R. Mathieu

Uppsala University, Uppsala, Uppsala, Sweden

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Publications (162)395.67 Total impact

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    ABSTRACT: Polycrystalline samples of the triple perovskites Ba3ZnRu2-xIrxO9 (x = 0, 1, 2) are prepared by calcination of stoichiometric amounts of BaCO3, ZnO, RuO2, and Ir (900 °C, 12 h) and subsequent heating in O2 atmosphere (1150 °C, 72 h).
    No preview · Article · Feb 2016 · ChemInform
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    Matthias Hudl · Roland Mathieu · Per Nordblad
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    ABSTRACT: A unidirectional anisotropy appears in field cooled samples of dilute magnetic alloys at temperatures well below the cusp temperature of the zero field cooled magnetization curve. Magnetization measurements on a Cu(13.5 at% Mn) sample show that this anisotropy is essentially temperature independent and acts on a temperature dependent excess magnetization, ΔM. The anisotropy can be partially or fully transferred from being locked to the direction of the cooling field at lower fields to becoming locked to the direction of ΔM at larger fields, thus instead appearing as a uniaxial anisotropy. This introduces a deceiving division of the anisotropy into a superposition of a unidirectional and a uniaxial part. This two faced nature of the anisotropy has been empirically scrutinized and concluded to originate from one and the same exchange mechanism: the Dzyaloshinsky-Moriya interaction.
    Full-text · Article · Jan 2016 · Scientific Reports
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    ABSTRACT: We report the remarkable phase separation behaviour in La0.67Sr0.33MnO3 doped with Bi3+ ion at La site. The temperature dependent resistivity and magnetization of La0.67-xBixSr0.33MnO3 (x>0) show the presence of phase separation of ferromagnetic metallic and charge ordered antiferromagnetic insulating phases. Markedly, the field dependant magnetization studies of La0.67-xBixSr0.33MnO3 (x=0.3) show the metamagnetic nature of ferromagnetic metallic state implying the competition of coexisting ferromagnetic metallic and charge ordered antiferromagnetic phases. The electron spin resonance and exchange bias studies of La0.67-xBixSr0.33MnO3 (x=0.4 and 0.5) substantiate the coexistence of ferromagnetic clusters in antiferromagnetic matrix.
    Full-text · Article · Jan 2016 · Journal of Magnetism and Magnetic Materials
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    ABSTRACT: Two kinds of nanocomposites of transition metal oxides were synthesized and investigated. Each nanocomposite comprises nanoparticles of La0.67Ca0.33MnO3 and CoFe2O4 in similar volume fractions, however arranged with different morphologies. The temperature-dependent magnetic and electrical properties of the two systems are found to greatly differ, suggesting different degrees of interaction and coupling of their constituents. This is confirmed by magnetic field-dependent experiments, which reveal contrasted magnetization reversal and magnetoresistance in the systems. We discuss this morphology-physical property relationship, and the possibility to further tune the magnetism and magneto-transport in such nanocomposites.
    No preview · Article · Dec 2015 · Nanoscale
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    ABSTRACT: The complex nature of magnetic ordering in the spinel Co2TiO4 is investigated by analyzing the temperature and magnetic field dependence of its magnetization (M), specific heat (Cp), and ac magnetic susceptibilities χ′ and χ″. X-ray diffraction of the sample synthesized by the solid-state reaction route confirmed the spinel structure whereas x-ray photoelectron spectroscopy shows its electronic structure to be Co2TiO4=[Co2+][Co3+Ti3+]O4. From analysis of the temperature dependence of the dc paramagnetic susceptibility, the magnetic moments μ(A)=3.87μB and μ(B)=5.19μB on the A and B sites are determined with μ(B) in turn yielding μ(Ti3+)=1.73μB and μ(Co3+)=4.89μB. Analysis of the dc and ac susceptibilities combined with the weak anomalies observed in the Cp vs T data shows the existence of a quasi-long-range ferrimagnetic state below TN∼47.8K and a compensation temperature Tcomp∼32K, the latter characterized by sign reversal of magnetization with its magnitude depending on the applied magnetic field and the cooling protocol. Analysis of the temperature dependence of M (field cooled) and M (zero field cooled) data and the hysteresis loop parameters is interpreted in terms of large spin clusters. These results in Co2TiO4, significantly different from those reported recently in isostructural Co2SnO4=[Co2+][Co2+Sn4+]O4, warrant further investigations of its magnetic structure using neutron diffraction.
    Preview · Article · Dec 2015 · Physical Review B
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    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.
    No preview · Article · Nov 2015 · Nanotechnology
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    ABSTRACT: Static and dynamical magnetic properties of Fe nanoparticles (NPs) embedded in non-magnetic (Ag) and antiferromagnetic (Cr) matrices with a volume filling fraction (VFF) of 10% have been investigated. In both Fe@Ag and Fe@Cr nanocomposites, the Fe NPs have a narrow size distribution, with a mean particle diameter around 2 nm. In both samples, the saturation magnetization reaches that of Fe bulk bcc, suggesting the absence of alloying with the matrices. The coercivity at 5 K is much larger in Fe@Cr than in Fe@Ag as a result of the strong interaction between the Fe NPs and the Cr matrix. Temperature-dependent magnetization and ac-susceptibility measurements point out further evidence of the enhanced interparticle interaction in the Fe@Cr system. While the behaviour of Fe@Ag indicates the presence of weakly interacting magnetic monodomain particles with a wide distribution of blocking temperatures, Fe@Cr behaves like a superspin glass produced by the magnetic interactions between NPs.
    No preview · Article · Oct 2015 · Philosophical Magazine
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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.
    No preview · Article · Oct 2015 · Solid State Sciences
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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.
    No preview · Article · Sep 2015 · Journal of Physics D Applied Physics
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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.
    No preview · Article · Jun 2015
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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.
    No preview · Article · May 2015 · Materials Chemistry and Physics
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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.
    No preview · Article · Mar 2015 · Journal of Physics Condensed Matter
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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.
    Full-text · Article · Mar 2015 · Chemistry of Materials
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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).
    No preview · Article · Mar 2015 · ChemInform
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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.
    No preview · Article · Mar 2015 · Physical Review B
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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.
    Full-text · Article · Feb 2015
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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.
    Full-text · Article · Dec 2014 · Journal of Magnetism and Magnetic Materials
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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.
    No preview · Article · Dec 2014 · Solid State Sciences
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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.
    Full-text · Article · Nov 2014 · Journal of Magnetism and Magnetic Materials
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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.
    Full-text · Article · Oct 2014 · Applied Surface Science

Publication Stats

2k Citations
395.67 Total Impact Points


  • 2001-2016
    • Uppsala University
      • Department of Engineering Sciences
      Uppsala, Uppsala, Sweden
  • 2002-2007
    • KTH Royal Institute of Technology
      • • Department of Microelectronics and Applied Physics (MAP)
      • • Department of Materials Science and Engineering (MSE)
      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