Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V22: Magnetic Phase Transitions I |
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Sponsoring Units: DCMP Chair: Kyle Shen, Cornell University Room: D163 |
Thursday, March 24, 2011 8:00AM - 8:12AM |
V22.00001: Pressure-induced antiferromagnetism in pure CeFe2 Jiyang Wang, Thomas Rosenbaum, Yejun Feng, Rafael Jaramillo, Sara Haravifard CeFe2 is a ferromagnet that exhibits antiferromagnetic fluctuations in its ground state at low temperature. We use x-ray diffraction to measure directly the emergence of antiferromagnetic order in pure CeFe2 at high pressure. We present an analysis of both the magnetic and lattice symmetries in the newly discovered high pressure phase, and compare our results to those from doped CeFe2 systems. This comparison provides insights into the roles of pressure and chemical doping in driving the magnetic quantum phase transition. [Preview Abstract] |
Thursday, March 24, 2011 8:12AM - 8:24AM |
V22.00002: Transition-metal dihalide $MX_2$ as magnetoelectric multiferroics Shinichiro Seki, Takashi Kurumaji, Shintaro Ishiwata, Hiroyuki Matsui, Yoshinori Tokura, Hiroshi Murakawa, Yusuke Tokunaga, Yoshio Kaneko, Tatsuo Hasegawa Magnetoelectric properties were investigated for transition-metal dihalide $MX_2$, which turns out to be the first example of non-chalcogen based spiral-spin induced multiferroics. We discovered the emergence of ferroelectric polarization ($P$) in helimagnetic state for several compounds such as CuCl$_2$ with edge-shared $S=1/2$ chain and MnI$_2$ with stacked triangular lattice. In the latter material, in-plane magnetic field ($H$) was found to induce the rearrangement of six possible multiferroic domains. With every $60^{\circ}$-rotation of $H$ around the $c$-axis, discontinuous $120^{\circ}$-flop of $P$-vector is observed as a result of the flop of magnetic modulation vector ($q$). In-plane $H$ also alters the stable direction of $q$-vector from original $q \parallel \langle 1\bar{1}0 \rangle$ to $q \parallel \langle 110 \rangle$ above 3 T, which leads to significant change of $P$-flop patterns under rotating $H$. At the critical field region ($\sim$ 3T), due to the enhanced $q$-flexibility, $P$-vector smoothly rotates clockwisely twice while $H$ rotates counter-clockwisely only once. [Preview Abstract] |
Thursday, March 24, 2011 8:24AM - 8:36AM |
V22.00003: Infrared spectroscopy of phonons and electromagnons in multiferroic TbMnO$_3$ R.P.S.M. Lobo, R. Schleck, R.L. Moreira, H. Sakata We measured the temperature dependent infrared reflectivity spectra of TbMnO$_3$ with the electric field of light polarized along each of the three crystallographic axes. We analyzed the effect, on the phonon spectra, of the different phase transitions occurring in this material. We show that the antiferromagnetic transition at $T_N$ renormalizes the phonon parameters along the three directions. Our data indicate that the electromagnon, observed along the $a$ direction, has an important contribution in building the dielectric constant. We show that this electromagnon spectral weight comes only from a few phonons which can be clearly identified. We also determined that only one phonon, observed along the $c$-axis, has anomalies at the ferroelectric transition. This phonon is built mostly from Mn vibrations, suggesting that Mn displacements are closely related to the formation of the ferroelectric order. [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 8:48AM |
V22.00004: Ultrafast pump-probe spectroscopy of multiferroic TbMnO$_{3}$ Jingbo Qi, Li Yan, Stuart Trugman, Jianxin Zhu, Alexander Balatsky, Quanxi Jia, Antoinette Taylor, Rohit Prasankumar TbMnO$_{3}$, exhibiting simultaneously both magnetic and ferroelectric phases, is an excellent multiferroic candidate for demonstrating the strong coupling between different degrees of freedom, i.e. spin, orbital and charge order. Previously, ultrafast optical pump-probe spectroscopy has proven to be an ideal technique for unraveling the interplay between different orders in the time domain. In this work, we used this technique to study ultrafast dynamics in multiferroic TbMnO$_{3}$. At low temperatures, we initially observed an extraordinarily slow rising process, with a timescale of tens of picoseconds, followed by another decay process with a relaxation time of hundreds of picoseconds. An analysis of these two processes as a function of temperature reveals the influence of the magnetic and ferroelectric phase transitions on carrier dynamics in TbMnO$_{3}$, which agrees well with other experimental results. [Preview Abstract] |
Thursday, March 24, 2011 8:48AM - 9:00AM |
V22.00005: Photoinduced Femtosecond Formation of Ferromagnetism in a Strongly Correlated Antiferromagentic Manganite Tianqi Li, Aaron Patz, Jiaqiang Yan, Thomas Lograsso, Jigang Wang There has been strong current interest to manipulate collective spins and even induce magnetic phase transitions in their highly \textit{non-equilibrium, non-thermal }states at \textit{femtosecond} time scales. Such processes offer opportunities to exceed the upper limit of the magnetic switching speed (0.1-10 GHz) in modern magneto-optical recording industry and magnetic storage/logic devices. One prominent system to explore such femtosecond magnetism is strongly correlated manganites, which are truly ``responsive'' near the phase boundary, exhibiting extreme sensitivity to external stimuli, such as light, electric and magnetic fields. Using ultrafast two-color magnetic circular dichroism spectroscopy, we have observed a substantial photoinduced magnetization enhancement in Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ within 180 fs above a threshold pump fluence and at low temperature. Such a photoinduced critical behavior vanishes at elevated temperature. These results clearly show a photoinduced ultrafast antiferromagnetic to ferromagnetic phase transition, demonstrating particularly, that one can reveal a hidden, thermally inaccessible ground state at fs time scales. [Preview Abstract] |
Thursday, March 24, 2011 9:00AM - 9:12AM |
V22.00006: Polaron Glass in La$_{0.35}$Pr$_{0.275}$Ca$_{0.375}$MnO$_3$ Mark Burkhardt, M.A. Hossain, S. Sarkar, J. St\"ohr, Y.-D. Chuang, A.G. Cruz Gonzalez, A. Doran, A. Scholl, A.T. Young, Y.J. Choi, S.-W. Cheong Manganite compounds in the La$_{1-x-y}$Pr$_y$Ca$_x$MnO$_3$ series are known for exhibiting phase separation over a large temperature range. We combined the x-ray photoemission electron microscopy (PEEM) and resonant elastic soft x-ray scattering (REXS) techniques to study the interplay between the ferromagnetic and charge-ordered/antiferromagnetic phases, respectively, in La$_{0.35}$Pr$_{0.275}$Ca$_{0.375}$MnO$_3$. We found a polaronic glassy state at intermediate temperatures, when the material is dominated by charge- and orbital-order domains. When the sample is cooled below T$_{\mathrm{C}}$, the magnetization increases, accompanied by a relaxation of the lattice deformations that accompany the polaron glass. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:24AM |
V22.00007: The role of oxygen in the colossal magnetoresistance in manganites M.A. Hossain, M.H. Burkhardt, S. Sarkar, H. Ohldag, S. de Jong, A. Scholl, A.T. Young, A. Doran, Y.-D. Chuang, D. Dessau, J. Mitchell, H.A. D\"{u}rr, J. St\"{o}hr We have used Low temperature Photo-Emission Electron Microscopy (PEEM) measurements on the bi-layered manganite compound La$_ {1.8}$Sr$_{1.2}$Mn$_2$O$_7$ to explore the origin of the colossal magnetoresistance (CMR) effect. It is generally agreed that CMR cannot be explained by double exchange only, and that other interactions mediated by oxygen atoms such as polarons must be important. We have imaged the magnetic domain structure by x-ray magnetic circular dichroism PEEM spectro-microscopy at both the O and Mn sites. By probing the ferromagnetic domain formation below T$_c$, we find that the insulator-to-metal transition is mediated by magnetic interactions involving a strong magnetic moment on oxygen. The spatially resolved oxygen K-edge XMCD signal reveals the role of the in-plane and out-of- plane e$_g$ orbitals in the magnetic transitions and thereby sheds light on the very origin of CMR. [Preview Abstract] |
Thursday, March 24, 2011 9:24AM - 9:36AM |
V22.00008: Momentum-Space Dichotomy in the Metal-Insulator Transition in doped EuO Daniel Shai, Alexander Melville, John Harter, Eric Monkman, Dawei Shen, Darrell Schlom, Kyle Shen EuO possesses a wide variety of remarkable properties, most which can be accessed only upon carrier doping. In addition to its large ferromagnetic moment (S = 7/2), doped EuO exhibits a metal-insulator transition with a change in resistivity of over $10^{13}$ and highly spin polarized carriers. Furthermore, the ferromagnetic Curie temperature can be enhanced from 69 K in undoped EuO to over 200 K in carrier doped EuO. We present angle-resolved photoemission studies of Eu$_{1-x}$Gd$_x$O thin films which elucidate the electronic structure and mechanism of the metal-insulator transition. Our ARPES studies verify that the exchange coupling between the Eu 4f moments and the delocalized Eu 5d states pushes the bottom of the majority-spin conduction band through $E_F$ below $T_C$. We also reveal a surprising dichotomy between the delocalized carriers at the Brillouin zone boundary below $T_C$, and localized carriers around the zone center above $T_C$ which are responsible for the respective low-temperature ferromagnetic metallic and high-temperature paramagnetic semiconducting behaviors observed in transport measurements. [Preview Abstract] |
Thursday, March 24, 2011 9:36AM - 9:48AM |
V22.00009: Anharmonic Lattice Effect in the Giant Negative Thermal Expansion Antiperovskite Cu$_{1-x}$Sn$_{x}$NMn$_{3}$ Peng Tong, Despina Louca, Graham King, Anna Llobet, Bosen Wang, Yuping Sun The antiperovskite ANMn$_{3}$ (A: transitional metals and semiconducting elements) often shows a large, discontinuous volume contraction at the magnetic transition associated with a large negative thermal expansion (NTE). The NTE property was initially observed in Cu$_{1-x}$Ge$_{x}$NMn$_{3}$ where Ge-doping broadens the discontinuous volume contraction. It was recently reported that although the average symmetry is cubic in the solid solutions, locally, the symmetry is tetragonal with the I4/mcm symmetry of the end member GeNMn$_{3}$. We investigated the local structure of a ``Ge''-free NTE system, the Cu$_{1-x}$Sn$_{x}$NMn$_{3}$ with $x$= 0.1 and 0.5. On average, both compounds are cubic at all temperatures. At base temperature, the local structure is cubic as well. As the temperature rises however, local lattice distortions evidenced by the splitting of the Mn-Cu and Mn-Sn bonds are observed. This local distortion can be described by the I4/mcm symmetry but this symmetry is different from the P4/mmm symmetry of SnNMn$_{3}$. The splitting of the Mn-Cu and Mn-Sn bond gives rise to a local lattice anharmonicity that may in turn be significant in the NTE behavior in this class of compounds. [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:00AM |
V22.00010: Magneto-elastic coupling in molecule-based materials: [Ru$_2$(O$_2$CMe)$_4$]$_3$[Cr(CN)$_6$] and Mn[N(CN)$_2$]$_2$ T.V. Brinzari, Q.-C. Sun, J.L. Musfeldt, L.-C. Tung, Y.J. Wang, J. Liu, M.-H. Whangbo, J.S. Miller, J.L. Manson, J.A. Schlueter We measured the infrared vibrational response of two prototypical molecule-based magnets, [Ru$_2$(O$_2$CMe)$_4$]$_3$[Cr(CN)$_6$] and Mn[N(CN)$_2$]$_2$. We find that both temperature and magnetic field driven transitions impact spin-lattice interactions in these materials. For instance, through the N\'eel transition, Cr--CN stretching and bending modes as well as Ru-O stretching mode in [Ru$_2$(O$_2$CMe)$_4$]$_3$[Cr(CN)$_6$] display sudden frequency shifts and a strong hysteresis that reveal local structure changes around Cr and Ru centers in response to magnetic ordering. On the other hand, the dicyanamide ligands in Mn[N(CN)$_2$]$_2$ display pronounced sensitivity to the 30 T magnetic quantum critical transition, in line with our calculations that point toward the importance of N--C--N and C--N--C angles for the mediation of Mn$\cdots$Mn spin exchange interactions. [Preview Abstract] |
Thursday, March 24, 2011 10:00AM - 10:12AM |
V22.00011: Neutron diffraction study of the magnetic order in magnetic coordination polymer CuF$_{2}$(H$_{2}$O)$_{2}$(pyz) (pyz=pyrazine) CuiHuan Wang, Mark D. Lumsden, Andrew D. Christianson, John A. Schlueter The new linear chain coordination polymer CuF$_{2}$(H$_{2}$O)$_{2}$(pyz)(pyz=pyrazine) provides an interesting example for the study of low dimensional physics. CuF$_{2}$(H$_{2}$O)$_{2}$(pyz) has a monoclinic structure where the Cu$^{2+}$ ions form a 2 dimensional (2D) square lattice in the bc-plane. Short range magnetic order appears below 10 K followed by a transition to long range antiferromagnetic order with T$_{N} \quad \sim $ 2.6 K. To further understand the magnetic behavior of CuF$_{2}$(H$_{2}$O)(pyz), we have performed neutron diffraction experiments on deuterated single crystals CuF$_{2}$(D$_{2}$O)$_{2}$(d$_{4}$-pyz). Below 2.6 K we observe magnetic Bragg peaks which are consistent with the propagation vector (1/2 1 0). Refinement of the data shows that the magnetic moment lies in the ac-plane. Fitting the temperature dependence of the magnetic order parameter to a power-law form in the reduced temperature range of 1-T/T$_{N}$ = 0.01-0.4 yields a critical exponent, $\beta $, of 0.25$\pm $0.01. This result is consistent with the expectation for a 2D XY model where $\beta $=0.23 [1]. \\[4pt] [1] S. T. Bramwell and P. C. W. Holdsworth, J. Phys.: Condens. Matter. \textbf{5} L53-L59 (1993). [Preview Abstract] |
Thursday, March 24, 2011 10:12AM - 10:24AM |
V22.00012: Valence instability of Eu in EuPd$_3$B$_x$ ($ 0 < x < 0.53$) Andreas Leithe-Jasper, Roman Gumeniuk, Miriam Schmitt, Wilder Carillo-Cabrera, Walter Schnelle, Christoph Geibel, Helge Rosner In a joint theoretical and experimental study large series of intermetallic compounds EuPd$_3$B$_x$ and GdPd$_3$B$_x$ are characterized by X-ray diffraction, metallography, EPMA and chemical anlysis assessing the range of formation up to $x < 0.53$ and $x < 0.42$, respectively. Density functional based electronic structure calculations predict a valence change in EuPd$_3$B$_x$ above $x = 0.19(0.02)$ from a non-magnetic Eu$^{3+}$ state into a magnetic Eu$^{2+}$ state which is reflected in a discontinuity of the lattice parameter. Consistent with the calculations X-ray diffraction data show a kink in the lattice parameter at $x = 0.22(0.02)$. X-ray absorption spectroscopy measurements assign this kink to a transition into a heterogeneous mixed valence state of Eu. The influence of external pressure on the valence instability will be discussed. [Preview Abstract] |
Thursday, March 24, 2011 10:24AM - 10:36AM |
V22.00013: Efficient First-principles Wang-Landau Calculations G. Brown, A. Rusanu, Kh. Odbadrakh, M. Eisenbach, D.M. Nicholson The Wang-Landau (WL) method of finding the density of states g(E) contributing to the partition function Z(kT) is useful for determining thermodynamic properties from first-principles energy calculations for magnetic systems. Since DFT calculations require significant computer resources, it is important to make the convergence of the WL method to a self-consistent g(E) as efficient as possible. We present approaches for making accurate initial estimates of g(E) based on similar Hamiltonians or estimates of g(E) for a different number of atoms. These approaches can include workstation-based calculations using classical Heisenberg Hamiltonians based on exchange parameters calculated from initial data from DFT WL calculations. In addition, we annunciate serveral insights we have gained into the convergence of the WL method. For instance, the minimum curvature of the calculated g(E) is limited by the update parameter and the maximum energy step of the Markov chain. This material is based upon work supported as part of the Center for Defect Physics, an Energy Frontier Research Center funded by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences. This research used resources of the Oak Ridge Leadership Computing Facility at Oak Ridge National Laboratory, which is supported by the Office of Science of the Department of Energy. [Preview Abstract] |
Thursday, March 24, 2011 10:36AM - 10:48AM |
V22.00014: First principles calculations of magnetic properties of Fe and $\textrm{Fe}_3\textrm{C}$ at finite temperature Markus Eisenbach, Gregory Brown, Aurelian Rusanu, Don M. Nicholson We demonstrate a method to investigate finite temperature magnetism from first principles that harnesses massively parallel computers to obtain the free energy, specific heat, magnetization, susceptibility, and other quantities as function of temperature by combining classical Wang-Landau Monte-Carlo calculations with a first principles electronic structure code that allows the energy calculation of constrained magnetic states. Here we will present our calculations of finite temperature properties such as specific heat, magnetization and susceptibility of Fe and $\textrm{Fe}_3\textrm{C}$ using this approach where we find the Curie temperatures to be in good agreement with experiment at 980K and 425K respectively. This work was conducted at Oak Ridge National Laboratory (ORNL), which is managed by UT-Battelle for the U.S. Department of Energy (US DOE) under contract DE-AC05-00OR22725 and sponsored in parts by the Center for Nanophase Material Sciences, Scientific User Facilities Division, the Center for Defect Physics, an Energy Frontier Research Center funded by the US DOE Office of Basic Energy Sciences and by the US DOE Office of Energy Efficiency and Renewable Energy, Industrial Technologies Program. This research used resources of the Oak Ridge Leadership Computing Facility at ORNL, which is supported by the US DOE, Office of Science. [Preview Abstract] |
Thursday, March 24, 2011 10:48AM - 11:00AM |
V22.00015: Fist Principles Approach to the Magneto Caloric Effect: Application to Ni$_2$MnGa Khorgolkhuu Odbadrakh, Don Nicholson, Aurelian Rusanu, Markus Eisenbach, Gregory Brown, Boyd Evans III The magneto-caloric effect (MCE) has potential application in heating and cooling technologies. In this work, we present calculated magnetic structure of a candidate MCE material, Ni$_2$MnGa. The magnetic configurations of a 144 atom supercell is first explored using first-principle, the results are then used to fit exchange parameters of a Heisenberg Hamiltonian. The Wang-Landau method is used to calculate the magnetic density of states of the Heisenberg Hamiltonian. Based on this classical estimate, the magnetic density of states is calculated using the Wang Landau method with energies obtained from the first principles method. The Currie temperature and other thermodynamic properties are calculated using the density of states. The relationships between the density of magnetic states and the field induced adiabatic temperature change and isothermal entropy change are discussed. This work was sponsored by the Laboratory Directed Research and Development Program (ORNL), by the Mathematical, Information, and Computational Sciences Division; Office of Advanced Scientific Computing Research (US DOE), and by the Materials Sciences and Engineering Division; Office of Basic Energy Sciences (US DOE). [Preview Abstract] |
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