Rare earth compounds

Hypersonic flight involves extremely high velocities and gas temperatures with the attendant necessity for thermal protection systems (TPS). New high temperature materials are needed for these TPS systems. A systematic investigation of the thermodynamics of potential materials revealed that low oxidation rate materials, which form pure scales of SiO 2 , Al 2 O 3 , Cr 2 O 3 , or BeO, cannot be utilized at temperatures of 1800 • C (and above) due to disruptively high vapor pressures which arise at the interface of the base material and the scale. Vapor pressure considerations provide significant insight into the relatively good oxidation resistance of ZrB 2 -and HfB 2 -based materials at 2000 • C and above. These materials form multi-oxide scales composed of a refractory crystalline oxide (skeleton) and a glass component, and this compositional approach is recommended for further development. The oxidation resistance of ZrB 2 -SiC and other non-oxide materials is improved, to at least 1600 • C, by compositional modifications which promote immiscibility in the glass component of the scale. Other candidate materials forming high temperature oxides, such as rare earth compounds, are largely unexplored for high temperature applications and may be attractive candidates for hypersonic TPS materials.

The orientation dependence of characteristic x-ray emissions have been used to determine specific site occupations of Rare Earth additions in epitaxially grown films of Y. 7Smn fiLu" -FecO,,-A theoretical formulation based on the assumption of highly localized inner shell excitations was used not only to predict specific site sensitive orientations, but also to refine experimentally observed data employing a constrained least squares analysis to give probabilities for the occupation of the RE additions in the different crystallographic sites. Thus, it has been shown that in this compound the preference for the RE additions is a predominantly octahedral occupation with a probability _> 95%. Some of the assumptions and limitations of the technique have also been discussed.

Neutron-diffraction techniques have been used to study the interplay between superconductivity and magnetism in HoNi2B2C (Tc=8 K). The experimental results, obtained on single crystals, show that below approximately 4.7 K, this compound is in a simple antiferromagnetic state that coexists with superconductivity. Between approximately 4.7 and 6 K, an incommensurate modulated magnetic structure has been found. This observation strongly suggests that pair breaking associated with this incommensurate magnetic structure is responsible for the deep minimum in Hc2 and the near-reentrant behavior observed in this compound at approximately 5 K.

An investigation of the x-ray resonant magnetic scattering ͑XRMS͒ across the series of heavy rare-earth elements, coupled with first-principles calculations, has revealed that spin-orbit coupling in the 5d band plays a critical role in the systematics of the XRMS and x-ray magnetic circular dichroism branching ratio at the L edges of magnetic rare-earth compounds.

The total heat capacity has been resolved into lattice, electronic, magnetic, and Schottky components. The Schottky contributions agree well with the calculated values based on the observed s~litting of the ground-state manifold of the rare earth ions occupying sites of S4 symmetry In the Th 3 P 4 structure. The observed splitting is obtained from an analysis of the hot bands in the absorption spectrum and from direct observation of the Stark levels in the far infrared. The Stark levels (all doublets) for Ce 2 S 3 eF s /2) are 0,185, and 358 cm-I ; for Nd z S 3 (4/ 9 / 2 ), they are 0, 76, 150, 180, and 385 cm -I. For La Z S 3 , which has no Schottky or magnetic contributions to the heat capacity, the thermal data can be extrapolated to ° K. The entropy for La 2 S 3 at 298.15 K (as SOIR) is 19.51. Schottky and magnetic ordering at lower temperatures in Ce Z S 3 , Nd 2 S 3 , and Gd 2 S 3 preclude such extrapolation techniques. Therefore the entropy at 298.15 K for these compounds {SO-S°(7 K)}IR, is 21. 34, 22.38, and 20.05, respectively.

The Louisiana Accelerator Center at the University of Louisiana at Lafayette houses National Electrostatics Corporation's 1.7 MV Tandem Pelletron® Accelerator with Cs sputter ion, duoplasmatron, and rf charge exchange ion sources. The machine has been stable for many years, mostly producing beams of protons and 4 He ions. Despite heavy use, necessary maintenance has been confined to servicing the ion source, vacuum pumps, and one tank opening. To support the development of new experimental programs, the Oxford triplet MeV ion microprobe has been reconfigured and upgraded. This includes direct, and off-axis STIM detection capabilities, Amptek SDD X-ray detector, OMDAQ2007 data collection software, and GeoPIXE™ data analysis. The ion implantation beamline was also upgraded with cryo-pumping and a Kelvin probe developed to measure ion bombardment induced changes in surface potential. Radiation exposure during space activities is a persistent theme in the work of the Center. A new beamline has been constructed that uses a quadrupole triplet as a condenser lens to deliver controlled, ultra-low proton fluxes to simulate space radiation effects on biological tissues. Future plans include the development of MeV-SIMS using a Time-of-Flight mass spectrometer. Development of the Louisiana Accelerator Center (LAC), formerly known as the Acadiana Research Laboratory, as a major research center started in the mid 1970's at the University of Southwestern Louisiana (now University of Louisiana at Lafayette). These developments included two major acquisition, a High Voltage Engineering Corporation (HVEC) Model KN 3000 van de Graaff accelerator system was obtained from NASA's Johnson Space Flight Center in Houston, Texas and the contents of machine and metrology shops were acquired from NASA's Michaud fabrication facility in New Orleans. This equipment was housed in a 4000 ft 2 bare metal building. Around the early 1980's, a shielded accelerator building was constructed and the KN accelerator was moved into this new facility which, combined with the metal structure, provided over 12000 ft 2 of laboratory, machine shop, and office space.

The three-dimensional (3D) magnetism present in Nd&Cu04 and Pr2Cu04 has been examined in neutron-scattering experiments. Antiferromagnetic long-range order of the Cu'+ moments devel- ops below T& =255 K in both compounds. In Nd2Cu04 a series of magnetic phase transitions in which the Cu'+ spins reorient has also been observed below T&, while in Pr2Cu04 the 3D Cu + spin structure is stable. No changes in the crystalline structure of Nd&Cu04 associated with the spin- reorientation transitions were found. Induced ordering of the rare-earth magnetic moments has been observed in both compounds. The Nd + (Pr +) magnetic moments align parallel (antiparallel) to Cu + moments which are nearest neighbors along the c axis, and the ordered moments of the rare-earth ions were determined to be -1. 3p& for Nd'+ at 0.4 K and -0. 08p& for Pr'+ at 8 K. The temperature-dependent Pr'+ moment is shown to be proportional to the Cu'+ moment times the bulk susceptibility. In this paper we also discuss the role of the XY anisotropy in the 3D order- ing.

It is shown by model calculations that the dispersion of the diagonal part of the dielectric constant near the plasma edge in a metallic crystal has a strong influence on the magneto-optical properties. The plasma edge leads to resonancelike peaks in the Kerr rotation and Kerr ellipticity spectra and to a strong enhancement of the magnitude of the Kerr effect. The sharp and very large peaks observed in the Kerr effect of several transition-metal and rare-earth compounds are not necessarily related to sharp magneto-optical transitions, but could be a result of the presence of the plasma edge at the same fre- quency.

The electronic structure of thin films of the organic infrared emitters Yb-and Er-tris(8-hydroxyquinoline), YbQ 3 and ErQ 3 , and the related materials GdQ 3 and DyQ 3 , on Ag (1 1 1) is investigated using synchrotron radiation excited photoemission, and X-ray absorption at the C and N K edges. Anti-resonant photoemission is used to suppress the 4f emission. We find that, apart from a shift in the oxygen-derived highest occupied molecular orbital (HOMO), and small differences in the X-ray absorption spectra, the results are similar to those of the well-known visible light emitter aluminium tris(8-hydroxyquinoline), AlQ 3 . Given the documented differences in the photoluminscence spectra compared to AlQ 3 , this implies that the visible light emission from YbQ 3 and ErQ 3 is dominated by triplet states, which are not accessible to the spectroscopies used here. The alignment of the organic HOMO with the Ag Fermi level is shown to be essentially identical to that of AlQ 3 on Ag, suggesting that the wealth of published band alignment studies for AlQ 3 may be directly applicable to these materials. YbQ 3 and ErQ 3 are found to be highly susceptible to radiation damage, particularly at photon energies <100 eV.

We apply ab initio quantum-chemical methods to calculate correlation effects on cohesive properties of GdN, thereby extending the recently proposed incremental method to rare-earth compounds. Our calculated values are in reasonable agreement with the experimental cohesive energy ͑86.0%͒ and the experimental lattice constant ͑102.0%͒. Furthermore, we calculate a bulk modulus of 140.3 GPa. Taking into account estimates for the effect of a better basis both at the one-particle level and at the many-particle level, we even reach 98.5% of the experimental cohesive energy and 101.3% of the experimental lattice constant. For this estimate, we obtain a bulk modulus of 163.8 GPa. ͓S0163-1829͑98͒07304-4͔

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Superconducting critical magnetic fields Hc2(T), magnetoresistance and the Hall effect have been investigated for Gd 1_xPrxBa2Cu307 y single crystals grown in Pt crucibles. Several essential differences of the properties of the system, based on nonmagnetic y3+ were found. The anisotropy of Hoe(T) was found to be ~ 6, i.e. two times higher than for the (Y-Pr)-123 system. Highly anisotropic negative magnetoresistance and non-linear dependence of the Hall voltage on magnetic field were observed at low temperatures.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Calcium lanthanum sulfide powders were prepared by reacting methoxides of calcium and lanthanum with H2S. Alkoxide mixtures of various La/Ca ratios dispersed in methanol were reacted with H2S at 25" to 85°C and the amorphous gels obtained after the removal of methanol were heat-treated in H2S for full sulfurization at various temperatures. Up to 500"C, cubic LaS2 was the only crystalline phase present. Calcium lanthanum sulfide phase started to form at 550°C. In the temperature range of 650" to 750"C, single-phase calcium lanthanum sulfide powders were obtained for La/Ca = 2.41 and 2.68.

Magnetic and neutron diffraction measurements were carried out in order to study the spontaneous and induced spin-reorientation (SR) transition of the ''easy axis-easy plane'' type in the poly and single-crystalline samples of the hexagonal Tm 2 Fe 17. We have determined the temperature dependence of the lattice parameters and the angle between the c-axis and the magnetic moment of the Tm-subsystem. We also find that the SR transition is accompanied by a large (about 20%) magnetization change of the Tm subsystem. In order to induce such a SR transition with the external magnetic field, m 0 H cr ¼ 5 T is necessary to be applied along the hard-magnetization direction (the a-axis) at 4.2 K. The H cr value decreases with an increasing temperature. The magnetization measurements demonstrate that at 10 K the saturation magnetization along the easy-magnetization direction (the c-axis) is smaller than that along the hardmagnetization direction. Based on this observation, we believe that Fe-subsystem of Tm 2 Fe 17 is likely to have magnetization anisotropy.

We present the first evidence by acoustic measurements of H tunneling and trapping by 0 atoms in a rare-earth-hydride system. The YO00027HOO]6 alloy presents three new elastic-energy-loss peaks most likely due to H trapped by 0 between 1.4 and 330 K. The peak at the lowest temperature is due to tunneling of the trapped H; its relaxation rate strongly depends on temperature, indicating a dominant contribution of multiphonon processes already at a few kelvin. The other two peaks, with activation energies of 0.14 and 0.22 eV, can be attributed to jumps of H in the distorted environment of the 0 atom.

Single crystals of stoichiometric lanthanide rare earth compounds, RETiNbO,, doped with Nd"+ and Er'* ions have been grown by the laser heated pedestal growth technique. Excited state absorption resulting in energy upconverted fluorescence are observed for temperatures up to 298 K. The upconversion process is efficient because of weak nonradiative decay rate whilst energy transfer and concentration quenching processes are important in these materials, 0 1997 Elsevier Science B.V.

The local structures of molten CeCl 3 and NdCl 3 have been investigated by XAFS at SPring-8 with the K-absorption edges of Ce and Nd. The nearest-neighbour Ln 3+-Cl − distances and coordination numbers,n Cl Ln , are found to be 2.81Å and 6.52 for CeCl 3 , and 2.73Å and 6.96 for NdCl 3 , respectively. The coordination numbers are >6, implying thereby that the local structures around the lighter lanthanide ions are not exactly octahedral, but somewhat distorted. The XAFS technique using the 3rd generation X-ray sources is a powerful tool to elucidate the local structure around the lanthanide ions in the light rare earth compounds.

Energy transfer from several semiconducting organic molecules and polymers to rareearth complexes in the form of spin-cast films is reported. Energy transfer is observed in poly(N-vinylcarbazole) (PVK) films doped with various europium and samarium complexes. Polystyrene films containing the hole-transporting organic molecule 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) and the electron-transporting molecule N,N′-bis(3-methylphenyl)-N,N-diphenylbenzidine also show energy transfer to the europium complexes, but not to the samarium ones. We show that energy transfer from PBD to the europium complexes is more efficient than from PVK. These films are semiconducting and easy to process, so they are candidates for monochromatic light-emitting diodes.

We present the first evidence by acoustic measurements of H tunneling and trapping by 0 atoms in a rare-earth-hydride system. The YO00027HOO]6 alloy presents three new elastic-energy-loss peaks most likely due to H trapped by 0 between 1.4 and 330 K. The peak at the lowest temperature is due to tunneling of the trapped H; its relaxation rate strongly depends on temperature, indicating a dominant contribution of multiphonon processes already at a few kelvin. The other two peaks, with activation energies of 0.14 and 0.22 eV, can be attributed to jumps of H in the distorted environment of the 0 atom.

We investigated the effect of spin polarization on the structural properties and gradient of electric field (EFG) on Sn, In, and Cd impurity in RSn 3 (R=Sm, Eu, Gd) and RIn 3 (R=Tm, Yb, Lu) compounds. The calculations were performed selfconsistently using the scalar-relativistic full potential linearized augmented plane wave method. The local density approximations (LDA) and generalized gradient approximation without spin polarization (GGA) and with spin polarization (GGA+SP) to density functional theory were applied. In addition to that we performed some calculations within open core treatment (GGA+open core). It is clearly seen that GGA+SP is successful in predicting the larger lattice parameter and the dramatic drop of EFG for R=(Eu, Yb) relative to other rare earth compounds. This is an indication that spin splitting generated by spin polarization without any modification, is capable of treating properly the highly correlated f electrons in these systems.

The compounds Ce(Cu 1 À x Ni x) 4 Ga crystallize in the hexagonal CaCu 5-type structure for the whole doping range 0 r x r 1. The border compounds CeCu 4 Ga and CeNi 4 Ga represent a heavy fermion and fluctuating valence systems, respectively. We report on the studies of the valence evolution in Ce(Cu 1 À x Ni x) 4 Ga employing the X-ray photoemission spectroscopy (XPS) and magnetic susceptibility measurements. The photoemission of the Ce 3d peaks shows a gradual decrease of the occupation of the f states with Ni content. Simultaneously, the hybridization strength and the low temperature magnetic susceptibility are reduced. Within the valence band spectrum a transition from the dominance of the Cu 3d to the dominance of the Ni 3d states is well visible with the traces of the Ce 4f 1 states for up to x ¼ 0.5.

The optical spectroscopic properties of layered oxychalcogenide semiconductors LnCuOCh (Ln=La, Pr, and Nd; Ch=S or Se) on epitaxial films were thoroughly investigated near the fundamental energy band edges. Free exciton emissions were observed for all the films between 300 and ∼30 K. In addition, a sharp emission line, which was attributed to bound excitons, appeared below ∼80 K. The free exciton energy showed a nonmonotonic relationship with lattice constant and was dependent on lanthanide and chalcogen ion substitutions. These results imply that the exciton was confined to the (Cu2Ch2)2− layer. Anionic and cationic substitutions tune the emission energy at 300 K from 3.21 to 2.89 eV and provide a way to engineer the electronic structure in light-emitting devices.

Copper nuclear magnetic resonance (NMR) has been observed at plane and chain Cu sites in PrBa2Cu30p. The field-swept NMR signal from plane Cu sites disappears due to magnetic ordering below-280 K. In the antiferromagnetic state (T 1.4 K) quadrupole-split NMR spectra from plane-site Cu nuclei can be fit using an axial electric-field gradient (vg 17+2 MHz) and an internal field 0;,t 65.2~0.2 kOe directed at an angle 79'+ 1' to the c axis. The observation of plane-Cu magnetism is consistent with the absence of doped holes on Cu02 planes in PrBa2Cu30p, which supports the view that hole filling is important in suppressing superconductivity in Yl-,Pr, Ba2Cu307. Copper antiferromagnetism plays an important role in the electronic properties and superconductivity of the high-T, cuprates. The systems YBa2Cu306+"and La2-Cu04q"are insulators and exhibit long-range antiferromagnetism for zero or low doped-hole concentrations, but become metallic and superconducting with increasing hole concentration from the added oxygen. ' Previous nuclear magnetic resonance (NMR) experimentsz in Y~-"Pr"BazCu307 provided evidence that Pr doping, like oxygen depletion, reduces the CuOz-plane hole concentration. This Rapid Communication reports NMR studies of the end compound PrBazCu307, which is insulating 'and which was found in recent @SR experiments to exhibit plane-Cu antiferromagnetic order. The superconducting properties of most RBa2Cu307 compounds (R lanthanide) isomorphic to YBazCu307 are remarkably insensitive to the large magnetic moments

Structure D 2000 Crystal Structure of the CeIr 3 Compound.-The structure of the title compound is determined by single crystal XRD for the first time. CeIr 3 crystallizes in the PuNi 3 type structure (space group R3m, Z = 9) and belongs to the Ln 2n+n M 4m+5n family, where m and n are the numbers of MgCu-and CaCu 5-type slabs, resp., and m = n = 1 for CeIr 3 .

he crystal structure of the Nd(Cd,Si) compound (C2/m space group, a510.969(3) A, b54.061(2) A, c57.038(4) A, b 5 2 3 3 21 129.05(1)8, V5243.49 A , r 57.931 g / cm , m 529.89 mm , R50.0390 for 323 reflections with F .4s(F)) was determined from o o single-crystal X-ray diffraction (Laue, rotation and Weissenberg methods and automatic diffractometer Philips PW 1100, Mo Ka radiation).

Strong-coupling models for the electronic structure of La2Cu04 are derived from the localdensity-functional results in two successive stages of renormalization. First, a three-band Hubbard model is derived with parameters explicitly calculated from first principles using a constrained density-functional approach and a mean-field fit to the Cu-0 pdo bands. Second, exact diagonalization studies of finite clusters within the three-band Hubbard model are used to select and map the low-energy spectra onto effective one-band Hamiltonians, e.g. , the Heisenberg, one-band Hubbard, or "t-t'-J" model. At each stage, calculated observables are in quantitative agreement with experiment.

SmCo 5 samples were investigated under the following conditions: 'as-sintered' at 1150 • C (low coercivity-0.1-0.5 T), heat-treated at 850-880 • C (coercivity ∼2.5-3.0 T) and after heat treatment at 750 • C during 25 days (low coercivity-0.1-0.5 T). Transmission electron microscopy investigation showed that the increase of coercivity at 850-880 • C is not due to second phases precipitated at grain boundaries in the samples nor to the elimination of stacking faults or dislocations. After the heat treatment during 25 days at 750 • C, no clear evidence of eutectoid decomposition of SmCo 5 phase was found, but a slight change of lattice parameters was detected.

Cerium I. 3 XANES (x-ray-absorption near-edge-structure) spectra were analyzed to separate Ce moment contributions and mixed valence (MV) in complex magnetic silicides CePd2 Mn"Si2 (0 x 2). The Ce valence mixing does not vary linearly with x, but increases rapidly for x 1.5. The associated moment collapse correlates with pronounced deviations of the unit-cell volume from Vegard law and the onset of structural instability. Reorientation of [001] Mn 3d antiferromagnetic order for x & 2 appears to rapidly suppress the weak Ce valence mixing coexisting with antiferromagnetic order in CeMn2Si2.

Relaxation phenomena have become a major concern in the physics of spin glasses. There are certain resemblances of these relaxation properties to those of ordinary glasses. In this work, we compare the relaxation properties of spin glasses near the freezing temperature with those of glasses near the glass transition temperature. There are similarities between the two types of glasses. Moreover, the relaxation properties of many glasses and spin glasses are in conformity with two coupled ‘‘universality’’ relations predicted by a recent model of relaxations in condensed matter.

S toichiometric intermetallic compounds have always been touted for their attractive chemical, physical, electrical, magnetic and mechanical properties, but few practical uses have materialized because they are brittle at room temperature 1-4. Here we report on a large family of fully ordered, stoichiometric binary rare-earth intermetallic compounds with high ductility at room temperature. Although conventional wisdom calls for special conditions, such as non-stoichiometry, metastable disorder or doping to achieve some ductility in intermetallic compounds at room temperature, none of these is required in these unique B2 rare-earth compounds. Ab initio calculations of YAg, YCu and NiAl crystal defect energies support the observed deformation modes of these intermetallics. Intermetallic compounds consist of two or more metals bonded with specific stoichiometries (for example,Ni 3 Al) by mixed metallic,covalent and ionic bonding. Their chemical, physical, electrical, magnetic and mechanical properties are often superior to those of ordinary metals,but their enormous potential to improve engineering performance remains largely unused because they are brittle and fracture easily at room temperature. Tens of thousands of binary intermetallic compounds are known, but only a few dozen show appreciable room-temperature ductility.That ductility is usually achieved by contrivances such as testing at high temperature 5 or in zero-humidity atmospheres 6 ; using material that deviates from exact stoichiometry 7 , or has a metastable disordered structure 8 ; adding dopants 9 such as boron; or exploiting martensitic transformations to achieve ductility 10. We have discovered high ductilities in twelve fully ordered, completely stoichiometric RM intermetallic compounds (where R = a rare earth and M = a main group or transition metal). These RM phases are line compounds that do not deviate from ideal stoichiometry and crystallize in the CsCl (B2)-type structure. All of the compounds were tested at room temperature in normal-humidity air. Furthermore, they have no third element additions being made from high purity M (99.99 at.%) and R (99.8 at.%). Another 120 B2 RM phases exist (see Supplementary Information, Fig. S1), most of which are also probably ductile.It is also remarkable that their ductile nature remained hidden for so long;the first rare-earth B2 compounds,RMg,were reported in 1933 11. Here we report on a family of rare-earth intermetallic compounds with high ductility and high fracture-toughness at room temperature. Our study has focused on YAg, YCu and DyCu, but a preliminary examination of nine other RM compounds, CeAg, ErAg, ErAu, ErCu, ErIr, HoCu, NdAg, YIn and YRh, showed that all are ductile. Polycrystalline specimens of YAg,YCu and DyCu were produced by arcmelting the pure elements to form compounds that are 50.0at.% Y or Dy and 50.0 at.% Ag or Cu. X-ray diffraction (which included a full profile Rietveld analysis), optical metallography, and electron microscopy confirmed that the specimens were single-phase with the fully ordered B2 crystal structure. Transmission electron microscopy (TEM) analysis of a hot-rolled (700°C) DyCu fracture-toughness specimen indicated that the grain size ranged between 0.14 and 0.34 µm. Although this is

Fine structural determinations are utilized to illustrate the advantage of the use of mixed oxides as starting materials rather than mixtures of separated oxides, with respect to the expected performance of yttrium-europium oxide red phosphor for lamps. X-ray synchrotron diffraction experiments show how improved homogeneity is achieved, inducing better processability and higher performance through easier access to crystallographically pure materials and the regularity of the repartition of the dopants.

57R TiFe Co (R5Y, Dy, Er) systems have been studied by Fe Mossbauer spectroscopy at 295 and 78 K. The composition 112x x dependencies of hyperfine interaction parameters and the relative occupancies of Fe crystal sites by Co atoms were derived from the spectra. It was found that Co atoms strongly avoid the 8i sublattice and strongly prefer the 8f and 8j sites for the three studied systems. Some anomalies in behaviour of the quadrupole interaction were observed for ErTiFe Co system which can be attributed to spin 112x x reorientation transition.

Polycrystalline Er Y Fe B (x50.0, 0.5, 1.0, 1.5) compounds have been studied by Fe Mossbauer spectroscopy and differential 22x x 14 scanning calorimetry (DSC) in the temperature range 80-370 K. The spin reorientation phenomenon has been studied extensively by narrow step temperature scanning in the vicinity of the spin reorientation temperature. Using the procedure of subtracting the Mossbauer spectra taken for the same compound at different temperatures, it was possible to follow the influence of transition on the shape of spectra. From this procedure it was concluded that in the region of transition, each subspectrum splits into two Zeeman sextets, which are characterised by different hyperfine fields and quadrupole splittings. A consistent way of describing the Mossbauer spectra was proposed. The composition and temperature dependencies of hyperfine interaction parameters and subspectra contributions were derived from experimental spectra. Endothermic DSC peaks were observed for all compounds studied which correspond to the transition from basal plane to axial easy magnetisation direction. The spin reorientation temperatures and the enthalpies of transitions were established from the DSC data. A spin arrangement diagram was constructed and spin reorientation temperatures obtained by different methods were compared.

A polarization Cu K-edge x-ray-absorption near-edge structure study has been carried out on Pr2-.,Ce.,Cu04 single crystals. The spectra for x-ray polarization vector E nearly parallel to the crystal c axis suggest that electrons contributed by Ce doping are initially localized at the Cu site. The spectra for E perpendicular to the c axis exhibit an almost rigid edge shift to lower energies upon Ce doping. This suggests that the unoccupied in-plane Cu 4p states shift to lower energies. Therefore, the Ce doping donates electrons to the Cu site and also shifts the unoccupied 4p band. We propose that the upper unoccupied band consisting of predominantly Cu 3d states shifts downward and eventually joins the initially localized states near the Fermi level and thus, forms the conduction band in the n-type superconductor.

Strong-coupling models for the electronic structure of La2Cu04 are derived from the localdensity-functional results in two successive stages of renormalization. First, a three-band Hubbard model is derived with parameters explicitly calculated from first principles using a constrained density-functional approach and a mean-field fit to the Cu-0 pdo bands. Second, exact diagonalization studies of finite clusters within the three-band Hubbard model are used to select and map the low-energy spectra onto effective one-band Hamiltonians, e.g. , the Heisenberg, one-band Hubbard, or "t-t'-J" model. At each stage, calculated observables are in quantitative agreement with experiment.

The three-dimensional (3D) magnetism present in Nd&Cu04 and Pr2Cu04 has been examined in neutron-scattering experiments. Antiferromagnetic long-range order of the Cu'+ moments develops below T& =255 K in both compounds. In Nd2Cu04 a series of magnetic phase transitions in which the Cu'+ spins reorient has also been observed below T&, while in Pr2Cu04 the 3D Cu + spin structure is stable. No changes in the crystalline structure of Nd&Cu04 associated with the spinreorientation transitions were found. Induced ordering of the rare-earth magnetic moments has been observed in both compounds. The Nd + (Pr +) magnetic moments align parallel (antiparallel) to Cu + moments which are nearest neighbors along the c axis, and the ordered moments of the rare-earth ions were determined to be-1. 3p& for Nd'+ at 0.4 K and-0. 08p& for Pr'+ at 8 K. The temperature-dependent Pr'+ moment is shown to be proportional to the Cu'+ moment times the bulk susceptibility. In this paper we also discuss the role of the XY anisotropy in the 3D ordering.

Crystalline KY(WO 4) 2 thin layers doped with different rare-earth ions were grown on b-oriented, undoped KY(WO 4) 2 substrates by liquid-phase epitaxy employing a low-temperature flux. The ternary chloride mixture of NaCl, KCl, and CsCl with a melting point of 480 C was used as a solvent. X-ray diffraction, differential-interference contrast microscopy, and profilometry confirmed that the layers were oriented in [0 1 0] direction with a thickness of 5-10 mm. Tb 3+ , Dy 3+ , or Yb 3+ ions were incorporated into the KY(WO 4) 2 host, and their distribution coefficients were estimated by electron-probe microanalysis. The layers were crack-free and solvent inclusions could not be found, although the presence of secondary phases in the solution has been observed. The high optical quality of the layers was confirmed by the demonstration of passive and active planar waveguiding.

The anisotropic resistivity p"and Hall coefficient R~"of La2-"M"Cu04 are analyzed as a function of x, using a rigid-band treatment of the previously given linearized augmented planewave bands of tetragonal La2Cu04. The experimental p""(T) is used to extract At, =3.8, which permits a conventional electron-phonon interpretation of the high T,. The Hall coefficients are qualitatively explained, including sign discrepancies in published measurements. In cubic metals, band theoretic analysis of transport properties has given insight into electron-phonon interactions and superconducting properties. ' Here we present a similar analysis for the new superconducting materials based on La2CuO4 with T, = 40 K. A new feature of this analysis is that, unlike materials studied previously, these have highly anisotropic transport behavior. Our conclusion is that a consistent picture of both transport and superconductivity can be based on the standard electronphonon model, using available density-functional band structures. This conclusion agrees with the calculations of Weber, and Pickett, Krakauer, Papaconstantopoulos, and Boyer, and is not directly contradicted by any experimental evidence.

We have synthesized for the first time, two new compounds of the R3Pt23Si11 series with R = Pr and R = Nd. They both crystallize in the same face-centered cubic structure (Fm-3m space group) as the La, Ce and Yb compounds. The lattice parameters are 16.8634(3) A and 16.8493(4) A for Pr3Pt23Si11 and Nd3Pt23Si11, respectively. From magnetic and heat capacity measurements, we found that Nd3Pt23Si11 presents a magnetic transition at 1.6 K, while Pr3Pt23Si11 displays temperature-independent Van Vleck paramagnetism down to at least 450 mK. At higher temperatures (50 to 300 K), the inverse magnetic susceptibility follows the Curie-Weiss law in both compounds, with θP of the order of -14 K.

We have synthesized new compounds of the R 3 Pt 23 Si 11 series with R¼ Gd, Tb, Dy, Ho and Er, and studied their magnetic and thermodynamic properties. X-ray powder diffraction characterization confirms that all these compounds crystallize in the same face-centered cubic structure, space group Fm3m, as Ce 3 Pt 23 Si 11. They all present a ferromagnetic order at low temperatures. The Curie temperatures range between 42 71 K and 3.39 7 0.05 K for Gd 3 Pt 23 Si 11 and Er 3 Pt 23 Si 11 respectively. For all compounds the value of the spontaneous moment at 2 K is far below the value expected for the saturated moment of the R 3 þ ions, except for the Gd compound. The estimations of the magnetic entropy at T C remain smaller than the full entropy of the fundamental multiplet, R lnð2J þ 1Þ. Both reductions, of the moment and of the magnetic entropy, are ascribed to the crystal field effects. In the paramagnetic phase the thermal variation of the suceptibilities follows a Curie-Weiss law with Curie constants in good agreement with the theoretical values.

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Giant magnetoresistance (GMR), which was until recently confined to magnetic layered and granular materials, as well as doped magnetic semiconductors, occurs in manganate perovskites of the general formula Ln 1-x A x MnO 3 (Ln) rare earth; A) divalent ion). These manganates are ferromagnetic at or above a certain value of x (or Mn 4+ content) and become metallic at temperatures below the curie temperature, T c. GMR is generally a maximum close to T c or the insulator-metal (I-M) transition temperature, T im. The T c and %MR are markedly affected by the size of the A site cation, 〈r A 〉, thereby affording a useful electronic phase diagram when T c or T im is plotted against 〈r A 〉. We discuss GMR and related properties of manganates in polycrystalline, thin-film, and single-crystal forms and point out certain commonalities and correlations. We also examine some unusual features in the electrontransport properties of manganates, in particular charge-ordering effects. Charge ordering is crucially dependent on 〈r A 〉 or the e g band width, and the charge-ordered insulating state transforms to a metallic ferromagnetic state on the application of a magnetic field.

We report results of magnetic and transport measurements on single crystals of a new rare earth series RIr Ga (R5La, Pr, Nd, Sm, and 2 Gd through Tm). These compounds appear to crystallize in the triple-hexagonal Na As structure with space group P6 cm. Magnetic 3 3 ordering temperatures show deviation from De Gennes scaling. These compounds are an interesting example of a quasi-2D structure in which there are anisotropic magnetic interactions.