Search

The growth of single crystals of the single layered manganites with compositions La1−xSr1+xMnO4 (0⩽x⩽0.6) by the floating zone technique is reported. Several neutron and X-ray scattering techniques were used to characterize the samples. Based on these data, the growth parameters were optimized with regard to structural (i.e. mosaic spread) and chemical homogeneity of the crystals. In addition, the neutron data show strong changes of the antiferromagnetic ordering as a function of the doping level x, i.e. upon increasing the mean Mn valence in La1−xSr1+xMnO4.

The intriguing, earlier observed, temperature shift of the X-band ESR magnetic field H res was studied for powder samples of linear Cu 3 EtBz·2MeOH and Cu 3 BuNaEs·2MeOH trimeric Cu(II) complexes. For Cu 3 BuNaEs·2MeOH, H res changes dramatically at ∼100 K, where a crystallographic phase transition was found by X-ray diffraction. For Cu 3 EtBz·2MeOH, ESR measurements were performed under high pressure. It was observed that H res decreases with increasing pressure but the linewidth increases. Far-infrared absorption measurements were performed as a function of temperature for Cu 3 EtBz·2MeOH and Cu 3 BuNaEs·2MeOH. No clear change of vibration frequencies with temperature was observed. These results agree with the earlier interpretation that the temperature shift of H res is caused by the thermal averaging of ESR lines due to the short spin-lattice relaxation time.

Superstructure reflections due to charge ordering have been observed in single crystals using X-ray diffraction. The critical temperatures for the ordering process are 150 K and 180 K in samples with Sr concentrations of and , respectively. For x =0.125 the charge order transition correlates with a drastic resistivity enhancement, while a much smaller change of the resistivity is found for x = 0.15. It is argued that the onset of charge ordering in the sample with x = 0.125 is related to the suppression of co-operative Jahn-Teller distortions which is initiated by rising ferromagnetism.

We have studied the thermal conductivity κ of stripe ordering La2–xSrxNiO4. In particular, long range stripe order is well established for x = 1/3. For this composition κ is enhanced in the charge ordering phase and exhibits a clear maximum at low T. Even small deviations from this stochiometry cause a strong change of κ: the low T-maximum is suppressed and no anomaly is observed at high T. Our data confirm that the stripe phase is most stable for x = \(\tfrac{{\text{1}}}{{\text{3}}}\). Moreover, we find clear differences in the thermal conductivity for x > \(\tfrac{{\text{1}}}{{\text{3}}}\)and for x < \(\tfrac{{\text{1}}}{{\text{3}}}\), respectively, which indicates different properties of the stripe ordered phase in these two regions of the phase diagram.

We have investigated the temperature dependence of the crystal structure of Ca1.5Sr0.5RuO4 by powder and single-crystal neutron diffraction in the temperature range from 1.5 to 300 K. Both an anomaly in the temperature dependence of the c lattice parameter and the observation of characteristic superstructure reflections indicate a continuous transition from the I41/acd structure, in which the corner-sharing RuO6 octahedra are rotated about the c axis, to a low-temperature phase with an additional rotation of the octahedra around an axis lying in the RuO2 planes. The occurrence of this additional tilt distortion coincides with the previously observed change of the magnetic properties, from the nearly ferromagnetic state to the antiferromagnetic correlated state.

By inelastic neutron scattering, we have analyzed the magnetic correlations in the paramagnetic metallic region of the series Ca${}_{2\ensuremath{-}x}$Sr${}_{x}$RuO${}_{4}$, $0.2\ensuremath{\leqslant}x\ensuremath{\leqslant}0.62$. We find different contributions that correspond to two-dimensional ferromagnetic fluctuations and to fluctuations at incommensurate wave vectors ${Q}_{1}^{\mathrm{IC}}=(0.11,0,0)$, ${Q}_{2}^{\mathrm{IC}}=(0.26,0,0)$, and ${Q}_{\ensuremath{\alpha}\ensuremath{\beta}}^{\mathrm{IC}}=(0.3,0.3,0)$. These components constitute the measured response as a function of the Sr concentration $x$, of the magnetic field, and of the temperature. A generic model is applicable to metallic Ca${}_{2\ensuremath{-}x}$Sr${}_{x}$RuO${}_{4}$ close to the Mott transition, in spite of their strongly varying physical properties. The amplitude, characteristic energy, and width of the incommensurate components vary only slightly as functions of $x$, but the ferromagnetic component depends sensitively on concentration, temperature, and magnetic field. While ferromagnetic fluctuations are very strong in Ca${}_{1.38}$Sr${}_{0.62}$RuO${}_{4}$ with a low characteristic energy of 0.2 meV at $T=1.5$ K, they are strongly suppressed in Ca${}_{1.8}$Sr${}_{0.2}$RuO${}_{4}$, but reappear upon the application of a magnetic field, and form a magnon mode above the metamagnetic transition. The inelastic neutron scattering results document how the competition between ferromagnetic and incommensurate antiferromagnetic instabilities governs the physics of this system.

The magnetoelastic coupling in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ and in Ca$_{1.5}$Sr$_{0.5}$RuO$_4$ has been studied combining high-resolution dilatometer and diffraction techniques. Both compounds exhibit strong anomalies in the thermal-expansion coefficient at zero and at high magnetic field as well as an exceptionally large magnetostriction. All these structural effects, which are strongest in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$, point to a redistribution of electrons between the different $t_{2g}$ orbitals tuned by temperature and magnetic field. The temperature and the field dependence of the thermal-expansion anomalies in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ yield evidence for a critical end-point lying close to the low-temperature metamagnetic transition; however, the expected scaling relations are not well fulfilled., Comment: 17 pages, 7 figures

The crystal structure of ${\mathrm{Ca}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{RuO}}_{4}$ with $0.2\ensuremath{\le}x\ensuremath{\le}1.0$ has been studied by diffraction techniques and by high resolution capacitance dilatometry as a function of temperature and magnetic field. Upon cooling in zero magnetic field, the crystal structure and the octahedra shrink along the $c$ direction and elongate in the $a$ and $b$ planes, whereas the opposite occurs upon cooling at high field ($x=0.2$ and 0.5). These findings yield evidence for an orbital rearrangement driven by temperature and magnetic field, which accompanies the metamagnetic transition at low temperature. The temperature and magnetic-field dependencies are found to be governed by the same energy scale.

Strong resonant enhancements of the charge-order and spin-order superstructure-diffraction intensities in La1.8Sr0.2NiO4 are observed when x-ray energies in the vicinity of the Ni L2,3 absorption edges are used. The pronounced photon-energy and polarization dependences of these diffraction intensities allow for a critical determination of the local symmetry of the ordered spin and charge carriers. We found that not only the antiferromagnetic order but also the charge-order superstructure resides within the NiO2 layers; the holes are mainly located on in-plane oxygens surrounding a Ni2+ site with the spins coupled antiparallel in close analogy to Zhang-Rice singlets in the cuprates.

We studied the crystal and magnetic structure of Ca2RuO4 by different diffraction techniques under high pressure. The observed first order phase transition at moderate pressure (0.5 GPa) between the insulating phase and the metallic high pressure phase is characterized by a broad region of phase coexistence. The following structural changes are observed as function of pressure: a) a discontinuous change of both the tilt and rotation angle of the RuO6-Octahedra at this transition, b) a gradual decrease of the tilt angle in the high pressure phase (p>0.5 GPa) and c) the disappearance of the tilt above 5.5GPa leading to a higher symmetry structure. By single crystal neutron diffraction at low temperature, the ferromagnetic component of the high pressure phase and a rearrangement of antiferromagnetic order in the low pressure phase was observed., 7 pages, 5 figures

Strong resonant enhancements of the charge-order and spin-order superstructure-diffraction intensities in La1.8Sr0.2NiO4 are observed when x-ray energies in the vicinity of the Ni L2,3 absorption edges are used. The pronounced photon-energy and polarization dependences of these diffraction intensities allow for a critical determination of the local symmetry of the ordered spin and charge carriers. We found that not only the antiferromagnetic order but also the charge-order superstructure resides within the NiO2 layers; the holes are mainly located on in-plane oxygens surrounding a Ni2+ site with the spins coupled antiparallel in close analogy to Zhang-Rice singlets in the cuprates.

The crystal and magnetic structure of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{1+x}\mathrm{Mn}{\mathrm{O}}_{4}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.6)$ has been studied by diffraction techniques and high resolution capacitance dilatometry. There is no evidence for a structural phase transition related to octahedron tilting like those found in isostructural cuprates or nickelates, but there are significant structural changes induced by the variation of temperature and doping which we attribute to a rearrangement of the orbital occupation.

The crystal structure of Ca(2-x)Sr(x)RuO(4) with 0.2or = xor = 1.0 has been studied by diffraction techniques and by high resolution capacitance dilatometry as a function of temperature and magnetic field. Upon cooling in zero magnetic field, the crystal structure and the octahedra shrink along the c direction and elongate in the a and b planes, whereas the opposite occurs upon cooling at high field (x = 0.2 and 0.5). These findings yield evidence for an orbital rearrangement driven by temperature and magnetic field, which accompanies the metamagnetic transition at low temperature. The temperature and magnetic-field dependencies are found to be governed by the same energy scale.

The phase-diagram of Ca2-xSrxRuO4 has been studied by several diffraction techniques as function of concentration, temperature, pressure and magnetic field. The substitution of Sr through the smaller Ca induces a series of structural phase transitions with a strong impact on the physical properties. The spin triplet superconductor Sr2RuO4 exhibits an undistorted crystal structure, and the Mott-insulator Ca2RuO4 strong structural distortions. For intermediate structural distortions samples stay metallic but with outstanding properties. Throughout the phase diagram we find a close coupling between the crystal structure on one side and magnetic and electronic behavior on the other side.

We have studied the magnetic excitations in Ca2-xSrxRuO4, x=0.52 and 0.62, which exhibit an anomalously high susceptibility and heavy mass Fermi liquid behavior. Our inelastic neutron scattering experiments reveal strongly enhanced magnetic fluctuations around an incommensurate wave-vector (0.22,0,0) pointing to a magnetic instability. The magnetic fluctuations show no correlation in the c direction and also along the RuO2 planes the signal is extremely broad, Deltaq=0.45 A(-1). These fluctuations can quantitatively account for the high specific heat coefficient and relate to the high macroscopic susceptibility. The magnetic scattering is attributed to the d(xy) band, the active band for spin triplet superconductivity in Sr2RuO4.

In Sr2RuO4 the spin excitation spectrum is dominated by incommensurate fluctuations at q = (0.3 0.3q(z)), which arise from Fermi-surface nesting. We show that upon Ti substitution, known to suppress superconductivity, a short range magnetic order develops with a propagation vector (0.307 0.307 1). In Sr2Ru0.91Ti0.09O4, the ordered moment points along the c direction. This finding shows that superconducting Sr2RuO4 is extremely close to an incommensurate spin density wave instability.

The phase diagram of ${\mathrm{Ca}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{RuO}}_{4}$ has been studied by neutron diffraction on powder and single-crystalline samples. The experiments reveal antiferromagnetic order and structural distortions characterized by tilts and rotations of the ${\mathrm{RuO}}_{6}$ octahedra. There is strong evidence that the structural details of the isovalent samples tune the magnetic as well as the electronic behavior. In particular we observe for low Sr concentration a metal-insulator transition associated with a structural change and magnetic ordering.

We have studied the influence of a magnetic field on the thermodynamic properties of Ca 2 - x Sr x RuO 4 in the intermediate metallic region with tilt and rotational distortions ( 0.2 ⩽ x ⩽ 0.5 ) . We find strong and anisotropic thermal expansion anomalies at low temperatures, which are suppressed and even reversed by a magnetic field. The metamagnetic transition of Ca 1.8 Sr 0.2 RuO 4 is accompanied by a large magnetostriction. Furthermore, we observe a strong magnetic-field dependence of c p / T and that can be explained by magnetic fluctuations.

We discuss the magnetic excitations in several 214-ruthenates as observed by inelastic neutron scattering. In the spin-triplet superconductor Sr2RuO4 the magnetic excitation spectrum is dominated by incommensurate peaks arising from strong nesting between quasi-one-dimensional bands. These excitations are found to freeze out through minor substitution of Ru by Ti. One may conclude that Sr2RuO4 itself is already very close to spin density wave ordering. Sr2RuO4 seems to exhibit also additional, much weaker magnetic excitations; these are still incommensurate but situated much closer to the zone-center. Similar almost ferromagnetic fluctuations become strongly enhanced and dominant by replacing Sr through Ca.

Superstructure reflections at low temperatures in La1−xSr x MnO3 single crystals indicating charge order in samples containing 12.5% and 15% of Sr have been observed. The positions of the reflections in reciprocal space suggest a fourfold unit cell referring to the cubic perovskite structure. The onset temperatures for charge ordering are 150 K and 180 K for x=0.125 and x=0.15, respectively. Especially in the sample with the lower Sr concentration, the onset is correlated to a jump in resistivity measured on the same sample.