Project:
Elastic and magnetoelastic behavior of Fe-Ga alloys
Project: Lattice
dynamics of high temperature superconductor The mechanism of high-temperature superconductivity in the cuprates is not known. The electron-phonon interactions giving rise to the BCS mechanism in normal superconductors are insufficient to explain the large transition temperatures. In place of the phonon mechanism, alternate scenarios, such as spin fluctuation mediated superconductivity, have been proposed. However, not much is known about the behavior of the electron-lattice interaction in the presence of strong electron correlation, thus phonons may still play a role in high-temperature superconductivity. Our neutron scattering results have shown evidence for abnormally large electron-lattice interactions of certain oxygen phonons (the planar half-breathing modes). Evidence for these electron-lattice interactions is also seen by photoemission. The effect of such strong interactions on superconductivity is under investigation. The graph shows the development of soft half-breathing modes at 70 meV upon doping into the metallic phase. Project:
Magnetism and magnetoelastic coupling in rare-earth
germanides
Gd5Ge4 is the end compound of the Gd5(Si,Ge)4 alloy series. This alloy series is known for is large magnetocaloric and magnetoelastic properties. We have been studying the magnetic structure of these compounds using magnetic resonant x-ray scattering at the mu-CAT sector of the Advanced Photon Source. The incident radiation of synchrotron radiation was linearly polarized perpendicular to the vertical scattering plane (σ-polarized). In this configuration the resonant magnetic scattering, arising from electric dipole transitions (E1, from the 2p-to-5d states), rotates the plane of linear polarization into the scattering plane (π-polarization). In contrast, charge scattering does not change the polarization of the scattered photons (σ-σ scattering). Pyrolytic graphite PG(006) was used as a polarization analyzer to suppress the charge background relative to the magnetic scattering signal. The left figure shows θ-scan through the (030) magnetic peak at 10 K (filled circles) and 145 K (open circles) and energy scans at 10 K (filled circles) and 145 K (open circles) through the magnetic peak. The data were measured at an azimuth angle 30° using aluminum attenuator with 0.41 transmission. The dashed line represents the position of the Gd L2 absorption edge (E=7.934 keV). Project: Electron-Phonon Coupling in La1-xSrxFeO3-δ Investigators: Jie Ma, Sung Chang, Jiaqiang Yan, Rob McQueeney
La1-xSrxFeO3-δ compounds have been studied for many years due to their potential for use in a wide variety of applications, such as catalysts, membrane materials, and high temperature fuel cell electrodes. For x=2/3, La1/3Sr2/3FeO3-δ is rhombohedral at room temperature, but its exact structure at low temperatures is still not very clear. La1/3Sr2/3FeO3-δ undergoes magnetic ordering at TN~200K below which charge ordering (CO) is gradually developed with charge disproportionation, 2Fe4+=>Fe3++Fe5+. The charge disproportation is completed by 20K. The above image is the charge ordering sequence of [Fe3+Fe3+Fe5+Fe3+Fe3+Fe5+…] along the body diagonal [111]c. Now we are making these samples and using the thermal gravity (TG) and titration to determine the oxygen content. Because the oxygen decides the number of the holes, it plays an important role in the structure of these compounds. Then we use neutron diffraction to study them, and investigate spin waves and magnetic excitation by the inelastic scattering.
Project: Neutron diffraction studies of rare-earth intermetallic compounds
Project: Spin and Orbital Ordering in Y1-xLaxVO3 Perovskites Investigators: Jiaqiang Yan, Sung Chang, Jerel Zarestky, Ovidiu Garlea, Bella Lake, and Robert McQueeney. Collaborators: J.-S. Zhou, and J. B. Goodenough, the University of Texas at Austin, Yang Ren, Advanced Photon Source (APS), Argonne National Laboratory (ANL), J. G Cheng, Y. Sui, and W.H. Xu, Harbin Institute of Technology, Anna LIobet, Lujan Neutron Scattering Center, Los Alamos National Laboratory (LANL), Joseph Fieramosca, Intensed Pulsed Neutron Source (IPNS), ANL
The strong coupling between spin, orbit, and lattice in transition metal oxides induces many fascinating physical properties, such as high-Tc superconductivity, CMR, ferroelectricity. RVO3 perovskites all exhibit an intriguing sequence of orbital and magnetic orderings below a Too and a TN, respectively. They provide an opportunity to explore cooperative orbital ordering among the p-bonding t orbitals in the absence of s-bonding e electrons and the relationship of this orbital order to long-range magnetic ordering. At lowest temperatures, LaVO3 exhibits G-type OO and C-type SO; while the ground state of YVO3 is C-type OO and G-type SO. With the aid of an IR image furnace, we successfully synthesized single phase Y1-xLaxVO3 (0 £ x £ 1) and characterized the physical properties which include magnetic susceptibility, thermal conductivity, specific heat, and low-temperature structure. The magnetic structure was characterized by both neutron powder diffraction and single crystal diffraction experiments. Based on those measurements, a phase diagram was proposed for Y1-xLaxVO3 as shown in Fig 1. Obviously, a critical behavior was observed at x = 0.20 where the ground state suddenly changes from pure C-type OO for x<0.20 to a mix of C-OO and G-OO for x>0.20.
Project: Structural Anomalies induced by Spin-state transition in PrCoO3 Investigators: Jiaqiang Yan, Sung Chang, Bella Lake, and Robert McQueeney Collaborators: Yang Ren, Advanced Photon Source (APS), ANL, J.-S. Zhou, and J. B. Goodenough, the University of Texas at Austin, S. Short, and J. D. Jorgensen, Intensed Pulsed Neutron Source (IPNS), ANL
Figure 1 shows (a) the Crystal structure of PrCoO3 with anisotropic thermal motions of O atoms in ellipsoid shapes, and (b) the temperature dependence of the anisotropic thermal parameters for the two oxygen atoms. The spin-state transition in RCoO3 perovskites has been continuously attracting much attention since the 1950’s. A Low-Spin (LS)—Intermediate Spin (IS)—High Spin (HS) model has been widely used to explain the observed physical properties. The IS Co(III), with the electronic configuration of t5e1, is Jahn-Teller active. For LaCoO3, the rhombohedral symmetry allows only a single Co-O bond length, which rules out the possibility of the long-range orbital-ordering. However, Maris et al. concluded based on a high-resolution x-ray diffraction study that the symmetry is lowered to be monoclinic, where significant coherent JT distortions of the CoO6 octahedra were found. To overcome the symmetry restriction, we chose to study a similar compound PrCoO3 with the Pbnm symmetry which is compatible with Jahn-Teller distortion.
We studied the structure anomalies induced by spin-state transition in PrCoO3 with powder neutron diffraction in the temperature range from 12 to 600 K. We found no evidence for static orbital ordering of e-orbitals from thermally populated IS Co3+ in PrCoO3. The large anisotropic atomic displacement parameters (ADPs) of the oxygen atoms signal significant bond-length fluctuations associated with the spin-state transition. Project: Spin Dynamics of LiNiPO4 and LiFePO4 Investigators: Jiying Li, Ovidiu Garlea, David Vaknin, Jerel Zarestky
Figure 1: (a) Atomic structure of LiMPO4. The M2+ ions form buckled layers stacked perpendicular to the [100] crystallographic direction. (b) Spin arrangement of the two Fe2+ layers in LiFePO4, the intra-plane nearest and next-nearest neighbor interactions J1 and J2 and inter-plane nearest neighbor inaction J┴ are labeled (spin along c-axis direction for LiNiPO4). LiNiPO4 and LiFePO4 are antiferromagnetic insulators, belong to the olive family of lithium orthophosphates LiMPO4 (M = Mn, Fe, Co, Ni, Cu). These group of materials have a space group of Pnma, as shown in Figure 1 (a), with M2+ ion occupy the center of a slightly distorted MO6 octahedron that shares oxygen anions with a tetrahedral PO4 forming a closely packed oxygen framework. The M2+ ions form puckered layers stacked along the (001) crystallographic axis. Thus, the LiMPO4 exhibits highly anisotropic properties intermediate to 2D and 3D systems. The previous studies on LiMPO4 are mostly concerned with their magnetoelectric properties and more recently the promising application as high potential cathodes for lithium rechargeable battery. In understanding these properties, it is very important to know the spin exchange paths among the M2+ ions. As shown in Figure 1 (b), the spin couplings between the inter-plane nearest-neighboring M2+ are through M2+-O-M2+ oxygen bond, the inter-plane next-nearest-neighbor couplings are through M2+-O-O-M2+, and the intro-plane couplings are through M2+-O-P-O-M2+ bonding. We proposed to study the spin wave excitations of LiFePO4 and LiNiPO4 using inelastic neutron scattering, determine the values for all the spin interactions, thus to further understand the magnetic properties of the system. The findings will also help to reveal the origin of the novel magnetic commensurate-incommensurate magnetic phase transition discovered in LiNiPO4 (D. Vaknin, et. al., PRL, 92 (2004), 207201/1). Project: Magnetoelectric Properties of LiMPO4 (M = Ni, Fe, Co) Investigators: Jiying Li, David Vaknin, Jerel Zarestky
Figure 1: ME coefficients of LiCoPO4 and LiNiPO4 versus temperature measured by the dynamic technique. The ME coefficients were measured under 5 kOe magnetic field.
Magnetoelectric effect (ME) is a coupled two-field effect, in which the application of a magnetic field induces linear change of electric polarization and application of electric field induces linear change of magnetization. Since the birth of the ME effect in 1894, more than 80 compounds have been identified displaying the ME effects. Among these compounds, ferroelectrics/ferromagnetic/ferroelastic boracites or multiferroics are all known to have spontaneous polarization or spontaneous magnetization. Lithium olivine phosphate LiMPO4 (M = Mn, Co, Fe, Ni), on the other hand, is a special class of compounds in the ME materials. They are antiferrimagnet with collinear ground state and no spontaneous polarization or spontaneous magnetization has been reported. These compounds also show considerable large ME coefficients. We proposed to study the origins of the magnetoelectric effects in LiMPO4 using single crystal x-ray scattering under magnetic fields. The findings will shed light on the origin of the ME effects in the lithium olivine phosphate and also help explain the anomalous temperature dependence of ME effect for LiNiPO4, as shown in the Figure 1, comparing with the other members in the olive family of lithium orthophosphates. Project: Magnetic excitations in the magnetic molecule {Cr8} Investigators: Ovidiu Garlea, Jiying Li, David Vaknin, Bella Lake, Jerel Zarestky, Paul Kögerler, Marshall Luban, Steve Nagler (ONL), Yiming Qiu (NIST)
Fig. 1. (a) Structure of a {Cr8} molecule with one benzoate (C6H5COO–) ligand highlighted (Cr: green, O: red, C: grey) (b) Scheme of the connectivity of the eight numbered spin centers in {Cr8} via the exchange pathways, i.e. four m3-oxo centers and 12 carboxylate bridges. (c) The symmetry of the system leads to four distinct exchange parameters (black, grey, dotted, dashed).
The magnetic molecule {Cr8} has a chemical formula of [Cr8O4(C6H5COO)16] · 4 CH3CN = {Cr8}. This molecule contains eight s = 3/2 CrIII centers, and their interactions are thought to be described by a Heisenberg model with isotropic exchange interactions between pairs of centers connected via both benzoate and oxo bridges. In particular, a central Cr4O4 cube is extended by four outer Cr positions spanning a tetrahedron. Each outer Cr is coordinated to one of the four oxo groups as well as to the three nearest Cr positions of the Cr4O4 core via three carboxylate groups. Based on the different exchange pathways, four distinct exchange constants (J1, J2, J3, and J4) are necessary to characterize the system. Referring to Fig. 1b/c, these exchange constants are defined by different number and type of the ligands linking the spin centers. We studied the magnetic excitations at 0.1 K to 30 K under magnetic fields up to 9 Tesla. The findings will provide important input information for the determination of the microscopic Hamiltonian of this complex system and help the understanding of fundamental problems in magnetism on the nanoscale level.
Project: Inelastic neutron scattering study of charge order in La1-xSrxFeO3-δ perovskite Investigators: J. Ma, J.-Q. Yan, S. Chang, and R. McQueeney
La1/3Sr2/3FeO3-δ is mixed valent. Around 210 K, charge disproportionation occurs according to 2Fe4+=>Fe3++Fe5+, and the constant ratio of Fe3+ and Fe5+ ions is 2:1. These different iron valences are observed to order along the body diagonal [111]c, resulting in a change in crystal structure and antiferromagnetic ordering. The stability of the [111] ordering depends on the balance of electrostatic, magnetic, and elastic interactions. In order to determine the relative importance of these interactions, we study the effect of simultaneous charge and magnetic ordering on the phonon and spin wave excitations by inelastic neutron scattering.
Project: Compositional variation of the phonon dispersion curves of bcc Fe-Ga alloys Investigators: J. L. Zarestky, V. O. Garlea, T. A. Lograsso, D. L. Schlagel, and C. Stassis
Because of the practical applications of
magnetostrictive (MS) materials, considerable effort is being
spent in the development of simple alloys with large MS
coefficients. Fe‑Ga alloys are particularly promising, since,
in spite of the complexity of their phase diagram, they can be
obtained as practically single-phase bcc alloys. As a result,
their physical properties have been extensively studied by a
variety of experimental techniques. The results of sound
velocity measurements1 showed that, although the
elastic constant C44 is independent of Ga
composition, the shear elastic constant C' = ½(C11
- C12) decreases linearly with increasing Ga
concentration and extrapolates to zero at approximately 26 at.
% Ga. The decrease in C' also correlates with the
increase of the magnetostrictive constant
l100 Inelastic neutron scattering techniques were used to measure the phonon dispersion curves of bcc Fe-Ga alloys as a function of Ga composition. Samples of 10.8, 13.3, 16.0, and 22.5 at. % Ga were measured. A 28.8 at. % Ga sample, in which approximately 50% of the sample was in the ordered DO3 phase, was also studied. The phonon frequencies of every branch were found to decrease significantly with increasing Ga concentration, however, the softening was most pronounced for the T2[xx0] branch (fig. 1), the branch associated with C'. The dip in the L[xxx] branch in the vicinity of x = ⅔, a feature frequently observed in bcc metals and associated with an instability toward the w-phase, was also found to become deeper with increasing Ga concentration. For the 22.5 at. % Ga sample, new branches appeared, an effect associated with the increase in the number of atoms per unit cell. All measurements were performed on the HB1A and HB1 triple-axis spectrometers at HFIR. The concentration dependence of the shear elastic constant C' = ½(C11 - C12), calculated from the slope of the T2[xx0] branch (Fig. 2), was found to agree with the results of sound velocity measurements.
Project: Lattice dynamics and phonon softening in Ni-Mn-Al Heusler alloys Investigators: X. Moya, Li. Mañosa, A.Planes, T. Krenke, M. Acet, E. F. Wasserman, V. O. Garlea and J. L. Zarestky Although the research in Heusler magnetic shape memory alloys has received a considerable interest in recent years, the vast majority of studies dealt with the prototype Ni-Mn-Ga alloy. In particular, inelastic scattering and elastic constant experiments have evidenced an unusual lattice dynamical behavior. This behavior has been accounted for by ab initio calculations. Recently, these calculations have been extended to the Ni-Mn-Al system. The purpose of our work is to experimentally determine the lattice dynamics of this Heusler shape memory alloy by means of neutron scattering.
We have measured the low-lying phonon dispersion curves of a Ni54Mn23Al23 alloy (fig. 1). Results are in agreement with ab initio calculations. We have found that the frequencies of the TA2 modes are relatively low. The branch has an anomaly at q=0.33 which softens on decreasing temperature. In addition, we have found elastic satellites (fig. 2). Some of these satellites arise from an antiferromagnetic ordering while others have been identified as precursors of a low temperature martensitic phase.
Project: Metamagnetic transitions in TbPtIn intermetallic compound Investigators: V.O. Garlea, J. L. Zarestky, J. Schefer, E. Morosan, S.L. Bud’ko, P.C. Canfield, C. Stassis
The interesting physical properties exhibited by rare-earth intermetallic compounds have motivated, in recent years, a considerable effort toward the synthesis of new classes of these compounds. In particular, TbPtIn, a ternary rare-earth intermetallic compound, recently synthesized in the Ames Laboratory, displays a very rich magnetic phase diagram. The compound crystallizes in the hexagonal system (space group P-62m ) and consist of layers of Tb-Pt perpendicular to the c-axis, separated by Pt-In layers, an interesting structure for the study of indirect exchange interaction between Tb atoms. The rare-earth sites form a triangular lattice and in the case of antiferromagnetic coupling between nearest neighbors, this topology induces a frustration of the magnetic interaction.
Neutron diffraction experiments were performed on single crystals as well as on powder samples. In the absence of an externally applied magnetic field, the compound orders, below approximately 47 K, in an antiferromagnetic structure with propagation vector k = (½, 0, ½). The magnetic peak intensity versus temperature curves exhibit an inflection point at approximately 27K and saturates below approximately 20K, in agreement with magnetization measurements, Fig. 1. In order to sort out possible magnetic structures we applied group theory (representation analysis method). There are two possible magnetic structure models which give a satisfactory fit to the experimental data. In both models the magnetic moments lie in the basal plan, have the same magnitude and make an angle of 120º with respect to each other. Measurements performed in the a-c scattering plane, at 4.2K, with a magnetic field applied along the [-120] direction, revealed a transition from the zero field antifferomagnetic state to a state with a net ferromagnetic component, at approximately 20kG, followed by another transition to a state with a larger ferromagnetic component, at approximately 35kG (see Fig2.). Field cooled and zero field cooled measurements were also performed with the magnetic field applied along the [-110] direction. Two metamagnetic transitions were observed: one at 20kG and the other at 50kG. Measurements in magnetic field allowed us to identify the correct model for the magnetic structure (Fig 2, inset).
Investigators: V. O. Garlea, S. E. Nagler, J. L. Zarestky, C. Stassis, D.Vaknin, P. Kögerler, D. F. McMorrow, C. Niedermayer, D. A.Tennant, B. Lake, Y. Qiu, M. Exler, J. Schnack, M. Luban.
Magnetic molecules are confined assemblies of finite numbers of interacting spin centers, and are ideal systems for exploring fundamental issues in nanomagnetism. The experimental determina-tion of the magnetic excitation spectrum is pivotal for comparison with theoretical models, and the inelastic neutron scattering technique has been most effective in this regard.
Low temperature inelastic neutron scattering studies have been performed to characterize the low energy magnetic excitation spectrum of the magnetic nanomolecule {Mo72Fe30} [1]. This unique highly symmetric cluster features spin frustration and is one of the largest discrete magnetic molecules studied to date by inelastic neutron scattering. The thirty s=5/2 FeIII ions, embedded in a spherical polyoxomolybdate molecule, occupy the vertices of an icosi-dodecahedron and are coupled via nearest-neighbor antiferromagnetic interactions. While the complete quantum Heisenberg Hamiltonian of {Mo72Fe30}, with a Hilbert space dimension of 630, cannot be solved, a solvable approximate version has been derived for this system [2]. The system has an S = 0 ground state while the lower section of the excitation spectrum features a set of parallel rotational bands (Fig.1), where the discrete energies are proportional to S(S+1) and the energy gap separating the two lowest rotational bands is 5J. We found that the solvable three-sublattice model for describing the frustrated spin system accounts well for the overall energy scale and qualitative temperature dependence of our experimental observations. The principal mode observed at about 0.65 meV can be understood as arising from transitions between the two lowest rotational bands. Applied magnetic fields in the 0 - 7 Tesla range modify the intensity of the main excitation, and bring about satellite excitations whose energies depend linearly on the field (see Fig 2). The results obtained in various magnetic fields confirmed the magnetic origin of the spectrum but challenge the existing theoretical models.
Project: Magnetic excitations study of the cubane-like magnetic cluster {Cr8} Investigators: V.O. Garlea, S.E. Nagler, J. Li, P. Kögerler, J.L. Zarestky, L. Engelhardt, M. Luban, Y. Qiu, and D. Vaknin
Recently we have initiated a systematic study of the cubane-type magnetic molecule {Cr8}, with the chemical formula [Cr8O4(C6H5COO)16]· 4CH3CN. This molecule contains eight s = 3/2 CrIII paramagnetic centers that are connected via both benzoate and oxo bridges. A central Cr4O4 cube is extended by four outer Cr positions spanning a tetrahedron. Each outer Cr ion is coordinated to one of the four oxo groups as well as to the three nearest Cr positions of the Cr4O4 core via three carboxylate groups. The cluster structure is shown in Fig.1. Despite the complexity of the molecule, symmetry considerations lead us to expect that thee distinct exchange constants are sufficient to characterize the system. Referring to Fig. 1, these exchange constants are defined by different type of the ligands linking the spin centers.
Inelastic neutron scattering
studies of the cubane-like cluster
Numerical diagonalization of the Hamiltonian with the three distinct antiferromagnetic exchange couplings yields a singlet ground state (S = 0) and two distinct triplet excited states (S = 1) consistent with the two excitations observed experimentally. The change in intensity of the two excitation peaks as a function of magnetic field at base temperature (~100 mK) can also be explained qualitatively by the theoretical spectrum scheme
Project: Neutron diffraction studies of the Tb5SixGe4-x magnetoelastic compounds Investigators: V. O. Garlea, J. L. Zarestky, C. Y. Jones, L.-L. Lin, D. L. Schlagel, T. A.Lograsso, A. O. Tsokol, V. K. Pecharsky, K. A. Gschneidner Jr., C. Stassis
Systematic research performed over the last decade reveals that the family of R5SixGe4-x pseudo-binary alloys, where R is a rare earth metal, exhibit some startling magneto-responsive properties associated both with their layered crystal structures and the combined magnetic- martensitic transformations below room temperature. These include the giant magneto-caloric effect, colossal magnetostriction and giant magnetoresistence.
Progress in understanding the properties of these compounds has been hindered by difficulties in growing large, high-quality single crystals. Recently single crystals of Tb5SixGe4-x were grown at the Ames Laboratory and this motivated us to initiate a neutron diffraction study of these compounds as a function of temperature. Detailed analyses of the magneto-martensitic transfor-mations were performed on two different Tb5SixGe4-x stoichiometries, i.e., with x=2.2 and 2.5. The room temperature crystal structure of these compounds can be described as the stacking of identical, well defined blocks of atoms [Tb5M4], where M = Si or Ge atoms, perpendicular to the b axis (see Fig. 1a). We have confirmed that the large magnetocaloric effect in Tb5SixGe4-x is related to a first-order phase transition between the high-temperature monoclinic-paramagnetic and the low temperature orthorhombic-ferromagnetic states of the material, which occurs at approximately 120 K. Between 120 K and 75 K, the measurements show a canted ferromagnetic structure with the a-axis being the easy magnetization direction (Fig 1b). Examination of the single crystal data shows that a second magnetic transition occurs at approximately 75 K. Below this temperature the components of the magnetic moment along the a- and c-axes order ferromagnetically while the components along the b-axis remain antiferromagnetically aligned (Fig 2c). A satisfactory fit of neutron diffraction data was obtained by mixing the basis vectors of two irreducible representations associated with P212121, a subgroup of the Pnma space group.
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with increasing Ga concentration.
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