H. Kierspel

University of Cologne, Köln, North Rhine-Westphalia, Germany

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Publications (23)56.67 Total impact

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    ABSTRACT: We have investigated a method for substituting oxygen with nitrogen in EuO thin films, which is based on molecular beam epitaxy distillation with NO gas as the oxidizer. By varying the NO gas pressure, we produce crystalline, epitaxial EuO_(1-x)N_x films with good control over the films' nitrogen concentration. In-situ x-ray photoemission spectroscopy reveals that nitrogen substitution is connected to the formation Eu3+ 4f6 and a corresponding decrease in the number of Eu2+ 4f7, indicating that nitrogen is being incorporated in its 3- oxidation state. While small amounts of Eu3+ in over-oxidized Eu_(1-delta)O thin films lead to a drastic suppression of the ferromagnetism, the formation of Eu3+ in EuO_(1-x)N_x still allows the ferromagnetic phase to exist with an unaffected Tc, thus providing an ideal model system to study the interplay between the magnetic f7 (J=7/2) and the non-magnetic f6 (J=0) states close to the Fermi level.
    Applied Physics Letters 04/2012; 100(16). · 3.79 Impact Factor
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    ABSTRACT: The origin of the Curie temperature enhancement of EuO upon Gd doping is studied by using soft x-ray absorption spectroscopy on epitaxial pure and Gd-doped EuO thin films. Temperature- and doping-dependent changes in the oxygen K edge spectra provide information about the correlation of magnetism and lattice. Band-structure calculations reveal that these spectral changes as well as the increase of the Curie temperature to 125 K for Gd-doped EuO cannot be explained by electron doping alone. The compression of the crystal lattice due to the incorporation of the smaller Gd3+ ions plays also an important role.
    Physical review. B, Condensed matter 02/2012; 85(8). · 3.77 Impact Factor
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    ABSTRACT: We report on our study on the influence of the growth conditions on the europium/oxygen stoichiometry, morphology, magnetic properties, and electrical conductivity of EuO thin films. SQUID magnetometry and x-ray photoelectron spectroscopy were utilized as complementary techniques to determine the oxygen content of EuO1±x thin films grown by molecular beam epitaxy with and without the employment of the so-called Eu distillation process. We found indications for phase separation to occur in Eu-rich as well as in over-oxidized EuO for films grown at substrate temperatures below the Eu distillation temperature. Only a fraction of the excess Eu contributes to the metal-insulator transition in Eu-rich films grown under these conditions. We also observed that the surfaces of these films were ill defined and may even contain more Eu excess than the film average. Only EuO films grown under distillation conditions are guaranteed to have the same magnetic and electrical properties as stoichiometric bulk EuO, and to have surfaces with the proper Eu/O stoichiometry and electronic structure.
    Physical review. B, Condensed matter 10/2011; 84(15). · 3.77 Impact Factor
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    ABSTRACT: We have measured field and temperature dependent magnetization of YbInNi4 to elucidate the nature of the magnetic transition at 3 K. For small fields we find magnetic order as previously reported. In contrast to former reports, however, our high resolution magnetization measurements down to 500 mK indicate dominating antiferromagnetic exchange interactions. We discuss the presence of geometrical frustration.
    01/2011;
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    ABSTRACT: We have succeeded in preparing high-quality Gd-doped single-crystalline EuO films. Using Eu-distillation-assisted molecular beam epitaxy and a systematic variation in the Gd and oxygen deposition rates, we have been able to observe sustained layer-by-layer epitaxial growth on yttria-stabilized cubic zirconia (001). The presence of Gd helps to stabilize the layer-by-layer growth mode. We used soft x-ray absorption spectroscopy at the Eu and Gd M4,5 edges to confirm the absence of Eu3+ contaminants and to determine the actual Gd concentration. The distillation process ensures the absence of oxygen vacancies in the films. From magnetization measurements we found the Curie temperature to increase smoothly as a function of doping from 70 K up to a maximum of 125 K. A threshold behavior was not observed for concentrations as low as 0.2%.
    Physical review. B, Condensed matter 03/2009; 80(8). · 3.77 Impact Factor
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    ABSTRACT: We have succeeded in growing epitaxial and highly stoichiometric films of EuO on yttria-stabilized cubic zirconia (YSZ) (001). The use of the Eu-distillation process during the molecular beam epitaxy assisted growth enables the consistent achievement of stoichiometry. We have also succeeded in growing the films in a layer-by-layer fashion by fine tuning the Eu vs. oxygen deposition rates. The initial stages of growth involve the limited supply of oxygen from the YSZ substrate, but the EuO stoichiometry can still be well maintained. The films grown were sufficiently smooth so that the capping with a thin layer of aluminum was leak tight and enabled ex situ experiments free from trivalent Eu species. The findings were used to obtain recipes for better epitaxial growth of EuO on MgO (001). Comment: 10 pages, 15 figures
    Physical review. B, Condensed matter 02/2009; · 3.77 Impact Factor
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    ABSTRACT: We report thermal-expansion, lattice-constant, and specific-heat data of the series La1-xAxCoO3 for 0
    Physical review. B, Condensed matter 01/2009; 79(22). · 3.77 Impact Factor
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    ABSTRACT: We report thermal-expansion, lattice-constant, and specific-heat data of the series La_1-xA_xCoO_3 for 0<= x <= 0.30 with A = Ca, Sr, and Ba. For the undoped compound LaCoO_3 the thermal-expansion coefficient alpha(T) exhibits a pronounced maximum around T=50K caused by a temperature-driven spin-state transition from a low-spin state of the Co^{3+$ ions at low towards a higher spin state at higher temperatures. The partial substitution of the La^{3+} ions by divalent Ca^{2+}, Sr^{2+}, or Ba^{2+} ions causes drastic changes in the macroscopic properties of LaCoO3. The large maximum in alpha(T) is suppressed and completely vanishes for x> 0.12. For A = Ca three different anomalies develop in alpha(T) with further increasing x, which are visible in specific-heat data as well. Together with temperature-dependent x-ray data we identify several phase transitions as a function of the doping concentration x and temperature. From these data we propose an extended phase diagram for La_1-xCa_xCoO_3. Comment: 9 pages, 7 figures
    01/2008;
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    ABSTRACT: We present a study of the structure, the electric resistivity, the magnetic susceptibility, and the thermal expansion of La1−xEuxCoO3. LaCoO3 shows a temperature-induced spin-state transition around 100 K and a metal-insulator transition around 500 K. Partial substitution of La3+ by the smaller Eu3+ causes chemical pressure and leads to a drastic increase of the spin gap from about 190 K in LaCoO3 to about 2000 K in EuCoO3, so that the spin-state transition is shifted to much higher temperatures. A combined analysis of thermal expansion and susceptibility gives evidence that the spin-state transition has to be attributed to a population of an intermediate-spin state without orbital degeneracy for x<0.5 and with orbital degeneracy for larger x. In contrast to the spin-state transition, the metal-insulator transition is shifted only moderately to higher temperatures with increasing Eu content, showing that the metal-insulator transition occurs independently from the spin-state distribution of the Co3+ ions. Around the metal-insulator transition the magnetic susceptibility shows a similar increase for all x and approaches a doping-independent value around 1000 K, indicating that well above the metal-insulator transition the same spin state is approached for all x.
    Physical review. B, Condensed matter 05/2004; 71(1). · 3.77 Impact Factor
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    ABSTRACT: We present an investigation of the influence of structural distortions in charge-carrier doped La1-xMxCoO3 by substituting La3+ with alkaline-earth metals of strongly different ionic sizes, that is, M=Ca2+, Sr2+, and Ba2+, respectively. We find that both the magnetic properties and the resistivity change nonmonotonically as a function of the ionic size of M. Doping La1-xMxCoO3 with M=Sr2+ yields higher transition temperatures to the ferromagnetically ordered states and lower resistivities than doping with either Ca2+ or Ba2+ having a smaller or larger ionic size than Sr2+, respectively. From this observation we conclude that the different transition temperatures and resistivities of La1-xMxCoO3 for different M (of the same concentration x) do not only depend on the varying chemical pressures. The local disorder due to the different ionic sizes of La3+ and M2+ plays an important role, too.
    Physical review. B, Condensed matter 03/2004; 69(9). · 3.77 Impact Factor
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    ABSTRACT: Measurements of the thermal conductivity as a function of temperature and magnetic field in the 2D dimer spin system SrCu2(BO3)(2) are presented. In zero magnetic field the thermal conductivity along and perpendicular to the magnetic planes shows a pronounced double-peak structure as a function of temperature. The low-temperature maximum is drastically suppressed with increasing magnetic field. Our quantitative analysis reveals that the heat current is due to phonons and that the double-peak structure arises from pronounced resonant scattering of phonons by magnetic excitations.
    Physical Review Letters 08/2001; 87(4):047202. · 7.73 Impact Factor
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    ABSTRACT: Large single crystals of S=12 quasi-one-dimensional antiferromagnetic spin system NaV2−zTizO5 (0⩽z⩽0.06) have been successfully grown by a flux method. We present growth conditions together with a characterization of the single crystals by means of X-ray powder diffraction, energy dispersive X-ray (EDX) analysis and magnetic susceptibility measurements.
    Journal of Crystal Growth 01/2000; 210(4):646-650. · 1.55 Impact Factor
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    ABSTRACT: Measurements of the thermal-expansion coefficient, the magnetostriction, and the specific heat of Zn-doped CuGeO3 single crystals (x<~3.3%) in magnetic fields up to 16 T are presented. Measuring the lattice constant as a function of temperature, magnetic field, and doping concentration allows us to determine the concentration and field dependence of the averaged spin-Peierls order parameter. A strong reduction upon doping is observed and interpreted in terms of solitonlike defects of the dimerization. From these data the relationship between the averaged spin-Peierls order parameter in doped CuGeO3 and the transition temperature TSP is extracted. The specific heat at low temperatures is dominated by doping induced low-energy excitations. The temperature and field dependence of the corresponding contribution to the specific heat is discussed in a model with random magnetic exchange constants. Studying the thermodynamic properties as a function of an external field, we determine the magnetic field vs temperature phase diagrams of Cu1-xZnxGeO3.. Several systematic changes upon doping are revealed. In particular, we find a strong enhancement of disorder in doped CuGeO3, when entering the incommensurate phase with increasing field which we attribute to an additional randomness of the order-parameter phase. Moreover, this field-induced phase transition between dimerized and incommensurate phases is accompanied by a pronounced increase of the antiferromagnetic ordering temperature TN. A phenomenological description assuming a coupling between the spin-Peierls and the antiferromagnetic order parameters is presented, which allows us to interpret simultaneously both field and concentration dependences of TN.
    Physical Review B 02/1999; 59(10). · 3.66 Impact Factor
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    ABSTRACT: We have measured specific heat, magnetization, and electrical transport properties of La0.875Sr0.125-MnO3 single crystals in magnetic fields up to 14 T. In addition to a ferromagnetic transition two structural phase transitions are observed with one of the latter associated with charge ordering. The low temperature charge ordered state is found to be stabilized by an applied magnetic field, what cannot be understood within the scope of the double exchange mechanism, but only in terms of a subtle interplay between Jahn-Teller effect, magnetism, and charge ordering. Our results demonstrate the importance of spin-lattice and charge-lattice interactions in the manganites.
    Physical Review Letters 01/1999; 82(1):185-188. · 7.73 Impact Factor
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    ABSTRACT: We present high-resolution measurements of the specific heat and the thermal expansion of the inorganic spin-Peierls cuprate CuGeO3 doped with 1.4% Zn and 0.5% Si, respectively. Both compounds exhibit rather well-defined spin-Peierls transitions at about 11 K, which cause anomalies in both the specific-heat and the thermal-expansion coefficients. By comparing these anomalies we derive very large pressure dependencies of the transition temperatures. In particular, the pressure dependencies of the doped compounds are significantly larger than those in pure CuGeO3. From this difference we conclude that the suppression of the spin-Peierls transition in CuGeO3 by doping with Zn or Si is significantly reduced under pressure.
    Physical Review B 07/1997; 56(2). · 3.66 Impact Factor
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    ABSTRACT: The coefficients of the thermal expansion parallel (alphaa,b) and perpendicular (alphac) to the CuO2 planes as well as the specific heat (cp) of a Bi2Sr2CaCu2O8+delta single crystal have been measured. A clear anomaly at Tc is present in alphaa,b whereas no corresponding anomaly could be observed in alphac. The anomalies of cp as well as alphaa,b show pronounced fluctuations of the superconducting order parameter and in addition a small mean-field-like increase of alphaa,b below Tc can be resolved, where the relative size of the anomaly is much larger than in the specific heat. Comparing the anomalies of alphaa,b and cp an uniaxial pressure dependence for pressure parallel to the CuO2 planes of dTc/dpa,b = 0.9(1) K/GPa is determined for Bi2212, which seems to represent the 'intrinsic' pressure dependence of the high-Tc cuprates. Besides the anomaly at Tc we observe a jump-like increase of the thermal expansion parallel to the c-axis at about 120 K indicating a structural instability of Bi2Sr2CaCU2O8+delta at this temperature.>
    Physica C Superconductivity 01/1996; 262:177-186. · 0.72 Impact Factor
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    ABSTRACT: The coefficients of the thermal expansion parallel (αa,b) and perpendicular (αc) to the CuO2 planes as well as the specific heat (cp) of a Bi2Sr2CaCu2O8+δ single crystal have been measured. A clear anomaly at Tc is present in αa,b whereas no corresponding anomaly could be observed in αc. The anomalies of cp as well as αa,b show pronounced fluctuations of the superconducting order parameter and in addition a small mean-field-like increase of αa,b below Tc can be resolved, where the relative size of the anomaly is much larger than in the specific heat. Comparing the anomalies of αa,b and cp an uniaxial pressure dependence for pressure parallel to the CuO2 planes of dTc/dpa,b = 0.9 (1) K/GPa is determined for Bi2212, which seems to represent the “intrinsic” pressure dependence of the high-Tc cuprates. Besides the anomaly at Tc we observe a jump-like increase of the thermal expansion parallel to the c-axis at about 120 K indicating a structural instability of Bi2Sr2CaCu2O8+δ at this temperature.
    Physica C-superconductivity and Its Applications - PHYSICA C. 01/1996; 262(3):177-186.
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    ABSTRACT: We present high-resolution dilatometry measurements of a La1.87Sr0.13CuO4-single crystal parallel and perpendicular to the CuO2-planes in magnetic fields up to 14 Tesla. We find a strong suppression of the thermal expansion anomalies for magnetic fields parallel to the c-axis. In contrast, a magnetic field perpendicular to the c-axis affects the anomalies only weakly.
    Physica C Superconductivity 12/1994; · 0.72 Impact Factor
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    ABSTRACT: High resolution specific heat measurements of Bi2Sr2CaCu2O8+x superconductors in magnetic fields up to 16 Tesla are presented. In a polycrystalline sample the magnetic field leads to a drastic suppression and a pronounced broadening of the anomaly at Tc. In contrast, our measurements on a high quality single crystal for fields parallel to the CuO2-layers do not show any significant suppression of the specific heat at Tc. Moreover, the specific heat above Tc, which is attributed to superconducting fluctuations, decreases. As a consequence the anomaly at Tc appears to become sharper with increasing magnetic field.
    Physica C Superconductivity 01/1994; 235:1765-1766. · 0.72 Impact Factor
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    ABSTRACT: The europium valency in hexagonal EuPtP, as examined byL III-x-ray absorption and Moessbauer experiments, is a function of temperature and changes from 2.16 at 295 K to 2.40 at 4 K. In the region of the strongest temperature dependence of the valency, the compound undergoes two first order phase transitions atT 1=235 K andT 2=190 K, characterized by discontinuities in the lattice constants and in the magnetic susceptibility. In the europium Moessbauer spectra, several temperature dependent absorption lines were found, indicating a complex europium valency behaviour. Furthermore, we observe antiferromagnetic order at 8.6 K.
    Zeitschrift für Physik B Condensed Matter 05/1989; 74(2):227-232.