Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session A11: Magnetic Phase Transitions |
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Sponsoring Units: DMP Chair: Oleg Starykh, University of Utah Room: Colorado Convention Center Korbel 1F |
Monday, March 5, 2007 8:00AM - 8:12AM |
A11.00001: Competing magnetic fluctuations in Sr$_3$Ru$_2$O$_7$ probed by Ti doping David Fobes, J. Hooper, M. Zhou, N. Dang, Z.Q. Mao, M.H. Fang, C.M. Feng, Z.A. Xu, M.H. Yu, C.J. O'Connor, G.J. Xu, N. Andersen, M. Salamon The bilayered ruthenate Sr$_3$Ru$_2$O$_7$ shows itinerant metamagnetic quantum criticality which has been cited as a textbook example. In this talk we report the effect of nonmagnetic Ti$^{4+}$ impurities on the electronic and magnetic properties of this material. Small amounts of Ti suppress the characteristic peak in magnetic susceptibility near 16 K and result in a sharp upturn in specific heat. The metamagnetic quantum phase transition and related anomalous features are quickly smeared out by small amounts of Ti. These results provide strong evidence for the existence of competing magnetic fluctuations in the ground state of Sr$_3$Ru$_2$O$_7$. Ti doping suppresses the low temperature antiferromagnetic interactions that arise from Fermi surface nesting, leaving the system in a state dominated by ferromagnetic fluctuations. [Preview Abstract] |
Monday, March 5, 2007 8:12AM - 8:24AM |
A11.00002: Effect of Magnetic Field on Electronic Nematic Order in a Bilayer System: Application to Sr$_{3}$Ru$_{2}$O$_{7}$ Christoph Puetter, Hyeonjin Doh, Hae-Young Kee Recent experiments on the bilayer compound Sr$_{3}$Ru$_{2}$O$_{7}$ suggest the existence of an electronic liquid-crystal phase. A possible explanation for the unusual behavior observed in this material is provided by an electronic nematic theory. Within this framework, a bilayer system undergoes multiple phase transitions and exhibits strong transport anisotropy. The model also incorporates an external in-plane magnetic field to study the effect on metamagnetic transitions and anisotropic transport. Details of our numerical calculations will be presented. [Preview Abstract] |
Monday, March 5, 2007 8:24AM - 8:36AM |
A11.00003: Magnetic Phase Transition and Magnetic Structure of Ca$_{3}$Ru$_{2}$O$_{7}$ Z. Qu, J. Peng, T.J. Liu, F. Etienne, D. Fobes, Z.Q. Mao, W. Bao Ca$_{3}$Ru$_{2}$O$_{7}$ shows exciting physical properties, including a bulk spin-valve behavior and orbital ordering.$^{[1-3]}$ We have investigated the magneto-transport properties and the magnetic structure of this material using high-quality Ca$_{3}$Ru$_{2}$O$_{7}$ single crystals grown by a floating-zone (FZ) method. From magnetoresistivity measurements, we observe that the previously reported metamagnetic transition at $\sim $6T for $H//a$ axis consists of two separate transitions occurring at 5.9 and 6.5T, respectively. The first transition is extremely sharp with the transition width less than 1 Gauss, corresponding to the bulk spin-valve behavior, while the second transition has a finite width which is likely associated with the change of orbital polarization. Our elastic neutron scattering measurements on FZ-grown Ca$_{3}$Ru$_{2}$O$_{7}$ single crystals confirm the magnetic structure suggested by previous works,$^{[2, 4]}$ i.e., the magnetic moments align ferromagnetically within the double layers and antiferromagnetically between the double layers. 1. X.N. Lin et al., Phys. Rev. Lett., 95, 017203 (2005). 2. D.J. Singh and S. Auluck, Phys. Rev. Lett., 96, 097203 (2006). 3. J.F. Karpus, et al., Phys. Rev. Lett., 93, 167205 (2004). 4. Y. Yoshida, et al., Phys. Rev. B, 72, 054412 (2005). [Preview Abstract] |
Monday, March 5, 2007 8:36AM - 8:48AM |
A11.00004: Specific Heat of (Ca$_{1-x}$Sr$_{x})_{3}$Ru$_{2}$O$_{7}$ Single Crystals V. Varadarajan, S. Chikara, V. Durairaj, X.N. Lin, G. Cao, J.W. Brill We have measured the specific heat of crystals of (Ca$_{1-x}$Sr$_{x})_{3}$Ru$_{2}$O$_{7 }$using ac- and relaxation-time calorimetry. Special emphasis was placed on the characterization of the N\'{e}el (T$_{N}$=56 K) and structural (T$_{c}$ = 48 K) phase transitions in the pure, x=0 material. While the latter is believed to be first order, detailed measurements under different experimental conditions suggest that all the latent heat (with L $\sim $ 0.3 R) is being captured in a broadened peak in the effective heat capacity. The specific heat has a mean-field-like step at T$_{N}$, but its magntitude ($\Delta $c$_{P }\sim $ R) is too large to be associated with a conventional itinerant electron (e.g. spin-density-wave) antiferromagnetic transition, while its entropy is too small to be associated with full ordering of localized spins. The T$_{N}$ transition broadens with Sr substitution while its magnitude decreases slowly. On the other hand, the entropy change associated with the T$_{c}$ transition decreases rapidly with Sr substitution and is not observable for our x=0.58 sample. [Preview Abstract] |
Monday, March 5, 2007 8:48AM - 9:00AM |
A11.00005: In-plane anisotropy of magnetoresistivity of tri-layered ruthenate Sr$_4$Ru$_3$O$_{10}$ Z.Q. Mao, M. Zhou, D. Fobes, H.Q. Yuan, M. Salamon The tri-layered ruthenate Sr$_4$Ru$_3$O$_{10}$ exhibits intriguing magnetic properties; its ferromagnetic transition at $T_c$ $\approx$ 105 K is followed by an additional magnetic phase transition at $T^*$ $\approx$ 50 K [1,2]. Below $T^*$, a first order metamagnetic transition is induced by a magnetic field applied in the plane. We have recently measured the in-plane angular dependence of magnetoresistivity of this material at various magnetic fields and temperatures. Our data reveal that the in-plane anisotropy of magnetoresistivity undergoes a transition from two-fold to four-fold symmetry across the metamagnetic transition of Sr$_4$Ru$_3$O$_{10}$. Such a transition can be well interpreted in terms of a multiple-band effect which involves the coexistence of ferromagnetic and metamagnetic bands.\\* $[1]$ G. Cao $et$ $al.$, Phys. Rev. B \textbf{68}, 174409 (2003). \\* $[2]$ Z.Q. Mao $et$ $al.$, Phys. Rev. Lett. \textbf{96}, 077205 (2006). [Preview Abstract] |
Monday, March 5, 2007 9:00AM - 9:12AM |
A11.00006: Long Range Order in Orbital Model Wenlong You, Guangshan Tian, Haiqing Lin We investigate the existence of N\'eel type long range order (LRO) in an orbital model which is highly anisotropic and frustrated. The model originated from magnetic materials such as LaMnO$_3$ where orbital degrees of freedom play important role. In the system described by the two-fold degenerate $e_g$ orbitals, due to the Kugel-Khomskii superexchange, the orbital degrees of freedom are represented by quantum pseudo-spin 1/2 operators. By applying the reflection-positivity method developed by Dyson, Lieb, and Simon, and adopting appropriate numerical variational method to obtain good estimations on the energy density and correlation functions, we are able to rigorously prove the existence of long range order in this orbital model on the square lattice. [Preview Abstract] |
Monday, March 5, 2007 9:12AM - 9:24AM |
A11.00007: Orbital order and spin waves in the Kugel-Khomskii model Tamar Pereg-Barnea, Wei-Cheng Lee, Allan MacDonald The Kugel-Khomskii model, introduced in the seventies, attempts to describe transition metal oxides in which orbital degeneracy plays an important role in ground state properties. The model provides a qualitative description of pseudocubic perovskites like LaTiO$_3$ and YTiO$_3$ in which the three $t_{2g}$ d-orbitals are thought to be active at low energy. We investigate the cubic $t_{2g}$ Kugel-Khomskii model in the limit of strong electron-electron interaction (on-site Hubbard U). We use perturbation theory with small hopping parameter (t/U) to derive an effective large pseudospin Hamiltonian with 6 degrees of freedom on each site (2 spins X 3 orbitals). In this model the $t_{2g}$ orbital structure combined with cubic symmetry leads to hopping that depends on both the orbital label and the bond direction. We find the classical (mean-field) ground state manifold systematically and derive a spin wave theory to account for quantum fluctuations. The theory proceeds beyond leading order in order to capture the coupling between the spin and orbital degrees of freedom of the system. This approach leads to better understanding of the quantum-mechanical ground state, it's energy and symmetries. [Preview Abstract] |
Monday, March 5, 2007 9:24AM - 9:36AM |
A11.00008: Optical spin waves in magnetite R.J. McQueeney, M. Yethiraj, W. Montfrooij, S. Chang, T.G. Perring, P. Metcalf, J.M. Honig For the last 70 years, the microscopic origin of the Verwey transition in magnetite (Fe$_{3}$O$_{4})$ was thought to be charge-ordering, although this has been disputed of late, bringing renewed interest in this system. The spinel structure of magnetite contains two different iron sites; A (stable valence, Fe$^{3+})$ and B (mixed valence, Fe$^{2.5+})$, with charge ordering of Fe$^{2+}$/Fe$^{3+}$ species occurring on the B-site. As the spin waves are expected to be sensitive to charge ordering, the optical spin waves were measured above and below the Verwey transition by inelastic neutron scattering. The optical spin waves propagating on the A-site sublattice ($\sim $115 meV) are unchanged at the transition. The spin waves propagating on the B-site sublattice ($\sim $75 meV) are $\sim $5 meV stiffer and broader in the metallic phase. The results are interpreted as evidence of B-site double exchange in the metallic phase. [Preview Abstract] |
Monday, March 5, 2007 9:36AM - 9:48AM |
A11.00009: Electronic Raman scattering in Magnetite Lev Gasparov, G. Guntherodt, K.-Y. Choi, H. Berger, L. Forro Raman spectra of optimally doped magnetite (Fe$_{3}$O$_{4})$ single crystals reveal broad electronic background extending up to 900 wavenumbers ($\sim $110 meV). Redistribution of this background is observed when sample is cooled below the Verwey transition temperature (T$_{V}$= 123K). In particular, spectra of the low temperature phase show diminished background below 300 wavenumbers followed by an enhancement of the electronic background between 300 and 400 wavenumbers with subsequent decrease of the background below 400 wavenumbers. Such redistribution may be assigned to an opening of the charge gap at about 350 wavenumbers (43 meV). The value of the gap is within the range of the photoemission data on freshly fractured magnetite sample. [Preview Abstract] |
Monday, March 5, 2007 9:48AM - 10:00AM |
A11.00010: Resonant x-ray scattering of the Bi$_{1-x}$Sr$_{x}$MnO$_{3}$ (x$\le $0.5) charge-ordered phases Joaquin Garcia, Gloria Subias, M.C. Sanchez, Premek Beran, J. L. Garc\'ia-Munoz, M. Nevriva Charge-orbital ordering (CO-OO) in Bi$_{1-x}$Sr$_{x}$MnO$_{3}$ (x=0.3, 0.5) have been studied by resonant x-ray scattering (RXS) at the Mn K edge. Strong resonances were observed at the Mn K-edge for weak superstructure (h00), (0k0) and forbidden (h/200), (0k/20) reflections with h, k odd within the \textit{ab} plane (\textit{Ibmm} setting) in both single crystals. Additional (hk0) and (hk/20) with k odd have also been studied. The azimuth angle and polarization dependence of the resonant intensity for this set of reflections point out to a structural transition at the T$_{COO}$ that stabilizes an checkerboard ordering of two non-equivalent Mn atoms with different local geometrical structures and a very small charge segregation for both x=0.5 and x=0.3 compounds. We can conclude that A$_{1-x}$B$_{x}$MnO$_{3}$ tends to order in a checkerboard pattern independently of the nature of the A and B atoms and for x even far from 0.5. Furthermore, the electronic states of the two non-equivalent Mn atoms are far from the ionic (+3 and +4) species. [Preview Abstract] |
Monday, March 5, 2007 10:00AM - 10:12AM |
A11.00011: Effect of Charge Ordering on Phonon Spectra of La$_{1/3}$Sr$_{2/3}$FeO$_{3-\sigma }$ Jie Ma, S. Chang , J.-Q. Yan, F. Trouw, M. Hehlen, R. J. McQueeney La$_{1/3}$Sr$_{2/3}$FeO$_{3-\delta }$ (LSFO) compounds are reported to have an unusual magneto-structural transition at low temperatures. Below $\sim $210$K$, it is proposed that charge disproportionation occurs according to 3Fe$^{3.67+}=>$2Fe$^{3+}$+Fe$^{5+}$, and the different iron valences order in the pattern 3+, 3+, 5+ along the body diagonal [111]$_{c}$.$_{ }$Simultaneously, LSFO undergoes antiferromagnetic ordering and a slight distortion in crystal structure from cubic to rhombohedral. Inelastic neutron scattering was used to determine the effect of the charge ordering on the phonon spectra. We find that the high frequency oxygen phonons ($\sim $80 meV) soften above the transition by several meV. The result and relationship between the charge ordering and the phonon softening are discussed. [Preview Abstract] |
Monday, March 5, 2007 10:12AM - 10:24AM |
A11.00012: Pressure-Induced Quantum Criticality in Cr Rafael Jaramillo, Y. Feng, T.F. Rosenbaum, J.J. Pluth, G. Srajer, J. Lang, Z. Islam Diamond anvil cell high-pressure techniques are used in concert with high resolution magnetic x-ray diffraction to probe the quantum critical regime of the elemental itinerant antiferromagnet Chromium. The antiferromagnetic order is suppressed by an applied pressure of $\sim $6 GPa in the zero-temperature limit. We perform high-resolution measurements of both the charge- and spin-density-wave order parameters as the system is tuned through this magnetic phase transition. The results illustrate the effects of quantum fluctuations and enhanced dimensionality on a canonical correlated electron system. [Preview Abstract] |
Monday, March 5, 2007 10:24AM - 10:36AM |
A11.00013: High Pressure Probes of Magnetic Quantum Phase Transitions Y. Feng, R. Jaramillo, C.T. Seagle, D.M. Silevitch, T.F. Rosenbaum, D.L. Heinz Magnetic susceptibility and electrical transport measurements in concert with diamond anvil cell techniques permit access to magnetic quantum critical points impossible to access by other means. We combine a continuously variable high-pressure cell with an optical system for in situ pressure calibration and Raman capability that can be cooled to pumped helium temperatures. With this system, we investigate the P-T phase diagram of the spin-density wave antiferromagnet chromium. We discuss its quantum critical behavior along with the possibility of alternative correlated phases as the magnetic order is suppressed in the zero-temperature limit. [Preview Abstract] |
Monday, March 5, 2007 10:36AM - 10:48AM |
A11.00014: Influence of the antiferromagnetic spin density wave on the magnetoresistance of Cr Yeong-Ah Soh, Ravi Kummamuru We have performed magnetotransport measurements on Cr films that are 350, 56, 43 and 18 nm thick. The magnetoresistance with the field perpendicular to the film plane shows a clear increase below the Neel temperature and is accompanied by an anomalous negative magnetoresistance at the Neel temperature. The orbital magnetoresistance satisfies the Kohler's rule in the paramagnetic state but violates it in the Neel state. The Hall resistance shows temperature dependence in the paramagnetic state, which was previously suggested to be indicative of a pseudogap [1]. We explain the above phenomena by the evolution of the electronic structure due to the formation of antiferromagnetic spin density wave, the influence of antiferromagnetic domain walls, and the existence of more than one scattering time. [1] ``Quantum phase transition in a common metal'', A. Yeh, Y-A. Soh, J. Brooke, G. Aeppli, T. F. Rosenbaum, and S. M. Hayden, Nature (London) \textbf{419}, 459 (2002). [Preview Abstract] |
Monday, March 5, 2007 10:48AM - 11:00AM |
A11.00015: Thermomagnetic Studies of K$_{2}$NaCrO$_{8}$ Saritha Nellutla, Mekhala Pati, Kwanyong Choi, Younjung Jo, Naresh Dalal, Luis Balicas, Johan van Tol, Daniel Pajerowski, Byoung Hee Moon, Yoonseok Lee, Mark Meisel, Yasumasa Takano There has been renewed interest in the search for new model quantum spin systems that can exhibit BEC of magnons. K$_{2}$NaCrO$_{8}$ is one of the simplest spin systems available since $S=\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ and $I$ = 0. Specific heat ($C_{p})$ measurements indicate that this material orders antiferromagnetically at $T_{N} \quad \sim $ 1.7 K in zero-field [1]. Application of an external magnetic field pushes the $C_{p}$ maximum to lower temperatures. Torque and AC susceptibility measurements show that the transition temperature is rapidly suppressed around 7.4 T, with no hysteretic behavior, implying the presence of a quantum phase transition. Measurements are underway to map the phase boundary in the $T \quad \to $ 0 K, B $\sim $ 7.4 T region and extract the critical exponent ($\alpha )$ from the relation $k_{B}T_{c} \quad \approx $ (B$_{c}$-B)$^{\alpha }$ . [1] B. Cage, N. S. Dalal, \textit{Chem. Mater.} \textbf{13}, 881 (2001). [Preview Abstract] |
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