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Zero-field {mu}{sup +}SR study of the collossal magnetoresistance material La{sub 0.67}Ca{sub 0.33}MnO{sub 3}

Description: Zero-field {mu}{sup +}SR and resistivity experiments on La{sub 0. 67}Ca{sub 0.33}MnO{sub 3} powder shows that the ferromagnetic transition temperature (Tc = 274 K) and resistivity peak temperature coincide to within 1K, about 10 K higher that T{sub c} determined from the bulk magnetization. The sublattice magnetization v{sub mu}(T) is well described for T {<=} T{sub c} by (1-T/T{sub c}){sup beta}, where {beta} = 0.345 {+-} 0.015. Unusual relaxational dynamics suggest a wide distribution of Mn-ion correlation times. These results are discussed in terms of the effects of polarons on the spin and charge dynamics.
Date: April 1, 1996
Creator: Heffner, R.H.; Le, L.P. & Hundley, M.F.
Partner: UNT Libraries Government Documents Department

Substitutional effects on the electronic transport of the Kondo insulator Ce{sub 3}Bi{sub 4}Pt{sub 3}

Description: The resistivity {rho} and thermoelectric power S of the doped Kondo insulator (Ce{sub 1-x}La{sub x}){sub 3}Bi{sub 4}Pt{sub 3} are examined to determine the effects of doping on the narrow gap exhibited by this compound. With increasing La concentration the energy gap progressively disappears in both {rho} and S and band-like transport develops below 25 K. The T-0 transport energy gap as determined from either {rho} or S scales with the single-impurity Kondo energy scale T{sub K} as determined from magnetic susceptibility measurements, independently of x for x {le} 0.25. This result strongly suggests that the gap arises from band hybridization that is driven by Kondo-like many-body correlations rather than from single-electronic interactions.
Date: April 1, 1994
Creator: Hundley, M. F.; Canfield, P. C.; Thompson, J. D. & Fisk, Z.
Partner: UNT Libraries Government Documents Department

Pressure dependence of magnetic order in single crystalline CePtGa{sub 1-x}

Description: The authors present measurements of the susceptibility, the specific heat and the resistivity under hydrostatic pressure on a single crystals of an antiferromagnetically ordered Kondo compound CePtGa{sub 1{minus}x} (T{sub N} = 4.2K). They observe a positive temperature response of T{sub N} on application of hydrostatic pressure up to 1.7 GPa.
Date: October 1, 1997
Creator: Modler, R.; Moshopoulou, E.G. & Hundley, M.F.
Partner: UNT Libraries Government Documents Department

Growth-related magnetic and physical structures in CMR films

Description: Scanning tunneling microscopy (STM), atomic force microscopy (AFM), and magnetic force microscopy (MFM) have proven to be powerful tools for revealing property-sensitive structures in magnetic materials. With the renewed interest in perovskite films as materials for read-heads in high density magnetic data storage, the same challenges faced by high temperature superconductor (HTS) film fabrication are repeated for these materials. To begin addressing these challenges, we used vapor phase epitaxy to fabricate La (Sr, Ca,) based manganate films on single crystal perovskite substrates under different conditions and characterized them with scanning probe microscopies, x-ray diffraction, and temperature-dependent magnetization and resistivity measurements (M(T) and {rho}(T)). The as-grown films were polygranular with grain sizes increasing with increasing temperature (T). The post-deposition annealed films consisted of coalesced layers with improved transport properties. The room temperature magnetic structure of the Sr-based films appeared to be related to defects and/or strain.
Date: September 1, 1997
Creator: Hawley, M.E.; Brown, G.W. & Hundley, M.F.
Partner: UNT Libraries Government Documents Department

Characterization of advanced electronic materials

Description: This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). Our goal has been to extend the Laboratory`s competency in nuclear and advanced materials by characterizing (measuring and interpreting) physical properties of advanced electronic materials and in this process to bridge the gap between materials synthesis and theoretical understanding. Attention has focused on discovering new physics by understanding the ground states of materials in which electronic correlations dominate their properties. Among several accomplishments, we have discovered and interpreted pressure-induced superconductivity in CeRh{sub 2}Si{sub 2}, boron content in UBe{sub 13-x}B{sub x} and the origin of small gaps in the spin and charge excitation spectra of Ce{sub 3}Bi{sub 4}Pt{sub 3}, and we provided seminal understanding of large magnetoresistive effects in La{sub 1-x}Ca{sub x}MnO{sub 3}. This work has established new research directions at LANL and elsewhere, involved numerous collaborators from throughout the world and attracted several postdoctoral fellows.
Date: August 1, 1997
Creator: Arko, A.J.; Heffner, R.H. & Hundley, M.F.
Partner: UNT Libraries Government Documents Department

Effects of localized holes on charge transport, local structure and spin dynamics in the metallic state of CMR La{sub 1{minus}x}Ca{sub x}MnO{sub 3}

Description: The authors review resistivity, x-ray-absorption fine-structure (XAFS) and muon spin relaxation ({mu}SR) data which provide clear evidence for localized holes causing polaron distortion and unusual spin dynamics below {Tc} in ``colossal magnetoresistive`` (CMR) La{sub 1{minus}x}Ca{sub x}MnO{sub 3}. Resistivity measurements for x = 0.33 under an applied field H have shown that ln[{rho}(H,T)] {infinity}-M, where M is the magnetization. The XAFS data show a similar functional dependence for the polaron distortions on M The data from these two measurements are interpreted in terms of some fraction of the available holes x remaining localized and some increasing fraction becoming delocalized with increasing M Finally, this polaron-induced spatial inhomogeneity yields anomalously slow, spatially inhomogeneous spin dynamics below {Tc}, as shown in the {mu}SR data. These experiments individually probe the charge, lattice and spin degrees of freedom in this CMR system and suggest that the polarons retain some identity even at temperatures significantly below {Tc}.
Date: March 1, 1998
Creator: Heffner, R.H.; Hundley, M.F. & Booth, C.H.
Partner: UNT Libraries Government Documents Department

Effects of doping on hybridization gapped materials

Description: Doping studies are presented on three materials exhibiting hybridization gaps: Ce{sub 3}Bi{sub 4}Pt{sub 3}, U{sub 3}Sb{sub 4}Pt{sub 3}, and CeRhSb. In the case of trivalent La, Y, or Lu substituting for Ce or U, there is a suppression of the low temperature gap and an increase in the electronic specific heat, {gamma}. In the case of tetravalent Th substitutions for U there is no change in {gamma} and in the case of tetravalent Zr substitution for Ce in CeRhSb, there is an enhanced semiconductor-like behavior in the electrical resistance. These results are discussed in the light of a simple model of hybridization gapped systems. 12 refs., 3 figs.
Date: June 5, 1991
Creator: Canfield, P.C.; Thompson, J.D.; Hundley, M.F.; Lacerda, A. & Fisk, Z.
Partner: UNT Libraries Government Documents Department

Interplay between electronic transport and magnetic order in ferromagnetic magnetic manganite thin films

Description: The transition metal oxides La{sub 1{minus}x}A{sub x}MnO{sub 3} (A = Ba, Ca, or Sr) order ferromagnetically with Curie temperatures ranging from as low as 50 K to well above room temperature. Magnetic order in these compounds results in a concomitant metal-insulator transition. The feature displayed by the manganites that is most important technologically is the extremely large negative magnetoresistance that achieves its largest values near the magnetic ordering temperature. Qualitatively, this colossal magnetoresistance (CMR) phenomenon involves the suppression of the relatively sharp maximum in the resistivity that is centered at T{sub C}. When considered collectively, the anomalous temperature-dependent transport properties, the CMR effect, and the magnetically ordered ground state indicate that a novel interplay between magnetism and electronic transport occurs in the manganites. General features of the magnetic-field and temperature-dependent electrical resistivity and magnetization as displayed by PLD-grown thin films are examined. Particular emphasis is placed on what these measurements tell us about the conduction process both above and below the magnetic ordering temperature.
Date: May 1, 1997
Creator: Hundley, M.F.; Neumeier, J.J.; Heffner, R.H.; Jia, Q.X.; Wu, X.D. & Thompson, J.D.
Partner: UNT Libraries Government Documents Department

Observation of large low field magnetoresistance in ramp-edge tunneling junctions based on doped manganite ferromagnetic electrodes and a SrTiO{sub 3} insulator

Description: The authors report the fabrication of ferromagnet-insulator-ferromagnet junction devices using a ramp-edge geometry based on (La{sub 0.7}Sr{sub 0.3})MnO{sub 3} ferromagnetic electrodes and a SrTiO{sub 3} insulator. The multilayer thin films were deposited using pulsed laser deposition and the devices were patterned using photolithography and ion milling. As expected from the spin-dependent tunneling, the junction magnetoresistance depends on the relative orientation of the magnetization in the electrodes. The maximum junction magnetoresistance (JMR) of 30% is observed below 300 Oe at low temperatures (T < 100 K).
Date: July 1, 1998
Creator: Kwon, C.; Jia, Q.X.; Fan, Y.; Hundley, M.F.; Reagor, D.W.; Hawley, M.E. et al.
Partner: UNT Libraries Government Documents Department

Volume-based considerations for the metal-insulator transition of CMR oxides

Description: The sensitivity of {rho} [electrical resistivity] to changes in volume which occur through: (1) applied pressure, (2) variations in temperature, and (3) phase transitions, is evaluated for some selected CMR oxides. It is argued that the changes in volume associated with phase changes are large enough to produce self pressures in the range of 0.18 to 0.45 GPa. The extreme sensitivity of the electrical resistivity to pressure indicates that these self pressures are responsible for large features in the electrical resistivity and are an important component for occurrence the metallicity below {Tc}. It is suggested that this is related to a strong volume dependence of the electron phonon coupling in the CMR oxides.
Date: March 1, 1998
Creator: Neumeier, J. J.; Hundley, M. F.; Cornelius, A. L. & Andres, K.
Partner: UNT Libraries Government Documents Department

Magnetostriction and thermal expansion of the Kondo semiconductor Ce{sub 3}Bi{sub 4}Pt{sub 3}

Description: We report dilatometric thermal expansion ({alpha}) and magnetostriction ({lambda}) measurements on the Kondo semiconductor Ce{sub 3}Bi{sub 4}Pt{sub 3} and its non-magnetic analog La{sub 3}Bi{sub 4}Pt{sub 3} in fields to 100 kOe. The magnetic contribution to the thermal expansion of Ce{sub 3}Bi{sub 4}Pt{sub 3} displays a broad maximum centered at 50 K, close to the temperature where the 4f specific heat is a maximum. The linear magnetostriction is anomalously large in Ce{sub 3}Bi{sub 4}Pt{sub 3}, with values that are characteristic of mixed-valent compounds ({lambda}{sub {perpendicular}} = 3.26x10{sup {minus}5}, {lambda}{sub {parallel}} = -6.24x10{sup {minus}5} in 100 kOe at 4 K). The volume magnetostriction is positive and a factor of ten smaller than the linear coefficients ({lambda}{sub V} = 2.75x10{sup {minus}6} in 100 kOe at 4 K). The volume magnetostriction is temperature-dependent, and peaks at 50 K. The data are considered in terms of a Grueneisen analysis that links the temperature-dependent magnetic susceptibility, thermal expansion, magnetostriction, bulk modulus, and specific heat of Ce{sub 3}Bi{sub 4}Pt{sub 3} via temperature-dependent electronic and magnetic scaling parameters.
Date: May 1, 1995
Creator: Hundley, M.F.; Neumeier, J.J.; Thompson, J.D.; Lacerda, A. & Canfield, P.C.
Partner: UNT Libraries Government Documents Department

Observation of spin-dependent tunneling and large magnetoresistance in La{sub 0.7}Sr{sub 0.3}MnO{sub 3}/SrTiO{sub 3}/La{sub 0.7}Sr{sub 0.3}MnO{sub 3} ramp-edge junctions

Description: The authors have fabricated ferromagnet-insulator-ferromagnet tunneling junctions using a ramp-edge geometry based on (La{sub 0.7}Sr{sub 0.3})MnO{sub 3} ferromagnetic electrodes and a SrTiO{sub 3} insulator. Pulsed laser deposition was used to deposit the multilayer thin films and the devices were patterned using photolithography and ion milling. As expected from the spin-dependent tunneling, the junction magnetoresistance is dependent on the relative orientation of the magnetization in the electrodes. A junction magnetoresistance (JMR) as large as 30% is observed at low temperatures and low fields. In additino, they have found that JMR is reduced at high temperatures (T > 100 K) and decreases monotonically with increasing field at high fields (0.5 T < H < 1 T). Possible causes for these are also discussed.
Date: December 31, 1997
Creator: Kwon, C.; Jia, Q.X.; Fan, Y.; Hundley, M.F. & Reagor, D.W.
Partner: UNT Libraries Government Documents Department

Residual stress, mechanical behavior and electrical properties of Cu/Nb thin-film multilayers

Description: Effect of compositional wavelength (modulation) on residual stress, electrical resistivities and mechanical properties of Cu/Nb thin-film multilayers sputtered onto single-crystal Si substrates, was evaluated. Electrical resistivities were measured down to 4 K using a standard 4-point probe. Differential specimen curvature was used to determine residual stress, and a microprobe was used to obtain hardness and elastic modulus. Profilometry, ion-beam analysis and TEM were used. Hardness of the Cu-Nb multilayers increased with decreasing compositional wavelength so that the layered structures had hardness values in excess of either constituent and the hardness predicted by the rule of mixtures. A peak in net residual compressive stress of the multilayers was observed at a compositional wavelength of 100 nm. No resistivity plateau was observed within the composition wavelength range studied.
Date: September 1995
Creator: Griffin, A. J., Jr.; Hundley, M. F.; Jervis, T. R.; Kung, H. H.; Scarborough, W. K.; Walter, K. C. et al.
Partner: UNT Libraries Government Documents Department

Transport and magnetism correlations in thin-film ferromagnetic oxides

Description: In order to determine the {Tc}-dependence of the colossal magnetoresistance (MR) exhibited by the ferromagnetic La{sub 0.7}M{sub 0.3}MnO{sub 3+{sigma}} (M = Ba, Ca, Sr) system, the authors examine the magnetic-field and temperature dependent resistivity and magnetization of a series of thin films that were grown via pulsed-laser deposition. The films had magnetic ordering temperatures (T{sub C}) ranging from 150 to 350 K; all samples displayed a large negative MR that is largest near {Tc}. The magnitude of a given sample`s MR at {Tc} inversely correlates with {Tc}; samples with a low {Tc} display significantly larger MR values than do samples with large {Tc}`s. The quantity {rho}({Tc})/{rho}(4 K), the amount by which the resistivity is reduced by full ferromagnetic order, is an activated function of {Tc} with an activation energy E{sub a} = 0.1 eV. These results indicate that the magnitude of the CMR effect in a given specimen is controlled not by {rho}({Tc}), but by {Tc} via the ratio {rho}({Tc})/{rho}(4 K). Phenomenological scaling relationships are also reported that link {rho}(H,T) to both H and M(H, T).
Date: September 1, 1995
Creator: Hundley, M.F.; Neumeier, J.J.; Heffner, R.H.; Jia, Q.X.; Wu, X.D. & Thompson, J.D.
Partner: UNT Libraries Government Documents Department

Response of CeRh{sub 2}Si{sub 2} to pressure

Description: Under atmospheric pressure, CeRh{sub 2}Si{sub 2} orders antiferromagnetically at T{sub N} = 35 K, with magnetic entropy S = R{ell}n2 associated with the ordered groundstate. Application of modest pressure ({approx}9 kbar) to CeRh{sub 2}Si{sub 2} suppresses T{sub N} to near zero Kelvin, increases its Sommerfeld coefficient of specific heat by nearly a factor of 3.5 and induces some form of superconductivity below 400 mK that is depressed by a magnetic field at a rate of {minus}80 mK/kG.
Date: September 1, 1995
Creator: Movshovich, R.; Hundley, M.F.; Thompson, J.D.; Smith, J.L.; Graf, T.; Mandrus, D. et al.
Partner: UNT Libraries Government Documents Department

Barium potassium bismuth oxide: Synthesis and physical properties

Description: A series of compounds Ba/sub 1/minus/x/K/sub x/BiO/sub 3/ have been prepared and characterized over the range of compositions ranging from x = 0.3 to 0.5. A neutron powder diffraction analysis has been carried out for the composition x = 0.4 at room temperature and at 10 K. Examination of the superconducting properties as a function of x indicates superconductivity occurs over a narrow range of compositions close to x = 0.4, with T/sub c/ of 29 K. Specific heat measurements indicate conventional electron-phonon interactions may play a role in promoting superconductivity in Ba/sub .6/K/sub .4/BiO/sub 3/. Magnetization loops and examination of the time dependent magnetization indicate that Ba/sub .6/K/sub .4/BiO/sub 3/ has a low value of J/sub c/ due to very weak pinning. 18 refs., 6 figs.
Date: January 1, 1989
Creator: Ott, K.C.; Hundley, M.F.; Kwei, G.H.; Maley, M.P.; McHenry, M.E.; Peterson, E.J. et al.
Partner: UNT Libraries Government Documents Department

Doping and pressure study of U sub 3 Sb sub 4 Pt sub 3

Description: The effects of doping and pressure on the U{sub 3}Sb{sub 4}Pt{sub 3} system have been studied. Substitution of either trivalent Y or Lu for U causes significant changes in temperature dependence of the electrical resistance and magnitude of the linear coefficient of the specific heat, {gamma}. However, doping with tetravalent Th causes little change in {gamma}, even though it affects the electrical resistance in a manner similar to that seen in the cases of Lu and Y. Finally, the application of hydrostatic pressures up to 16.5 kbar causes no significant change in the electrical resistance. 10 refs., 4 figs.
Date: January 1, 1991
Creator: Canfield, P.C.; Lacerda, A.; Thompson, J.D.; Sparn, G.; Beyermann, W.P.; Hundley, M.F. et al.
Partner: UNT Libraries Government Documents Department

Possible heavy fermion behavior and field-induced transitions in new rare earth platinum gallium compounds

Description: We have discovered a new family of rare-earth-platinum-gallium compounds RGaPt (R = La, Ce, Pr, Nd, Sm, Gd, Er, Yb, and Y) that have been grown as single crystals. These compounds have been studied through electrical resistance, dc magnetic susceptibility and, in a few cases, by specific heat and single-crystal x-ray diffraction. From growth considerations, preliminary structural analysis and magnetic susceptibility measurements, which will be discussed in the next sections, the chemical formula for the family seems to be R{sub 2-{delta}}Pt{sub 4}Ga{sub 8}. For the Ce-compound, experiments of specific heat at {sup 3}He temperatures reveal a possibly heavy- fermion state with a large electronic term of order 1 J/mole-Ce K{sup {minus}2}. Additional interesting members of this family are the Pr and Nd compounds, that display a large jump in their magnetization near 20 kOe and 30 kOe, respectively, at 7 K. We present an account of our preliminary results on the series and concentrate on the striking properties of the Ce and Pr-based compounds. 8 refs., 3 figs. , 2 tabs.
Date: January 1, 1991
Creator: Lacerda, A.; Canfield, P.C.; Beyermann, W.P.; Hundley, M.F.; Thompson, J.D.; Sparn, G. et al.
Partner: UNT Libraries Government Documents Department

Magnetotransport measurements in magnetic fields up to 50T in SmB{sub 6} and FeSi

Description: We have measured the transverse magnetoresistance in high-quality single-crystalline samples of SmB{sub 6} and FeSi at 4K in applied magnetic fields to 50T and in the temperature range 4K < T < 150K in magnetic fields to 18T. The magnetoresistance in SmB6 at 4K decreases quadratically and reaches about {minus}45% at 50T. The compound FeSi reveals three distinct features in the magnetoresistance: a negative magnetoresistance for fields below 5T, a kink at around 10T, and a broad maximum near 3OT.
Date: December 31, 1993
Creator: Lacerda, A.; Graf, T.; Sarrao, J. L.; Mandrus, D.; Hundley, M. F.; Thompson, J. D. et al.
Partner: UNT Libraries Government Documents Department

Evidence for a coherence gap in Ce sub 3 Bi sub 4 Pt sub 3

Description: In this paper we examine the physical properties of the f-electron system Ce{sub 3}Bi{sub 4}Pt{sub 3}. In the majority of cases f-moment compounds are metallic, and, depending upon the relative strength of competing magnetic interactions, have a superconducting, paramagnetic, or antiferromagnetic ground state. In rare instances f-electron compounds have a narrow-gapped insulating ground state instead. This gap is thought to result from hybridization between conduction and f electrons. We have measured the magnetic susceptibility, low-temperature specific heat, resistivity, Hall effect, and thermoelectric power of Ce{sub 3}Bi{sub 4}Pt{sub 3} and find that this material is non-metallic at all temperatures. Isostructural lanthanum substitution for cerium suppresses the gap and produces transport and thermodynamic behavior that is characteristic of a relatively heavy Kondo impurity system. Further, the energy scale at which coherence develops is roughly equal to the energy gap in this compound. This suggests that the gap in Ce{sub 3}Bi{sub 4}Pt{sub 3} appears to stem from the same requirements of lattice periodicity necessary for coherence to manifest itself in metallic systems. Hence, there is strong evidence that the insulating behavior displayed by Ce{sub 3}Bi{sub 4}Pt{sub 3} results form the presence of a Kondo coherence-induced gap. 15 refs., 4 figs.
Date: January 1, 1990
Creator: Hundley, M.F.; Canfield, P.C.; Thompson, J.D. Fisk, Z. (Los Alamos National Lab., NM (USA)) & Lawrence, J.M. (California Univ., Irvine, CA (USA))
Partner: UNT Libraries Government Documents Department

High field magnetotransport and specific heat in YbAgCu{sub 4}

Description: The electrical resistivity ({rho}) and magnetoresistance of polycrystalline YbAgCu{sub 4} have been measured at temperatures between 25 mK and 300 K, and at magnetic fields (B) to 18 T. The magnetoresistance ({rho}(B){minus}{rho}(0))/{rho}(0) is positive at all temperatures below 200 K and reaches its maximum of 60% at 18 T and 25 mK. The field- and temperature-dependent resistivity does not scale in a simple way. The opposite magnetoresistance behaviors at ambient and high pressure can be explained qualitatively by crystal-field effects lifting the degeneracy of the J = 7/2 groundstate. The linear coefficient of specific heat ({gamma}) measured at fields to 10 T shows a quadratic field dependence. The authors do not find a linear relation between {gamma}{sup 2} and A, the T{sup 2}-coefficient of the temperature-dependent resistivity, with field as the implicit parameter.
Date: July 1, 1994
Creator: Lacerda, A.; Graf, T.; Hundley, M. F.; Thompson, J. D.; Gajewski, D.; Canfield, P. C. et al.
Partner: UNT Libraries Government Documents Department

Science and technology of reduced-dimensional magnetic materials

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). This work involved the synthesis of single crystal and thin film samples of magnetoresistive manganites (LaMnO{sub 3} doped with Ca and Sr) and the characterization of their electronic transport properties to understand the underlying physical mechanisms responsible for the colossal magnetoresistance (CMR) of these materials. The experimental program was supplemented by a modeling effort that sought to develop microscopic mathematical models of the observed phenomena. The authors succeeded in finding an important relation between the magnetization and resistivity in these materials, which helps to explain the importance of lattice distortions accompanied by clusters of ferromagnetic spins (called spin-lattice polarons) in the CMR phenomena. In addition, they developed rudimentary tunnel junctions of CMR-insulator-CMR multilayers that will lead to possible applications of these materials as magnetic sensors.
Date: December 31, 1998
Creator: Heffner, R.H.; Bishop, A.R.; Hundley, M.F.; Jia, Q.; Neumeier, J.J.; Trugman, S.A. et al.
Partner: UNT Libraries Government Documents Department

Cerium heavy-fermion compounds near their T = 0 magnetic-non-magnetic boundary

Description: Measurements of the temperature-dependent specific heat and thermal expansion coefficient near a T = 0 magnetic-non-magnetic boundary, accessed in CeRh{sub 2}Si{sub 2} by application of pressure and in CeRh{sub 2{minus}x}Ru{sub x}Si{sub 2} at ambient pressure by chemical substitution, emphasize the role of disorder in producing non-Fermi-liquid behavior. Interestingly, superconductivity also develops near this boundary in some crystallographically-ordered Ce-based heavy-fermion compounds.
Date: December 1, 1997
Creator: Thompson, J.D.; Hundley, M.F.; Movshovich, R.; Sarrao, J.L.; Graf, T.; Uwatoko, Y. et al.
Partner: UNT Libraries Government Documents Department

Local and average crystal structure and displacements of La{sup 11}B{sub 6} and EuB{sub 6} as a function of temperature

Description: Measurements of both the average crystal structure from Rietveld refinement of neutron powder diffraction (NPD) data and the local structure from La L{sub III}-edge x-ray-absorption fine-structure (XAFS) are presented for a La{sup 11}B{sub 6} sample as a function of temperature ({approx}10-320 K). These data are compared to XAFS results on a EuB{sub 6} sample. The single-site La and B positional distribution widths and the La-B and La-La bond length distribution widths and their temperature dependence are compared. This comparison allows an estimate of the La and B site displacements, and we find that these sublattices are only slightly correlated with each other. Moreover, while the temperature dependence of the displacement parameters of the average sites from diffraction fit an Einstein model well, the temperature dependence of the La-B bond length distribution width requires at least two vibrational frequencies, corresponding to the La and B frequencies of the individual sites. XAFS data on EuB{sub 6} indicate that the situation is the same in the Eu compound. In addition, comparisons between data taken below and above the ferromagnetic transition temperature for EuB{sub 6} place stringent limits on the lattice involvement in the associated metal-insulator transition and the ensuing large magnetoresistance effect. This lack of lattice involvement in the magnetoresistance transition is in sharp contrast to the strong lattice involvement observed in the colossal magnetoresistance lanthanum manganese perovskites.
Date: January 30, 2001
Creator: Booth, C.H.; Sarrao, J.L.; Hundley, M.F.; Cornelius, A.L.; Kwei, G.H.; Bianchi, A. et al.
Partner: UNT Libraries Government Documents Department