R. Mathieu

Uppsala University, Uppsala, Uppsala, Sweden

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Publications (156)361.86 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The question of the dominant interparticle magnetic interaction type in random closely packed assemblies of different diameter (6.2-11.5 nm) bare maghemite nanoparticles (NPs) is addressed. Single-particle magnetic properties such as particle anisotropy and exchange bias field are first of all studied in dilute (reference) systems of these same NPs, where interparticle interactions are neglible. Substantial surface spin disorder is revealed in all particles except the smallest, viz. for diameters d = 8-11.5 nm but not for d = 6.2-6.3 nm. X-ray diffraction analysis points to a crystallographic origin of this effect. The study of closely packed assemblies of the d [Formula: see text] 8 nm particles observes collective (superspin) freezing that clearly appears to be governed by interparticle dipole interactions. However, the dense assemblies of the smallest particles exhibit freezing temperatures that are higher than expected from a simple (dipole) extrapolation of the corresponding temperatures found in the d [Formula: see text] 8 nm assemblies. It is suggested that the nature of the dominant interparticle interaction in these smaller particle assemblies is superexchange, whereby the lack of significant surface spin disorder allows this mechanism to become important at the level of interacting superspins.
    Nanotechnology 11/2015; 26(47):475703. DOI:10.1088/0957-4484/26/47/475703 · 3.82 Impact Factor

  • Philosophical Magazine 10/2015; DOI:10.1080/14786435.2015.1090640 · 1.83 Impact Factor
  • P. Beran · S.A. Ivanov · P. Nordblad · S. Middey · A. Nag · D.D. Sarma · S. Ray · R. Mathieu ·
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    ABSTRACT: The triple perovskites Ba3ZnRu2-xIrxO9 with x Combining double low line 0, 1, and 2 are insulating compounds in which Ru(Ir) cations form a dimer state. Polycrystalline samples of these materials were studied using neutron powder diffraction (NPD) at 10 and 295 K. No structural transition nor evidence of long range magnetic order was observed within the investigated temperature range. The results from structural refinements of the NPD data and its polyhedral analysis are presented, and discussed as a function of Ru/Ir content.
  • Tapati Sarkar · Sergey A Ivanov · G V Bazuev · Per Nordblad · Roland Mathieu ·
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    ABSTRACT: Synthesis and crystal structure, magnetization and heat capacity measurements of phase pure polycrystalline TmVO3 are reported. TmVO3 was stabilized in the orthorhombic structure by thermal treatment of the precursor TmVO4 in a reducing atmosphere. Magnetization and heat capacity measurements reveal the presence of several successive structural and magnetic phase transitions in this compound. At T = 108 K, the sample undergoes a transition from a paramagnetic state to an antiferromagnetic state, followed by a second transition at 78 K which is related to spin and orbital reorientation. The heat capacity measurements reveal the presence of a third transition in the paramagnetic state (at T = 175 K), which corresponds to a structural phase transition and orbital ordering. At low temperatures (∼15 K) and weak fields, there is an anomaly in the magnetization, which may be associated with antiferromagnetic short range ordering of the Tm3+ ions.
    Journal of Physics D Applied Physics 09/2015; 48(34):345003. DOI:10.1088/0022-3727/48/34/345003 · 2.72 Impact Factor
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    ABSTRACT: Strong spin-orbit coupling (SOC) effects of heavy $d$-orbital elements have long been neglected in describing the ground states of their compounds thereby overlooking a variety of fascinating and yet unexplored magnetic and electronic states, until recently. The spin-orbit entangled electrons in such compounds can get stabilized into unusual spin-orbit multiplet $J$-states which warrants severe investigations. Here we show using detailed magnetic and thermodynamic studies and theoretical calculations the ground state of Ba$_3$ZnIr$_2$O$_9$, a 6$H$ hexagonal perovskite is a close realisation of the elusive $J$~=~0 state. However, we find that local Ir moments are spontaneously generated due to the comparable energy scales of the singlet-triplet splitting driven by SOC and the superexchange interaction mediated by strong intra-dimer hopping. While the Ir ions within the structural Ir$_2$O$_9$ dimer prefers to form a spin-orbit singlet state (SOS) with no resultant moment, substantial interdimer exchange interactions from a frustrated lattice ensure quantum fluctuations till the lowest measured temperatures and stabilize a spin-orbital liquid phase.
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    ABSTRACT: Spinel-type compounds of Fe–Ni–Sb–O system were synthesized as polycrystalline powders. The crystal and magnetic properties were investigated using X-ray and neutron powder diffraction, Mössbauer and X-ray absorption spectroscopy and magnetization measurements. The samples crystallize in the cubic system, space group Fd – 3 m. The distribution of cations between octahedral and tetrahedral sites was refined from the diffraction data sets using constraints imposed by the magnetic, Mössbauer and EDS results and the ionic radii. The cation distribution and the temperature dependence of the lattice parameter (a) and the oxygen positional parameter (u) were obtained. A chemical formula close to Fe0.8Ni1.8Sb0.4O4 was determined, with Sb5+ cations occupying octahedral sites, and Fe3+ and Ni2+ occupying both tetrahedral and octahedral sites. Fe3+ mainly (85/15 ratio) occupy tetrahedral sites, and conversely Ni2+ mainly reside on octahedral ones.
    Materials Chemistry and Physics 05/2015; 158. DOI:10.1016/j.matchemphys.2015.03.051 · 2.26 Impact Factor
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    ABSTRACT: In the spinel Co2SnO4, coexistence of ferrimagnetic ordering below TN ≃ 41 K followed by a spin glass state below TSG ≃ 39 K was proposed recently based on the temperature dependence of magnetization M(T) data. Here new measurements of the temperature dependence of the specific heat CP(T), ac-susceptibilities χ'(T) and χ″(T) measured at frequencies between 0.51 and 1.2 kHz, and the hysteresis loop parameters (coercivity HC(T) and remanence MR(T)) in two differently prepared samples of Co2SnO4 are reported. The presence of the Co(2+) and Sn(4+) states is confirmed by x-ray photoelectron spectroscopy (XPS) yielding the structure: Co2SnO4 = [Co(2+)][Co(2+)Sn(4+)]O4. The data of CP versus T shows only an inflection near 39 K characteristic of spin-glass ordering. The analysis of the frequency dependence of ac-magnetic susceptibility data near 39 K using the Vogel-Fulcher law and the power-law of the critical slowing-down suggests the presence of spin clusters in the system which is close to a spin-glass state. With a decrease in temperature below 39 K, the temperature dependence of the coercivity HC and remanence MR for the zero-field cooled samples show both HC and MR reaching their peak magnitudes near 25 K, then decreasing with decreasing T and becoming negligible below 15 K. The plot of CP/T versus T also yields a weak inflection near 15 K. This temperature dependence of HC and remanence MR is likely associated with the different magnitudes of the magnetic moments of Co(2+) ions on the 'A' and 'B' sites and their different temperature dependence.
    Journal of Physics Condensed Matter 03/2015; 27(16):166001. DOI:10.1088/0953-8984/27/16/166001 · 2.35 Impact Factor
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    ABSTRACT: The thermal decomposition of acetylacetonate precursors is one of the most employed syntheses to prepare high quality colloidal magnetic nanoparticles. In this paper, an advanced version of this synthetic approach was developed to prepare cobalt ferrite nanoparticles, introducing for the first time a rigorous control on one commonly neglected reaction parameter, that is, the residual oxygen content in the reaction environment. A new concept derived from the statistical analysis of S(T)EM images, i.e., the so-called aspects maps, was introduced: this tool has allowed us to clearly identify the optimal value of pressure to produce particles with an average size ∼19 nm and with a very narrow size distribution (polydispersity 0.4 nm −1). The magnetic properties of this sample were also analyzed, and a strong improvement of the magnetization reversal mechanism, which is a critical issue for several technological applications, was observed. ■ INTRODUCTION Advanced synthesis approaches, necessary to achieve a strict control of the structural, morphological, and chemical properties of nanomaterials, are at the basis of a reproducible manipulation of their unique physical behavior. Nowadays, this is one of the most difficult problems faced by nanotechnology. In fact, any advanced application that takes advantage of nanoparticle (NP) systems will also rely on the achievement of such control. This is true in particular for magnetic nano-particles that are unique and complex physical objects whose properties can greatly differ from their parent massive materials.
    Chemistry of Materials 03/2015; 27(6). DOI:10.1021/cm5038815 · 8.35 Impact Factor
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    ABSTRACT: Phase-pure AM1/3W8/3O9 (A: K, Rb, Cs; M: CrIII, FeIII) are prepared by calcination and sintering of stoichiometric mixtures of Cr2O3, Fe2O3, alkali carbonates, and WO3 (800 °C, 12 h).
    ChemInform 03/2015; 46(13):no-no. DOI:10.1002/chin.201513015
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    ABSTRACT: Stoichiometric polycrystalline samples of LnFe2/3Mo1/3O3(Ln=Nd,Pr,Ce,La) have been prepared by solid-state reaction and studied by means of x-ray and neutron powder diffraction as well as Mössbauer spectroscopy and magnetic measurements. All samples were found to be of single phase and to have Pnma symmetry with valence state +3 of Fe and Mo. It is demonstrated that the B-site cations of LnFe2/3Mo1/3O3 in accord with LnFeO3 order in a G-type antiferromagnetic structure with the magnetic moments aligned along the b axis. However, with significantly lower Néel temperatures than their LnFeO3 parent compounds. The Fe-O-Fe bond lengths and bond angles and thus the magnitude of the antiferromagnetic superexchange interaction are found to systematically change with the ionic radius of Ln such that TN increases with increasing radius. Only the CeFe2/3Mo1/3O3 compound experiences a low temperature spin reorientation from alignment along the b axis to the a axis.
    Physical Review B 03/2015; 91(9). DOI:10.1103/PhysRevB.91.094418 · 3.74 Impact Factor
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    ABSTRACT: We present an experimental set-up permitting Raman and luminescence spectroscopy studies in a commercial Physical Properties Measurement System (PPMS) from Quantum Design. Using this experimental set-up, gaseous, liquid and solid materials, in bulk or thin film form, may be investigated. The set-up is particularly suitable for the study of the spin-lattice coupling in strongly correlated oxide materials utilizing several different stimuli, e.g. magnetic and electric fields, high pressure and low temperatures. Details for the Raman extension, sample holder assembly and optical design, as well as data acquisition and measurement routine are described. Finally, we present exemplary results collected using the set-up, measured on reference materials, as well as on a correlated transition metal oxide.
    02/2015; 3(1). DOI:10.1140/epjti/s40485-015-0014-x
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    ABSTRACT: STM based magnetotransport measurements of epitaxial La0.7Sr0.3MnO3 31 nm thick films with and without an internal LaMnO3 layer (0-3.1 nm thick) grown on Nb doped SrTiO3 are presented. The measurements reveal two types of low field magnetoresistance (LFMR) with a magnitude of ~0.1-1.5%. One LFMR contribution is identified as a conventional grain boundary/domain wall scattering through the symmetric I-V characteristics, high dependence on tip placements and insensitivity to introduction of LaMnO3 layers. The other contribution originates from the reverse biased Nb doped SrTiO3 interface and the interface layer of La0.7Sr0.3MnO3. Both LFMR contributions display a field dependence indicative of a higher coercivity (~200 Oe) than the bulk film. LaMnO3 layers are found to reduce the rectifying properties of the junctions, and sub-micron lateral patterning by electron beam lithography enhances the diodic properties, in accordance with a proposed transport model based on the locality of the injected current.
    Journal of Magnetism and Magnetic Materials 12/2014; 374. DOI:10.1016/j.jmmm.2014.08.089 · 1.97 Impact Factor
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    ABSTRACT: The structure and magnetic properties of hexagonal tungsten bronzes AM1/3W8/3O9 (A - K+, Rb+, Cs+; M - Cr3+, Fe3+) have been investigated. Pure ceramic samples were synthesized by solid-state reaction. The samples have been studied by X-ray powder diffraction in combination with magnetic measurements. The compounds crystallize in hexagonal space group P63/mcm. The substitution of magnetic ions into the WO6 octahedra yields dilute antiferromagnetic Cr3+-O2--Cr3+ (or Fe3+-O2--Fe3+) superexchange interaction causing the appearance of short-range magnetic order at low temperatures. The antiferromagnetic character of the interaction is supported by negative values of the derived Curie-Weiss temperatures, θCW. The magnitude of θCW is found to decrease with increasing ionic radius of the A cation.
    Solid State Sciences 12/2014; 40(13). DOI:10.1016/j.solidstatesciences.2014.12.012 · 1.84 Impact Factor
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    ABSTRACT: We present a comprehensive study of the thickness dependence of static and magneto-dynamic magnetic properties of La0.7Sr0.3MnO3. Epitaxial pulsed laser deposited La0.7Sr0.3MnO3/SrTiO3(001) thin films in the range from 3 unit cell (uc) to 40 uc (1.2–16 nm) have been investigated through ferromagnetic resonance (FMR) spectroscopy and SQUID magnetometry at variable temperature. Magnetodynamically, three different thickness, d , regimes are identified: 20 uc ≲d≲d uc where the system is bulk like, a transition region 8 uc ≤d≲20≤d≲20 uc where the FMR linewidth and the position depend on thickness and d=6 uc which displays significantly altered magnetodynamic properties, while still displaying bulk magnetization. Magnetization and FMR measurements are consistent with a nonmagnetic volume corresponding to ~4 uc. We observe a reduction of Curie temperature (TC) with decreasing thickness, which is coherent with a mean field model description. The reduced ordering temperature also accounts for the thickness dependence of the magnetic anisotropy constants and resonance fields. The damping of the system is strongly thickness dependent, and is for thin films dominated by thickness dependent anisotropies, yielding both a strong two-magnon scattering close to Tc and a low temperature broadening. For the bulk like samples a large part of the broadening can be linked to spread in magnetic anisotropies attributed to crystal imperfections/domain boundaries of the bulk like film.
    Journal of Magnetism and Magnetic Materials 11/2014; 369:197–204. DOI:10.1016/j.jmmm.2014.06.038 · 1.97 Impact Factor
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    ABSTRACT: Fe@Au core-shell nanoparticles (NPs) exhibit multiple functionalities enabling their effective use in applications such as medical imaging and drug delivery. In this work, a novel synthetic method was developed and optimized for the synthesis of highly stable, monodisperse Fe@Au NPs of average diameter similar to 24 nm exhibiting magneto-plasmonic characteristics. Fe@Au NPs were characterized by a wide range of experimental techniques, including scanning (transmission) electron microscopy (S(T)EM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), dynamic light scattering (DLS) and UV-vis spectroscopy. The formed particles comprise an amorphous iron core with a crystalline Au shell of tunable thickness, and retain the superparamagnetic properties at room temperature after formation of a crystalline Au shell. After surface modification, PEGylated Fe@Au NPs were used for in vitro studies on olfactory ensheathing cells (OECs) and human neural stem cells (hNSCs). No adverse effects of the Fe@Au particles were observed post-labeling, both cell types retaining normal morphology, viability, proliferation, and motility. It can be concluded that no appreciable toxic effects on both cell types, coupled with multifunctionality and chemical stability make them ideal candidates for therapeutic as well as diagnostic applications.
    Applied Surface Science 10/2014; 316(1):171–178. DOI:10.1016/j.apsusc.2014.07.081 · 2.71 Impact Factor
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    ABSTRACT: Magnetization dynamics of a model superspin glass system consisting of nearly monodispersed close-packed maghemite particles of diameter 8 nm is investigated. The observed non-equilibrium features of the dynamics are qualitatively similar to those of atomic spin glass systems. The intrinsic relaxation function, as observed in zero-field–cooled magnetization relaxation experiments, depends on the time the sample has been kept at constant temperature (ageing). Accompanying low-field experiments show that the archetypal spin glass characteristics —ageing, memory and rejuvenation— are reproduced in this dense system of dipolar-dipolar interacting superspins.
    EPL (Europhysics Letters) 10/2014; 108(1). DOI:10.1209/0295-5075/108/17004 · 2.10 Impact Factor
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    ABSTRACT: The effect of Zn substitution on the magnetic and magnetocaloric properties of Cu1−xZnxFe2O4 (x=0.6, 0.7, 0.8) ferrites over a wide temperature range has been investigated. The polycrystalline samples were synthesized using the solid-state reaction at sintering temperature 1050 °C (1323 K) for 2 h and has been characterized by SQUID magnetometry. Magnetization versus temperature showed that all samples exhibit a paramagnetic to ferromagnetic transition with decreasing temperature. The Curie temperature Tc is found to decrease from 373 K for x=0.6 to 140 K for x=0.8 as well as the saturation magnetization Ms which shifts from 100 to 44 emu/gm. The magnetocaloric effect was obtained by measuring a family of M–H curves at set temperature intervals and calculating the entropy change, ΔS for this system using the Maxwell relation. The ΔS of all samples increased with increasing applied field and showed a maximum around their respective Tc. The entropy change (ΔS) decreased with increasing Zn content, whereas the relative cooling power (RCP) slightly increased. The large RCP and ΔS found in Zn substitution Cu–Zn ferrites will be interesting for magnetic refrigeration near room temperature.
    Journal of Magnetism and Magnetic Materials 10/2014; 367:75–80. DOI:10.1016/j.jmmm.2014.04.070 · 1.97 Impact Factor
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    ABSTRACT: Monodisperse iron nanoparticles of size range 5–19 nm with controlled crystallinity and magnetic properties are synthesized via thermal decomposition of iron pentacarbonyl in the presence of an appropriate ligand. Crystalline body-centered cubic iron nanoparticles synthesized in the presence of didodecyldimethylammonium bromide exhibit a high saturation magnetization and room temperature ferromagnetic behavior with a remarkable increased stability in the aqueous phase in comparison to amorphous nanoparticles synthesized in the presence of other ligands.
    Particle and Particle Systems Characterization 10/2014; 31(10). DOI:10.1002/ppsc.201400032 · 3.08 Impact Factor
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    ABSTRACT: Temperature-dependent crystallographic and magnetic studies on stoichiometric single crystals of LiCu2O2 are reported. The magnetic properties are similar to earlier findings demonstrating antiferromagnetic ordering below 25 K. Evidence of magnetoelastic coupling is observed in the thermal expansion along the c-direction; not only at the low temperature antiferromagnetic transitions, but an anomalous behavior of the thermal expansion indicate magnetoelastic coupling also to the magnetic ordering related to a weak spontaneous magnetic moment appearing at 150 K. Ac-susceptibility measurements at different frequencies and superposed dc- fields are employed to further characterize this magnetic anomaly.
    Solid State Sciences 08/2014; 34. DOI:10.1016/j.solidstatesciences.2014.05.014 · 1.84 Impact Factor
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    ABSTRACT: Nanoparticles of iron and iron oxide are widely explored in several biomedical and technological applications. We report on the magnetic properties of amorphous Fe/Fe–O core/shell nanoparticles compared to those of a reference system with crystalline Fe–O nanoparticles. These nanoparticles are prepared by thermal decomposition of iron precursor, where the amorphous and crystalline nature of core and shell is determined by the choice and concentration of the ligand. The crystalline system exhibits a blocking temperature higher than 300 K and negligible exchange bias effect. In contrast, the amorphous systems display large exchange bias, and collective magnetic behavior at low temperatures, with features of magnetic frustration and disorder reminiscent of those observed in spin glass and superspin glass systems. We discuss the origin of the dynamical magnetic behavior of the amorphous particles and study the dependence of the exchange bias field on the cooling field.
    07/2014; 1(3):036103. DOI:10.1088/2053-1591/1/3/036103

Publication Stats

2k Citations
361.86 Total Impact Points


  • 2001-2015
    • Uppsala University
      • Department of Engineering Sciences
      Uppsala, Uppsala, Sweden
  • 2007
    • KTH Royal Institute of Technology
      • Department of Microelectronics and Applied Physics (MAP)
      Tukholma, Stockholm, Sweden
  • 2006
    • Japan Science and Technology Agency (JST)
      • Exploratory Research for Advanced Technology (ERATO)
      Edo, Tōkyō, Japan
  • 2005
    • The University of Tokyo
      • Department of Applied Physics
      白山, Tōkyō, Japan
  • 2003
    • National Institute of Advanced Industrial Science and Technology
      • Nanoelectronics Research Institute
      Tsukuba, Ibaraki, Japan