K. McEwen | University College London (original) (raw)
Papers by K. McEwen
Journal of Physics F: Metal Physics, 1975
... Rec. B10 197%93 Jordan R G and Jones D W 1973 J . Less-Common Metals 31 125-41 Nigh H E, Legv... more ... Rec. B10 197%93 Jordan R G and Jones D W 1973 J . Less-Common Metals 31 125-41 Nigh H E, Legvold S and Spedding F H 1963 Phys. Rev. 132 1092-7 Roeland LW, Cock G J and Lindgird PA 1975a J . Phys.-C: Solid St. Phys. ...
Physica B: Condensed Matter, 1990
ABSTRACT
Journal of Magnetism and Magnetic Materials, 1986
35 are reported for T 2 3 K. The magnetic structure of these dhcp alloys is simpler than that of ... more 35 are reported for T 2 3 K. The magnetic structure of these dhcp alloys is simpler than that of pure Nd: only satellites with q,, = 0.13 r,,Ir, were found. The structure has moments on both the hexagonal and cubic sites.
Journal of Physics: Condensed Matter, 2000
ABSTRACT We have performed powder neutron diffraction experiments to examine the magnetic orderin... more ABSTRACT We have performed powder neutron diffraction experiments to examine the magnetic ordering in Ce3Pd20Ge6, which exhibits ferroquadrupolar ordering at TQ1≈1.2 K and antiferromagnetic ordering at TN2 = 0.75 K. In the series of isostructural cubic rare-earth (R) compounds R3Pd20X6 (X = Si, Ge; space group Fmm), it is common that at the Néel temperature TN1 the magnetic moments at the 8c site (simple cubic sublattice) order in an antiferromagnetic sequence with propagation vector k1 = [1,1,1] and at TN2 the magnetic moments at the 4a site (face-centred cubic sublattice) order with a propagation vector k2 = [0,0,1]. In Ce3Pd20Ge6 the Néel temperature TN1 is replaced by TQ1 and at the 8c site a possible ordered cerium moment is found to be reduced to an undetectable small value down to the lowest measured temperature T = 0.05 K. In contrast, at the 4a site the cerium moments order below TN2 with the expected antiferromagnetic structure. The moments align perpendicular to the propagation vector k2 = [0,0,1] with a saturation value µ2(4a) = (1.1±0.1) µB/Ce that is reasonably large for the crystal field ground-state quartet Γ8. Our results provide indirect experimental evidence that only the quadrupolar moments of cerium at the 8c site are involved in the phase transition at TQ1.
Physica B: Condensed Matter, 2005
ABSTRACT We have measured the magnetic susceptibility and magnetisation of the double-hexagonal c... more ABSTRACT We have measured the magnetic susceptibility and magnetisation of the double-hexagonal close-packed (DHCP) phase of NpPd3 from T=2–300K in magnetic fields up to 7T. Our results clearly indicate the presence of two phase transitions in this compound, at 10 and 30K. At higher temperatures, the susceptibility exhibits Curie–Weiss behaviour, with an effective moment of 2.8μB/Np atom. This result implies that the Np ions are trivalent, with a 5f4 electronic configuration. Below the 30K transition, the magnetisation of NpPd3 displays hysteresis in fields below 0.1T with a residual ferromagnetic moment of the order of 0.06μB/Np atom.
Journal of Magnetism and Magnetic Materials, 2007
In the last couple of decades there has been considerable interest in the unique properties of ma... more In the last couple of decades there has been considerable interest in the unique properties of matter in the vicinity of a quantum critical point, and therefore also in the mechanisms through which quantum phase transitions are generated. UPd 3 is a canonical example of a system displaying long range antiferroquadrupolar order. We are investigating the evolution of the quadrupolar order in UPd 3 with neptunium doping. We have found that the order is sensitive to low Np concentrations without the lattice being strongly disrupted, and anticipate that at some critical concentration the quadrupolar transition will be suppressed to zero temperature producing a quadrupolar quantum critical point. Here we present susceptibility and resistivity data for (U 0:95 Np 0:05
Journal of Applied Physics, 2012
We performed inelastic neutron scattering spectroscopy on the Heusler intermetallic compound UPd2... more We performed inelastic neutron scattering spectroscopy on the Heusler intermetallic compound UPd2Sn. The presence of a clear intramultiplet transition between the two lowest-lying 5f electronic states of uranium was clarified, and is consistent with predictions made in the framework of a localized-electron model. The excess molar specific heat is well reproduced by a Schottky curve corresponding to the experimentally measured energy gap if a moderately enhanced electronic contribution is taken into account.
Journal of Magnetism and Magnetic Materials, 1992
Physical Review B, 1997
We have studied the temperature dependence of the photoemission spectra of La 1Ϫx Sr x MnO 3 (xϭ0... more We have studied the temperature dependence of the photoemission spectra of La 1Ϫx Sr x MnO 3 (xϭ0.0, 0.2, and 0.4͒ and found that the spectral line shape dramatically changes in the entire valence-band region, particularly for xϭ0.2 and 0.4. By contrast, the spectra of La 0.6 Sr 0.4 CoO 3 show no significant temperature dependence. From comparison between the temperature-and composition-(x) dependent spectral changes and the temperature-composition phase diagram of La 1Ϫx Sr x MnO 3 , we suggest that the changes are related to the degree of hole localization on oxygen p orbitals, which is influenced by electron-lattice coupling and magnetic correlations. ͓S0163-1829͑97͒04238-0͔
The effect of the crystal field levels on the magnetoresistance of Pr is calculated numerically o... more The effect of the crystal field levels on the magnetoresistance of Pr is calculated numerically on a simple model and agreement is shown with experimental measurements at 4.2K up to 20 tesla. Results at higher temperatures are discussed.
Journal of Physics F: Metal Physics, 1983
ABSTRACT The authors evaluate the magnetoelastic contributions to the coefficients in the free en... more ABSTRACT The authors evaluate the magnetoelastic contributions to the coefficients in the free energy which determine whether the ground state of neodymium is single Q, double Q or triplet Q. They also develop a model free energy which leads to a second-order polarisation and wavevector reorientation phase transition at a temperature somewhat below the Neel transition.
Physica B: Condensed Matter, 2002
Crystal field (CF) excitations in the ternary intermetallic uranium compound U 3 Pd 20 Si 6 have ... more Crystal field (CF) excitations in the ternary intermetallic uranium compound U 3 Pd 20 Si 6 have been measured using inelastic neutron scattering. Three inelastic peaks were observed at energy transfers B12, B23 and B32 meV. This is direct evidence that the 5f electrons in this material are well localized. From this result and the bulk properties of U 3 Pd 20 Si 6 ; possible CF level schemes are discussed. The strong magnetic excitation below the antiferromagnetic phase transition temperature of 19 K is also reported.
Journal of Physics: Condensed Matter, 1995
ABSTRACT The magnetization of NdCu2 has been measured from 0.3 to 7.0 K, in fields up to 12 T. Th... more ABSTRACT The magnetization of NdCu2 has been measured from 0.3 to 7.0 K, in fields up to 12 T. The correspondence of the magnetization with a zero-field neutron diffraction study is considered. A model is proposed in order to explain the various magnetic phases observed in the magnetization data.
Journal of Magnetism and Magnetic Materials, 1988
With spallation neutron sources, it is now feasible to observe intermultiplet transitions in the ... more With spallation neutron sources, it is now feasible to observe intermultiplet transitions in the rare-earth metals at energies up to about 1 eV. As well as measuring the dipolar ./ + J + 1 transition in metallic praseodymium and neodymium. we have also identified several non-dipolar. inter-term transitions in Pr. The energies of the inter-term transitions are significantly reduced by approximately 50 meV from their free-ion values. This is ascribed to an increase in the conduction-electron screening of the on-site Coulomb interaction between 4f electrons in rare-earth metals.
Physical Review B, 2002
The physical properties of Tm x La 1Ϫx Te for xϭ0.05 and 0.5 have been investigated by electrical... more The physical properties of Tm x La 1Ϫx Te for xϭ0.05 and 0.5 have been investigated by electrical resistivity, magnetic susceptibility, magnetization, and specific heat measurements and by inelastic neutron scattering. It is shown that the Tm ion experiences a large crystalline electric field ͑CEF͒ in a metallic environment at a low concentration of xϭ0.05. The level scheme for xϭ0.05 is determined to be ⌫ 7 Ϫ⌫ 8 (10 meV) Ϫ⌫ 6 (17.6 meV) by neutron scattering, which is consistent with the bulk properties. However, this scheme is completely opposite to the one predicted from the point charge model. Furthermore, since the Tm ions in an insulating system of Tm x Yb 1Ϫx Te do not show such a big CEF, the present result indicates that the conduction electrons play an important role in determining the strength of the CEF. The ⌫ 7 →⌫ 8 excitation is split into some peaks. This is associated with the local structure around the Tm ions: when the number of other Tm ions in the first-nearest-neighbor rare-earth sites increases, the excitation energy decreases. At xϭ0.5, the CEF excitation disappears and only a broad quasielastic scattering is observed. The excitation spectrum is only reproduced by two Gaussian spectral functions both centered at zero energy.
Journal of Physics: Condensed Matter, 2003
ABSTRACT We have made inelastic neutron scattering measurements on the doped Kondo insulators Ce(... more ABSTRACT We have made inelastic neutron scattering measurements on the doped Kondo insulators Ce(Rh0.8Pd0.2)Sb and (Ce0.7Th0.3)RhSb. Previous bulk property measurements showed non-Fermi liquid (NFL) behaviour for both compositions. In our inelastic neutron scattering data these two compounds also show E/T scaling behaviour in the dynamical spin susceptibility with different exponents, which is considered a characteristic feature of NFL systems. We discuss how these two different exponents might be related to different microscopic mechanisms of the NFL behaviour in the bulk properties as well as the E/T scaling behaviour seen in the dynamical susceptibility.
Journal of Physics: Condensed Matter, 2002
We have measured the Knight shift and inhomogeneous line broadening of positive muons implanted i... more We have measured the Knight shift and inhomogeneous line broadening of positive muons implanted in monocrystalline UPd3 from 2 K up to 300 K. We find two components in the transverse-field (Hext = 0.6 T) precession signal with amplitude ratio 2:1, which is independent of temperature up to 300 K. The two signals are associated with two different muon sites
Journal of Physics: Condensed Matter, 2003
The x-ray resonant scattering cross-section of K 2 ReCl 6 has been studied for energies near the ... more The x-ray resonant scattering cross-section of K 2 ReCl 6 has been studied for energies near the L edges of rhenium. Below the Néel temperature of T N ∼ 11 K, additional peaks appear when the photon energy is tuned to the L II or L III edges. The peaks are found to rotate the incident (linear) polarization, and have a Lorentzian lineshape in energy. The peaks are ascribed to resonant magnetic scattering, where a dipolar transition connects 2p-5d states. At resonance the scattering amplitude is of order ∼r 0 per atom, roughly two orders of magnitude greater than that of the bare magnetic scattering.
Journal of Applied Physics, 1996
We have performed a detailed study of the magnetization of Y0.6U0.4Pd3 (Tf∼10 K), which shows the... more We have performed a detailed study of the magnetization of Y0.6U0.4Pd3 (Tf∼10 K), which shows thermal, transport, and magnetic properties typical of a spin-glass system. Strong irreversibility is observed among our zero-field-cooled and field-cooled measurements in fields as large as 30 kOe. Thus, the freezing temperature displays an unusual weak dependence with field. We observe a very slow temporal dependence
Journal of Physics F: Metal Physics, 1975
... Rec. B10 197%93 Jordan R G and Jones D W 1973 J . Less-Common Metals 31 125-41 Nigh H E, Legv... more ... Rec. B10 197%93 Jordan R G and Jones D W 1973 J . Less-Common Metals 31 125-41 Nigh H E, Legvold S and Spedding F H 1963 Phys. Rev. 132 1092-7 Roeland LW, Cock G J and Lindgird PA 1975a J . Phys.-C: Solid St. Phys. ...
Physica B: Condensed Matter, 1990
ABSTRACT
Journal of Magnetism and Magnetic Materials, 1986
35 are reported for T 2 3 K. The magnetic structure of these dhcp alloys is simpler than that of ... more 35 are reported for T 2 3 K. The magnetic structure of these dhcp alloys is simpler than that of pure Nd: only satellites with q,, = 0.13 r,,Ir, were found. The structure has moments on both the hexagonal and cubic sites.
Journal of Physics: Condensed Matter, 2000
ABSTRACT We have performed powder neutron diffraction experiments to examine the magnetic orderin... more ABSTRACT We have performed powder neutron diffraction experiments to examine the magnetic ordering in Ce3Pd20Ge6, which exhibits ferroquadrupolar ordering at TQ1≈1.2 K and antiferromagnetic ordering at TN2 = 0.75 K. In the series of isostructural cubic rare-earth (R) compounds R3Pd20X6 (X = Si, Ge; space group Fmm), it is common that at the Néel temperature TN1 the magnetic moments at the 8c site (simple cubic sublattice) order in an antiferromagnetic sequence with propagation vector k1 = [1,1,1] and at TN2 the magnetic moments at the 4a site (face-centred cubic sublattice) order with a propagation vector k2 = [0,0,1]. In Ce3Pd20Ge6 the Néel temperature TN1 is replaced by TQ1 and at the 8c site a possible ordered cerium moment is found to be reduced to an undetectable small value down to the lowest measured temperature T = 0.05 K. In contrast, at the 4a site the cerium moments order below TN2 with the expected antiferromagnetic structure. The moments align perpendicular to the propagation vector k2 = [0,0,1] with a saturation value µ2(4a) = (1.1±0.1) µB/Ce that is reasonably large for the crystal field ground-state quartet Γ8. Our results provide indirect experimental evidence that only the quadrupolar moments of cerium at the 8c site are involved in the phase transition at TQ1.
Physica B: Condensed Matter, 2005
ABSTRACT We have measured the magnetic susceptibility and magnetisation of the double-hexagonal c... more ABSTRACT We have measured the magnetic susceptibility and magnetisation of the double-hexagonal close-packed (DHCP) phase of NpPd3 from T=2–300K in magnetic fields up to 7T. Our results clearly indicate the presence of two phase transitions in this compound, at 10 and 30K. At higher temperatures, the susceptibility exhibits Curie–Weiss behaviour, with an effective moment of 2.8μB/Np atom. This result implies that the Np ions are trivalent, with a 5f4 electronic configuration. Below the 30K transition, the magnetisation of NpPd3 displays hysteresis in fields below 0.1T with a residual ferromagnetic moment of the order of 0.06μB/Np atom.
Journal of Magnetism and Magnetic Materials, 2007
In the last couple of decades there has been considerable interest in the unique properties of ma... more In the last couple of decades there has been considerable interest in the unique properties of matter in the vicinity of a quantum critical point, and therefore also in the mechanisms through which quantum phase transitions are generated. UPd 3 is a canonical example of a system displaying long range antiferroquadrupolar order. We are investigating the evolution of the quadrupolar order in UPd 3 with neptunium doping. We have found that the order is sensitive to low Np concentrations without the lattice being strongly disrupted, and anticipate that at some critical concentration the quadrupolar transition will be suppressed to zero temperature producing a quadrupolar quantum critical point. Here we present susceptibility and resistivity data for (U 0:95 Np 0:05
Journal of Applied Physics, 2012
We performed inelastic neutron scattering spectroscopy on the Heusler intermetallic compound UPd2... more We performed inelastic neutron scattering spectroscopy on the Heusler intermetallic compound UPd2Sn. The presence of a clear intramultiplet transition between the two lowest-lying 5f electronic states of uranium was clarified, and is consistent with predictions made in the framework of a localized-electron model. The excess molar specific heat is well reproduced by a Schottky curve corresponding to the experimentally measured energy gap if a moderately enhanced electronic contribution is taken into account.
Journal of Magnetism and Magnetic Materials, 1992
Physical Review B, 1997
We have studied the temperature dependence of the photoemission spectra of La 1Ϫx Sr x MnO 3 (xϭ0... more We have studied the temperature dependence of the photoemission spectra of La 1Ϫx Sr x MnO 3 (xϭ0.0, 0.2, and 0.4͒ and found that the spectral line shape dramatically changes in the entire valence-band region, particularly for xϭ0.2 and 0.4. By contrast, the spectra of La 0.6 Sr 0.4 CoO 3 show no significant temperature dependence. From comparison between the temperature-and composition-(x) dependent spectral changes and the temperature-composition phase diagram of La 1Ϫx Sr x MnO 3 , we suggest that the changes are related to the degree of hole localization on oxygen p orbitals, which is influenced by electron-lattice coupling and magnetic correlations. ͓S0163-1829͑97͒04238-0͔
The effect of the crystal field levels on the magnetoresistance of Pr is calculated numerically o... more The effect of the crystal field levels on the magnetoresistance of Pr is calculated numerically on a simple model and agreement is shown with experimental measurements at 4.2K up to 20 tesla. Results at higher temperatures are discussed.
Journal of Physics F: Metal Physics, 1983
ABSTRACT The authors evaluate the magnetoelastic contributions to the coefficients in the free en... more ABSTRACT The authors evaluate the magnetoelastic contributions to the coefficients in the free energy which determine whether the ground state of neodymium is single Q, double Q or triplet Q. They also develop a model free energy which leads to a second-order polarisation and wavevector reorientation phase transition at a temperature somewhat below the Neel transition.
Physica B: Condensed Matter, 2002
Crystal field (CF) excitations in the ternary intermetallic uranium compound U 3 Pd 20 Si 6 have ... more Crystal field (CF) excitations in the ternary intermetallic uranium compound U 3 Pd 20 Si 6 have been measured using inelastic neutron scattering. Three inelastic peaks were observed at energy transfers B12, B23 and B32 meV. This is direct evidence that the 5f electrons in this material are well localized. From this result and the bulk properties of U 3 Pd 20 Si 6 ; possible CF level schemes are discussed. The strong magnetic excitation below the antiferromagnetic phase transition temperature of 19 K is also reported.
Journal of Physics: Condensed Matter, 1995
ABSTRACT The magnetization of NdCu2 has been measured from 0.3 to 7.0 K, in fields up to 12 T. Th... more ABSTRACT The magnetization of NdCu2 has been measured from 0.3 to 7.0 K, in fields up to 12 T. The correspondence of the magnetization with a zero-field neutron diffraction study is considered. A model is proposed in order to explain the various magnetic phases observed in the magnetization data.
Journal of Magnetism and Magnetic Materials, 1988
With spallation neutron sources, it is now feasible to observe intermultiplet transitions in the ... more With spallation neutron sources, it is now feasible to observe intermultiplet transitions in the rare-earth metals at energies up to about 1 eV. As well as measuring the dipolar ./ + J + 1 transition in metallic praseodymium and neodymium. we have also identified several non-dipolar. inter-term transitions in Pr. The energies of the inter-term transitions are significantly reduced by approximately 50 meV from their free-ion values. This is ascribed to an increase in the conduction-electron screening of the on-site Coulomb interaction between 4f electrons in rare-earth metals.
Physical Review B, 2002
The physical properties of Tm x La 1Ϫx Te for xϭ0.05 and 0.5 have been investigated by electrical... more The physical properties of Tm x La 1Ϫx Te for xϭ0.05 and 0.5 have been investigated by electrical resistivity, magnetic susceptibility, magnetization, and specific heat measurements and by inelastic neutron scattering. It is shown that the Tm ion experiences a large crystalline electric field ͑CEF͒ in a metallic environment at a low concentration of xϭ0.05. The level scheme for xϭ0.05 is determined to be ⌫ 7 Ϫ⌫ 8 (10 meV) Ϫ⌫ 6 (17.6 meV) by neutron scattering, which is consistent with the bulk properties. However, this scheme is completely opposite to the one predicted from the point charge model. Furthermore, since the Tm ions in an insulating system of Tm x Yb 1Ϫx Te do not show such a big CEF, the present result indicates that the conduction electrons play an important role in determining the strength of the CEF. The ⌫ 7 →⌫ 8 excitation is split into some peaks. This is associated with the local structure around the Tm ions: when the number of other Tm ions in the first-nearest-neighbor rare-earth sites increases, the excitation energy decreases. At xϭ0.5, the CEF excitation disappears and only a broad quasielastic scattering is observed. The excitation spectrum is only reproduced by two Gaussian spectral functions both centered at zero energy.
Journal of Physics: Condensed Matter, 2003
ABSTRACT We have made inelastic neutron scattering measurements on the doped Kondo insulators Ce(... more ABSTRACT We have made inelastic neutron scattering measurements on the doped Kondo insulators Ce(Rh0.8Pd0.2)Sb and (Ce0.7Th0.3)RhSb. Previous bulk property measurements showed non-Fermi liquid (NFL) behaviour for both compositions. In our inelastic neutron scattering data these two compounds also show E/T scaling behaviour in the dynamical spin susceptibility with different exponents, which is considered a characteristic feature of NFL systems. We discuss how these two different exponents might be related to different microscopic mechanisms of the NFL behaviour in the bulk properties as well as the E/T scaling behaviour seen in the dynamical susceptibility.
Journal of Physics: Condensed Matter, 2002
We have measured the Knight shift and inhomogeneous line broadening of positive muons implanted i... more We have measured the Knight shift and inhomogeneous line broadening of positive muons implanted in monocrystalline UPd3 from 2 K up to 300 K. We find two components in the transverse-field (Hext = 0.6 T) precession signal with amplitude ratio 2:1, which is independent of temperature up to 300 K. The two signals are associated with two different muon sites
Journal of Physics: Condensed Matter, 2003
The x-ray resonant scattering cross-section of K 2 ReCl 6 has been studied for energies near the ... more The x-ray resonant scattering cross-section of K 2 ReCl 6 has been studied for energies near the L edges of rhenium. Below the Néel temperature of T N ∼ 11 K, additional peaks appear when the photon energy is tuned to the L II or L III edges. The peaks are found to rotate the incident (linear) polarization, and have a Lorentzian lineshape in energy. The peaks are ascribed to resonant magnetic scattering, where a dipolar transition connects 2p-5d states. At resonance the scattering amplitude is of order ∼r 0 per atom, roughly two orders of magnitude greater than that of the bare magnetic scattering.
Journal of Applied Physics, 1996
We have performed a detailed study of the magnetization of Y0.6U0.4Pd3 (Tf∼10 K), which shows the... more We have performed a detailed study of the magnetization of Y0.6U0.4Pd3 (Tf∼10 K), which shows thermal, transport, and magnetic properties typical of a spin-glass system. Strong irreversibility is observed among our zero-field-cooled and field-cooled measurements in fields as large as 30 kOe. Thus, the freezing temperature displays an unusual weak dependence with field. We observe a very slow temporal dependence