Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L15: Magnetism Experiment and Theory |
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Sponsoring Units: GMAG Chair: Dave Pappas, National Institute of Standards and Technology, Boulder Room: Colorado Convention Center Korbel 4E |
Tuesday, March 6, 2007 2:30PM - 2:42PM |
L15.00001: Precise measurements of dynamic susceptibility near Curie temperature Matthew D. Vannette, Athena Safa-Sefat, Joerg Schmalian, Sergey L. Bud'ko, Paul C. Canfield, Rusian Prozorov We report tunnel diode resonator measurements of the 10 MHz dynamic magnetic susceptibility in the vicinity of the Curie temperature on a variety of ferromagnetic compounds with $T_{c}$ ranging from 4.5 K for CeVSb3 to 325 K for LaMn2Ge2. The outstanding sensitivity of the technique allows for a detection of the magnetic signal at pico-emu level. A sharp peak in susceptibility in the critical region is rapidly suppressed by applying relatively weak DC magnetic fields. Measurements are compared to specific heat, resistivity, and DC magnetization data. Results of scaling analysis in the critical fluctuations region are presented. [Preview Abstract] |
Tuesday, March 6, 2007 2:42PM - 2:54PM |
L15.00002: Anomalous magnetoresistance and quadratic Hall effect in a time-reversal symmetry breaking state without net magnetization Benedikt Binz, Ashvin Vishwanath Recently, the authors proposed a magnetic structure that breaks time reversal symmetry in the absence of net magnetization as an explanation for the high pressure ``partially ordered'' state of MnSi. The structure has a magnetic octupolar moment, but no net dipole moment. Based on symmetry, we demonstrate that this leads to anomalous magneto-transport properties: a magnetoresistivity which is linear and a Hall conductance which is quadratic in the applied magnetic field. Field cooling procedures for obtaining single domain samples are also discussed. The anomalous effects are elaborated in the case of three geometries chosen to produce experimentally unambiguous signals of this unusual magnetic state; e.g., it is predicted that a field in z-direction induces an anisotropy in the x-y plane. Another geometry leads to a Hall voltage which is parallel to the magnetic field. [Preview Abstract] |
Tuesday, March 6, 2007 2:54PM - 3:06PM |
L15.00003: Structure and magnetic properties on Y$_{0.26}$Sr$_{0.72}$CoO$_{3-\delta }$ single crystal Yufeng Zhang, Sho Sasaki, Mitsuru Izumi We studied structural and magnetic properties on a single crystal of Y$_{0.33}$Sr$_{0.67}$CoO$_{3-\delta}$ in nominal composition. Single crystal was grown by the floating-zone method with radiation heating. Both electron-probe microanalysis (EPMA) and inductively coupled plasma (ICP) spectroscopy indicated the obtained single crystal was Y$_{0.26}$Sr$_{0.72}$CoO$_{3-\delta}$ in composition. The iodometric titration gives the oxygen content of 2.95. These results show that almost all Co ions are in trivalent state. The dc magnetization increases remarkably to reach a maximum at 301 K, then falls abruptly with decreasing temperature after both cooling and zero-field cooling. These behaviors were interpreted as a successive transition in which with decreasing temperature there is a transition from paramagnetic to ferromagnetic and eventually the antiferromagnetic properties appear at low temperatures. This should be associated with a successive spin state transition of Co ions [1, 2]. The detailed structural study and magnetic anisotropy will be reported and discussed with respect to a possible spin state change of Co ions. [1] Y. F. Zhang et al., accepted by Phys Rev. B for publication. [2] Y. F. Zhang et al., published in JMMM. [Preview Abstract] |
Tuesday, March 6, 2007 3:06PM - 3:18PM |
L15.00004: Magnetic ordering in PrBCO by MaxEnt Muon-Spin Research H. Pham, L. Rafik, C. Boekema Muon-Spin Research ($\mu $SR) is used to probe the magnetic ordering of PrBa2Cu3O7. The $\mu $SR PrBCO data are analyzed using the Maximum Entropy (ME) technique, a spectral analysis tool more sensitive than Fourier transformation. [1] At low temperature and zero applied field, muons are localized and their Larmor spin-precessions map the internal magnetic fields. For temperatures well below room temperature, ME-$\mu $SR analysis yields two unique frequencies ($\sim $1.8 MHz and $\sim $2.4 MHz) corresponding to two different magnetic field regions (13 mT and 18 mT ) probed by the muon. Previous studies [2, 3] using Fourier analysis and curve fitting have shown only one broad frequency signal. We seek to confirm our new improved findings through dipole field search calculations and ME-$\mu $SR simulations at these near-zero $\mu $SR frequencies. [1] JC Lee \textit{et al,} J Appl Phys 95 (2004) 6906; AIP/APS www: Virtual J Applications of Superconductivity, June 2004 V6 Iss11; S. Alves \textit{et al,} Phys Rev \textit{Rapid Comm }B49 (1994) 12396. [2] WK Dawson \textit{et al,} J Appl Phys 69 (1991) 5385. [3] DW Cooke \textit{et al}, Phys Rev B41 (1990) 4801. [Preview Abstract] |
Tuesday, March 6, 2007 3:18PM - 3:30PM |
L15.00005: ABSTRACT WITHDRAWN |
Tuesday, March 6, 2007 3:30PM - 3:42PM |
L15.00006: Observation of Magnetic Excitations in Na$_3$RuO$_4$ using Inelastic Neutron Scattering J.T. Haraldsen, M.B. Stone, M.D. Lumsden, T. Barnes, R. Jin, S.E. Nagler, J.W. Taylor, F. Fernandez-Alonso We report results on magnetic excitations observed in polycrystalline sodium ruthenate (Na$_3$RuO$_4$) in an inelastic neutron scattering study. Previous work has suggested that this material consists of relatively isolated tetramers of S=3/2 Ru(V) ions, and a Heisenberg antiferromagnet Hamiltonian was proposed. We compare predictions for the neutron inelastic structure factor in this model with our observations, and suggest future studies that might clarify apparent discrepancies between this model and our results. [Preview Abstract] |
Tuesday, March 6, 2007 3:42PM - 3:54PM |
L15.00007: High Frequency EPR studies of an antiferromagnetic supramolecular grid Saiti Datta, Stephen Hill, Oliver Waldmann, Victoria Milway, Laurie Thompson The magnetic anisotropy of an antiferromagnetic Mn(II)-[3 x 3] grid was studied by High Frequency Electron Paramagnetic Resonance(HFEPR). The ground state level crossing was observed from various temperature dependent studies. Frequency dependent studies illustrate a breaking of the $\Delta $S~=~0 EPR selection rule due to the strong mixing induced by the comparable exchange and magneto-anisotropy energy scales within the grid. In addition, the field-orientation dependence of the energy gap between the ground and first excited state was studied. All of these studies have contributed to a better understanding of this supramolecular grid, which represents a promising candidate to observe the long sought after quantum tunneling of the N\'{e}el vector. [Preview Abstract] |
Tuesday, March 6, 2007 3:54PM - 4:06PM |
L15.00008: Latent heat and magnetocaloric properties of MnAs, CaMn$_{2}$Sb$_{2}$ and Mn-based compounds Ana Lima Sharma, Angelo Gomes, Sergio Gama The intense search for new magnetic materials for magnetic refrigeration has brought out several aspects of the magnetocaloric effect (MCE) behavior and interest in the underlined physics. We investigated the behavior of the magnetocaloric effect of materials such as MnAs and alloys, CaMn$_{2}$Sb$_{2}$ and Mn-based compound which have been a fruitful playground for competing interactions that lead to a variety of magnetic ordering such as spin liquid behavior, glass phase, etc. The MCE of those systems We extracted The MCE of those systems from heat capacity data and magnetization data using Maxwell's relations and we also investigated the latent heat involved in magnetic and structural phase transitions using a differential scanner calorimeter. Our results can discussed using a mean field approach in a quenched (or frozen) disorder parameters frame. In the present work, the MCE will be present as characterization tool for materials which exhibt unusual magetic ordering phases. [Preview Abstract] |
Tuesday, March 6, 2007 4:06PM - 4:18PM |
L15.00009: Coexistence of a long-range and short-range ordered state in MnV$_2$O$_4$ Seung-Ho Baek, Kwang-Yong Choi, Arneil Reyes, Phillip Kuhns, N. Dalal, Haidong Zhou, Chris Wiebe We report $^{51}$V zero-field NMR of manganese vanadate spinel, MnV$_2$O$_4$ together with both ac and dc magnetization measurements. The zero-field NMR spectrum consists of multiple lines distributed from 240 MHz to 320 MHz. Its temperature dependence reveals that the ground state is formed by the delicate balance between long-range ferrimagnetic order and second short-range order, which causes reentrant-spin-glass-like behavior. The unusual ground state is ascribed to the competition between Mn$^{2+}$ and V$^{3+}$ exchange couplings and the orbital ordering of the V site induced by the structural phase transition. [Preview Abstract] |
Tuesday, March 6, 2007 4:18PM - 4:30PM |
L15.00010: Internal Magnetostatic Potentials of Magnetization-Graded Hexagonal Ferrites C. Sudakar, J.V. Mantese, A.L. Micheli, R. Naik, G. Srinvasan, S.P. Alpay, G. Lawes Compositionally graded ferromagnetic material offer novel functional properties that have promising device applications. Our investigations of the internal magnetic field induced by a spatially varying magnetization in a compositionally graded ferromagnet will be presented. We discuss results on a hexagonal ferrite sample, with a variation in saturation magnetization of 30 emu/g over a sample thickness of 2.5 mm. The DC magnetization shows a small anisotropy depending on the direction of the externally applied magnetic field relative to the grading direction. This contribution from a grading induced magnetic field is more pronounced in AC susceptibility measurements, which show small gradient dependent shift in M$\prime $ and M$\prime \prime $. We find a shift in magnetic properties corresponding to an internal magnetic field of 30 Oe, which is significantly lower than the predicted value of approximately 1900 Oe. The reasons for this difference and the implications of domains as the possible origin for the small H$_{int}$ will be will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 4:30PM - 4:42PM |
L15.00011: Magneto-Optical Approach in Characterization on Ferrites in Millimeter Waves Konstantin Korolev, Mohammed Afsar Complex magnetic permeability and dielectric permittivity characteristics of yttrium iron garnets (YIG), aluminum and gadolinium substituted YIG and nickel ferrite materials in broad band millimeter wave frequency range have been obtained. The measurements have been done using free-space quasi-optical millimeter wave spectrometer in magnetic fields up to 1 T. A set of backward wave oscillators have been used as sources of high power coherent radiation, tunable in 30 -- 120 GHz frequency range. Magneto-optical approach has been successfully employed for the separation of dielectric and magnetic effects and simultaneous determination of complex dielectric permittivity and magnetic permeability of ferrite materials. Frequency dependences of real and imaginary parts of magnetic permeability of ferrites have been obtained. Strong frequency dependence of complex magnetic permeability in external magnetic field has been found for these ferrites materials. [Preview Abstract] |
Tuesday, March 6, 2007 4:42PM - 4:54PM |
L15.00012: Optical Probe of Magnetization Dynamics in SrRuO3 Thin Films M.C. Langner, C.L.S. Kantner, L.M. Martin, Y.-H. Chu, M. Huijben, R. Ramesh, J. Orenstein We report all-optical measurements of ferromagnetic resonance (FMR) in the oxide ferromagnet SrRuO3 using the time-resolved magneto-optic Kerr effect. A Kerr signal at the FMR frequency is triggered by a laser-induced change in the direction of the magnetocrystalline anisotropy. We measure the subsequent precession of the magnetization vector as a function of temperature and laser intensity. We perform measurements in very thin films, with thicknesses from 2-10 nm, and show that the cooling of the film is limited by the boundary resistance between the film and substrate. This cooling of the SRO film via the substrate varies linearly with the film thickness, allowing us to separate thermal effects from the recovery kinetics. [Preview Abstract] |
Tuesday, March 6, 2007 4:54PM - 5:06PM |
L15.00013: Density functional studies of magnetic behavior of layered Ag(II) fluorides Sefa Dag, Vincent Meunier, William A. Shelton, Douglas Scalapino We present an extensive study of the structural and magnetic properties of layered Ag(II) fluoride Cs$_{2}$AgF$_{4}$ by using density functional theory within the local spin density approximation. We find that this material is well described as a two-dimensional ferromagnetism. Our investigations of a number of ground state properties are in good agreement with reported experiments. We also predict Jahn-Teller distortions in the low pressure phase and find that elastic anisotropy and the electric field created by the displacement of F ions does not modify the orbital order and origin of ferromagnetism. [Preview Abstract] |
Tuesday, March 6, 2007 5:06PM - 5:18PM |
L15.00014: Electronic Structure, Chemical Bonding and Magnetic Properties in the Intermetallic Series Sc$_{2}$Fe(Ru$_{1-x}$Rh$_{x}$)$_{5}$B$_{2}$ from First Principles German Samolyuk, Boniface Fokwa, Richard Dronskowski, Gordon Miller First-principles, density-functional studies of the electronic structure, chemical bonding, ground-state magnetic ordering and exchange-interaction parameters have been performed for the entire Sc$_{2}$Fe(Ru$_{1-x}$Rh$_{x})_{5}$B$_{2}$ series of magnetic compounds. The results indicate that their magnetic properties depend in an extremely sensitive way on the degree of band filling and bandwidth. Continuous substitution of Ru by Rh changes the ground state from an antiferromagnet to a ferromagnet, as well as increases the effective spin moment caused by filling the bands with five additional electrons per formula unit together with a narrowing of the 4d band. The correlations between the character of the chemical bonding and the resulting exchange couplings are discussed. Trends for the macroscopic ordering temperatures are correctly reproduced. [Preview Abstract] |
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