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  • Nader Yaacoub
  • Nirina Randriantonandro
  • Jean-Marc Grenèche
  • Yvan Labaye
  • Florent Calvayrac
  • Rémi Busselez

Exploring the magnetic disorder in Ultrahin Iron Oxide Hollow Nanoparticules

Magnetic nanoparticles are of special interest in the field of storage information and nanomedicine (hyperthermia, medical imagery, …). Due to the nanometric size of the particles they can be guided using a magnetic field. Unfortunately, reducing the size of the nanomagnets leads to magnetic instability incompatible to technological applications. Modifying the shape of the nanoparticles (core-shell particles, Hollow particles, …) lead to an exaltation of surface/interface effects and an enhancement of the magnetic anisotropy and the apparition of dynamical effect (superparamagnetic). As an example, by use of in-field Mössbauer spectrometry, we have shown the existence of complex non-collinear magnetic structure in iron oxide hollow nanoparticles. Indeed, this structure consists in a ferrimagnetic layer of few atomic planes confined between two layers with canted structure resulting from two antiferromagnetic coupled speromagnetic structures. Such a magnetic structure leads to an increase of the magnetic coupling between the interfacial moments of the magnetic phases. The exchange bias coupling finally tends to increase the magnetic anisotropy of the nanoparticles and in definitive the magnetic stability of the nanostructure.


Properties of Hollow magnetic nanoparticle.

Related papers

  • Sayed, N. Yaacoub, Y. Labaye, R. S. Hassan, G. Singh, P. A. Kumar, J. M. Greneche, R. Mathieu, G. C. Hadjipanayis, E. Agostinelli and D. Peddis, Surface Effects in Ultrathin Iron Oxide Hollow Nanoparticles: Exploring Magnetic Disorder at the Nanoscale, J. Phys. Chem. C, 2018, 122, 7516–7524.
  • Muscas, N. Yaacoub, G. Concas, F. Sayed, R. Sayed Hassan, J. M. Greneche, C. Cannas, A. Musinu, V. Foglietti, S. Casciardi, C. Sangregorio and D. Peddis, Evolution of the magnetic structure with chemical composition in spinel iron oxide nanoparticles, Nanoscale, 2015, 7, 13576–13585.
  • Gaudisson, R. Sayed-Hassan, N. Yaacoub, G. Franceschin, S. Nowak, J.-M. Grenèche, N. Menguy, P. Sainctavit and S. Ammar, On the exact crystal structure of exchange-biased Fe 3 O 4 –CoO nanoaggregates produced by seed-mediated growth in polyol, CrystEngComm, 2016, 18, 3799–3807.
  • Sayed, Y. Labaye, R. Sayed Hassan, F. El Haj Hassan, N. Yaacoub and J. M. Greneche, Size and thickness effect on magnetic structures of maghemite hollow magnetic nanoparticles, Journal of Nanoparticle Research, 2016, 18, 279.
  • Prado, N. Daffé, A. Michel, T. Georgelin, N. Yaacoub, J.-M. Grenèche, F. Choueikani, E. Otero, P. Ohresser, M.-A. Arrio, C. Cartier-dit-Moulin, P. Sainctavit, B. Fleury, V. Dupuis, L. Lisnard and J. Fresnais, Enhancing the magnetic anisotropy of maghemite nanoparticles via the surface coordination of molecular complexes, Nature Communications, 2015, 6, 10139.


Rare Earth free permanent magnets

Among currently investigated rare-earth-free magnets, ferromagnetic τ-MnAl is a potential candidate having promising intrinsic magnetic properties.  Mn(Fe)AlC was synthesized by mechanical alloying method. Effects of carbon on microstructure and magnetic properties were systematically investigated. It was found that high purity of τ-MnAl(C) could be obtained at 2 at.% C doping, showing clearly stabilizing effect of carbon. Mn54.2Al43.8C2 has the best magnetic properties: magnetization at 2T M2T = 414 kA.m-1, remanent magnetization Mr = 237 kA.m-1, coercivity HC = 229 k.Am-1, and |BH|max = 11.2 kJ.m-3. HC increased inversely with the crystallite size of τ phase and proportionally with C content. Moreover, first principle calculation showed both stabilizing effect and preferable interstitial positions of carbon in tetragonal   τ-MnAl. Mn51-xFexAl47C2 (x= 0.25, 0.5, 1, 2, 4, 6) alloys were also synthesized by mechanical alloying method, showing the high purity of τ phase up to 2 at.% Fe doping. Adding of Fe on MnAl(C) reduced both magnetization and TC but likely increased slightly HC. 57Fe Mössbauer spectrometry at 300K was used to probe local environment in ε-, τ-, β-, and γ2-MnFeAl(C). In which, γ2-, ε-, and β-MnFeAl(C) exhibited a quadrupolar structure while τ -Mn50.5Fe0.5Al47C2 spectrum showed a rather complex magnetic hyperfine splitting. The interaction between Fe and Mn examined by in-field Mössbauer measurement at 10 K and 8 T showed a non-collinear magnetic structure between Fe and Mn with different canting angles at different sites.  Hyperfine field of MnFeAl alloy calculated by Wien2k supported both magnetic properties and Mössbauer results.

Changes in magnetic moment with substitution. Mössbauer spectra showing ferrimagnetic order. Magnetization curve of substituted phases

Related papers

  • Tang Nguyen, F. Calvayrac, A. Bajorek and N. Randrianantoandro, Mechanical alloying and theoretical studies of MnAl(C) magnets, Journal of Magnetism and Magnetic Materials, 2018, 462, 96–104

Classical Magnetic models : towards a multi-scale approach

Phenomenological magnetic models are able to bridge a gap between the microscopic magnetic properties and local organization of magnetic sites from a first part and the overall magnetic properties of bulk or nanosized materials from a second part.

In the case of Heisenberg model, the magnetic Hamiltonian is governed by few critical parameters, namely the magnetic moments of sites, the exchange energy between neighbors, the volume anisotropy constant  and for finite size objects, the surface anisotropy constant and interface anisotropy constants.

Accuracy of the results emerging from the classical model is then intimately related to the theoretical or experimental determination of the underlying magnetic parameters and microscopic site organization of the system. This level of knowledge is not necessarily fulfilled in the case of complex structures or unusual compounds. Using a numeric multi-scale approach coupling ab-initio, molecular dynamics and Heisenberg simulations, access to a finer level of description. From one part the use of molecular dynamics simulation, permits to relax the structure of nano-sized or bulk systems and then to reproduce the magnetic sites position and distribution in a more realistic way. From another side, adjustments of ab-initio calculations and Heisenberg model on a large statistical sample of representative local environments allow us to determine the  microscopic magnetic parameters.


Left: Influence of dopamine functionalization on magnetic properties. Right: Influence of structure relaxation on magnetic properties

Related papers

  • Brymora and F. Calvayrac, Surface anisotropy of iron oxide nanoparticles and slabs from first principles: Influence of coatings and ligands as a test of the Heisenberg model, Journal of Magnetism and Magnetic Materials, 2017, 434, 14–22.
  • Sayed, Y. Labaye, R. Sayed Hassan, F. El Haj Hassan, N. Yaacoub and J. M. Greneche, Size and thickness effect on magnetic structures of maghemite hollow magnetic nanoparticles, Journal of Nanoparticle Research, 2016, 18, 279.
  • Nehme, Y. Labaye, R. Sayed Hassan, N. Yaacoub and J. M. Greneche, Modeling of hysteresis loops by Monte Carlo simulation, AIP Advances, 2015, 5, 127124.


Multiferroic Materials

Multiferroic materials constitute a class of multifunctional materials presenting in the same time coupled properties in terms of ferromagnetic-ferroelectric and ferroelastic order. On a fundamental scale, the nature of the interactions and in particular the magnetoelastic coupling mechanism is not fully understood despite the amount of experiments. On a technological level, this class of material is interesting in the field of information storage or for microelectromechanical systems development. Among multiferroic materials, BiFeO3 is one is one of the few which presents in the same time both ferroelectric and magnetic order above 300K. However despite this unique property, BiFeO3 also presents a magnetic helicoïdal structure of Fe3+ magnetic moments, this magnetic structure leads to weak magnetoelectric coupling. A drastic increase of magnetoelectric coupling can be obtained through the breakdown of helicoïdal spirale and the formation of an antiferromagnetic linear structure of type G leading to a non-zero mean magnetization.

Multiples ways can be followed to breakdown this helicoidal structure among them:

  1. Reducing the size of the material dimension to a value lower than the cycloid period (~64 nm). This can be obtained by a polyol method for synthetizing BiFeO3 nanoparticles.
  2. Substituting Bi3+ and Fe3+ by Ti4+ and Zr4+ using a ceramic synthesis in order to partially break helicoïdal structure of Fe3+ magnetic moments


MEB picture of BiFeO3 particles obtained with polyol processModification of helicoïdal magnetic order and apparition of ferromagnetic order

Spin disorder versus Exchange bias coupling in magnetic nanoparticles with complex architecture

An important relevant features of the size effect in magnetic nanoparticles (NPs) is the occurrence of non-collinear spin structures (spin canting, magnetic frustration, spin disorder). Indeed, the non-collinear spin structures could strongly modify the magnetic properties of the magnetic NPs, for example as it was shown recently by us which found a strong correlation between complex spin disorder and exchange bias (EB) features in iron oxide hollow NPs. Although the macroscopic model of EB effect existed for nearly six decades, the microscopic origin of this phenomenon is still requiring further investigation in some specific systems at nanoscales. Even in the case of very well-studied and investigated nanomaterials such as spinel ferrites, a full understanding of the correlation between spin structure, surface/interface effect and EB coupling is still lacking. In this context, we propose to investigate this phenomenon in systems presenting high spin disorder like Hollow NPs with different sizes and shell thickness and extended this study to a system of shell/shell that consists of two magnetic phases: a ferrimagnetic phase and an antiferromagnetic one. The idea is to discriminate the EB resulting from surface spin disorder from that resulting from spins interface coupling between two different magnetic phases. Figure below show the Mössbauer spectra for hollow (g-Fe2O4) and shell/shell (g-Fe2O4/NiO) NPs with same external diameter and thickness (15 nm and 7.5 nm). As a preliminary result, we observe that the spin disorder is bigger in the shell/shell than in the hollow one. On the other hand the EB observed in shell/shell NPs is bigger than that observed in hollow NPs. This raises the question about the effect of the antiferromagnetic phase on the spin disorder and the EB coupling in this kind of nanoscale systems ? Finally, In order to avoid the effect of dipolar interaction and study its effects on spin disorder and EB we propose to disperse these systems in non magnetic matrix.

In-field Mössbauer spectra of magnetic NPs with different architectures: γ-Fe2O3, (hollow) and γ-Fe2O3/NiO (Shell/shell)

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