríguez, Diego Alba Venero, Cristina Echevarría–Bonet, Philipp Bender, Björn Fåk, Jesús A. Blanco & Luis Fernández Barquín
Antiferromagnetic materials are receiving renewed interest on behalf of their potential for information technologies. Recent reports have also revealed how the physics governing such magnetic arrangements and their excitations become more complex compared to traditional ferromagnetic materials, especially at the nanoscale. Here, we address two main issues that are of prime interest to their technological transfer. First, using small-angle neutron scattering, we show the existence of a magnetic helix-like super-structure in a polycrystalline TbCu2 alloy, preserved at both bulk and nanoparticle ensembles of 8 nm. Second, using inelastic neutron scattering, we elucidate the magnetic excitons and the crystalline electric field energy level schemes of TbCu2 in bulk and nanoparticle ensembles. This allows to understad the effect of the surface broken symmetry on the quantum energy levels at the nanoscale, so as the key role of interfacial effects on the propagation of magnetic excitations. Our research provides insights for the realization of magnetic moment dynamics models based on complex nanometric super-structures, and for nanoparticles to be integrated in spintronics and information technology applications.
Incorporation of Tb and Gd improves the diagnostic functionality of magnetotactic bacteria
Lucía Gandarias*, Elizabeth M. Jefremovas, David Gandia, Lourdes Marcano, Virginia Martínez-Martínez, Pedro Ramos-Cabrer, Daniel M. Chevrier, Sergio Valencia, L. Fernández Barquín, M. Luisa Fernández-Gubieda, Javier Alonso, Ana García-Prieto and Alicia Muela
Magnetotactic bacteria are envisaged as potential theranostic agents. Their internal magnetic compass, chemical specificity and natural motility enable these microorganisms to behave as nanorobots, as they can be tracked and guided towards specific regions and activated to generate a therapeutic response. Here we provide additional diagnostic functionalities to magnetotactic bacteria Magnetospirillum gryphiswaldense MSR-1 while retaining their intrinsic capabilities. These additional functionalities are achieved by incorporating Tb or Gd to the bacteria by culturing them in Tb/Gd supplemented media. The incorporation of Tb provides luminescence properties, enabling potential applications of bacteria as biomarkers. The incorporation of Gd turns bacteria into dual contrast agents for magnetic resonance imaging, since Gd adds T1 contrast to the existing T2 contrast of unmodified bacteria. Given their potential clinical applications, the diagnostic ability of the modified MSR-1 has been successfully tested in vitro in two cell models, confirming their suitability as fluorescent markers (Tb-MSR-1) and dual contrast agents for MR1 (Gd-MSR-1).
https://trebuchet.public.springernature.app/get_content/d9c0efb6-876a-4899-90d6-f7492dc3ffff
Dual disorder‑driven magnetic dynamics in GdCu2 superantiferromagnetic nanoparticles
E. M. Jefremovas, P. Svedlidh and L. Fernández Barquín
The spin dynamics in magnetically-disordered GdCu2 nanoparticles, varying the nanoparticle size in the range 53 to 7 nm, has been scrutinized. Dynamic XAC susceptibility measurements have revealed the existence of dissipation at Tg = 18 K, which is associated to the spin freezing transition, for all the ensembles. Besides, the superantiferromagnetic ensembles (<D> = 24 nm) also showcase a dissipation contribution close to the vicinity of the Néel transition, TN = 40.2 K. This dissipation, that takes the form of two humps located at Td1 = 33.5(5) K and Td2 = 40.0(5) K, is associated to uncompensated antiferromagnetic moments. Time-dependent phenomena (ageing and memory effects) are only evidenced below the spin freezing transition, evidencing that solely this low–temperature disordered phase is driven by the frustration of RKKY exchange interactions. Consequently, GdCu2 nanoparticles display a dual disorder-driven magnetic dynamics, which are the one ascribed to the magnetically-frustrated moments located at the nanoparticle surface; and that of uncompensated antiferromagnetic moments located within the nanoparticle core.
https://trebuchet.public.springernature.app/get_content/d9c0efb6-876a-4899-90d6-f7492dc3ffff
Magnetic order and disorder environments in superantiferromagnetic NdCu2 nanoparticles
E. M. Jefremovas, P. Svedlidh, F. Damay, D. Alba Venero, A. Michels, J. A. Blanco and L. Fernández Barquín
Magnetic nanoparticles display two symmetry environments: One associated to the core, assumed to maintain an arrangement very similar to the bulk, and the one related to the surface, where the bulk symmetry breaks and a disorder dynamics is usually settled. However, the precise determination on how a complex antiferromagnetic structure (helix-like) is affected by the size reduction and the surface disorder is yet to be scrutinized. In this article, we have produced two ensembles of NdCu2 nanoparticles (18 and 13 nm) to scrutinize the static magnetic structure and spin dynamics of both surface and core magnetic moments.
Modifying the magnetic response of magnetotactic bacteria: incorporation of Gd and Tb ions into the magnetosome structure
E. M. Jefremovas, L. Gandarias, L. Marcano, A. Gacía-Prieto, I. Orue, A. Muela, M. L. Fdez-Gubieda, L. Fernández Barquín and J. Alonso
May magnetotactic bacteria and rare earth (RE) ions get along with each other? Definitely, yes. In this work we have successfully doped magnetosomes synthesised by magnetotactic bacteria Magnetospirillum gryphiswaldense with Gd3+ and Tb3+ ions. The magnetic properties of the doped bacteria get altered by the substitution of Fe3+ ions by those RE3+. This boosts the potential applications of magnetotactic bacteria in the biomedical and theragnostic field.
https://pubs.rsc.org/en/content/articlepdf/2022/NA/D2NA00094F
This article has been selected as front cover of vol. 4 and included in the 2022 Popular Advances collection (including articles which have been very well received by the community) by Journal Editors.
Spin dynamics in magnetic nanoparticles
PhD. Thesis, E. M. Jefremovas
The Thesis work studies the spin dynamics in ensembles of magnetic nanoparticles, where the magnetism arises from 4f (GdCu2, NdCu2, TbxR1-xCu2) and 3d (γ-Fe2O3, Fe3O4) orbitals. The analisis of a rich variety of magnetic order and disorder states provides a deep understanding on the fundamentals beneath the interactions among the magnetic moments.
https://www.dropbox.com/s/cq61764lq3oixvt/tesis_deposito_21_Octobre.pdf?dl=0
Observation of surface magnons and crystalline electric field shifts in superantiferromagnetic NdCu2 nanoparticles
E. M. Jefremovas, M. de la Fuente Rodríguez, F. Damay, B. Fåk, A. Michels, J. A. Blanco, and L. F. Barquín
Check out our work on NdCu2 nanoparticles. Thanks to the use of Inelastic neutron scattering, we have been able to observe the crystalline electric field splitting, and the magnon excitations, in these 4f ensembles of nanoparticles. We have also been able to observe such excitations taking place at the nanoparticle surface, which is unprecedent in 4f ensembles of nanoparticles. https://journals.aps.org/prb/abstract/10.1103/PhysRevB.104.134404
Nanoflowers Versus Magnetosomes: Comparison Between Two Promising Candidates for Magnetic Hyperthermia Therapy
E. M. Jefremovas, L. Gandarias, I. Rodrigo, L. Marcano, C. Grüttner, J. A. García, E. Garayo, I. Orue, A. García-Prieto, A. Muela, M. Fdez-Gubieda, J. Alonso and L. F. Barquín
Magnetic Fluid Hyperthermia mediated by iron oxide nanoparticles is one of the most promising therapies for cancer treatment. Among the different candidates, magnetite and maghemite nanoparticles are on top due to both their performance and their biocompatibility. Nonetheless, up to date, the literature comparing the heating efficiency of magnetite and maghemite nanoparticles of similar size is scarce. To fill this gap, here we provide a comparison between commercial Synomag Nanoflowers (pure maghemite) and bacterial magnetosomes (pure magnetite) synthesized by the magnetotactic bacterium Magnetospirillum gryphiswaldense of <D> ≈ 40–45 nm. Both types of nanoparticles exhibit a high degree of crystallinity and an excellent degree of chemical purity and stability. The structural and magnetic properties in both nanoparticle ensembles have been studied by means of X–Ray Diffraction, Transmission Electron Microscopy, X–Ray Absorption Spectroscopy, and SQUID magnetometry. The heating efficiency has been analyzed in both systems using AC magnetometry at several field amplitudes (0–88 mT) and frequencies (130, 300, and 530 kHz).
Exploring the Different Degrees of Magnetic Disorder in TbxR1−xCu2 Nanoparticle Alloys
E. M. Jefremovas, M. de la Fuente Rodríguez, J. Alonso, J. Rodríguez, J.. I. Espeso, I. Puente-Orench, D. P. Rojas, A. García-Prieto, M. L. Fdez-Gubieda, L. Rodríguez and L. F. Barquín
Recently, potential technological interest has been revealed for the production of magnetocaloric alloys using Rare-Earth intermetallics. In this work, three series of TbxR1−xCu2 (R ≡ Gd, La, Y) alloys have been produced in bulk and nanoparticle sizes via arc melting and high energy ball milling. Rietveld refinements of the X-ray and Neutron diffraction patterns indicate that the crystalline structure in all alloys is consistent with TbCu2 orthorhombic Imma bulk crystalline structure. The analyses of the DC-magnetisation (MDC) and AC-susceptibility (χAC) show that three distinct degrees of disorder have been achieved by the combination of both the Tb3+ replacement (dilution) and the nanoscaling. These disordered states are characterised by transitions which are evident to MDC, χACand specific heat. There exists an evolution from the most ordered Superantiferromagnetic arrangement of the Tb0.5La0.5Cu2 NPs with Néel temperature, TN∼ 27 K, and freezing temperature, Tf ∼ 7 K, to the less ordered weakly interacting Superparamagnetism of the Tb0.1Y0.9Cu2 nanoparticles (TN absent, and TB ∼ 3 K). The Super Spin Glass Tb0.5Gd0.5Cu2 nanoparticles (TN absent, and Tf ∼ 20 K) are considered an intermediate disposition in between those two extremes, according to their enhanced random-bond contribution to frustration.
Investigating the Size and Microstrain Influence in the Magnetic Order/Disorder State of GdCu2 Nanoparticles
E. M. Jefremovas, J. Alonso, M. de la Fuente Rodríguez, J. Rodríguez, J. I. Espeso, D. P. Rojas, A. García-Prieto, M. L. Fdez-Gubieda, and L. F. Barquín
A series of GdCu2 nanoparticles with controlled sizes ranging from 7 nm to 40 nm has been produced via high-energy inert-gas ball milling. Rietveld refinements on the X-ray diffraction measurements ensure that the bulk crystalline Imma structure is retained within the nanoparticles, thanks to the employed low milling times ranging from t = 0.5 to t = 5 h. The analysis of the magnetic measurements shows a crossover from Superantiferromagnetism (SAF) to a Super Spin Glass state as the size decreases at NP size of ⟨D⟩ ≈ 18 nm. The microstrain contribution, which is always kept below 1%, together with the increasing surface-to-core ratio of the magnetic moments, trigger the magnetic disorder. Additionally, an extra contribution to the magnetic disorder is revealed within the SAF state, as the oscillating RKKY indirect exchange achieves to couple with the aforementioned contribution that emerges from the size reduction. The combination of both sources of disorder leads to a maximised frustration for ⟨D⟩≈ 25 nm sized NPs.
Collaborations
MuMag2022: a software tool for analyzing magnetic field dependent unpolarized small-angle neutron scattering data of bulk ferromagnets
Michael P. Adams*, Mathias Bersweiler, Elizabeth M. Jefremovas* and Andreas Michels*
Work in collaboration with the Magnetic Neutron Scattering Group of the University of Luxembourg.
The MATLAB-based software tool MuMag2022 is presented for the analysis of magnetic-field-dependent unpolarized small-angle neutron scattering (SANS) data of bulk ferromagnets such as elemental nanocrystalline ferromagnets, magnetic nanocomposites or magnetic steels. On the basis of the micromagnetic theory for the magnetic SANS cross section, the program analyzes unpolarized total (nuclear and magnetic) SANS data within the approach-to-saturation regime. The main features of MuMag2022 are the estimation of the exchangestiffness constant, and of the strength and spatial structure of the magnetic anisotropy field and the magnetostatic field due to longitudinal magnetization fluctuations. MuMag2022 is open source and available as a standalone executable for Windows at https://mumag.uni.lu.
Suppression of ferromagnetic order in CuO/Cu2O nanocomposites
R Das, J Alonso, EM Jefremovas, L Fernández Barquín, PK Ngoc, HT Nguyen, DT Viet, PV Vinh, AT Duong
Work in collaboration with Dr. R. Das (Faculty of Materials Science and Engineering, Phenikaa University, Hanoi 12116, Vietnam). With increasing the Cu2O content, a progressive dissappearance of the ferromagnetic phase evidenced by single phase CuO nanoparticles was revealed, all along with the increase of an antiferromagnetic contribution at low temperatures (2-5 K). When the amount of Cu2O is increased to 91%, this AF phase is also suppressed. These magnetic changes showcase the relevance of the interface effects introduced by the Cu2O phase in CuO/Cu2O nanocomposites.
Spontaneous Formation of Core@shell Co@Cr Nanoparticles by Gas Phase Synthesis
Jimena Soler-Morala, E. M. Jefremovas, Lidia Martínez, Álvaro Mayoral, Elena H Sánchez, Jose A De Toro, Elena Navarro, Yves Huttel
This work presents the gas phase synthesis of CoCr nanoparticles using a magnetron-based gas aggregation source. The effect of the particle size and Co/Cr ratio on the properties of the nanoparticles is investigated. In particular, we report the synthesis of nanoparticles from two alloy targets, Co90Cr10 and Co80Cr20. In the first case, we observe a size threshold for the spontaneous formation of a segregated core@shell structure, related to the surface to volume ratio. When this ratio is above one, a shell cannot be properly formed, whereas when this ratio decreases below unity the proportion of Cr atoms is high enough to allow the formation of a shell. In the latter case, the segregation of the Cr atoms towards the surface gives rise to the formation of a shell surrounding the Co core. When the proportion of Cr is increased in the target (Co80Cr20), a thicker shell is spontaneously formed for a similar nanoparticle size. The magnetic response was evaluated, and the influence of the structure and composition of the nanoparticles is discussed. An enhancement of the global magnetic anisotropy caused by exchange bias and dipolar interactions, which enables the thermal stability of the studied small particles up to relatively large temperatures, is reported.
Unusual magnetic behaviour of binary YbNi3 alloy
D. P. Rojas, J. I. Espeso, J. Rodríguez, E. M. Jefremovas
This work presents the gas phase synthesis of CoCr nanoparticles using a magnetron-based gas aggregation source. The effect of the particle size and Co/Cr ratio on the properties of the nanoparticles is investigated. In particular, we report the synthesis of nanoparticles from two alloy targets, Co90Cr10 and Co80Cr20. In the first case, we observe a size threshold for the spontaneous formation of a segregated core@shell structure, related to the surface to volume ratio. When this ratio is above one, a shell cannot be properly formed, whereas when this ratio decreases below unity the proportion of Cr atoms is high enough to allow the formation of a shell. In the latter case, the segregation of the Cr atoms towards the surface gives rise to the formation of a shell surrounding the Co core. When the proportion of Cr is increased in the target (Co80Cr20), a thicker shell is spontaneously formed for a similar nanoparticle size. The magnetic response was evaluated, and the influence of the structure and composition of the nanoparticles is discussed. An enhancement of the global magnetic anisotropy caused by exchange bias and dipolar interactions, which enables the thermal stability of the studied small particles up to relatively large temperatures, is reported.
https://www.sciencedirect.com/science/article/abs/pii/S0304885319313642
Elizabeth M Jefremovas 