Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G29: Cooperative Phenomena in Magnetism I |
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Sponsoring Units: GMAG Chair: Yang Zhao, National Institute of Standards and Technology Room: 206A |
Tuesday, March 3, 2015 11:15AM - 11:27AM |
G29.00001: A minimal tight-binding model for ferromagnetic canted bilayer manganites Christopher Lane, M. Baublitz, H. Lin, H. Hafiz, R.S. Markiewicz, B. Barbiellini, Z. Sun, D.S. Dessau, A. Bansil Half-metallicity in materials has been a subject of extensive research due to its potential for applications in spintronics. Ferromagnetic manganites have been seen as a good candidate, and aside from a small minority-spin pocket observed in La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ $(x=0.38)$, transport measurements show that ferromagnetic manganites essentially behave like half metals. Here we develop robust tight-binding models to describe the electronic band structure of the majority as well as minority spin states of ferromagnetic, spin-canted antiferromagnetic, and fully antiferromagnetic bilayer manganites. Both the bilayer coupling between the MnO$_2$ planes and the mixing of the $|x^2 - y^2>$ and $|3z^2 - r^2>$ Mn 3d orbitals play an important role in the subtle behavior of the bilayer splitting. Effects of $k_z$ dispersion are included. [Preview Abstract] |
Tuesday, March 3, 2015 11:27AM - 11:39AM |
G29.00002: ABSTRACT WITHDRAWN |
Tuesday, March 3, 2015 11:39AM - 11:51AM |
G29.00003: $1/(N-1)$ expansion, NRG, NCA, and exact $T\to \infty$ limit for the Green's function of an SU($N$) Anderson impurity Akira Oguri, Rui Sakano We study the Green's function of the $N$-orbital Anderson impurity in a wide range of the Coulomb interaction $U$, frequency $\omega$, and temperature $T$, carrying out the calculations with the $1/(N-1)$ expansion [1], the numerical renormalization group (NRG), the non-crossing approximation (NCA), and the exact expression that is available at high temperatures or the high-bias limit of a nonequilibrium steady state [2]. Comparisons of these approaches are made, specifically, for the $N=4$ particle-hole symmetric case. The $1/(N-1)$ expansion is a new large $N$ approach based on the perturbation theory in $U$ [1], and is complementary to the NCA which uses the power series expansion in the hybridization matrix element $V$. The calculations with this approach are carried out up to order $1/(N-1)^2$, which reasonably capture the fluctuations beyond the random phase approximation (RPA) especially at low energies. We also discuss the $N$ dependence for $N > 4$. [1] A.O., R.\ Sakano, and T.\ Fujii, PRB {\bf 84}, 113301 (2011).\\[4pt] [2] A.O. and R.\ Sakano, PRB {\bf 88}, 155424 (2013). [Preview Abstract] |
Tuesday, March 3, 2015 11:51AM - 12:03PM |
G29.00004: High Pressure Transport Studies of NdIn$_{3}$ Kenneth Purcell, David Graf, Takao Ebihara NdIn$_{3}$ is a cubic antiferromagnetic metal that orders with a Neel temperature of 5.9 K and belongs to a family of rare earth intermetallic compounds RIn$_{3}$ that have a cubic AuCu$_{3}$-type crystal structure. At 0.5 K and the magnetic field applied in 100 direction, NdIn$_{3}$ exhibits metamagnetic transitions at 7.8 T and 8.9 T before entering a field induced paramagnetic state at 11.1 T. We report high pressure transport studies of single crystal NdIn$_{3}$ and the effect that pressure has on the Neel temperature, critical field, and metamagnetic transitions observed in the magnetoresistance. Comparisons to the behavior of the pressure induced superconductor CeIn$_{3}$ will be discussed. [Preview Abstract] |
Tuesday, March 3, 2015 12:03PM - 12:15PM |
G29.00005: Giant proximity effect and critical opalescence in EuS Timothy Charlton, Silvia Ramos, Jorge Quintanilla, Andreas Suter, Jagadeesh Moodera The proximity effect is a type of wetting phenomenon where an ordered state, usually magnetism or superconductivity, ``leaks'' from one material into an adjacent one over some finite distance. For superconductors, the characteristic range is of the order of the coherence length, usually hundreds of nm. Nevertheless much longer, ``giant'' proximity effects have been observed in cuprate perovskite junctions. Such giant proximity effects can be understood by taking into account the divergence of the pairing susceptibility in the non-superconducting material when it is itself close to a superconducting instability: a superconducting version of critical opalescence. Since critical opalescence occurs in all second order phase transitions, giant proximity effects are expected to be general, therefor there must be a giant ferromagnetic proximity effect. Compared to its superconducting counterpart, the giant ferromagnetic proximity effect has the advantage that the order parameter (magnetization) can be observed directly. We have fabricated Co/EuS thin films and measured the magnetization profiles as a function of temperature using the complementary techniques of low energy muon relaxation and polarized neutron reflectivity. Details of the proximity effect near $T_C^{EuS}$ will be presented. [Preview Abstract] |
Tuesday, March 3, 2015 12:15PM - 12:27PM |
G29.00006: Hole Properties In and Out of Magnetization Plateaus in 2-d Antiferromagnet Imam Makhfudz, Pierre Pujol We study the signatures of magnetization plateaus and the presence or absence of Goldstone modes in terms of their effects on the physics of holes in hole-doped two-dimensional antiferromagnet defined on square lattice. Holes with quadratic dispersion around Fermi point existing at infinitesimally small doping and linear dispersion around nearly circular Fermi surface at finite but low doping are investigated. They are coupled to an effective gauge field, generated by the spin sector, which subsequently mediates interaction between the holes. We find that out-of-plateaus case leads to algebraically decaying long-range interaction between fermionic holes with both Coulombic and dipolar forms, whereas in-plateaus case leads to short-range (local) interaction. We show that the spectral peak is significantly broadened in the out-of-plateaus case, while the spectral weight is still sharply-peaked in the in-plateau case. This conclusion holds in both infinitesimally small doping limit and in the more realistic finite doping case. We also extend the result obtained for 1-D system where finite hole doping gives rise to a shift in the magnetization value of the plateaus. Reference: arXiv:1411.1713 [Preview Abstract] |
Tuesday, March 3, 2015 12:27PM - 12:39PM |
G29.00007: Emergence of New Magnetic Plateaus in Sheets of Magnetic Dimers S. Haravifard, D. Graf, A. Feiguin, C.D. Batista, J.C. Lang, D.M. Silevitch, G. Srajer, H.A. Dabkowska, B.D. Gaulin, S.W. Tozer, T.F. Rosenbaum SrCu$_{2}$(BO$_{3}$)$_{2}$ has corner-sharing Cu$^{2+}$ spin 1/2 dimers lying on a square lattice, corresponding to the two-dimensional Shastry-Sutherland model. At low temperatures and fields exceeding 20 T, there is a Bose-Einstein condensation of triplet excitations of the dimers. At higher magnetic fields, plateaus have been observed in the magnetization, which have been interpreted in terms of preferred filling of the singlet ground state with increasing densities of triplet excitations. We apply hydrostatic pressures up to 2.5 GPa to study the movement and suppression of the quantized magnetic plateaus identified at ambient pressure as well as the emergence of new superstructure configurations. [Preview Abstract] |
Tuesday, March 3, 2015 12:39PM - 12:51PM |
G29.00008: Magnetism in RFe$_2$Ge$_2$ (R = Y, Lu, Tb) as seen by $^{57}$Fe M\"ossbauer spectroscopy Sergey L. Bud'ko, Marcos A. Avila, Xiaoming Ma, Sheng Ran, Hyunsoo Kim, Paul C. Canfield Magnetic properties of RFe$_2$Ge$_2$ (R = Y, Lu, Tb) were studied using temperature dependent (4.6 - 300 K) M\"ossbauer spectroscopy and specific heat capacity. In the paramagnetic state all three compounds have similar evolution of the isomer shift and quadrupolar splitting. Magnetic phase transitions can be detected both in LuFe$_2$Ge$_2$ and TbFe$_2$Ge$_2$, with an indication of very small iron magnetic moment in the ordered state for LuFe$_2$Ge$_2$. The Debye temperature evaluated from the temperature-dependent isomer shift for these three compounds is significantly higher that inferred from specific heat capacity measurements, that might indicate presence of magnetic correlations. The results will be compared with the bulk measurements. [Preview Abstract] |
Tuesday, March 3, 2015 12:51PM - 1:03PM |
G29.00009: Quadrupolar order triggered by magnetic order in GdB$_{4}$ Hoyoung Jang, B.Y. Kang, B.K. Cho, J.-S. Lee Various electronic and magnetic properties and phase transition of Gd and Gd compounds have been studied. For example, 4f delocalization of Gd metal was revealed by resonant inelastic x-ray scattering study with high pressure condition [1]. Meantime, extremely large positive magnetoresistance in GdB4 was reported [2] -- which is unexpected in the general Gd configuration. We investigated GdB$_{4}$ by resonant soft x-ray scattering (RSXS) experiment at Gd M-edge. In (100) reflection, huge RSXS signal was observed, which corresponds to the reported antiferromagnetic (AFM) order by neutron scattering measurement [3]. Besides, unexpected RSXS signal was also detected in different photon polarization, which cannot be explained by the reported AFM structure. The energy profile and order parameter show the unexpected signal which has strong coupling but different origin with the AFM order. We deduce that the unexpected signal comes from Gd quadrupolar order, which is triggered by magnetostriction through the AFM order. \\[4pt] [1] B. R. Maddox et al. PRL 96, 215701 (2006).\\[0pt] [2] B. K. Cho et al. JAP 97, 10A923 (2005).\\[0pt] [3] J. A. Blanco et al. PRB 73, 212411 (2006). [Preview Abstract] |
Tuesday, March 3, 2015 1:03PM - 1:15PM |
G29.00010: Condensation of collective charge ordering in Chromium A. Singer, M. Marsh, S. Dietze, V.V. Uhl\' \i\v r, Y. Li, D.A. Walko, E.M. Dufresne, G. Srajer, M.P. Cosgriff, P.G. Evans, E.E. Fullerton, O.G. Shpyrko Here we report on the dynamics of the structural order parameter in a chromium film using synchrotron radiation in response to photo-induced ultra-fast excitations. Following transient optical excitations the effective lattice temperature of the film rises close to the N\'{e}el temperature and the charge density wave (CDW) amplitude is reduced. Although we expect the electronic charge ordering to vanish shortly after the excitation we observe that the CDW is never completely disrupted, which is revealed by its unmodified period at elevated temperatures. We attribute the persistence of the CDW to the long-lived periodic lattice displacement in chromium. The long-term evolution shows that the CDW revives to its initial strength within 1 ns, which appears to behave in accordance with the temperature dependence in equilibrium. This study highlights the fundamental role of the lattice distortion in charge ordered systems and its impact on the re-condensation dynamics of the charge ordered state in strongly correlated materials. [Preview Abstract] |
Tuesday, March 3, 2015 1:15PM - 1:27PM |
G29.00011: Direct probe of Fermi surface evolution at a pressure-induced quantum phase transition D.M. Silevitch, Yejun Feng, A. Palmer, Yishu Wang, T.F. Rosenbaum The nature of a material's Fermi surface is crucial to understanding its electronic, magnetic, optical, and thermal characteristics. Traditional measurements such as angle resolved photoemission spectroscopy and, de Haas-van Alphen quantum oscillations can be difficult to perform in the vicinity of a pressure-driven quantum phase transition. We demonstrate here that magnetic x-ray diffraction in combination with Hall effect measurements in a diamond anvil cell can provide valuable insight into the Fermi surface evolution in spin- and charge-density-wave systems near quantum phase transitions. In particular, we delineate the critical pressure and absence of Fermi surface reconstruction at the spin-flip transition in elemental chromium. [Preview Abstract] |
Tuesday, March 3, 2015 1:27PM - 1:39PM |
G29.00012: Factors influencing achievement of chemical order in tetragonal FeNi Nina Bordeaux, Ana Maria Montes-Arango, Jiaxing Liu, Katayun Barmak, Laura Henderson Lewis Chemically ordered ferromagnetic compounds with the L1$_{\mathrm{0}}$ structure have attracted wide interest for rare-earth-free permanent magnet applications. In particular, L1$_{\mathrm{0}}$-structured FeNi is a promising candidate due to the abundance and low cost of the constituent elements and high theoretical maximum energy product (BH)max $=$ 42 MGOe [1]. Synthesis of L1$_{\mathrm{0}}$ FeNi has been hindered by extremely sluggish kinetics below the equilibrium order-disorder temperature T$_{\mathrm{O/D}}=$ 320 ${^\circ}$ and the phase is known to form in meteorites over millions of years. In this work, the thermodynamic stability of the L1$_{\mathrm{0}}$ phase and kinetics of the L1$_{\mathrm{0}}\to $fcc magnetostructural phase transformation are quantitatively determined via magnetic and thermal measurements of bulk L1$_{\mathrm{0}}$ FeNi extracted from meteorites. Influences on phase transformation kinetics, including effects of magnetism, will be discussed. \\[4pt] [1] L. H. Lewis, et al., IEEE Mag Lett 5 (2014) 5500104. [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 1:51PM |
G29.00013: Phase Transitions on Random Lattices: How Random is Topological Disorder? Hatem Barghathi, Thomas Vojta We study the effects of topological (connectivity) disorder on phase transitions. We identify a broad class of random lattices whose disorder fluctuations decay much faster with increasing length scale than those of generic random systems, yielding a wandering exponent of $\omega=(d-1)/(2d)$ in $d$ dimensions. The stability of clean critical points is thus governed by the criterion $(d+1)\nu > 2$ rather than the usual Harris criterion $d\nu > 2$, making topological disorder less relevant than generic randomness. The Imry-Ma criterion is also modified, allowing first-order transitions to survive in all dimensions $d > 1$. These results explain a host of puzzling violations of the original criteria for equilibrium and nonequilibrium phase transitions on random lattices. We discuss applications, and we illustrate our theory by computer simulations of random Voronoi and other lattices. [Preview Abstract] |
Tuesday, March 3, 2015 1:51PM - 2:03PM |
G29.00014: Pressure Dependence of Magnetism in KCuF$_3$ Brian Nguyen, Alexander Thaler, Clarina Dela Cruz, Gregory MacDougall The perovskite KCuF$_3$ is a prototypical compound for studying both orbital ordering and 1-D Heisenberg antiferromagnetism. In the conventional Kugel-Khomskii picture, this material undergoes a Jahn-Teller distortion at T$_{JT}$=800~K causing long-range ordering of 3d Cu$^{2+}$ orbitals, effectively driving the spin interactions 1-D. A second magnetic transition to 3-D antiferromagnetism is observed at the N\'{e}el temperature, T$_N$=39~K. Recent data by Lee \emph{et al}. suggests a new interpretation where Cu$^{2+}$ orbitals do not order until a glassy structural transition at T$_s$=50~K, and this transition is a necessary precursor to 3-D ordering of spins. It was later shown that the glassy transition can be suppressed to zero temperature with pressures as low as P=7~kbar, implying that spin order should follow suit. Contrary to this prediction, single crystal neutron scattering measurements by our group demonstrated a moderate increase in T$_N$ with pressure, opposite of predictions. However, some evidence for uniaxial strain complicated interpretation. In an attempt to resolve this issue, we performed neutron powder diffraction measurements in helium gas and clamp pressure cells. We will present these data and discuss them in the context of the Lee \emph{et al}. model. [Preview Abstract] |
Tuesday, March 3, 2015 2:03PM - 2:15PM |
G29.00015: Neutron Study of the Magnetic Structures and Phase Transitions in RCuAs$_2$ (R=Pr, Nd, Tb, Dy, Ho, Yb) Yang Zhao, J. W. Lynn, Gohil S. Thakur, Zeba Haque, L.C. Gupta, A.K. Ganguli Neutron diffraction and inelastic scattering studies have been carried out on polycrystalline samples of the above titled materials as a function of temperature and applied magnetic field to determine the magnetic structures, order parameters, and overall spin dynamics. The space group of these compound is P4/nmm with typical (tetragonal) lattice parameters a $\sim$ 3.9 \AA\ and c $\sim$ 10.0 \AA. PrCuAs$_{2}$ develops commensurate magnetic order at T$_{N}$=6.4 K with an ordered moment of 1.24 $\mu _{B}$ oriented along the c-axis. The ordering wave vector is (0,0,1/2), with a $\Gamma _{3}^{1}$ representation. NdCuAs$_{2}$ orders at T$_{N}$=3.54 K with the same ordering wave vector (with the $\Gamma _{10}^{2}$ or $\Gamma _{9}^{2}$ representations), but with the moment direction in the a-b plane along [110] of magnitude 3.5 $\mu _{B}$. TbCuAs$_{2}$ and HoCuAs$_{2}$ exhibit incommensurate order at T$_{N}$=9.1 and 4.0 K, respectively. The results for the inelastic scattering will be discussed. [Preview Abstract] |
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