Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G45: Phase Transitions in Ruthenates |
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Sponsoring Units: GMAG Chair: Sung Chang, Ames Lab Room: Baltimore Convention Center 348 |
Tuesday, March 14, 2006 8:00AM - 8:12AM |
G45.00001: Time-resolved Kerr effect in SrRuO$_3$: observation of oscillatory dynamics M. Langner, C.L.S. Kantner, C.P. Weber, J. Orenstein , L.W. Martin, R. Ramesh We report measurements of magnetization dynamics in thin films of the perovskite transition metal oxide SrRuO$_3$, a metallic compound that is ferromagnetic below approximately 150 K. The dynamics of the magnetization vector, \overrightarrow{M}, were measured using the time-resolved magneto-optic Kerr effect. In this technique a pump laser pulse, at photon energy 1.5 eV, perturbs the magnet by reducing the magnitude of \overrightarrow{M} and changing the direction of the anisotropy field. The subsequent dynamics of \overrightarrow{M} are measured by detecting the rotation of the plane of polarization of a time-delayed probe beam that is reflected from the surface of the sample. Below $\sim$ 100 K, we observe a damped oscillation in the Kerr rotation with frequency 250 GHz. The damping decreases with decreasing temperature down to $\sim$ 50 K and remains constant below this temperature. We tentatively identify this oscillation as the \textit{q}=0 magnon, or ferromagnetic resonance frequency (FMR), of SrRuO3. The rather large value of the FMR frequency is consistent with the known large magnetocrystalline anisotropy of this compound. We will report measurements of \overrightarrow{M}(t) as a function of film thickness, residual resistance, and orientation of crystalline axes. [Preview Abstract] |
Tuesday, March 14, 2006 8:12AM - 8:24AM |
G45.00002: Itinerant Ferromagnetism in Cr Doped Perovskite Ruthenates V. Durairaj, E. Elhami, S. Chikara, X.N. Lin, A. Douglass, G. Cao, P. Schlottmann, E.S. Choi, R.P. Guertin We report results of structural, magnetic and transport properties of single crystal CaRu$_{1-x}$Cr$_{x}$O$_{3}$ (0$\le $x$\le $0.36) and SrRu$_{1-x}$Cr$_{x}$O$_{3}$ (0$\le $x$\le $0.20). Cr substitution as low as x=0.08 drives CaRu$_{1-x}$Cr$_{x}$O$_{3}$ from paramagnetic state to an itinerant ferromagnetic state with an abrupt jump in isothermal magnetization leading to a sizeable saturation moment ($\sim $0.4$\mu _{B}$/f.u. within the ab plane). Ferromagnetism occurs abruptly and reaches as high as T$_{C}$=123 K for x=0.22. The Cr-driven ferromagnetism is highly anisotropic, suggesting an important role for spin-orbit coupling. Lattice constant and magnetic measurements strongly support the valence of the Cr to be tetravalent (Cr$^{4+}$, 3d$^{2}$ configuration). The Cr substitution for Ru in SrRuO$_{3}$ (T$_{C}$=165 K) enhances the itinerant ferromagnetism, consistent with Cr-induced ferromagnetism in paramagnetic CaRuO$_{3}$, but without significant magnetic anisotropy. Preliminary pressure-dependent magnetization of CaRu$_{0.85}$Cr$_{0.15}$O$_{3}$ shows strong enhancement of the saturation magnetization (25{\%} for P$\sim $0.7 GPa). All results indicate a coupling of Ru 4d and Cr 3d electrons that is unexpectedly favorable for itinerant ferromagnetism which often exists delicately in the ruthenates. [Preview Abstract] |
Tuesday, March 14, 2006 8:24AM - 8:36AM |
G45.00003: Borderline magnetism in Sr$_{4}$Ru$_{3}$O$_{10}$: Impact of dilute La and Ca doping on itinerant metamagntism and ferromagnetism Shalinee Chikara, V. Durairaj, W.H. Song, Y.P. Sun, X.N. Lin, A. Douglass, G. Cao The triple-layered Sr$_{4}$Ru$_{3}$O$_{10}$ features ferromagnetic behavior with Tc=105 K along the c-axis (interlayer) and a first-order metamagnetic transition below 50 K in the basal plane [1]. The coexistence of ferromagnetism and metamagnetism suggests Sr$_{4}$Ru$_{3}$O$_{10}$ be on the borderline that separates its closest neighbors: the itinerant ferromagnet SrRuO$_{3}$ and the enhanced paramagnet Sr$_{3}$Ru$_{2}$O$_{7}$. We report our results on Sr$_{4}$Ru$_{3}$O$_{10 }$with Sr slightly substituted by La and Ca ions. The La doping drastically changes the magnetic and transport properties. In particular, it effectively suppresses Tc along the c-axis, but simultaneously induces ferromagnetic behavior in the basal plane with Tc=30 K, entirely different from that of the c-axis. Associated with these changes, the metamagnetism disappears in the basal plane but appears along the c-axis. In sharp contrast, the Ca doping significantly enhances the ferromagnetism along c-axis but drastically weakens the magnetic ordering in the basal plane where an unusual T$^{3/2}$-power law for the resisitivity is developed. The vastly different responses to the La and Ca doping, despite similar ionic radii of La$^{3+}$ and Ca$^{2+}$, highlight the role of the extra electron from the La ion added to the t$_{2g}$ orbitals. [1] G. Cao, et al., Phys. Rev. B \textbf{68} 174409 (2003) [Preview Abstract] |
Tuesday, March 14, 2006 8:36AM - 8:48AM |
G45.00004: Infrared Hall effect in SrRuO$_3$ and CaRuO$_3$ M.-H. Yang, G. Acbas, A. Markelz, J. Cerne, I. Ohkubo, P. Khalifah, H. Christen, D. Mandrus, Z. Fang The mid-infrared (MIR: 115-366 meV) Hall effect is studied in SrRuO$_3$ and CaRuO$_3$ films. In SrRuO$_3$, below 200 meV the MIR Hall response changes sign as a function of temperature near 120 K, similar to the dc Hall effect. Above 200 meV, no sign change occurs and the temperature dependence of the MIR Hall response is similar to the dc magnetization. On the other hand, in CaRuO$_3$, the MIR Hall effect is nearly independent of frequency above 100 meV, has a similar temperature dependence as the dc magnetic susceptibility, and does not change sign, unlike the dc Hall effect. In SrRuO$_3$, the complex Faraday and Kerr angles measured in the MIR at 10 K are in good qualitative and quantitative agreement with first-principles band calculations [Z. Fang et al., Science 2003]. [Preview Abstract] |
Tuesday, March 14, 2006 8:48AM - 9:00AM |
G45.00005: Oscillatory magnetoresistance periodic in $1/B$ and $B$ and specific heat of Ca$_{3}$Ru$_{2}$O$_{7}$ X.N. Lin, V. Durairaj, S. Chikara, A. Douglass, G. Cao, V. Varadarajan, V.A. Bondarenko, J.W. Brill, Z.X. Zhou, P. Schlottmann We report magnetoresistance oscillations in high magnetic fields, B, up to 45 T and over a wide range of temperature in the Mott-like system Ca$_{3}$Ru$_{2}$O$_{7}$ [1]. For B rotating within the \textit{ac}-plane, slow and strong Shubnikov-de Haas (SdH) oscillations periodic in 1/B are observed for T$\le $1.5 K in the presence of metamagnetism. These oscillations are highly angular dependent and intimately correlated with the spin-polarization of the ferromagnetic state. For B$\vert \vert $ [110], oscillations are also observed \textit{but periodic in B }(rather than 1/B)\textit{ which persist up to 15 K. } While the SdH oscillations are a manifestation of the presence of small Fermi surface (FS) pockets in the Mott-like system, the B-periodic oscillations, an exotic quantum phenomenon, may be a result of anomalous coupling of the magnetic field to the $t_{2g}$-orbitals that makes the extremal cross-section of the FS field-dependent. In addition, measurements of the specific heat show sharp anomalies at 48 K and 56 K, the former with a strong frequency dependence consistent with a sluggish first-order transition. Much less entropy is removed at the upper, second-order transition. All results will be presented and discussed along with comparisons drawn from impurity doped Ca$_{3}$Ru$_{2}$O$_{7}$. [1] XN Lin,et al.,Phys.Rev.Lett.95,017203(2005) [Preview Abstract] |
Tuesday, March 14, 2006 9:00AM - 9:12AM |
G45.00006: ARPES study on Ca$_{1.8}$Sr$_{0.2}$RuO$_{4}$ Madhab Neupane, A.K.P. Shekhran, Z.-H. Pan, J. -H. Ma, H. Ding, R. Jin, D. Mandrus Owing to the discovery of triplet superconductivity in Sr$_{2}$RuO$_{4}$, much effort has been devoted in the past few years to the understanding of the Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ family. These compounds exhibit a rich phase diagram which connects the p-wave superconductor Sr$_{2}$RuO$_{4}$ to the Mott insulator Ca$_{2}$RuO$_{4}$. Particularly, the x = 0.2 compound is at the boundary between a magnetic metal and a canted antiferomagnetic insulator. It has been suggested recently that this compound$_{ }$has d-electron heavy-fermion behavior due to modulations in the magnetic correlations induced by the structural distortion. For such behavior, flat electronic bands are expected. However, our angle resolved photoemission measurements observed only highly dispersive bands. [Preview Abstract] |
Tuesday, March 14, 2006 9:12AM - 9:24AM |
G45.00007: Investigation of the Competition between Structural and Ferromagnetic Transitions in GdRu$_2$ using the Mossbauer Effect Dermot Coffey, Michael DeMarco, Ryan Heary, Pei-Chun Ho, Todd Sayles, M. Brian Maple, Steve Toorongian, Michael Haka We use the $^{99}$Ru Mossbauer Effect(ME) to investigate magnetism in the Laves phase Ce$_{1-x}$Gd$_x$Ru$_2$ series. This series of compounds is superconducting for small $x$ and has been thought to be ferromagnetic for large $x$ on the basis of extrapolations of Curie-Weiss fits to the $\chi$(T) data. We have studied the temperature dependence of the ME spectra for a number of these compounds and found no evidence of a hyperfine magnetic field due to ferromagnetic order. In GdRu$_2$, a Curie-Weiss fit to the $\chi$(T) data above 120K yields a Curie-Weiss temperature of 93K. However, at $\sim 120K$, $\chi^{-1}$ deviates from a straight line fit. The ME spectra show that there is a qualitative change in the environment of the Ru nucleus in this temperature re gion pointing to a structural phase transition. Below this transition the width of the ME spectrum grows with decreasing temperature by 50$\%$ between 100K and 4.2K. We analyze this temperature dependence in terms of a slowing down of the fluctuations of the local field due to Gd moments. [Preview Abstract] |
Tuesday, March 14, 2006 9:24AM - 9:36AM |
G45.00008: Evidence for high temperature orbital fluctuations in La$_{4}$Ru$_{2}$O$_{10}$ Peter Khalifah, Brian Sales, David Mandrus, Qingzhen Huang, Raymond Osborn, Stephen Nagler, Mark Lumsden, Christopher Kendizora, Emil Bozin, Simon Billinge, Robert Cava Among ruthenates, the compound La$_{4}$Ru$_{2}$O$_{10}$ is unique in having a complete orbital ordering transition. Upon cooling below $T_{OO}$ = 160K, the magnetism of this compound is abruptly quenched. Initial crystallographic studies of this phase [1] showed that strong distortions in the Ru-O bond lengths appear only below $T_{OO}$. We have found distinctly different thermal conductivities of single-crystal La$_{4}$Ru$_{2}$O$_{10}$ above and below $T_{OO}$. At high temperatures, this material unexpectedly behaved as a thermal glass, providing strong evidence for dynamic orbital fluctuations in La$_{4}$Ru$_{2}$O$_{10}$. This is supported by a detailed analysis of the atomic displacement parameters (ADPs) of this material. We will discuss our current model for orbital ordering in La$_{4}$Ru$_{2}$O$_{10}$, the differences in the local and average structure of this material, and the reasons for the strong magnetoelastic coupling in this phase. [1] P. Khalifah, R. Osborn, Q. Huang, H. W. Zandbergen, R. Jin, Y. Liu, D. Mandrus, and R. J. Cava. ``Orbital ordering transition in La$_{4}$Ru$_{2}$O$_{10}$'', \textit{Science}, \textbf{297}, 2237-40 (2002). [Preview Abstract] |
Tuesday, March 14, 2006 9:36AM - 9:48AM |
G45.00009: Magnetic Field Induced Phases of Ca$_{2}$RuO$_{4}$ and Ca$_{3}$Ru$_{2}$O$_{7}$ J. F. Karpus, R. Gupta, H. Barath, S. L. Cooper, G. Cao We present a Raman scattering study of the magnetic field induced phases in the layered ruthenates Ca$_{2}$RuO$_{4}$ and Ca$_{3}$Ru$_{2}$O$_{7}$. Single-layer Ca$_{2}$RuO$_{4}$ has an antiferromagnetic ground state, is a paramagnetic (PM) insulator above T$_{N}$ = 113 K and is a PM metal above T$_{MI}$ = 357 K, while double-layer Ca$_{3}$Ru$_{2}$O$_{7}$ has an antiferromagnetic insulating ground state, is metallic above T$_{MI}$ = 48 K, and is paramagnetic above T$_{N}$ = 56 K. Applied fields oriented in the a-b planes of both these materials produce dramatic changes to both the magnetic and vibrational spectra; these changes are associated with field-induced changes in the Ru orbital populations of these materials. In this talk, we compare the field induced orbital, magnetic, and conducting phases observed in Ca$_{2}$RuO$_{4}$ and Ca$_{3}$Ru$_{2}$O$_{7}$. *Work supported by NSF DMR02-44502, NSF DMR02-40813, and DOE DEFG02-91ER45439. [Preview Abstract] |
Tuesday, March 14, 2006 9:48AM - 10:00AM |
G45.00010: Crystal structure, electronic and magnetic properties of SrRuO$_3$ under epitaxial strain Alexey Zayak, Karin Rabe, Xiangyang Huang, Jeffrey Neaton Using density functional theory, properties of SrRuO$_3$ have been investigated. We focused both on general properies of the orthorhombic distortion in SrRuO$_3$ and the effects of applying epitaxial constraints, whereby the influence of large in-plane strain resulting from coherent epitaxy have been isolated and investigated. The overall pattern of the structural relaxations revealed coherent distortions of the oxygen octahedra network which are sensitive to the magnetic order. The magnetic parameters exhibit substantial changes allowing us to discuss possibilities of magneto-structural tuning of the SrRuO$_3$-based thin film structures. [Preview Abstract] |
Tuesday, March 14, 2006 10:00AM - 10:12AM |
G45.00011: Spectroscopic Investigation on the Electronic Structure of Sr$_{2}$MO$_{4}$ (M=Ru, Rh, and Ir) S.J. Moon, M.W. Kim, K.W. Kim, T.W. Noh, J.-Y. Kim, J.-H. Park, I. Nagai, S.I. Ikeda, G. Cao We investigated the intriguing metal-insulator transition and the electronic structure change of the layered perovskite Sr$_{2}$MO$_{4}$ (M=Ru, Rh, and Ir) by optical and O 1s x-ray absorption (XAS) spectroscopy. Sr$_{2}$RuO$_{4}$ (four 4d electrons) is a superconductor. Sr$_{2}$RhO$_{4}$ (five 4d electrons) is a poor metal. Sr$_{2}$IrO$_{4}$ (five 5d electrons) is a small gap insulator. All the compounds have similar crystal structures, yet their unoccupied d-bands show intriguing change as the M is changed from Ru to Rh, and to Ir. In this presentation, we will discuss the possible origin of the electronic structure change in terms of the change in local structural deformation, Coulomb energy, electron occupation, and the orbital degeneracy by comparing the optical conductivity with XAS spectra. We will also suggest a possible link between the electronic structure change and the metal (Sr2RuO4)-insulator (Sr2IrO4) transition. [Preview Abstract] |
Tuesday, March 14, 2006 10:12AM - 10:24AM |
G45.00012: Metamagnetism and domain formation in Sr$_{3}$Ru$_{2}$O$_{7}$ Andrew Mackenzie, Rodolfo Borzi, Santiago Grigera, Robin Perry, Yoshiteru Maeno The bilayer perovskite Sr$_{3}$Ru$_{2}$O$_{7}$ offers the possibility of creating a quantum critical point by tuning the end-point of a line of first-order metamagnetic transitions towards zero temperature$^{1,2}$. In the purest samples, with residual resistivity less than 1 $\mu \Omega $cm, the quantum critical point is not reached, due to a bifurcation of the original first-order phase transition, possibly associated with the formation of a new phase $^{3,4}$. Here we present the results of new measurements which give strong evidence for the existence of magnetic domains throughout the region of the phase diagram enclosed by the bifurcated transition lines. The effect of these domains on the electrical resistivity can be controlled by varying the relative directions of current and the in-plane component of the applied magnetic field. [1] S.A. Grigera \textit{et al.}, Science Science \textbf{294}, 329 (2001). [2] S.A. Grigera \textit{et al.} Phys. Rev. B \textbf{67}, 214427 (2003). [3] S.A. Grigera \textit{et al.} Science \textbf{306}, 1155 (2004). [4] A.G. Green \textit{et al.} Phys. Rev. Lett. \textbf{95}, 086402 (2005). [Preview Abstract] |
Tuesday, March 14, 2006 10:24AM - 10:36AM |
G45.00013: Magnetic domain formation in the itinerant metamagnetic transition of Sr$_{4}$Ru$_{3}$O$_{10}$ Zhiqiang Mao, Meng Zhou, Joe Hooper, Minghui Yu, Charles O'Connor We have performed systematic investigation on the electronic transport properties of the triple-layered ruthenate Sr$_{4}$Ru$_{3}$O$_{10}$. We observed surprising anomalous features near its itinerant metamagnetic transition, including ultrasharp magnetoresistivity steps and a non-metallic temperature dependence in resistivity for upward field sweeps. These features suggest that the metamagnetic transition of Sr$_{4}$Ru$_{3}$O$_{10}$ occurs via a phase separation process with magnetic domain formation. We have also identified the phase separated regime in the magnetic field-temperature phase diagram. [Preview Abstract] |
Tuesday, March 14, 2006 10:36AM - 10:48AM |
G45.00014: Modification of the Fermi Surface of a Ferromagnetic Metal at a Metamagnetic Transition: Shubnikov de Haas Effect in High Quality Single Crystals of Sr$_{4}$Ru$_{3}$O$_{10}$ Youn-jung Jo, Luis Balicas, Naoki Kikugawa, Kevin Storr, Andrew Mackenzie, Zhiqiang Mao We performed a detailed electrical transport study at low temperatures and high magnetic fields in high quality single crystals of the tri-layered ferromagnetic metal Sr$_{4}$Ru$_{3}$O$_{10}$. We observed Shubnikov de Haas oscillations for several frequencies. The most prominent orbits have two-dimensional character and display frequencies $F_{\alpha }$ = 9959 T, $F_{\beta }$ = 3949 T, $F_{\gamma }$ = 1877 T corresponding respectively to 73.5, 29.2 and 13.9{\%} of the area of the orthorhombic first Brillouin zone (FBZ). Several additional orbits, mostly with frequencies below 2 kT have also been identified, whose spectral weight and frequencies are shifted by a metamagnetic transition induced by an in-plane field component. [Preview Abstract] |
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