Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session N38: Phase Transitions in Itinerant Magnets |
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Sponsoring Units: DCMP Chair: Sevnghun Lee, NIST Room: LACC 513 |
Wednesday, March 23, 2005 8:00AM - 8:12AM |
N38.00001: Influence of the Verwey transition on the magnon dispersion of magnetite R.J. McQueeney, M. Yethiraj, W. Montfrooij, J.S. Gardner, P. Metcalf, J. Honig Inelastic neutron scattering measurements of the magnon spectrum of magnetite (Fe$_{3}$O$_{4})$ were performed above and below the metal-insulator (Verwey) transition. Above the Verwey transition, the magnon dispersion behaves as expected for a classical Heisenberg ferrimagnet. Below T$_{V}$, a large gap (8 meV) forms in the middle of the acoustic magnon branch at \textbf{q}=(0,0,1/2) and E=43 meV. This wavevector corresponds to the main superlattice reflection of the low symmetry monoclinic structure that exists below T$_{V}$. It is plausible that the splitting is related to charge ordering occurring on the Fe spinel B-sites in the insulating phase. We examined this possibility by using Heisenberg models with large unit cells (up to 96 magnetic sites) to calculate the magnon dynamics when the superexchange is modified to reflect crystallographic symmetry lowering due to either atomic distortions or charge ordering. Neither of these models predicts the spin wave gap. Other physics is likely at play, such as strong magneto-elastic coupling, which may further complicate our understanding of the Verwey problem. [Preview Abstract] |
Wednesday, March 23, 2005 8:12AM - 8:24AM |
N38.00002: Double Exchange Model for Magnetic Hexaborides Vitor Pereira, Joao Lopes dos Santos, Eduardo Castro, Antonio Castro Neto A microscopic theory for rare-earth ferromagnetic hexaborides, such as Eu(1-x)Ca(x)B6, is proposed on the basis of the double-exchange Hamiltonian. In these systems, the reduced carrier concentrations place the Fermi level near the mobility edge, introduced in the spectral density by the disordered spin background. We show that the transport properties such as Hall effect, magnetoresistance, frequency dependent conductivity, and DC resistivity can be quantitatively described within the model. We also make specific predictions for the behavior of the Curie temperature, Tc, as a function of the plasma frequency. [Preview Abstract] |
Wednesday, March 23, 2005 8:24AM - 8:36AM |
N38.00003: Magnetic domains in itinerant metamagnets Benedikt Binz, Hans-Benjamin Braun, T. Maurice Rice, Manfred Sigrist Experimental results of the metamagnetism in Sr$_2$Ru$_2$O$_7$ give strong evidence for a new type of quantum critical behavior, namely a quantum critical end point [S. A. Grigera {\it et. al.}, Science {\bf 294}, 329 (2001)]. Our study shows that this behavior can be caused by band structure effects, e.g. by the vicinity of a van Hove singularity. Based on a mean field analysis, a phase diagram for the multi-layer ruthenates is developed as a function of magnetic field and band filling, showing the presence of a quantum critical end point which terminates a first-order phase boundary. At the end point, the Fermi level of the majority-spin band is precisely located at the Van Hove singularity. Thus, the system is rather susceptible to disorder effects. We investigate the possible appearance of Condon-domain like structures in a metamagnetic system. Condon-domains are the result of the coexistence of two magnetic phases at a first-order phase transition and may influence the motion of carriers via domain-wall scattering. We discuss the physical consequences and propose new test experiments. [Preview Abstract] |
Wednesday, March 23, 2005 8:36AM - 8:48AM |
N38.00004: Ultra-sharp jumps of magnetoresistivity in triple-layered ruthenate Sr$_4$Ru$_3$O$_{10}$ Zhiqiang Mao, Meng Zhou, Joe Hooper, David Fobes, Vladimir Golub, Charles O'Connor Sr$_4$Ru$_3$O$_{10}$ is the triple-layered member in the layered perovskite Ruddlesden-Popper series Sr$_{n+1}$Ru$_n$O$_{3n+1}$ with $n$ = 3. The magnetic properties of this compound are very anisotropic: it shows ferromagnetic behavior for $H$//$c$ and a metamagnetic transition for $H$// $ab$. We have performed systematic electronic transport property measurements under the field configuration $H$//$ab$ using high quality Sr$_4$Ru$_3$O$_{10}$ single crystals grown by a floating- zone method. We have observed very strong evidence for an inhomogeneous electronic state near the metamagnetic transition. The system phase separates into paramagnetic and ferromagnetic phases near or within the transition range. This phase separation process, together with the critical fluctuations occurring near the metamagnetic transition, results in very unusual transport properties: (1) the magnetoresistivity exhibits ultra-sharp jumps (width $<$ 1G) on the down sweep cycle of magnetic field, (2) The resistivity shows an non- metallic temperature dependence below 5K in the up-sweep cycle of field, and a drop in the down-sweep cycle. [Preview Abstract] |
Wednesday, March 23, 2005 8:48AM - 9:00AM |
N38.00005: Tunneling magnetoresistance studies of Sr$_3$Ru$_2$O$_7$ Joe Hooper, Meng Zhou, Zhiqiang Mao, Robin Perry, Yoshiteru Maeno Recent work has supported the existence a new type of field- tuned quantum phase transition (QPT) in the double layered ruthenate Sr$_3$Ru$_2$O$_7$. To further probe the physical properties near this QPT, we have performed planar tunneling measurements on Sr$_3$Ru$_2$O$_7$ single crystals. Our previously reported work revealed an unusual oscillation in tunneling magnetoresistance. We here report further characterization of this new phenomenon, showing that the oscillation has a systematic dependence on the tunnel barrier, temperature, and the field orientation. The oscillation pattern is identical even for different barrier materials (such as Al$_2 $O$_3$ and SiO), but is only prominent when the junction resistance is between roughly 15$\Omega$ and 1k$\Omega$. The oscillation shows a field orientation dependence for $H$//$c$ and $H$//$ab$, both in its pattern and its temperature dependence. The oscillation frequency for $H$//$ab$ appears to be smaller than that for $H$//$c$. We discuss possible origins of this unusual oscillation phenomenon in light of recent bulk measurements on Sr$_3$Ru$_2$O$_7$. [Preview Abstract] |
Wednesday, March 23, 2005 9:00AM - 9:12AM |
N38.00006: ARPES measurements of the 3-Dimensional Fermi Surface of LaRu$_2$Si$_2$ J.D. Denlinger, F. Wang, J.W. Allen, Kai Rossnagel, J.L. Sarrao LaRu$_2$Si$_2$ is important as the $f^0$ reference compound for heavy fermion systems CeRu$_2$Si$_2$ and CeRu$_2$Ge$_2$ which are hallmarks for the agreement between dHvA experiments and a renormalized-LDA description of the heavy mass quasiparticle Fermi surface (FS). Recent advancements in angle resolved photoemission (ARPES) capabilities including angular resolution and automation allow the measurement of finer electronic structure detail as well as the mapping of larger regions of k-space. These experimental improvements combined with photon energy dependent excitation are used to probe k$_{\perp}$-variations in the LaRu$_2$Si$_2$ FS topology. Normal emission band structure maps newly reveal closed FS contours of small Z-centered hole pockets, thereby refining the value of the crystal inner potential. Wide-angle FS maps at three different photon energies also show clear signatures of three-dimensionality and incomplete k$_{\perp}$-broadening, including (i) new evidence for an LDA-predicted narrow electron pocket at the P-point and (ii) an electron-like connectivity between FS contours centered around $\Gamma$ and the famous large hole-like 'pillow' pocket centered on Z. [Preview Abstract] |
Wednesday, March 23, 2005 9:12AM - 9:24AM |
N38.00007: Thermal Expansion and Magnetostriction of the Ising Antiferromagnet TbNi2Ge2 S.M. Hollen, G.M. Schmiedeshoff, S.L. Bud'ko, P.C. Canfield TbNi$_{2}$Ge$_{2}$ has been shown to be an extremely anisotropic, axial, Ising-like antiferromagnet. In zero field it enters incommensurate and commensurate antiferromagnetic states at 16.7 K and 9.6 K respectively; six additional metamagnetic phases have been observed in applied fields at 2 K. In this talk we present preliminary measurements of the thermal expansion and magnetostriction of this material along its c-axis from room temperature to 2 K and in magnetic fields (H$\vert \vert $c) to 14 T. This work was supported by the Director for Energy Research, Office of Basic Energy Sciences, US DOE and was partially supported by the NSF under DMR-0305397. [Preview Abstract] |
Wednesday, March 23, 2005 9:24AM - 9:36AM |
N38.00008: Unusual Spectral Weight Transfer in Temperature-dependent Optical Spectra of the Pyrochlore Nd$_{2}$Mo$_{2-x}$Ti$_{x}$O$_{7}$ Kyungwan Kim, M. W. Kim, T. W. Noh, M. Sato We investigated the optical conductivity spectra of the pyrochlore Nd$_{2}$Mo$_{2-x}$Ti$_{x}$O$_{7}$ single crystals of $x$ = 0.0, 0.1, and 0.3. Recently optical spectra of a ferromagnetic metal Nd$_{2}$Mo$_{2}$O$_{7}$ and a spin glass insulator Y$_{2}$Mo$_{2}$O$_{7}$ were understood in terms of the Orbitally Degenerate Hubbard Model (ODHM). According to the model, the ferromagnetic correlation of nearest neighbors gives the lowest energy optical transition and the metallic conductivity of Nd$_{2}$Mo$_{2}$O$_{7}$. Because Ti ions have no $d$-electron, the Ti substitution should decrease both of the carrier density and the magnetic moment of the system. The optical spectra of Ti-substituted samples showed strong decrease in the transition located below 1.0 eV. In the meanwhile, the temperature dependent optical spectra revealed an unusual spectral weight transfer. That is, the total spectral weight below the charge transfer energy from O 2$p$ to Mo 4$d$ states increased as temperature increased. And this unusual spectral behavior became more conspicuous as $x$ increased. Based on the ODHM, possible origins of the large spectral change and the relation of the magnetic and the electronic structures will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 9:36AM - 9:48AM |
N38.00009: Understanding Local Structure and Magnetically-Driven Phase Transitions in LiMO$_2$(M=V, Co) J. Cao, J.T. Haraldsen, K. Giesfeldt, J.L. Musfeldt, W. Tian, D. Mandrus, T. Barnes We report the variable temperature optical properties of high-quality, nearly stoichiometric LiVO$_2$ through the magnetic phase transition to probe the role of the lattice in this process. In contrast to the symmetry-consistent spectra of LiCoO$_2$, LiVO$_2$ shows extra vibrational structure in both low and high temperature phases. These extra peaks can not be accounted for within a traditional symmetry analysis, suggesting that the local structure and bulk structure are different. Leading phase transition mechanisms focus on trimer formation at low temperature phase as well as orbital ordering processes. Considering the Jahn-Teller local distortions in LiVO$_2$, a different picture of the magnetically-driven transition seems to emerge. [Preview Abstract] |
Wednesday, March 23, 2005 9:48AM - 10:00AM |
N38.00010: Spin structure probed by small-angle neutron scattering in Bi$_{0.125}$Ca$_{0.875}$MnO$_3$ Yuhai Qin, Trevor Tyson, Klaus Pranzas, Helmut Eckerlebe The manganite system Bi$_{1-x}$Ca$_x$MnO$_3$ possesses intriguing properties in the high calcium doping region. In this electron doped region (0.6$<$x$<$1), a ferromagnetic (FM) moment of $\sim $1.2 Bohr magnetons per Mn site is found for x $\sim $0.875. The magnetic moment per Mn site maintains a value $\sim $1/3 the theoretical limit even in fields a high as 60 T. The physical origin of this high moment region is not well understood. Various models including canted ferromagnetism and ferromagnetic clusters hosted by an antiferromagnetic background have been proposed. In order to understand the nature of magnetism in this system we have conducted small-angle neutron scattering (SANS) on Bi$_{0.125}$Ca$_{0.875}$MnO$_3$ polycrystalline samples. Both temperature and magnetic field dependent measurements were performed. Nontrivial spin structure was revealed in this system: cluster-like spin structure forms at temperatures above Tc. With a reduction in temperature, the clusters begin to be correlated and grow in size (and changing in shape) as Tc is approached. When an external magnetic field is applied, the clusters grow and the correlation is enhanced. The high moment suggests, that the spins inside the clusters are gradually aligned at temperature is educed or a magnetic filed is applied. This work is supported by NSF DMR-0209243 and NSF INT-0233316. [Preview Abstract] |
Wednesday, March 23, 2005 10:00AM - 10:12AM |
N38.00011: Evidence of Phase Separation on the Surfaces of La0.8MnO3-? Films Michael DeLeon, Trevor Tyson, Catherine Dubourdieu, Luana Margot, Joseph Dvorak Magnetic, transport, surface, and structural studies have been conducted on La$_{0.8}$MnO$_{3}$ films of thickness varying from 52 to 4127{\AA}. Bulk magnetization measurements reveal that maximum T$_{c}$ is obtained by 450{\AA}, though, maximum saturation moment per manganese is not attained until the thickest films. Thinner films have a reduced T$_{c}$. Synchrotron x-ray diffraction measurements on the films exhibit relaxation of the lattice and the onset of additional structural phases with thickness. XMCD versus temperature measurements, which measures magnetization of the top 50{\AA} of the films, gives strong correlation of transition temperatures for some of the films. Differing behavior at the surface however leads to some interesting insights. Surface magnetization is reduced with thickness beyond 450{\AA} as measured at 100K. Additionally, multiple transition temperatures are measured at the surface for one of the films, indicative of some startling phase separation. Additional measurements on local, surface, and nano-structure as well as transport characteristics will be conducted in evaluating this surface behavior in comparison to the bulk film characteristics. [Preview Abstract] |
Wednesday, March 23, 2005 10:12AM - 10:24AM |
N38.00012: Microscopic theory of multipole ordering in NpO$_2$ Katsunori Kubo, Takashi Hotta It has been a longstanding problem in physics of actinide compounds to determine the order parameter of the low-temperature ordered phase of NpO$_2$. Recently, several experimental facts have been found to be reconciled by assuming octupole ordering. To understand the origin of the octupole ordering, we construct an $f$-electron model on an fcc lattice based on a $j$-$j$ coupling scheme, and derive an effective multipole-interaction model. By analyzing the effective model numerically, we determine the interactions relevant to the ground state. Then, we apply mean field theory to the simplified model including only these interactions, and find that the longitudinal triple-$q$ $\Gamma_{5u}$ octupole order is realized in our model by the combined effects of multipole interactions and anisotropy of the $\Gamma_{5u}$ moment. We will discuss a possible relation between the present results and experimental observations for NpO$_2$. [Preview Abstract] |
Wednesday, March 23, 2005 10:24AM - 10:36AM |
N38.00013: Phonon-stabilized electron distributions in uranium Michael Manley, Roland Schulze, Cyril Opeil, Robert Hanrahan, James Smith Contrary to prevailing thinking, recent phonon measurements indicate that thermal electronic excitations may cause a thermodynamically significant softening of phonons in several actinides. A corollary of this effect is that these electronic excitations should be stabilized to higher energies by the entropy generated by phonon softening. Evidence in photoemission spectra will be presented that confirms this for alpha-uranium. While the overall electronic structure is unaffected by heating, electron distributions near the Fermi edge show a large temperature-dependent enhancement in good agreement with phonon-stabilized electron distributions predicted from phonon softening data. [Preview Abstract] |
Wednesday, March 23, 2005 10:36AM - 10:48AM |
N38.00014: Photoelectron spectroscopy of cubic actinide compounds John J. Joyce, Tomasz Durakiewicz, Gerry H. Lander, Clifford G. Olson, Martin T. Butterfield, Ela Guziewicz, Luis A. Morales, Kurt Mattenberger, Oscar Vogt Photoelectron spectroscopy (PES) was applied in investigating the electronic structure of single crystals of USb, NpSb, PuSb, UTe, NpTe and PuTe. Angular-resolved studies were perfomed on U compounds. The photoemission spectral features found within the valence band suggest all six materials contain comparable amounts of 5$f$ and conduction character. Some interesting and unexpected momentum dependent effects are observed in the angular-resolved studies of USb. In PuTe, we confirm the presence of a strong three-peak structure near E$_{F}$, which has been interpreted as the signature of an intermediate valence state in similar materials. Hybridization of the 5$f$ electrons with the conduction band is found within the series and the level of localization is shown to increase from Te to Sb. A surprising correlation between the binding energy of the peak bearing most of the 5$f$ spectral weight and the magnetic moment is discovered within the series, for which some explanations are suggested. [Preview Abstract] |
Wednesday, March 23, 2005 10:48AM - 11:00AM |
N38.00015: Localized magnetic excitation in the hybridization gap of YbAl$_{3}$ J.M. Lawrence, A.D. Christianson, E.A. Goremychkin, E.D. Bauer, J.L. Sarrao YbAl$_{3}$ is an intermediate valence (IV) compound which enters the coherent Fermi liquid phase below T$_{coh}$ = 50K.$^{[1]}$ We have recently measured the magnetic scattering on the MAPS spectrometer at ISIS using high-quality single crystals. For T $<$ 50K, the data can be fit as the sum of a background of nonmagnetic scattering and a pair of peaks at E$_{1}$ = 50meV and E$_{2}$ = 33meV which, in the extended zone scheme, scale with Q as the 4f form factor, as expected for magnetic scattering. The scattering near 50meV exhibits a peak in intensity near Q = (1.2, 0.5, 0.5) which also disperses somewhat with Q. Such Q-dependence is as expected for interband scattering across the hybridization gap in IV compounds. The scattering near 33meV, however, is independent of Q in both intensity and position and hence is the result of a spatially localized excitation. The energy of this excitation coincides with a deep minimum in the optical conductivity$^{[2]}$, and hence the excitation energy lies in the middle of the hybridization gap. Both the magnetic excitation and the deep minimum in the conductivity gradually disappear above 50K, indicating that they are properties of the renormalized ground state. 1 A. L. Cornelius, et al, Phys. Rev. Lett. 88 (2002) 117201. 2. H. Okamura et al, Journ. Phys. Soc. Japan 73 (2004) 2045. [Preview Abstract] |
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