Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session W30: Focus Session: Ruthenates |
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Sponsoring Units: DMP GMAG Chair: Zhiqiang Mao, Tulane University Room: 334 |
Thursday, March 19, 2009 11:15AM - 11:51AM |
W30.00001: Ca$_{3}$(Ru$_{1-x}$Cr$_{x}$)$_{2}$O$_{7}$: A new paradigm for spin valves Invited Speaker: A spin valve is a device structure whose electrical resistance can be manipulated by controlling the relative spin alignment of adjacent metallic, magnetic layers separated by nonmagnetic insulating layers. The spin valve effect is a quantum phenomenon so far only realized in multilayer thin films or heterostructures. Here we report a \textit{novel}, \textit{strong} spin valve effect existing in \textit{bulk} single crystals of Ca$_{3}$(Ru$_{1-x}$Cr$_{x})_{2}$O$_{7}$ having an anisotropic, bilayered crystal structure [1]. This discovery opens new avenues to understand the underlying physics of spin valves, and fully realize its potential in practical devices. \\[4pt] [1] G. Cao, V. Durairaj, S. Chikara, and L.E. DeLong and P. Schlottmann, \textit{Phys. Rev. Lett.} \textbf{100}, 016604 (2008) [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:03PM |
W30.00002: Antiferromagnetism and bulk spin valve effect in Ca$_{3}$(Ru$_{1-x}$Ti$_{x}$)$_{2}$O$_{7}$ J. Peng, T.J. Liu, Z. Qu, E. Vehstedt, B. Qian, D. Fobes, L. Spinu, W. Bao, Z.Q. Mao Ca$_{3}$Ru$_{2}$O$_{7}$ has generated growing interest. It shows an antiferromagnetic (AFM) transition at $T_{N}=56$ K, followed by a metal-insulator (MI) transition at $T_{MI}=48$ K $\left[1\right]$. Giant magnetoresistance (GMR) across its metamagnetic transition is also observed. We have determined the magnetic structures of Ca$_{3}$Ru$_{2}$O$_{7}$ under magnetic fields using neutron scattering $\left[2\right]$. Our results demonstrate that the GMR in this material originates from a bulk spin-valve effect, and clarify the origin for the puzzling observation that the GMR occurs under easy axis field alignment, while a colossal magnetoresistance appears with hard axis field alignment $\left[1\right]$. In addition, we have studied the effect of Ti doping on Ca$_{3}$Ru$_{2}$O$_{7}$. We find that Ti doping dramatically affects both the AFM and the MI transition by shifting them to much higher temperatures, e.g. $T_{N}=114$ K and $T_{MI}=107$ K for 10\% Ti doping. We will discuss possible origins for this remarkable doping effect. This work is supported by the NSF under grant DMR-0645305, the DOE under DE-FG02-07ER46358.\\ $\left[1\right]$ G.Cao \textit{et al}., Phys. Rev. Lett. \textbf{78}, 1751 (1997)\\ $\left[2\right]$ Wei Bao \textit{et al}., Phys. Rev. Lett. \textbf{100}, 247203 (2008) [Preview Abstract] |
Thursday, March 19, 2009 12:03PM - 12:15PM |
W30.00003: Antiferromagnetic Metallic State And Spin Valve Effect in Doped (Ca$_{1-x}$ A$_{x})_{3}$Ru$_{2}$O$_{7}$ (A = Sr, Ba) Single Crystals S. Chikara, O.B. Korneta, T.F. Qi, S. Parkin, G. Cao, W.P. Song, W.P. Crummett Bilayered Ca$_{3}$Ru$_{2}$O$_{7}$ is a highly anisotropic system [1] characterized by orbitally-driven colossal magnetoresistance$^{2}$ and an unusual antiferromagnetic metallic (AFM-M) state [2]. We report transport and thermodynamic properties of (Ca$_{1-x}$ A$_{x})_{3}$Ru$_{2}$O$_{7}$ (A = Sr, Ba) single crystals as a function of temperature and applied magnetic field. While Ba doping shows a far stronger impact, both Sr and Ba substitution for Ca induce a large array of interesting phenomena. Among them, a bulk spin-valve effect occurs in the AFM-M range, which is largely broadened due to the doping. This effect in bulk crystals is a novel phenomenon first observed in Ca$_{3}$(Ru$_{1-x}$Cr$_{x})_{2}$O$_{7}$ single crystals [3]. The spin-valve effect in (Ca$_{1-x}$A$_{x})_{3}$Ru$_{2}$O$_{7}$ single crystals opens new avenues to understand the underlying physics and realize the potential of spin valves in practical devices.\\[0pt] [1] G. Cao et al., \textit{PRL} \textbf{78}, 1751 (1997)\\[0pt] [2] X. N. Lin et al., \textit{PRL} \textbf{95}, 017203 (2005)\\[0pt] [3] G. Cao et al., \textit{PRL} \textbf{100}, 016604 (2008) [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W30.00004: Microscopic theory of metamagnetism and nematic order in Sr$_3$Ru$_2$O$_7$ Srinivas Raghu, Arun Paramekanti, Eun-Ah Kim, Steven Kivelson The bilayer ruthenate compound Sr$_3$Ru$_2$O$_7$ exhibits a remarkable set of low temperature electronic properties. In an externally applied magnetic field, ultra-pure crystals of the compound undergo a metamagnetic transition at a temperature which can be tuned towards zero as B $\parallel$ c approaches a critical value of $\sim$8T. This putative metamagnetic quantum critical point, however, is preempted by a nematic fluid phase with order one resistive anisotropy in the ab plane. In this talk, we consider the microscopic origins of metamagnetism and the accompanying nematic order, focusing primarily on the quasi-one-dimensional bands in a bilayer model. Making use of local Coulomb interactions in conjunction with the sharp divergence of the density of states near a van-Hove singularity, we construct a phase diagram which enables our system to traverse a metamagnetic transition into a nematic phase followed by a second metamagnetic transition into a phase which preserves C$_4$ rotational symmetry, with increasing magnetic field. We treat quantum nematic fluctuations in the vicinity of the metamagnetic transitions to 1-loop order and consider the extent to which nematic fluctuations can give rise to the observed ``pseudogap'' in the local density of states of this material. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W30.00005: Probing orbital-dependent magnetism in layered perovskite ruthenates through angle-dependent magnetoresistivity D. Fobes, T.J. Liu, Z. Qu, H.Q. Yuan, M. Salamon, M. Zhou, J. Hooper, Z.Q. Mao The orbital degree of freedom plays a very important role in layered perovskite ruthenates, leading to unusual magnetic states. The triple layered Sr$_{4}$Ru$_{3}$O$_{10}$ contains a ferromagnetic ground state, and undergoes a metamagnetic transition under moderate in-plane magnetic fields $\left[1\right]$. By analyzing the anisotropy in angle-dependent directional magnetoresistance measurements we can extract orbital information of magnetism. In Sr$_{4}$Ru$_{3}$O$_{10}$ we have found that below the metamagnetic transition the 4$d_{xy}$ orbitals are already polarized whereas the 4$d_{xz,yz}$ orbitals are not; \textit{$\rho $}$_{ab}$(H) exhibits 2-fold anisotropic symmetry indicating ferromagnetism and \textit{$\rho $}$_{c}$(H) exhibits anisotropy consistent with Fermi surface warping. Above the transition field, under polar rotation we observe a first order phase transition in \textit{$\rho $}$_{c}$(H) for angles close to the \textit{ab}-plane clearly indicating a polarization of the 4$d_{xz,yz}$ orbitals. Additionally, we have performed similar studies on Sr$_{3}$Ru$_{2}$O$_{7}$, and have observed preliminary evidence of orbital-dependent magnetic correlations. This work is supported by the NSF under grant DMR-0645305, the DOE under DE-FG02-07ER46358.\\ $\left[1\right]$ Cao \textit{et al}., Phys. Rev. B \textbf{68}, 174409 (2003) [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W30.00006: Electronic phase diagram in double layered ruthenates (Sr$_{1-x}$Ca$_x$)$_3$Ru$_2$O$_7$ Z.Q. Mao, Z. Qu, J. Peng, T.J. Liu, D. Fobes, B. Qian, L. Spinu We previously established a magnetic phase diagram for (Sr$_{1-x}$Ca$_x$)$_3$Ru$_2$O$_7$ ($0 \leq x \leq 1$) using high quality single crystals grown by a floating-zone method $\left[1\right]$. This phase diagram exhibits rich magnetic properties. The magnetic ground state ranges from an itinerant metamagnetic state $(0 \leq x < 0.08)$, to an unusual heavy-mass, nearly ferromagnetic (FM) state $(0.08 < x < 0.4)$, and finally to an antiferromagnetic (AFM) state $(0.4 < x \leq 1)$. In this talk we report the electronic properties of these magnetic states. We will show that the electronic and magnetic properties are strongly coupled in this system. The electronic ground state evolves from an AFM quasi-two-dimensional metal for $x = 1.0$, to an Anderson localized state for the AFM region $0.4 < x < 1.0$, and then to a weakly localized state, induced by magnetic scattering, for the nearly FM region $0.08 < x < 0.4$. When \textit{x} approaches the critical composition 0.08, the localization weakens and non-Fermi liquid (FL) behavior occurs. The system eventually transforms into a FL ground state when the magnetic ground state switches to the itinerant metamagnetic state for $x < 0.08$. These results demonstrate the delicate balance among the charge, spin, lattice and orbital degrees of freedom in ruthenates.\\ $\left[1\right]$ Z. Qu \textit{et al}., Phys. Rev B \textbf{78}, 180407 (2008). [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W30.00007: Inhomogeneous magnetic phases: a LOFF-like phase in Sr$_3$Ru$_2$O$_7$ Andrew Berridge, Andrew Green, Santiago Grigera, Ben Simons The phase diagram of Sr$_3$Ru$_2$O$_7$ contains a metamagnetic transition that bifurcates to enclose an anomalous phase with intriguing properties - a large resistivity with anisotropy that breaks the crystal-lattice symmetry. We propose that this is a magnetic analogue of the spatially inhomogeneous superconducting Fulde-Ferrel-Larkin-Ovchinnikov state. We show - through a Ginzburg- Landau expansion where the magnetisation transverse to the applied field can become spatially inhomogeneous - that a Stoner model with electronic band dispersion can reproduce this phase diagram and transport behaviour. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W30.00008: Crystal-Field Level Inversion in Lightly Mn-Doped Sr$_3$Ru$_2$O$_7$ Muhammed Hossain, Z. Hu, M.W. Haverkort, T. Burnus, C.F. Chang, S. Klein, J.D. Denlinger, H.-J. Lin, C.T. Chen, R. Mathieu, Y. Kaneko, Y. Tokura, S. Satow, H. Takagi, Y. Yoshida, A. Tanaka, I.S. Elfimov, G.A. Sawatzky, L.H. Tjeng, A. Damascelli Sr$_3$(Ru$_{1-x}$Mn$_x$)$_2$O$_7$, in which 4$d$-Ru is substituted by the more localized 3$d$-Mn, is studied by x-ray dichroism and spin-resolved density functional theory. We find that Mn impurities do not exhibit the same 4+ valence of Ru, but act as 3+ acceptors; the extra $e_g$ electron occupies the in-plane $3d_{x^2-y^2}$ orbital instead of the expected out-of-plane $3d_{3z^2-r^2}$. We propose that the 3$d$-4$d$ interplay, via the ligand oxygen orbitals, is responsible for this crystal-field level inversion and the material's transition to an antiferromagnetic, possibly orbitally ordered, low-temperature state. Published: Phys. Rev. Lett. 101, 016404 (2008). [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W30.00009: Strong spin-orbit coupling effects on the Fermi surface of Sr$_{2}$RuO$_{4}$ and Sr$_{2}$RhO$_{4}$ Andrea Damascelli, Maurits Haverkort, Ilya Elfimov, Hao Tjeng, George Sawatzky We present a first-principle study of spin-orbit coupling effects on the Fermi surface of Sr$_{2}$RuO$_{4}$ and Sr$_{2}$RhO$_{4}$ [1]. For nearly degenerate bands, spin- orbit coupling leads to a dramatic change of the Fermi surface with respect to non-relativistic calculations; as evidenced by the comparison with experiments on Sr$_{2}$RhO$_{4}$, it cannot be disregarded. For Sr$_{2} $RuO$_{4}$, the Fermi surface modifications are more subtle but equally dramatic in the detail: spin-orbit coupling induces a strong momentum dependence, normal to the RuO$_2$ planes, for both orbital and spin character of the low-energy electronic states. These findings have profound implications for the understanding of unconventional superconductivity in Sr$_{2}$RuO$_{4}$. [1] M.W. Haverkort {\it et al.}, Phys. Rev. Lett. \textbf{101}, 026406 (2008). [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W30.00010: Dimensionality and doping effect on the Core-level X-ray photoemission satellites in layered ruthenates. Haizhong Guo, Yi Li, Biao Hu, Rongying Jin, E.W. Plummer, Jiandi Zhang, D. Urbina, Tijiang Liu, David Fobes, Zhiqiang Mao Core-level photoelectron spectra of the layered perovskite crystal Sr$_{n+1}$Ru$_{n}$O$_{3n+1}$ (n = 1, 2, and 3) and Mn-doped Sr$_{3}$Ru$_{2}$O$_{7}$ are investigated by x-ray photoemission spectroscopy (XPS) techniques. The Sr 3$d$ and Ru 3$d$ core-level spectra exhibit a two-peak structure, screened and unscreened peaks, indicating strong correlation effects among Ru 4$d$ electrons. However, there are little changes of the core-level satellite features with n, suggesting the electron-electron correlation is mainly confined in the RuO$_{2}$ plane. On the other hand, doping of Mn will drastically affect the core-level spectral weight, reflecting the doping-induced metal-to-insulator transition in the doped system. The position of Ru-core levels remain the same, thus, indicating no doping-induced change of Ru valence. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W30.00011: Magnetic Ordering in Ba$_{2}$DyRuO$_{6}$ J. Lamsal, W. Yelon, H. Blackstead, M. Smylie, Q. Cai, W. James, J. Yang Magnetization measurements and neutron diffraction (ND) studies have been carried out on the double perovskite ruthenate, Ba$_{2}$DyRuO$_{6}.$ The low field magnetization data indicate ordering around 50K, a sharp rise below 8K and a possible transition around 25K. Rietveld analysis of ND data confirms antiferromagnetic ordering at 48K. The temperature dependence of the Ru moment appears to follow a Brillouin type curve down to the lowest temperature accessible, (12K), and there is no evidence for a transition around 25K. In contrast, the Dy moment is found to be proportional to the square of the Ru moment, implying that the Ru moment is the primary order parameter and that Dy ordering is driven by the Ru-Dy coupling. The ND experiment could not reach the temperature at which the magnetization rises sharply (8K), but following similar arguments, we suggest that this point represents the temperature at which the Dy-Dy interactions become larger than the Ru-Dy interactions, and may, thus, lead to a new structure. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W30.00012: Resistance noise in the bad metal SrRuO$_{3}$ Francoise Kidwingira, Michael Rozler, Gertjan Koster, Wolter Siemons, Rik Groenen, Malcolm Beasley SrRuO$_{3}$ (SRO) is a strongly correlated electrons system with some interesting properties. It is an itinerant ferromagnet below 150K and it transitions from a bad metal at high temperature to a Fermi Liquid at low temperature. In SRO thin films, there is evidence that even a slight presence of Ru deficiencies increases the degree of electron correlations [1]. Using Scanning Tunneling Potentiometry [2], we have studied the local transport properties of this material. The measured resistance has a noise level well above Johnson noise that depends both on the method of synthesis and on the voltage across the sample. We will attempt to characterize these resistance fluctuations with respect to the various unusual properties of the material. [1] W. Siemons \textit{et. al.}, Phys. Rev. B \textbf{76}, 075126 (2007) [2] M. Rozler and M. R. Beasley, Rev. Sci. Inst \textbf{79}, 073904 (2008) [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W30.00013: Nyquist noise as probe of hot-electron effects in the ferromagnetic insulating state of manganites Sudeshna Samanta, Arup K. Raychaudhuri Hole-doped rare-earth manganites (like La$_{1-x}$Ca$_{x}$MnO$_{3})$ in the ferromagnetic insulating (FMI) state show large non-linear conductance. Such non-linear conductance can arise due to hot-electron effect which originates from decoupling of the electron and lattice temperatures at high power level. The non-linear conductance manifests as electro-resistance or current induced resistance change. We report here low frequency temperature dependent noise measurement which allows us to estimate the electronic temperature by measuring Nyquist noise (``white noise'' in contrast to 1/f noise) in La$_{0.8}$Ca$_{0.2}$MnO$_{3}$ single crystals which has a distinct FMI state below 100K. The measurement was performed with low ac biasing current which was mixed with a high current density d.c that leads to electron heating. We observed that in the insulating state, above a certain input d.c power, the Nyquist noise increases by a large extent and this is coupled to the onset of non-linear conduction as signalled by the power dependence of the differential conductance. The experiment establishes a direct link between hot-electron effect and non-linear conductance. [Preview Abstract] |
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