R. Feyerherm - Academia.edu (original) (raw)

Papers by R. Feyerherm

Since April 2005 the Hahn-Meitner-Institute is operating two new beamlines for energy dispersive ... more Since April 2005 the Hahn-Meitner-Institute is operating two new beamlines for energy dispersive diffraction experiments (EDDI) and for (resonant) magnetic scattering (MAGS) at BESSY. The source for both beamlines is a superconducting 7 T multipole wiggler which provides hard X-ray photons with energies between 4 and 150 keV. The EDDI beamline uses the white beam and is intended for residual stress measurements on small samples as well as heavy engineering parts.The MAGS beamline delivers a focussed monochromatic beam with photon fluxes in the 10 12 (s 100 mA 0.1 % bandwidth) -1 range at energies from 4 to 30 keV. It is equipped for single crystal diffraction and resonant (magnetic) scattering experiments as well as for the study of thin films, micro-, and nanostructures in materials science.

Journal of Physics Condensed Matter

ABSTRACT Er5Ir4Si10 exhibits three phase transitions upon cooling below room temperature. At TCDW... more ABSTRACT Er5Ir4Si10 exhibits three phase transitions upon cooling below room temperature. At TCDW = 151 K a combined commensurate and incommensurate superstructure develops, that has been attributed to the formation of charge density waves (CDWs). At TLI = 60 K (LI = lock-in) the superstructure becomes commensurate, and at TN = 2.8 K a state with long-range antiferromagnetic order develops. In this contribution we report the results of high-intensity, high-resolution x-ray diffraction for the temperature region encompassing all four phases. We have found that above TCDW the critical scattering of the commensurate superlattice reflections persists up to much higher temperatures than the critical scattering of the incommensurate satellites. It is argued that this finding substantiates the hypothesis in which the mechanism of the CDW transition involves a structural transition towards a twofold superstructure. The superlattice reflections are found to be broader in the lock-in phase than above TLI. This suggests that the lock-in transition results in relatively small domains, that are responsible for the broadening of the reflections. Finally, the antiferromagnetic order is observed by resonant x-ray scattering. The commensurate superlattice reflections persist down to 1.87 K, and no effect of the magnetic transition on their positions or intensities is found. Thus the magnetic order and the CDW coexist below TN in this compound.

Defect and Diffusion Forum, 2008

ABSTRACT Solid state reactions between amorphous Si and crystalline Co have been investigated by ... more ABSTRACT Solid state reactions between amorphous Si and crystalline Co have been investigated by synchrotron radiation at Bessy Berlin, Germany . The multilayered samples with 10 periods of a Si 15 nm Co 15 nm layers were produced by magnetron sputtering and isothermally heat treated at temperatures between 523 and 593 K. From the time evolution of the XRD spectra first the growth rate of the CoSi phase as well as the decay rate of the Co layer we determined at 523 and 543 K . The kinetics were described by a power law; tk, and for the growth of CoSi k 0.65 while for the loss of the Co the k 0.77 was obtained, respectively. At higher temperatures at 573 and 593 K the formation and growth of the Co2Si layer, at the expense of the Co and already existing CoSi layers, was observed with exponents of about 1 for all the above kinetics. These results, together with the results of resistance kinetics measurements, in similar multilayered as well as bi layered samples at similar temperatures, providing similar exponents will be presented. Possibility of the interface reaction control and or the effect of the diffusion asymmetry which was recently published for the interpretation of solid state reactions with non parabolic kinetics on the nanoscale will be discussed

Journal of Physics: Conference Series, 2014

ABSTRACT Resonant magnetic x-ray scattering at the Gd L2 edge is used to investigate the magnetic... more ABSTRACT Resonant magnetic x-ray scattering at the Gd L2 edge is used to investigate the magnetic order of the Gd moments in the ferroelectric phase of multiferroic GdMnO3 at low temperatures and under magnetic fields. Our findings reaffirm the important role of the Gd moments in the symmetric magnetic exchange striction responsible for ferroelectricity in this compound.

Physical Review B, 1994

Muon-spin-rotation experiments have been carried out in single crystals of the heavy-fermion comp... more Muon-spin-rotation experiments have been carried out in single crystals of the heavy-fermion compound CeRu2Si2. The muon-stopping site has been determined from an analysis of the muon Knight shift. The zero-field relaxation rate of the muon polarization exhibits a small but significant increase below ~2 K which can be suppressed in longitudinal-field experiments. This result provides definite evidence for the occurrence

Journal of Physics-Condensed Matter, 1994

57Fe Mossbauer spectroscopy on several polymers containing ferrocene reveals two valence states o... more 57Fe Mossbauer spectroscopy on several polymers containing ferrocene reveals two valence states of iron. In addition to the structure SI (the ferrocene unit) a second structure SII is detected which at low temperatures has hyperfine parameters identical to those of ferrocene (Fe(II)) but which undergoes a reversible transformation to Fe(III) with rising temperature. The excitation energy for the valence transformation is found to be about 1.5 meV, the valence fluctuation rate is slower than 10-6 s up to 300 K. We attribute the mixed valent species to cross links in the polymer consisting of two ferrocene units.

Physica B, 1994

The field dependence of the laSR relaxation function in YbBiPt, together with an anisotropic resi... more The field dependence of the laSR relaxation function in YbBiPt, together with an anisotropic resistive anomaly recently discovered by Movshovich et al., indicate that both the Yb ions and the conduction electrons undergo magnetic ordering. New evidence is also presented for a change in Yb spin-ordering or spin-dynamics below about 0.1 K.

Zeitschrift Fur Physik B-Condensed Matter, 1995

Physical Review B, 1996

Muon spin rotation (muSR) in the non-Fermi-liquid (NFL) heavy-fermion alloys UCu_4Pd, UCu_3.5Pd_1... more Muon spin rotation (muSR) in the non-Fermi-liquid (NFL) heavy-fermion alloys UCu_4Pd, UCu_3.5Pd_1.5, and CeCu_5.9Au_0.1 has been used to probe the effect of disorder on the susceptibility \chi and to search for spin freezing in UCu_5-xPd_x. Any inhomogeneity in \chi due to disorder is directly mirrored in the muSR and NMR(O. O. Bernal et al./), Phys. Rev. Lett. 75, 2023 (1995).

Hyperfine Interactions, 1997

The muon‐spin‐relaxation technique has been utilized to characterize the magnetic properties of t... more The muon‐spin‐relaxation technique has been utilized to characterize the magnetic properties of the borocarbide compounds RNi2B2C ( R=\,Tm, Er, Ho and Dy), which exhibit coexistence of superconductivity and magnetism. Some general features of their magnetic ground‐state and unusual results obtained in the Ho, Tm and Dy systems are discussed.

Physical Review B, 1997

The interplay between superconductivity and magnetism in CeCu2Si2 has been investigated by means ... more The interplay between superconductivity and magnetism in CeCu2Si2 has been investigated by means of microprobe, muon spin rotation and relaxation (muSR), and specific-heat measurements on four slightly off-stoichiometric polycrystalline samples Ce1+xCu2+ySi2. Microprobe analysis reveals that within the errors (+/-3%) the main phases of all four samples exhibit the ideal stoichiometry 1:2:2 and their relative composition varies by less than 2%.

Physica B, 2000

The intermetallic compound U2Rh3Si5 exhibits a first-order transition at 25 K into a noncollinear... more The intermetallic compound U2Rh3Si5 exhibits a first-order transition at 25 K into a noncollinear antiferromagnetic structure. At the lowest temperature the antiferromagnetic structure can be destroyed by a field of 14 T. Using the two-axis spectrometer E4 at the Hahn-Meitner-Institute, we have studied the magnetic structure of U2Rh3Si5 for various temperatures and magnetic fields around the magnetic phase boundary.

Physical Review B, 2000

ABSTRACT We report the magnetic structure of the local moment magnetism in high-quality single cr... more ABSTRACT We report the magnetic structure of the local moment magnetism in high-quality single crystals of the intermetallic compound Er5Ir4Si10, which shows two charge-density-wave (CDW) transitions (TCDW(1)=155 K and TCDW(2)=55 K). Bulk measurements demonstrate that the system has an effective Curie-Weiss magnetic moment of 9.7μB and undergoes long-range antiferromagnetic (AFM) ordering at 2.8 K. The neutron-diffraction data establish the AFM structure of the Er3+ with unequal moments at different Er sites aligned along the c axis where also the CDW transitions occur.

Physica B, 2000

The antiferromagnetic structure of the purely organic free-radical 4,5-dimethyl-1,2,4-triazole-ni... more The antiferromagnetic structure of the purely organic free-radical 4,5-dimethyl-1,2,4-triazole-nitronyl-nitroxide (¹ , "0.33 K) was determined by neutron di!raction. The observed spin arrangement gives important information about the competing ferro-and antiferromagnetic exchange pathways in this compound.

The sodium ordering in NaxCoO2 in the vicinity of room temperature is rationalised at high x in t... more The sodium ordering in NaxCoO2 in the vicinity of room temperature is rationalised at high x in terms of phase transitions between square and striped phases. A complicated hexagon-of-hexagon diraction pattern observed for

Physical Review B, 2010

Synchrotron X-ray diffraction investigations of two single crystals of sodium cobaltate NaxCoO2 f... more Synchrotron X-ray diffraction investigations of two single crystals of sodium cobaltate NaxCoO2 from different batches with composition x = 0.525 − 0.530 reveal homogeneous incommensurate sodium ordering with propagation vector (0.53 0.53 0) at room-temperature. The incommensurate (qq0) superstructure exists between 220 K and 430 K. The value of q varies between q = 0.514 and 0.529, showing a broad plateau at the latter value between 260 K and 360 K. On cooling, unusual reversible phase-segregation into two volume fractions is observed. Below 220 K, one volume fraction shows the well-known commensurate orthorhombic x = 0.50 superstructure, while a second volume fraction with x = 0.55 exhibits another commensurate superstructure, presumably with a 6a×6a×c hexagonal supercell. We argue that the commensurate-to-incommensurate transition is an intrinsic feature of samples with Na concentrations x = 0.5 + δ with δ ≈ 0.03.

Since April 2005 the Hahn-Meitner-Institute is operating two new beamlines for energy dispersive ... more Since April 2005 the Hahn-Meitner-Institute is operating two new beamlines for energy dispersive diffraction experiments (EDDI) and for (resonant) magnetic scattering (MAGS) at BESSY. The source for both beamlines is a superconducting 7 T multipole wiggler which provides hard X-ray photons with energies between 4 and 150 keV. The EDDI beamline uses the white beam and is intended for residual stress measurements on small samples as well as heavy engineering parts.The MAGS beamline delivers a focussed monochromatic beam with photon fluxes in the 10 12 (s 100 mA 0.1 % bandwidth) -1 range at energies from 4 to 30 keV. It is equipped for single crystal diffraction and resonant (magnetic) scattering experiments as well as for the study of thin films, micro-, and nanostructures in materials science.

Journal of Physics Condensed Matter

ABSTRACT Er5Ir4Si10 exhibits three phase transitions upon cooling below room temperature. At TCDW... more ABSTRACT Er5Ir4Si10 exhibits three phase transitions upon cooling below room temperature. At TCDW = 151 K a combined commensurate and incommensurate superstructure develops, that has been attributed to the formation of charge density waves (CDWs). At TLI = 60 K (LI = lock-in) the superstructure becomes commensurate, and at TN = 2.8 K a state with long-range antiferromagnetic order develops. In this contribution we report the results of high-intensity, high-resolution x-ray diffraction for the temperature region encompassing all four phases. We have found that above TCDW the critical scattering of the commensurate superlattice reflections persists up to much higher temperatures than the critical scattering of the incommensurate satellites. It is argued that this finding substantiates the hypothesis in which the mechanism of the CDW transition involves a structural transition towards a twofold superstructure. The superlattice reflections are found to be broader in the lock-in phase than above TLI. This suggests that the lock-in transition results in relatively small domains, that are responsible for the broadening of the reflections. Finally, the antiferromagnetic order is observed by resonant x-ray scattering. The commensurate superlattice reflections persist down to 1.87 K, and no effect of the magnetic transition on their positions or intensities is found. Thus the magnetic order and the CDW coexist below TN in this compound.

Defect and Diffusion Forum, 2008

ABSTRACT Solid state reactions between amorphous Si and crystalline Co have been investigated by ... more ABSTRACT Solid state reactions between amorphous Si and crystalline Co have been investigated by synchrotron radiation at Bessy Berlin, Germany . The multilayered samples with 10 periods of a Si 15 nm Co 15 nm layers were produced by magnetron sputtering and isothermally heat treated at temperatures between 523 and 593 K. From the time evolution of the XRD spectra first the growth rate of the CoSi phase as well as the decay rate of the Co layer we determined at 523 and 543 K . The kinetics were described by a power law; tk, and for the growth of CoSi k 0.65 while for the loss of the Co the k 0.77 was obtained, respectively. At higher temperatures at 573 and 593 K the formation and growth of the Co2Si layer, at the expense of the Co and already existing CoSi layers, was observed with exponents of about 1 for all the above kinetics. These results, together with the results of resistance kinetics measurements, in similar multilayered as well as bi layered samples at similar temperatures, providing similar exponents will be presented. Possibility of the interface reaction control and or the effect of the diffusion asymmetry which was recently published for the interpretation of solid state reactions with non parabolic kinetics on the nanoscale will be discussed

Journal of Physics: Conference Series, 2014

ABSTRACT Resonant magnetic x-ray scattering at the Gd L2 edge is used to investigate the magnetic... more ABSTRACT Resonant magnetic x-ray scattering at the Gd L2 edge is used to investigate the magnetic order of the Gd moments in the ferroelectric phase of multiferroic GdMnO3 at low temperatures and under magnetic fields. Our findings reaffirm the important role of the Gd moments in the symmetric magnetic exchange striction responsible for ferroelectricity in this compound.

Physical Review B, 1994

Muon-spin-rotation experiments have been carried out in single crystals of the heavy-fermion comp... more Muon-spin-rotation experiments have been carried out in single crystals of the heavy-fermion compound CeRu2Si2. The muon-stopping site has been determined from an analysis of the muon Knight shift. The zero-field relaxation rate of the muon polarization exhibits a small but significant increase below ~2 K which can be suppressed in longitudinal-field experiments. This result provides definite evidence for the occurrence

Journal of Physics-Condensed Matter, 1994

57Fe Mossbauer spectroscopy on several polymers containing ferrocene reveals two valence states o... more 57Fe Mossbauer spectroscopy on several polymers containing ferrocene reveals two valence states of iron. In addition to the structure SI (the ferrocene unit) a second structure SII is detected which at low temperatures has hyperfine parameters identical to those of ferrocene (Fe(II)) but which undergoes a reversible transformation to Fe(III) with rising temperature. The excitation energy for the valence transformation is found to be about 1.5 meV, the valence fluctuation rate is slower than 10-6 s up to 300 K. We attribute the mixed valent species to cross links in the polymer consisting of two ferrocene units.

Physica B, 1994

The field dependence of the laSR relaxation function in YbBiPt, together with an anisotropic resi... more The field dependence of the laSR relaxation function in YbBiPt, together with an anisotropic resistive anomaly recently discovered by Movshovich et al., indicate that both the Yb ions and the conduction electrons undergo magnetic ordering. New evidence is also presented for a change in Yb spin-ordering or spin-dynamics below about 0.1 K.

Zeitschrift Fur Physik B-Condensed Matter, 1995

Physical Review B, 1996

Muon spin rotation (muSR) in the non-Fermi-liquid (NFL) heavy-fermion alloys UCu_4Pd, UCu_3.5Pd_1... more Muon spin rotation (muSR) in the non-Fermi-liquid (NFL) heavy-fermion alloys UCu_4Pd, UCu_3.5Pd_1.5, and CeCu_5.9Au_0.1 has been used to probe the effect of disorder on the susceptibility \chi and to search for spin freezing in UCu_5-xPd_x. Any inhomogeneity in \chi due to disorder is directly mirrored in the muSR and NMR(O. O. Bernal et al./), Phys. Rev. Lett. 75, 2023 (1995).

Hyperfine Interactions, 1997

The muon‐spin‐relaxation technique has been utilized to characterize the magnetic properties of t... more The muon‐spin‐relaxation technique has been utilized to characterize the magnetic properties of the borocarbide compounds RNi2B2C ( R=\,Tm, Er, Ho and Dy), which exhibit coexistence of superconductivity and magnetism. Some general features of their magnetic ground‐state and unusual results obtained in the Ho, Tm and Dy systems are discussed.

Physical Review B, 1997

The interplay between superconductivity and magnetism in CeCu2Si2 has been investigated by means ... more The interplay between superconductivity and magnetism in CeCu2Si2 has been investigated by means of microprobe, muon spin rotation and relaxation (muSR), and specific-heat measurements on four slightly off-stoichiometric polycrystalline samples Ce1+xCu2+ySi2. Microprobe analysis reveals that within the errors (+/-3%) the main phases of all four samples exhibit the ideal stoichiometry 1:2:2 and their relative composition varies by less than 2%.

Physica B, 2000

The intermetallic compound U2Rh3Si5 exhibits a first-order transition at 25 K into a noncollinear... more The intermetallic compound U2Rh3Si5 exhibits a first-order transition at 25 K into a noncollinear antiferromagnetic structure. At the lowest temperature the antiferromagnetic structure can be destroyed by a field of 14 T. Using the two-axis spectrometer E4 at the Hahn-Meitner-Institute, we have studied the magnetic structure of U2Rh3Si5 for various temperatures and magnetic fields around the magnetic phase boundary.

Physical Review B, 2000

ABSTRACT We report the magnetic structure of the local moment magnetism in high-quality single cr... more ABSTRACT We report the magnetic structure of the local moment magnetism in high-quality single crystals of the intermetallic compound Er5Ir4Si10, which shows two charge-density-wave (CDW) transitions (TCDW(1)=155 K and TCDW(2)=55 K). Bulk measurements demonstrate that the system has an effective Curie-Weiss magnetic moment of 9.7μB and undergoes long-range antiferromagnetic (AFM) ordering at 2.8 K. The neutron-diffraction data establish the AFM structure of the Er3+ with unequal moments at different Er sites aligned along the c axis where also the CDW transitions occur.

Physica B, 2000

The antiferromagnetic structure of the purely organic free-radical 4,5-dimethyl-1,2,4-triazole-ni... more The antiferromagnetic structure of the purely organic free-radical 4,5-dimethyl-1,2,4-triazole-nitronyl-nitroxide (¹ , "0.33 K) was determined by neutron di!raction. The observed spin arrangement gives important information about the competing ferro-and antiferromagnetic exchange pathways in this compound.

The sodium ordering in NaxCoO2 in the vicinity of room temperature is rationalised at high x in t... more The sodium ordering in NaxCoO2 in the vicinity of room temperature is rationalised at high x in terms of phase transitions between square and striped phases. A complicated hexagon-of-hexagon diraction pattern observed for

Physical Review B, 2010

Synchrotron X-ray diffraction investigations of two single crystals of sodium cobaltate NaxCoO2 f... more Synchrotron X-ray diffraction investigations of two single crystals of sodium cobaltate NaxCoO2 from different batches with composition x = 0.525 − 0.530 reveal homogeneous incommensurate sodium ordering with propagation vector (0.53 0.53 0) at room-temperature. The incommensurate (qq0) superstructure exists between 220 K and 430 K. The value of q varies between q = 0.514 and 0.529, showing a broad plateau at the latter value between 260 K and 360 K. On cooling, unusual reversible phase-segregation into two volume fractions is observed. Below 220 K, one volume fraction shows the well-known commensurate orthorhombic x = 0.50 superstructure, while a second volume fraction with x = 0.55 exhibits another commensurate superstructure, presumably with a 6a×6a×c hexagonal supercell. We argue that the commensurate-to-incommensurate transition is an intrinsic feature of samples with Na concentrations x = 0.5 + δ with δ ≈ 0.03.