Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session W45: Spin Structure in Magnetic Materials |
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Sponsoring Units: GMAG Chair: Mike Fitzsimmons, Los Alamos National Laboratory Room: Baltimore Convention Center 348 |
Thursday, March 16, 2006 2:30PM - 2:42PM |
W45.00001: Magnetism in the diluted induced moment system (La,Pr)$_{6}$Ni$_{2}$Si$_{3}$ R. W. McCallum, Y. Janssen, K. W. Dennis, P. C. Canfield Pr$_{6}$Ni$_{2}$Si$_{3}$ crystallizes in a complex hexagonal structure with two distinct Pr sites with no point symmetry, thus the 2J+1=9 ground state levels are CEF-split into singlets. Single-crystal magnetization and specific heat measurements suggest that this splitting is large compared to the ordering temperature. As CEF-induced singlets are nonmagnetic, any ordered magnetic moment is induced by magnetic interactions. Pr$_{6}$Ni$_{2}$Si$_{3}$ orders with a ferromagnetic component parallel to the c-axis at $\sim $40K, whereas La$_{6}$Ni$_{2}$Si$_{3}$ appears to be nonmagnetic. Upon substitution of Pr by La the ordering temperature decreases more rapidly than predicted by a rule of mixtures. For substitutions of more than 50{\%} La, no ordering is observed above 5 K. For 50{\%} La, both AC and DC magnetization measurements suggest superparamagnetic or spin glass type behavior. This behavior will be discussed in terms of clusters of Pr ions with size-dependent moment and anisotropy. [Preview Abstract] |
Thursday, March 16, 2006 2:42PM - 2:54PM |
W45.00002: Ferromagnetism in Ba$_2$NaOsO$_6$ Ann Erickson, Gordon Miller, Sumohan Misra, Zack Schlesinger, Raj Gupta, Ian Fisher Due to the extended nature of 5d orbitals, magnetism in systems of 5d electrons is uncommon. Here we present results of structural, thermodynamic and optical reflectivity experiments on single crystals of the novel magnetic material Ba$_2$NaOsO$_6 $. The material has a double perovskite structure, space group Fm-3m, with full occupancy of all sites. The osmium ions have a 7$^+$ valence, corresponding to a 5d$^1$ electron configuration. The effective moment at high temperatures is 1.10 $\mu_B$, somewhat less than the spin-only value due to spin-orbit coupling, with no apparent anisotropy. The Weiss temperatures are -11.0 and -12.4 $\pm$ 0.4 K for fields orinted along the [100] and [111] directions respectively. At 6.8 K a sharp anomaly in the heat capacity indicates the onset of long range magnetic order. The ordered state is characterized by a small ferromagnetic moment of just 0.2 $\mu_B$ per formula unit, with only a slight anisotropy, indicative of a helical magnetic structure. Infra red reflectivitity measurements confirm that the material is an insulator. [Preview Abstract] |
Thursday, March 16, 2006 2:54PM - 3:06PM |
W45.00003: Magnetic properties of (R, R', R",...)Ni$_2$Ge$_2$ solid solutions S.A. Law, S.L. Bud'ko, P.C. Canfield Anisotropic magnetic properties of single crystals of (R, R', R",...)Ni$_2$Ge$_2$ (R = rare earth) solid solutions will be presented. Whereas the magnetic ordering temperatures and the paramagnetic $\Theta$'s broadly follow de Gennes scaling there are some systematic deviations from this oversimplfied trend. In this talk we will examine these deviations and also discuss the apparent absence of a spin-glass state induced by random magnetic anisotropy for the most highly mixed samples. [Preview Abstract] |
Thursday, March 16, 2006 3:06PM - 3:18PM |
W45.00004: Temperature dependence of magnetization of hcp Gd A. Chernyshev, V. Pecharsky, K. Gschneidner, V. Antropov Careful magnetization measurements of high purity Gd single crystal have been performed. Our results confirm a strong deviation from the Bloch law for the magnetization at low temperatures, and we demonstrate that this effect is even stronger than had been previously measured on less pure Gd samples. We have analyzed the physical nature of this deviation qualitatively and quantitatively using known theoretical models and the newly obtained results. However, no fully satisfactory agreement between the new experimental data and existing theories has been reached. [Preview Abstract] |
Thursday, March 16, 2006 3:18PM - 3:30PM |
W45.00005: Magnetic phase separation in electron-doped Bi1-xCaxMnO3 systems Yuhai Qin, Trevor A. Tyson, Kenji Shimizu The manganite system Bi1-xCaxMnO3 possesses intriguing properties in the low bismuth 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 $1/3 the theoretical limit even in fields a high as 60 T. The physical origin of this high moment region is not well understood. Various models including canted ferromagnetism and ferromagnetic clusters hosted by an antiferromagnetic background have been proposed. In our previous work, we have conducted small-angle neutron scattering (SANS) on Bi0.125Ca0.875MnO3 polycrystalline samples as has revealed existence of FM clusters embedded in an AFM background. New 55Mn NMR results give more evidence supporting of this heterogeneous phase model: resonance signals from both AFM and FM phases were identified. More progress from multiple-temperature Bi-L3 edge XAFS measurements will be presented as well. This work is supported by NSF DMR-0209243 and NSF DMR-0512196. [Preview Abstract] |
Thursday, March 16, 2006 3:30PM - 3:42PM |
W45.00006: Physical Properties of Single Crystal EuIn$_{2}$P$_{2}$ and EuGa$_{2}$P$_{2}$ Newell Jensen, Samuel MaQuilon, Peter Klavins, Zachary Fisk, Cathie Condron, Jiong Jiang, Susan Kauzlarich Single crystals of EuIn$_{2}$P$_{2}$ and EuGa$_{2}$P$_{2}$ have been grown by a metal flux method.$^{ }$The EuIn$_{2}$P$_{2}$ material crystallizes in a new hexagonal structure type and orders magnetically at 24 K. The magnetic ordering is anisotropic suggesting a possible canted ferromagnetic magnetic structure. The temperature dependent resistivity data indicate semi-metallic behavior. Negative colossal magnetoresistance is observed at the ordering temperature. The gallium metal analogue, EuGa$_{2}$P$_{2}$, crystallizes in a related monoclinic structure and magnetically orders at a slightly higher temperature. Magnetization, resistivity and specific heat data are presented for both compounds. [Preview Abstract] |
Thursday, March 16, 2006 3:42PM - 3:54PM |
W45.00007: Structural and magnetic properties of single layered manganite Pr0.5Ca1.5MnO4 Songxue Chi, Pengcheng Dai, Feng Ye, Jaime Fernandez-Baca, Roland Mathieu, Yoshi Tokura, QingZhen Huang, Jeffrey Lynn High resolution neutron powder diffraction and elastic neutron scattering have been used to determine the lattice and magnetic structure of the single layer manganite Pr0.5Ca1.5MnO4. The system becomes charge/orbital ordered (CO-OO) near 300K and antiferromagnetically ordered with a Neel temperature ($T_N$) near 125K, which has CE-type (checkerboard like) structure in the Mn-O plane. At temperatures above $T_N$ but below $T_{CO-OO} $, we discovered an anomalous lattice response around 160K. We discuss the microscopic origin of this lattice distortion and its association with competing CO-OO and antiferromagnetic states. [Preview Abstract] |
Thursday, March 16, 2006 3:54PM - 4:06PM |
W45.00008: Magnetic structure of the Kondo lattice compound CeZn$_{0.6}$Sb$_2$ Y. Chen, J. W. Lynn, H. Lee, P. Klavins, Z. Fisk, S. Nakatsuji, W. Bao, J. Thompson, T. Park, R. Macaluso, J. Chan, B. Carter The new Kondo lattice compound CeZn$_{0.6}$Sb$_2$ has a tetragonal structure with space group P4/nmm and shows ferromagnetic behavior below 2.5 K. The Curie-Weiss temperature is 22 K along the tetragonal $ab$ plane, indicating ferromagnetic interactions in the plane. Along the $c$ axis, however, the Curie-Weiss temperature is -145 K, suggesting antiferromagnetic exchange interaction in this direction [1]. We determined the magnetic structure of CeZn$_{0.6}$Sb$_2$ using single crystal neutron diffraction. ($h$,0,$l$) and ($h$,$h$,$l$) scattering planes were investigated. We found CeZn$_{0.6}$Sb$_2$ orders ferromagnetically at T$_C$=2.5 K. The magnetic structure is collinear with a low temperature ordered Ce moment of 1.3 (1) $\mu_B$ that lies in the $ab$ plane. In addition, we measured the order parameter of the ferromagnetic transition. [1] Studies of the ferromagnetic Kondo lattice system of single crystal CeZnSb$_2$, H. Lee, S. Nakatsuji, Y. Chen, W. Bao, R. Macaluso, J. Chan, T. Park, B. Carter, P. Klavins, J. Thompson, Z. Fisk, BAPS, Session L41, 2005. [Preview Abstract] |
Thursday, March 16, 2006 4:06PM - 4:18PM |
W45.00009: APRES Study of Spin Spiral States in TlCo$_{2}$Se$_{2}$ Hae-Kyung Jeong, Tonica Valla, Rolf Berger, Peter Johnson, Kevin Smith The electronic structure of TlCo$_{2}$Se$_{2}$ has been measured using high resolution angle resolved photoemission spectroscopy (ARPES). TlCo$_{2}$Se$_{2}$ is thought to be a spin spiral system, but experimental evidence from the electronic structure supporting the spin spiral states has been lacking. The original indication of a spin spiral state came from neutron powder diffraction experiments, and was confirmed in a subsequent neutron diffraction study of single crystals. However, it was not possible to distinguish between the helical structure and the sine-modulated moments since the intensity of the magnetic pattern satellites from neutron diffraction was very low. We used high resolution ARPES to study the electronic structure of single crystals of TlCo$_{2}$Se$_{2}$ cleaved in ultra high vacuum. Our experimental results clearly show the existence of band crossings near the Fermi level that would support spin spiral states, and reveals the predicted quasi-two dimensional electronic structure. The Boston University program is supported in part by the Department of Energy under DE-FG02-98ER45680. [Preview Abstract] |
Thursday, March 16, 2006 4:18PM - 4:30PM |
W45.00010: Magnetic Ordering in Copper Pyrazine Perchlorate, a S=1/2 2D Quantum Heisenberg Antiferromagnet F. Xiao, C. P. Landee, M. M. Turnbull, M. Novak, S. Soriano Copper pyrazine perchlorate, Cu(Pz)$_{2}$(ClO$_{4})_{2}$, consists of antiferromagnetic layers of Cu$^{2+}$ ions bridged by neutral pyrazine molecules. The exchange strength within the layers is J/k$_{B}$ = 17.5 K; excellent isolation between layers is provided by the bulky perchlorate ions. Specific heat studies and muon spin resonance measurements [1] show the ordering temperature to be 4.25 K, corresponding [2] to an interlayer/intralayer exchange ratio of 1 x 10$^{-3}$. The specific heat data show no characteristic anomaly at T$_{N}$, only a broad contribution attributable to the short-range order within the layers. This result is consistent with recent theoretical predictions [3]. The excellent isolation results in the observation of field-induced XY-behavior in the magnetic susceptibility, as predicted elsewhere [4]. 1. J. Manson \textit{et al}, unpublished results. 2. C. Yasuda \textit{et al}, \textit{Phys. Rev. Lett.} \textbf{94}, 217201 (2005). 3. P. Sengupta \textit{et al}, \textit{Phys. Rev. B}, \textbf{68}, 094423 (2003). 4. A. Cuccoli \textit{et al}, \textit{Phys. Rev. B}, \textbf{68}, 060402 (2003). [Preview Abstract] |
Thursday, March 16, 2006 4:30PM - 4:42PM |
W45.00011: Magnetic field dependence of the order parameter in weakly ordered quasi-1D antiferromagnets Igor Zaliznyak, Andrei Savici, Beatrice Grenier, Louis-Pierre Regnault, Sergei Petrov We report neutron diffraction study of the antiferomagnetic order in two isostructural quasi-one-dimensional S=1 Heisenberg antiferromagnets, CsNiCl3 and RbNiCl3, in magnetic fields up to 15 T. These materials present model systems of Haldane spin chains with very similar in-chain exchange interaction, but coupled by the inter-chain exchange of different strengths. In both cases the inter-chain coupling is super-critical, so that the Haldane gap is suppressed and weak antiferromagnetic order appears below 4.84 K in CsNiCl3 and 11.1 K in RbNiCl3. In zero field the ordered magnetic moments in CsNiCl3 and RbNiCl3 are approximately 0.9 and 1.2 Bohr magnetons, respectively. We find that the antiferromagnetic order is enhanced by application of the magnetic field, as it is expected both for coupled Haldane chains and from the spin-wave theory arguments. However, we show that the spin-wave theory can not reproduce the observed behavior quantitatively. [Preview Abstract] |
Thursday, March 16, 2006 4:42PM - 4:54PM |
W45.00012: Neutron Diffraction Study of A-site Size and Variance Effect on the Spin and Orbital Ordering in RVO$_{3}$ Perovskites J.-Q. Yan, S. Chang, J. Zarestky, O. Garlea, R. McQueeney, A. Liobet, Y. Ren, J. Fieramosca RVO$_{3}$ members all exhibit an intriguing sequence of orbital and magnetic orderings below a Too and a T$_{N}$, respectively. Since only t-electrons are active in this system, they have been a prototype to study cooperative orbital ordering among the $\pi $-bonding t orbitals in the absence of $\sigma $-bonding e electrons and the relationship of this orbital order to long-range magnetic ordering. We have studied the A-site size and variance effect on the spin and orbital ordering in single phase (YLa)VO3 and (YLaLu)VO3 systems by magnetization, specific heat, thermal conductivity, and nenutron diffraction measurements. The results show that both A-site size and variance stabilize G-type magnetically, C-type orbitally ordered state before an electronic phase segregation takes place. The results will be presented with special emphasis on the neutron diffraction experiments for both single crystal and polycrystalline samples. [Preview Abstract] |
Thursday, March 16, 2006 4:54PM - 5:06PM |
W45.00013: Soft X-ray absorption Spectroscopic Investigation on Electronic Evolutions in SrFe$_{1-x}$Mo$_x$O$_{3\pm\delta}$ (0 $\le x \le$ 1) B.-K. Park, J.-H. Park, J.-Y. Kim, H.-J. Lee, J.H. Song, Y.H. Jeong, H.-J. Lin, C.T. Chen A double perovskite Sr$_2$FeMoO$_6$, which an alternative mixture of SrFeO$_3$ and SrMoO$_3$, is a half-metallic ferrimagnet with a high critical temperature $T_C$ $\sim$ 400K, although a certain amount of anti-site disorder diminishes the half-metallicity in a real system. SrFeO$_3$ and SrMoO$_3$, which have the ionic states of Fe$^{4+}$ and Mo$^{4+} $, are known to be an antiferromagnetic insulator and a non- magnetic metal, respectively. However, as they forms the double perovskite, the ionic states primarily form Fe$^{3+}$ (3$d^5_ {\uparrow}$) and Mo$^{5+}$ (4$d^1_{\downarrow}$) , and then the Fe 3$d$-Mo4$d$ hybridization makes a down spin band at the Fermi level. Hence the real ground state becomes a state with Fe$^{2+} $-Fe$^{3+}$ and Mo$^{5+}$-Mo$^{6+}$ mixed valences. It means that the valences of Fe and Mo can vary by two, i.e. from Fe$^ {4+}$ to Fe$^{2+}$ and from Mo$^{4+}$ to Mo$^{6+}$ in SrFe$_{1-x} $Mo$_x$O$_{3\pm\delta}$ (0$\le x \le$1), respectively. Here we presents the structural, electrical, and magnetic phase diagram and electronic evolutions in SrFe$_{1-x}$Mo$_x$O$_{3\pm\delta}$ (0$\le x \le$1). [Preview Abstract] |
Thursday, March 16, 2006 5:06PM - 5:18PM |
W45.00014: Electronic bandstructure of the NiMnSb(001) surface above and below the Fermi energy Juliet Correa, Christian Eibl, Georgi Rangelov, J\"{u}rgen Braun, Markus Donath The predicted 100\% spin polarization at the Fermi energy ($E_F$), together with the high Curie temperature (750K) of the half-Heusler alloy NiMnSb makes it interesting for technological applications. However, experimentally only 50\% surface polarization has been observed. Knowledge of the electronic structure of NiMnSb is key to improving our understanding of this material. We report on surface bandstructure measurements for the (001) surface of single crystal NiMnSb. The experimental techniques used are angle resolved ultraviolet photoemission spectroscopy (ARUPS)and inverse photoemission spectroscopy (ARIPES) to probe the bandstructure both below and above $E_F$. ARUPS and ARIPES are used quasi-simultaneously in a multi- chamber UHV-system to ensure well defined and equal sample preparation conditions. We see both non-dispersive (ARIPES) and dispersive (ARUPS) structures in the spectra, which stem from d-like bulk states. With our normal emission UPS data, we resolve the conflict in the various contradictory reports in the literature about the position of spectral features for both polycrystal and single-crystal samples. Finally, through comparison with theoretical calculations we have identified a surface state candidate. [Preview Abstract] |
Thursday, March 16, 2006 5:18PM - 5:30PM |
W45.00015: Contact-less measurements of Shubnikov-de Haas oscillations below N$\acute{e}$el temperature in single crystals SmAgSb$_{2}$ M. D. Vannette, R. Prozorov, S. L. Bud'ko, P. C. Canfield, B. N. Harmon Oscillations of a skin depth with magnetic field were measured in single crystals SmAgSb$_{2}$ by using radio-frequency resonant technique. Comparison with directly measured de Haas -- van Alphen and Shubnikov -- de Haas oscillations revealed additional details in the frequency spectra, probably due to high sensitivity of the measurements $\Delta \rho_{min} \approx 20\;{\rm{p}}\Omega \cdot {\rm{cm}} $. The temperature evolution of the frequency spectra was obtained. The correlation of the observed oscillations with calculated Fermi surface and possible influence of antiferromagnetic ordering are discussed. [Preview Abstract] |
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