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Measurements of electron paramagnetic resonance (EPR) at 9 and 35 GHz between 2 and 300 K in single crystals and powder samples of CuGeO3 are presented. Below 14 K a large decrease in the intensity of the Cu 2+ EPR signal is observed. The data can be interpreted as due to a spin-Peierls transition. However, an alternative interpretation in terms of a simple structural transition cannot be ruled out. An opening of an energy gap of ∼25 K is calculated from the analysis.

We report the observation of an unusual electron spin resonance (ESR) signal in single crystals of Eu2CuO4. The signal appears to be associated with a resonance mode of the CuO2 planes, similar to the midfield and low-field absorptions we have reported previously [Phys. Rev. B 41, 1934 (1990)]. However, it is only observed when the projection of the applied dc magnetic field in the CuO2 plane is within a few degrees of the 〈110〉 crystallographic direction. Additionally the sample must be field cooled in the CuO2 plane, but with a component of the cooling field perpendicular to the 〈110〉 ESR observation direction. Both the field for resonance and the linewidth exhibit a 1/cos θ dependence, where θ is the angle of the applied dc field between the c axis and the 〈110〉 observation direction. Additional constraints for observation of the resonance are that the microwave rf magnetic field must have a component in the CuO2 plane, but perpendicular to the dc field. The signal disappears above ∼215 K, which we assume is associated with the antiferromagnetic ordering temperature.

The results of recent inelastic neutron scattering investigations of the magnetic excitations in La2CuO4 are reviewed. Experiments were performed using a wide range of incident neutron energies. Our study of the paramagnetic phase is consistent with a picture for the magnetic dynamics which shows conventional spin waves at higher energies and an overdamped response due to spin wave interactions at lower energies. The evolution of the magnetic dynamics with doping in the La2-x(Ba, Sr)xCuO4 system is discussed. © 1991, American Institute of Physics. All rights reserved.

We have measured the two magnon Raman scattering spectrum of magnetic La2CuO4 at pressures of up to 100 kbar. Analysis of the moments of the two magnon line shape indicates that the renormalization parameters resulting from spin fluctuations are essentially pressure independent in this pressure range. Our results provide the first direct determination of the pressure dependence of the in plane exchange coupling constant J. The pressure dependence of J is compared with that of the Néel temperature and discussed in the context of recent theories for quasi two dimensional magnetic systems. © 1991, American Institute of Physics. All rights reserved.

Polarized and unpolarized neutron scattering techniques along with x-ray diffraction have been used to study the magnetic and structural properties of a single crystal of Nd2CuO4. Long-range magnetic order of the Cu moments develops at TN=245 K, with a simple antiferromagnetic configuration of spins as found in La2NiO4, while the spin directions may be either collinear or noncollinear. Additional abrupt transitions are observed at 75 and 30 K, in which spin reorientations take place. Bragg peaks associated with the crystal structure are found at the same positions as the magnetic Bragg peaks, and indicate that a small distortion of the basic tetragonal structure has occurred above 300 K. At low temperatures the Nd moments also order antiferromagnetically (TN=1.5 K), while an additional transition of a continuous nature is observed at 150 mK.

Magnetically induced ferroelectric polarization in rare-earth RMn2O5 manganites is believed to originate from the symmetric exchange striction associated with a specific antiferromagnetic phase in the low temperature (T) region and would be irrelevant with electropoling in the high-T paramagnetic-paraelectric phase region. In this work, we demonstrate that low-T pyroelectric polarization of GdMn2O5 single crystals along the b axis in the antiferromagnetic phase exhibits remarkable dependence on the electropoling history imposed in the high-T paramagnetic-paraelectric phase. In particular, the high-T electropoling results in a reversal of ferroelectric polarization in the low-T region, which can be flopped back by the electropoling being sustained in the low-T ferroelectric region. The existence of an electrically polarizable magnetic cluster state in the high-T paramagnetic-paraelectric region is proposed based on a combination of experimental observations and first-principles calculations. An intrinsic correlation between the low-T antiferromagnetic ordering and the high-T polarizable state is discussed. The present experiments unveil the emergent phenomena on multiferroicity of RMn2O5 and suggest an alternative scenario for electrocontrol of magnetism. [ABSTRACT FROM AUTHOR]

Piezoresponse force microscopy (PFM) has emerged as a powerful tool for research in ferroelectric and piezoelectric materials. While the vertical PFM (VPFM) mode is well understood and applied at a quantitative level, the lateral PFM (LPFM) mode is rarely quantified, mainly due to the lack of a practical calibration methodology. Here by PFM imaging on a LiNbO3 180° domain wall, we demonstrate a convenient way to achieve simultaneous VPFM and LPFM calibrations. Using these calibrations, we perform a full quantitative VPFM and LPFM measurement on a (001)-cut PbTiO3 single crystal. The measured effective piezoelectric coefficients def f 33 and def f 35 together naturally provide more information on a material's local tensorial electromechanical properties. The proposed approach can be applied to a wide variety of ferroelectric and piezoelectric systems. [ABSTRACT FROM AUTHOR]

Research into the susceptibility, magnetic specific heat and electron paramagnetic resonance spectrum of the geometrically frustrated antiferromagnetic ZnCr2O4 is discussed. Particular attention is given to the anomalous behaviour of the resonance intensity below 90 K.

Measurements of the structure of hexagonal RMnO3 [R = rare earths (Ho) and Y] for temperatures significantly above the ferroelectric transition temperature (TFE) were conducted to determine the nature of the transition. The local and long range structural measurements were complemented by ab initio molecular dynamics simulations. With respect to the Mn sites in YMnO3 and HoMnO3, we find no large atomic (bond distances or thermal factors), electronic structure changes, or rehybridization on crossing TFE from local structural methods. The local symmetry about the Mn sites is preserved. With respect to the local structure about the Ho sites, a reduction of the average Ho-O bond with increased temperature is found. Ab initio molecular dynamics calculations on HoMnO3 reveal the detailed motions of all ions. Above ∼900 K there are large displacements of the Ho, O3, and O4 ions along the z axis which reduce the buckling of the MnO3/O4 planes. The changes result in O3/O4 ions moving to toward central points between pairs of Ho ions on the z axis. These structural changes make the coordination of Ho sites more symmetric thus extinguishing the electric polarization. At significantly higher temperatures, rotation of the MnO5 polyhedra occurs without a significant change in electric polarization. The Born effective charge tensor is found to be highly anisotropic at the O sites but does not change appreciably at high temperatures. [ABSTRACT FROM AUTHOR]

The local structure of the parent and doped LaFeAsO1-xFx (pnictide) compounds were studied by x-ray absorption spectroscopy and density functional methods. In the doped system, the Fe-As and Fe-Fe correlations are both well modeled by an Einstein model. For the Fe-As bonds, the Einstein temperatures are identical for the doped (11%) and undoped samples but the doped sample is found to have a lower level of static disorder. Doping is found to increases the effective Einstein temperature of Fe-Fe atomic correlation. The results suggest that the onset of superconductivity in the F doped system may be related to enhanced magnetic interactions. Density functional calculations of the total charge density reveal strong bonding between neighboring As ions but metal-like behavior in the Fe layers. It is also seen directly that the replacement of oxygen by fluorine modifies the electron charge density mainly on the Fe sites. [ABSTRACT FROM AUTHOR]

We have investigated the details of phase separation in the electron-doped manganites Sm1-xCaxMnO3 (0.8≤x≤0.92) by using temperature dependent x-ray diffraction. The lattice of the low-temperature orthorhombic phase is distorted to various extents according to the strength of the coexisting monoclinic phase. Meanwhile, the room-temperature lattice becomes more distorted due to the increasing Jahn–Teller active Mn3+ ions by Sm3+ doping. On the other hand, the substitution of Ca2+ with Sm3+ introduces the Mn3+–Mn4+ pairs, which favor the double exchange ferromagnetism. As a result of the competition between the lattice distortion and electron doping level, the modified phase diagram have been presented, including the high-temperature Griffiths phase and the ground state with competing multiphases. [ABSTRACT FROM AUTHOR]

We report a sequent phase transition using high-resolution synchrotron x-ray scattering in a single crystal Nd1/2Sr1/2MnO3. By measuring the peak profile of Bragg reflections, upon cooling, we observed an increase in the width of the Bragg reflections around the Curie temperature (252 K) corresponding to the transition from a paramagnetic to a ferromagnetic state. Below approximately 200 K, dramatic changes in the width and integrated intensity were observed. Changes continued until the formation of charge ordering with q=(12,0,0) at TCO=152 K. This charge ordering was observed to be the first order transition with a large hysteresis width of 10 K. This sequent transition is understood by the formation of different magnetic domains at different temperature ranges as that observed by neutron powder diffraction. [ABSTRACT FROM AUTHOR]

Hexagonal HoMnO3 is a frustrated antiferromagnet (TN=72 K) ferroelectric (TC=875 K) with a rich magnetic phase diagram consisting of multiple temperature- and field-dependent phases. Previously observed anomalies in the dielectric constant at magnetic phase transitions indicate strong coupling between the ferroelectricity and magnetism. Neutron-diffraction measurements in a magnetic field reveal new intermediate-field phases at low temperatures. Inelastic neutron-scattering measurements are used to establish the primary magnetic interactions and demonstrate that the spin dynamics in HoMnO3 are well described by a simple two-dimensional nearest-neighbor Heisenberg antiferromagnetic exchange J=2.44 meV and a temperature-dependent anisotropy D. [ABSTRACT FROM AUTHOR]

Thin films of Sr[sub 2]FeMoO[sub 6] have been deposited on single-crystalline and polycrystalline SrTiO[sub 3] substrates by pulsed laser deposition from a stoichiometric target. In order to obtain high-quality films, the deposition parameters were systematically optimized. The films grown under optimized conditions show properties comparable to those of bulk Sr[sub 2]FeMoO[sub 6] single crystals. Surprisingly, polycrystalline films obtained by depositing on polycrystalline SrTiO[sub 3] substrates do not show any significant low-field magnetoresistance (MR), contrary to expectations for a spin-polarized material. However, after controlled annealing-induced alteration of the grain-boundary properties, a development of the low-field MR feature is clearly observed. The occurrence of the low-field MR is accompanied by nonlinearity in the current-voltage characteristics. These results are analyzed in the light of the prevalent understanding of the properties of this double perovskite material. [ABSTRACT FROM AUTHOR]

Compounds of the form A(2-x-y)B(x)Ce(y)CuO4, where A and B may be La, Pr, Nd, Sm, Eu, Gd and Y, exhibit weak ferromagnetism (WF) related to the canting of copper moments. The compounds have lattice parameters a less than a(0), where a(0) is approximately 3.91 angstroms. The rare earth ions present in the compounds do not influence magnetic absorption.

A combination of the x-ray absorption spectroscopy and density functional approaches is employed to explain the temperature-dependent local structure exhibited by the parent and doped LaFeAs[O.sub.1-x][F.sub.x] compounds. The effective Einstein temperature of the system is shown to significantly increase because of doping.

Temperature dependent x-ray diffraction is used for examining the details of phase separation in the electron-doped manganites [Sm.sub.1-x][Ca.sub.x]Mn[O.sub.3] (0.8

A sequential phase transition is described by using high-resolution synchrotron x-ray scattering in a single crystal [Nd.sub.1/2][Sr.sub.1/]Mn[O.sub.3]. The sequent transition is studied by the formation of different magnetic domains at different temperature ranges as that seen by neutron powder diffraction.

Studies charge-ordered stripes in La1-xCaxMnO3 with x>0.5. Use of electron diffraction and electron microscopy. Modulation wave vector; Incommensurability of the charge ordering.

A temperature dependent Raman scattering study in multiferroic single crystals RMn2O5 (R=Bi, Eu, and Dy) was performed. The Raman spectra were measured in the range from 150 to 450 cm-1 involving mostly Mn–O–Mn bending vibrations, complementing our previous work in a higher frequency range involving Mn–O stretching modes. A number of studied phonons present anomalous frequency behavior below a characteristic temperature, T*∼60–65 K, such as that found for the stretching modes. The sign and magnitude of such anomalous behavior appear to be correlated with the ionic radius of R, being softening for R=Bi and hardening for R=Eu and Dy in the range between TC/TN and T*. The anomalous phonon behaviors in both bending and stretching modes are consistent with an interpretation in terms of the spin-phonon coupling in a scenario of strong magnetic correlations. [ABSTRACT FROM AUTHOR]

Thermal expansion measurements were done on single-crystal RMn2O5 (R=Tb,Dy,Ho) along the principal crystallographic axes. Distinctive anomalies were observed in the linear thermal expansivities at critical temperatures marking the onset of long-range antiferromagnetic order (TN1), ferroelectricity (TC1), as well as at temperatures when anomalous changes in the polarization, dielectric constant, and spin wave incommensurability have been previously reported. These observations suggest that the coupling between the magnetic orders and the dielectric properties is mediated by the lattice distortion. In particular, a possible scenario as to how ferroelectricity arises and subsequently is altered as a result of the subtle changes in the highly frustrated magnetic order, is discussed in the light of the strong spin-lattice coupling observed in these materials. Neutron diffraction measurements on HoMn2O5 show a spin reorientation at 23 K corresponding to a step-like anomaly in the dielectric constant. [ABSTRACT FROM AUTHOR]

The crystallographic and magnetic properties of HoMn0.9957Fe0.01O3 powder have been studied by x-ray diffraction, superconducting quantum interference device measurements, and Mössbauer spectroscopy. It has a hexagonal space group P63cm with the lattice constants a0=6.139 and c0=11.402 Å at room temperature. The magnetic susceptibility follows a Curie–Weiss law behavior, where the Curie–Weiss temperature was determined to be θCW=-18 K. The Mössbauer spectra below TN=72 K exhibit the six-line patterns, which coalesce into two lines above TN. The electric quadrupole splitting value at TN was ΔEQ=1.79±0.01 mm/s. An important point can be accessed that the direct observation of a coupling for the electric and magnetic order parameters was possible by Mössbauer spectroscopy. [ABSTRACT FROM AUTHOR]

Investigates the electron spin resonance and magnetic properties in perovskite and pyrochlore manganites. Preparation of ceramic samples of La0.67Ca0.33MnO3; Description of the ferromagnetic interaction between manganese (Mn) ions in colossal magnetoresistence perovskites using double exchange interaction; Results of the investigation.

Presents a study that investigated the relationship between weak ferromagnetism, superconductivity, and lattice parameter in the A[sub2-x-y]B[subx]Ce[suby]CuO[sub4] (A,B...La, Pr, Nd, Sm, Eu, Gd, Y) compounds. Information on ac susceptibility as a function of temperature of Eu[sub1.8]Y[sub02]CuO[sub4]; Spectra for Sm[sub2-x]Y[subx]CuO[sub4] measured at 9Ghz and 263 Kelvin; Analysis of dc susceptibility measured at 10 Oe on a powder sample of La[sub1.1]Gd[0.75]Ce[sub0.15]CuO[sub4].

Magnetically induced ferroelectric polarization in rare-earth RMn2O5 manganites is believed to originate from the symmetric exchange striction associated with a specific antiferromagnetic phase in the low temperature (T) region and would be irrelevant with electropoling in the high-T paramagnetic-paraelectric phase region. In this work, we demonstrate that low-T pyroelectric polarization of GdMn2O5 single crystals along the b axis in the antiferromagnetic phase exhibits remarkable dependence on the electropoling history imposed in the high-T paramagnetic-paraelectric phase. In particular, the high-T electropoling results in a reversal of ferroelectric polarization in the low-T region, which can be flopped back by the electropoling being sustained in the low-T ferroelectric region. The existence of an electrically polarizable magnetic cluster state in the high-T paramagnetic-paraelectric region is proposed based on a combination of experimental observations and first-principles calculations. An intrinsic correlation between the low-T antiferromagnetic ordering and the high-T polarizable state is discussed. The present experiments unveil the emergent phenomena on multiferroicity of RMn2O5 and suggest an alternative scenario for electrocontrol of magnetism. [ABSTRACT FROM AUTHOR]

The crystallographic and magnetic properties of HoMn0.9957Fe0.01O3 powder have been studied by x-ray diffraction, superconducting quantum interference device measurements, and Mössbauer spectroscopy. It has a hexagonal space group P63cm with the lattice constants a0=6.139 and c0=11.402 Å at room temperature. The magnetic susceptibility follows a Curie–Weiss law behavior, where the Curie–Weiss temperature was determined to be θCW=-18 K. The Mössbauer spectra below TN=72 K exhibit the six-line patterns, which coalesce into two lines above TN. The electric quadrupole splitting value at TN was ΔEQ=1.79±0.01 mm/s. An important point can be accessed that the direct observation of a coupling for the electric and magnetic order parameters was possible by Mössbauer spectroscopy. [ABSTRACT FROM AUTHOR]