R. Mathieu

Uppsala University, Uppsala, Uppsala, Sweden

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Publications (141)299.26 Total impact

  • [Show abstract] [Hide abstract]
    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). · 2.26 Impact Factor
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    ABSTRACT: STM based magnetotransport measurements of epitaxial La$_{0.7}$Sr$_{0.3}$MnO$_3$ 32 nm thick films with and without an internal LaMnO$_3$ layer (0-8 nm thick) grown on Nb doped SrTiO$_3$ are presented. The measurements reveal two types of low field magnetoresistance (LFMR) with a magnitude of $\sim 0.1-1.5\%$. 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 LaMnO$_3$ layers. The other contribution originates from the reverse biased Nb doped SrTiO$_3$ interface and the interface layer of La$_{0.7}$Sr$_{0.3}$MnO$_3$. Both LFMR contributions display a field dependence indicative of a higher coercivity ($\sim$200 Oe) than the bulk film. LaMnO$_3$ 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.
    08/2014;
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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.
    Materials Research Express. 07/2014; 1(3):036103.
  • Particle and Particle Systems Characterization 05/2014; · 0.86 Impact Factor
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    ABSTRACT: We report results of magnetization and ac susceptibility measurements down to very low fields on a single crystal of the perovskite manganite, La$_{0.82}$Ca$_{0.18}$MnO$_3$. This composition falls in the intriguing ferromagnetic insulator region of the manganite phase diagram. In contrast to earlier beliefs, our investigations reveal that the system is magnetically (and in every other sense) single-phase with a ferromagnetic ordering temperature of $\sim$ 170 K. However, this ferromagnetic state is magnetically frustrated, and the system exhibits pronounced glassy dynamics below 90 K. Based on measured dynamical properties, we propose that this quasi-long-ranged ferromagnetic phase, and associated superspin glass behavior, is the true magnetic state of the system, rather than being a macroscopic mixture of ferromagnetic and antiferromagnetic phases as often suggested. Our results provide an understanding of the quantum phase transition from an antiferromagnetic insulator to a ferromagnetic metal via this ferromagnetic insulating state as a function of $x$ in La$_{1-x}$Ca$_x$MnO$_3$, in terms of the possible formation of magnetic polarons.
    02/2014; 4(1).
  • Materials Research Bulletin. 01/2014; 50:42-56.
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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 01/2014; 367:75–80. · 2.00 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. 01/2014;
  • Applied Surface Science 01/2014; 316:171–178. · 2.54 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 01/2014; 369:197–204. · 2.00 Impact Factor
  • Journal of Magnetism and Magnetic Materials 01/2014; · 2.00 Impact Factor
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    ABSTRACT: Endothermic chemical vapour transport (CVT) reactions of Ni2MSbO6 (M = Sc, In), using a temperature gradient of 1313 → 1233 K and HgCl2, HgBr2, PtCl2 or TeCl4 as transport agents, led to growth of Ni2InSbO6 single crystals in the millimetre range, whereas in the case of Ni2ScSbO6 an incongruent dissolution of the solid in the source region was observed, leading to the formation of single crystals of the ternary phase NiSb2O6 in the sink region. The crystal structures of the obtained crystals were refined from single crystal X-ray data with high precision [Ni2InSbO6: R3, Z = 3, a = 5.21640(10) Å, c = 14.0142(3) Å, 1279 structure factors, 33 parameters, R[F2 > 2σ(F2)] = 0.0189; NiSb2O6: P42/mnm, Z = 2, a = 4.63910(10) Å, c = 9.2182(2) Å, 500 structure factors, 19 parameters, R[F2 > 2σ(F2)] = 0.0145]. Ni2InSbO6 crystallizes in a corundum-related structure, NiSb2O6 in the trirutile structure type. Spontaneous polarization and the ferroelectric transition temperature were estimated from the atomic arrangement and cation displacement along the polar axis in Ni2InSbO6. Magnetic measurements on Ni2InSbO6 evidence an antiferromagnetic transition near TN = 74 K, with significant magnetic frustration.
    Crystal Research and Technology 12/2013; · 1.12 Impact Factor
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    ABSTRACT: The effects of Co2+ doping on the structural, magnetic and dielectric properties of the multiferroic frustrated antiferromagnet Mn3TeO6 have been investigated. Ceramic samples of the solid solution series Mn3-xCoxTeO6 were prepared by a solid-state reaction route. X-ray and neutron powder diffraction and electron microscopy techniques were combined with calorimetric, dielectric and magnetic measurements to investigate the dependence of the crystal structure and physical properties on temperature and composition. It is shown that the compounds with x up to 2.4 all adopt the trigonal corundum-related structure of pure Mn3TeO6 (space group R-3) in the temperature range 5-295 K and that the lattice parameters a and c and the unit-cell volume V decrease linearly with increasing Co2+ concentration. The low- temperature magnetic susceptibility and heat capacity data evidence the antiferromagnetic ordering of all samples. The Neel temperature linearly increases with Co2+ concentration x. Curie-Weiss fits of the high temperature susceptibility indicate that the magnetic frustration decreases with x. The derived magnetic structure of Mn3TeO6 can be described as an incommensurately modulated magnetic spin state with k = [0, 0, kz] and an elliptical spin- spiral order of spins within the chains of MnO6 octahedra. With increasing Co2+ concentration the propagation vector kz changes from 0.453 (x = 0) to 0.516 (x = 2.4). The magnetic anisotropy changes as well, leading to a reorientation of the spiral-basal plane. A possible coexistence of long-range order of electrical dipoles and magnetic moments in Mn3-xCoxTeO6 is discussed.
    Materials Research Bulletin. 10/2013; 50.
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    ABSTRACT: Several M3 TeO6 (M = Mn, Co, Ni, Cu) oxides order antiferromagnetically at low temperatures (near or below 60 K), while displaying interesting dielectric properties at high temperatures (ferroelectricity below 1000 K in M = Ni case). We have investigated and analyzed the structural and magnetic properties of Mn-doped Co3 TeO6 and Ni3 TeO6, which order antiferromagnetically at temperatures higher than their undoped counterparts.
    Physics of Condensed Matter 08/2013; 86(8). · 1.28 Impact Factor
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    ABSTRACT: Monte Carlo simulations of the dynamic magnetic behavior of an assembly of ferromagnetic core/antiferromagnetic shell nanoparticles are reported and compared with the experimental results on a system of Co nanoparticles in Mn matrix. Memory effects on low-field zero-field-cooled magnetization curves have been investigated. Our simulations show that the memory effects increase with the concentration and that both the interface exchange coupling and the dipolar interparticle interactions contribute to the observed dynamic behavior. In particular the interface exchange interaction provides an additive source for the frustration of the system resulting in an enhancement of the memory effect. The numerical data reproduce well the experimental results confirming the glassy behavior of the investigated nanoparticle systems.
    Physical Review B 08/2013; 88(14):140402. · 3.66 Impact Factor
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    ABSTRACT: We here confirm the occurrence of spin glass phase transition and extract estimates of associated critical exponents of a highly monodisperse and densely compacted system of bare maghemite nanoparticles. This system has earlier been found to behave like an archetypal spin glass, with e.g. a sharp transition from paramagnetic to non-equilibrium behavior, suggesting that this system undergoes a spin-glass phase transition at a relatively high temperature, $T_g$ $\sim$ 140 K.
    EPL (Europhysics Letters) 06/2013; 102(6). · 2.26 Impact Factor
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    ABSTRACT: The annealing-induced formation of (Mn, Ga)As nanocrystals in (Ga, Mn)As/GaAs superlattices was studied by x-ray diffraction, transmission electron microscopy and magnetometry. The superlattice structures with 50 Å thick (Ga, Mn)As layers separated by 25, 50 and 100 Å thick GaAs spacers were grown by molecular beam epitaxy at low temperature (250 ° C), and then annealed at high temperatures of 400, 560 and 630 ° C. The high-temperature annealing causes decomposition to a (Ga, Mn)As ternary alloy and the formation of (Mn, Ga)As nanocrystals inside the GaAs matrix. The nanocrystals are confined in the planes that were formerly occupied by (Ga, Mn)As layers for the up to 560 ° C annealing and diffuse throughout the GaAs spacer layers at 630 ° C annealing. The two-dimensionally confined nanocrystals exhibit a superparamagnetic behavior which becomes high-temperature ferromagnetism (∼350 K) upon diffusion.
    Journal of Physics Condensed Matter 04/2013; 25(19):196005. · 2.22 Impact Factor
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    ABSTRACT: Due to its polymorphism, Mn2FeSbO6 can be synthesized at high pressures and temperatures as a ferrimagnetic ilmenite or an antiferromagnetic perovskite. The structural phase transformation is discussed in detail, and magnetic structures are proposed for both phases. The high-pressure Mn2FeSbO6 polymorph is a rare example of A2B'B''��O6 perovskite with solely Mn cations on the A site. Fe and Sb cations are ordered on the B sites. Theoretical calculations for the perovskite phase suggest a complex magnetic structure, holding an electronic polarization.
    Physical Review B 01/2013; 87:014408. · 3.66 Impact Factor
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    ABSTRACT: (Ba1 − x Bix )(Mn0.5 + x/2Nb0.5 − x/2)O3 perovskite solid solutions have been prepared by solid-state reactions and their physicochemical properties have been investigated. We have studied the influence of bismuth substitution for barium cations on the phase composition of the samples. Their magnetic susceptibility and electrical conductivity have been measured as functions of temperature. The composition dependence of the antiferromagnetic ordering temperature is presented.
    Inorganic Materials 01/2013; 49(5). · 0.38 Impact Factor
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    ABSTRACT: We present a detailed structural study of tensile-strained La0.7Sr0.3MnO3 thin films. We use the substrate miscut to control the number of rhombohedral variants in the films and study the in-plane order and structural distortions. Using high-resolution X-ray diffraction, we demonstrate that step-edge induced lattice modulations occur in 4-variant films, whereas periodic twinning is the dominant in-plane order for 2-variant films. We show that the in-plane twinning angle is almost completely relaxed. However, the relaxation of shear strain by the out-of-plane twinning angle and the monoclinic distortion is only partial. Furthermore, the film thickness dependence of the domain width reveals that domain formation is a universal mechanism for shear strain relaxation. Finally, we show that the structural response to the transition from the paramagnetic to the ferromagnetic phase of La0.7Sr0.3MnO3 at 345 K is smaller in 4-variant films compared to 2-variant films.
    Philosophical Magazine 01/2013; 93:1549. · 1.60 Impact Factor

Publication Stats

1k Citations
299.26 Total Impact Points

Institutions

  • 2000–2014
    • Uppsala University
      • Department of Engineering Sciences
      Uppsala, Uppsala, Sweden
  • 2013
    • Polish Academy of Sciences
      • Institute of Physics
      Warszawa, Masovian Voivodeship, Poland
  • 2012
    • Indian Institute of Science
      • Department of Solid State and Structural Chemistry Unit
      Bengalore, State of Karnataka, India
  • 2007
    • KTH Royal Institute of Technology
      • Department of Microelectronics and Applied Physics (MAP)
      Tukholma, Stockholm, Sweden
  • 2004
    • The University of Tokyo
      • Department of Applied Physics
      Tokyo, Tokyo-to, Japan