J. Stankiewicz

University of Zaragoza, Caesaraugusta, Aragon, Spain

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Publications (24)58.49 Total impact

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    ABSTRACT: We present a study of granular Co-Pt multilayers by means of high-resolution transmission electron microscopy (HRTEM), extended x-ray absorption fine structure (EXAFS), SQUID-based magnetic measurements, anomalous Hall effect (AHE), and x-ray magnetic circular dichroism (XMCD). We describe these granular films as composed of particles with a pure cobalt core surrounded by an alloyed Co-Pt interface, embedded in a Pt matrix. The alloy between the Co and Pt in these granular films, prepared by room temperature sputter deposition, results from interdiffusion of the atoms. The presence of this alloy gives rise to a high perpendicular magnetic anisotropy (PMA) in the granular films, as consequence of the anisotropy of the orbital moment in the Co atoms in the alloy, and comparable to that of highly-ordered CoPt $L{1}_{0}$ alloy films. Their magnetic properties are those of ferromagnetically coupled particles, whose coupling is strongly temperature dependent: at low temperatures, the granular sample is ferromagnetic with a high coercive field; at intermediate temperatures the granular film behaves as an amorphous asperomagnet, with a coupling between the grains mediated by the polarized Pt, and at high temperatures, the sample has a superparamagnetic behavior. The coupling/decoupling between the grains in our Co-Pt granular films can be tailored by variation of the amount of Pt in the samples.
    Physical Review B 11/2014; 90(17). DOI:10.1103/PhysRevB.90.174421 · 3.66 Impact Factor
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    ABSTRACT: We have studied the structural and physical properties of the La2-xTbxCoMnO6 series. The crystal and magnetic structures of these compounds were determined by x-ray and neutron diffraction techniques. All samples belong to the family of double perovskites with space group P21/n, but the Co/Mn ordering is not perfect, and antisite defects are formed. The concentration of these defects increases for intermediate compositions, indicating that La/Tb disorder influences the Co/Mn arrangement. A ferromagnetic ground state is established due to the strong Mn(4+)-O-Co(2+) superexchange interaction. For the intermediate compositions and at low temperature, the Co/Mn ordering is accompanied by the ordering of Tb(3+) moments in the ab-plane, indicating a mutual polarization between both sublattices. Macroscopic magnetic properties reveal that Curie temperature decreases as Tb content increases in correlation with the increase of the structural distortion. All samples show semiconducting behaviour, and overall the electrical resistivity increases with decreasing La-content. The dielectric constant (ε') has a value of around 12 at low temperatures for all samples, revealing the lack of permanent dipoles. The temperature dependence of ε' on warming exhibits a strong increase that depends heavily on the frequency of the electric field. This effect is ascribed to non-intrinsic effects such as contacts or internal barrier-layers.
    Journal of Physics Condensed Matter 09/2014; 26(38):386001. DOI:10.1088/0953-8984/26/38/386001 · 2.22 Impact Factor
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    ABSTRACT: The magnetoelectric properties of the TbMn1-xScxO3 series have been studied at low temperatures by means of heat capacity, magnetic measurements and impedance spectroscopy. TbMnO3 exhibits as expected three transitions upon lowering the temperature corresponding to the magnetic ordering of the two sublattices (Mn and Tb) and the ferroelectric transition. Ferroelectricity disappears with Sc dilution for x > 0.1 because the non-collinear magnetic arrangement is destroyed. The dilution of Mn with a non-magnetic ion is also detrimental to the magnetic ordering of both Mn and Tb sublattices. The system evolves to a magnetic glassy state for the intermediate compositions. Formal TbScO3 shows Sc-deficiency and long range magnetic ordering of Tb(3+) moments in the ab-plane brought by the direct interaction between Tb(3+) ions. This ordering is different from the one found in TbMnO3 due to the lack of magnetic coupling between Tb- and Mn-sublattices. A small substitution of Sc by Mn in TbScO3 destroys the Tb ordering giving rise to a magnetic glass behaviour. This effect is ascribed to the partial polarization of Tb sublattice by the paramagnetic Mn which competes with the direct Tb-Tb exchange.
    Journal of Physics Condensed Matter 04/2013; 25(19):195601. DOI:10.1088/0953-8984/25/19/195601 · 2.22 Impact Factor
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    ABSTRACT: A sample holder design for high temperature measurements in a commercial MPMS SQUID magnetometer from Quantum Design is presented. It fulfills the requirements for the simultaneous use of the oven and reciprocating sample option (RSO) options, thus allowing sensitive magnetic measurements up to 800 K. Alternating current susceptibility can also be measured, since the holder does not induce any phase shift relative to the ac driven field. It is easily fabricated by twisting Constantan© wires into a braid nesting the sample inside. This design ensures that the sample be placed tightly into a tough holder with its orientation fixed, and prevents any sample displacement during the fast movements of the RSO transport, up to high temperatures.
    The Review of scientific instruments 06/2012; 83(6):066106. DOI:10.1063/1.4729380 · 1.58 Impact Factor
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    ABSTRACT: We report results of structural, magnetic and electrical measurements performed on the perovskite RE1/3Sr2/3FeO3 compounds (RE = rare earth). Samples with light RE (La, Pr or Nd) show a metal-insulator-like transition coupled to an antiferromagnetic ordering upon cooling. This transition is believed to be brought about by a charge ordering of Fe3+ and Fe5+ions in the low temperature phase accompanied by the concomitant structural transition. The replacing of light RE by heavy ones (Gd or Eu) gives rise to high-resistivity compounds with no metal-insulator transition. However, the magnetic transition remains at low temperatures. A previous Mössbauer study reports similar behaviour for RE=Eu compound. This suggests that variations in Mössbauer spectra may be of a magnetic origin in light RE compounds and not coming from a full charge disproportionation of Fe4+ as deduced previously.
    Journal of Physics Conference Series 01/2010; 200(1). DOI:10.1088/1742-6596/200/1/012015
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 11/2009; 40(46). DOI:10.1002/chin.200946011
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    ABSTRACT: We substitute La for Sr in the Sr2FeReO6 double perovskite to see how electron doping affects its properties. Sr2−xLaxFeReO6 compounds can be prepared up to x = 0.5. Beyond this value, a secondary phase is formed. The replacement of Sr by La leads to an increase of the unit cell volume and to a rise of the Curie temperature. However, both the saturated magnetic moment and the room-temperature magnetoresistance decrease with increasing the La content. The substituted compounds are magnetically hard and show maxima in the electrical resistivity at field values close to the coercive field. Our structural study reveals a low content of anti-site defects for all samples studied. The average Fe–O bond lengths remain almost constant upon doping whereas the Re–O distances increase as x increases. This suggests that electrons mainly go to the Re orbitals.Graphical abstract
    Solid State Sciences 05/2009; 11(9):1535-1541. DOI:10.1016/j.solidstatesciences.2009.06.026 · 1.68 Impact Factor
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    ABSTRACT: We report the growth of Sr2/3Ln1/3FeO3 single crystals (Ln=La, Pr, and Nd) by means of a floating zone melting method under a moderately high oxygen pressure (2–3 bar). We find that the boules are free of impurities or secondary phases but single crystals are only obtained using low growing rates. The La-based compounds crystallize in a rhombohedral structure, whereas crystals with Pr and Nd are orthorhombic. The as-grown crystals, which are oxygen deficient, exhibit smeared magnetic and resistive transitions upon cooling. The annealing at high oxygen pressure restores oxygen stoichiometry. The oxidized crystals show sharp resistive and magnetic transitions.
    Journal of Crystal Growth 06/2008; 310(13):3247–3250. DOI:10.1016/j.jcrysgro.2008.03.021 · 1.69 Impact Factor
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    ABSTRACT: The crystal and local structures of La1−xSrxFeO3−d (0<=x<=1) samples have been studied by x-ray diffraction and x-ray absorption spectroscopy techniques. The Fe-O bond length decreases with increasing x. Accordingly, the x-ray absorption near edge spectroscopy (XANES) spectra reveal a chemical shift of the iron K edge to higher energies. Both results agree with an Fe valence increase as La is substituted with Sr. Extended x-ray absorption fine structure spectroscopy and XANES show that the chemical state of Fe atoms in intermediate compositions can be described either by a bimodal distribution of formal Fe3+ and Fe4+ ions or by an Fe3.x+ intermediate valence. The large value of the Debye-Waller factors obtained for intermediate compositions indicates that hole doping produces local disorder around the Fe ions. These factors show unusually large values below the metal-insulator (MI) transition for x=2/3 or 3/4. We show that a significant charge disproportionation of the type 2Fe4+→Fe3++Fe5+ cannot account for the local structure observed below the MI transition temperature of these samples. We suggest that an electronic localization arises from an order-disorder transition between dynamic and static distortions, resulting in the opening of a gap at the Fermi level.
    Physical Review B 02/2008; 77(5):054107. DOI:10.1103/PhysRevB.77.054107 · 3.66 Impact Factor
  • J Stankiewicz, F Luis, A Camon, M Kroll, J Bartolome, W Blau
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    ABSTRACT: The average switching fields H-c of Fe nanowires with diameter D approximate to 12.5 nm prepared in porous alumina membranes have been measured down to 300 mK, using SQUID and micro-Hall magnetometry. On cooling from T = 300 to 5 K, H-c increases monotonically as predicted for thermally-activated magnetization-reversal models. Both the zero-temperature limit of H-c and the activation energy obtained are much smaller than those predicted for rotation at unison. Below 5 K, H-c shows a maximum, which we attribute to a crossover to a different reversal mechanism. (C) 2003 Elsevier B.V. All rights reserved.
    Journal of Magnetism and Magnetic Materials 05/2004; 272(Part 3, SI):1637-1639. DOI:10.1016/j.jmmm.2003.12.1065 · 2.00 Impact Factor
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    ABSTRACT: We study the magnetic properties of Co clusters with diameters between 0.8 and 3nm prepared by sequential sputtering of Co and Al2O3. The particle size distribution has been determined from the equilibrium susceptibility and magnetization data. Comparing this to the distribution of activation energies, obtained from the AC susceptibility, we have determined accurately the effective anisotropy constant Keff. We find that Keff is enhanced with respect to the bulk value and it increases as the average particle size decreases.
    Journal of Magnetism and Magnetic Materials 04/2002; 242:575-577. DOI:10.1016/S0304-8853(01)01119-2 · 2.00 Impact Factor
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    ABSTRACT: We study the magnetic properties of spherical Co clusters with diameters between 0.8 nm and 5.2 nm (25–7000 atoms) prepared by sequential sputtering of Co and Al2O3. The particle size distribution has been determined from the equilibrium susceptibility and magnetization data and it is compared with previous structural characterizations. The distribution of activation energies has been independently obtained from a scaling plot of the ac susceptibility. Combining these two distributions we have accurately determined the effective anisotropy constant Keff. We find that Keff is enhanced with respect to the bulk value and that it is dominated by a strong anisotropy induced at the surface of the clusters. Interactions between the magnetic moments of adjacent layers are shown to increase the effective activation energy barrier for the reversal of the magnetic moments. Finally, this reversal process is shown to proceed classically down to the lowest temperature investigated (1.8 K).
    Physical Review B 02/2002; 65(9). DOI:10.1103/PhysRevB.65.094409 · 3.66 Impact Factor
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    Acta Physica Polonica Series a 01/2002; 101(2):289. · 0.60 Impact Factor
  • J. Stankiewicz, L. Morellon, PA Algarabel, MR Ibarra
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    ABSTRACT: We have measured the Hall effect of polycrystalline Gd5(Si1.8Ge2.2) in the temperature range 4-360 K and in magnetic fields of up to 12 T. The Hall resistivity follows the magnetization of the material in the range studied. The anomalous Hall coefficient increases with increasing temperature and shows a sharp dip at the magnetostructural transition. Away from the dip, the Hall coefficient scales roughly with the square of the total resistivity, as expected if side-jump scattering dominates.
    Physical Review B 05/2000; 611(19):75345-12653. DOI:10.1103/PhysRevB.61.12651 · 3.66 Impact Factor
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    ABSTRACT: Magnetic and electrical transport properties of SmNi1-xCoxO3 perovskites, with 0<~x<~1, have been studied in the temperature range 5<~T<~700K. This series shows paramagnetic metal, paramagnetic insulator, antiferromagnetic insulator, and spin-glass-like insulator regions. Ferromagnetic interactions between different 3d ions show up at intermediate x concentrations, 0.3<~x<~0.8. We found a large negative magnetoresistance in the same region. A dynamic mechanism with spin memory, a type of double exchange interaction, can give rise to the observed magnetoresistance and spin-glass behavior. The role played by both electron-electron interactions and structural disorder in electrical transport properties of these systems is discussed.
    Physical Review B 05/1999; 59(22):14424-14430. DOI:10.1103/PhysRevB.59.14424 · 3.66 Impact Factor
  • J. Bartolomé, J. Stankiewicz
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    ABSTRACT: The electrical resistivity of R2Fe14C (R = Lanthanide series) polycrystalline samples was measured over the temperature range of 4–700 K. At low temperatures the scattering of electrons is caused by spin-wave modes involving all spins. In the paramagnetic region, T > Tc, the magnetic resistivity saturates. Finally, the total resistivity increases linearly with temperature due to phonon scattering.
    Journal of Magnetism and Magnetic Materials 05/1999; 196:776-778. DOI:10.1016/S0304-8853(98)00956-1 · 2.00 Impact Factor
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    ABSTRACT: SmNiO3 synthesized under high oxygen pressure has been studied from 1.8 to 650 K by means of both, DC magnetization and AC calorimetry. This compound shows a metal-insulator transition at 400 K and an antiferromagnetic ordering at lower temperatures. The DC magnetic susceptibility and the heat capacity clearly show the magnetic ordering at 220 K. In addition, a change in the slope of the M(T) curve is detected at the metal-insulator transition. The heat capacity measurements give an entropy content for the magnetic transition lower than the expected for a S=1/2 magnetic ion. This result suggests strong antiferromagnetic correlations above TN in this material.
    Journal of Magnetism and Magnetic Materials 05/1999; 196(1-3):541-542. DOI:10.1016/S0304-8853(98)00884-1 · 2.00 Impact Factor
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    ABSTRACT: Nd2Fe14-xSixB (x=0.25, 0.5, 0.75, 1.0, 1.5, 2.0) intermetallic compounds were investigated by magnetometry, 57Fe Mössbauer effect, electric resistivity, and differential scanning calorimetry mainly in the vicinity of the spin reorientation temperature, TSR. The Curie temperature, TC, and TSR were determined for different compositions and the spin phase diagram was constructed. 57Fe Mössbauer spectra showed noticeable changes in the reorientation region. For temperatures above the transition, the spectra were analysed using 6 Zeeman patterns associated with 6 inequivalent crystal sites in Nd2Fe14B according to site occupations (16k1:16k2:8j1:8j2:4c:4e). In the transition region, 12 subspectra had to be incorporated in the analysis. Electric resistivity measurements showed a typical metallic behaviour with no anomalies in the magnetic transition region. Differential scanning calorimetry measurements revealed broad endothermic two stage peaks around TSR.
    Acta Physica Polonica Series a 02/1999; · 0.60 Impact Factor
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    ABSTRACT: has been synthesized under high oxygen pressure from a precursor prepared by means of a sol-gel method. The material obtained is of single phase and has an orthorhombic crystal structure at room temperature. The magnetic, electrical and thermal properties have been measured over a wide temperature range. This compound exhibits a metal-insulator and a paramagnetic-antiferromagnetic transition at 400 K and at 220 K, respectively. The metal-insulator transition is of first order while the magnetic ordering is a second order transition as can be inferred from calorimetric measurements. In addition, we observe an anomaly in the resistivity near the Néel temperature indicating a change in the conduction mechanism at this temperature. Our results support local antiferromagnetic correlations above the Néel temperature for this compound.
    Journal of Physics Condensed Matter 12/1998; 11(2):405. DOI:10.1088/0953-8984/11/2/007 · 2.22 Impact Factor
  • J Pérez, J Stankiewicz, J Blasco, M Castro, J García
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    ABSTRACT: The perovskite oxides have been synthesized for under high oxygen pressure using a sol - gel method. Their structural, electrical, magnetic and calorimetric properties have been measured over a wide temperature range (4 - 300 K). All of the samples have an orthorhombic unit cell which does not change with Cu content. Nevertheless, their electrical properties change drastically with x from the very lowest values of x. The metal - insulator transition temperature decreases as x increases, up to x = 0.03; for the system is metallic for all temperatures. Moreover, the replacing of Ni with Cu atoms leads to magnetic disorder in the Ni sublattice.
    Journal of Physics Condensed Matter 12/1998; 8(49):10393. DOI:10.1088/0953-8984/8/49/028 · 2.22 Impact Factor

Publication Stats

510 Citations
58.49 Total Impact Points

Institutions

  • 1998–2014
    • University of Zaragoza
      • • Department of Condensed Matter Physics
      • • Faculty of Sciences (CIENCIAS)
      Caesaraugusta, Aragon, Spain
  • 1997–1998
    • Spanish National Research Council
      • Institut de Ciència de Materials de Barcelona
      Madrid, Madrid, Spain