Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session D19: Low Dimensional Magnetism and Spin Tunneling |
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Sponsoring Units: GMAG Chair: Enrique Del Barco, University of Central Florida Room: D170 |
Monday, March 21, 2011 2:30PM - 3:06PM |
D19.00001: GMAG Student Dissertation Award Talk: Effects of Nanoscale Structure on the Magnetism and Transport Properties of Chromium and Chromium-Aluminum Alloys Invited Speaker: Bulk Cr has an incommensurate spin density wave (ISDW) due to nesting of the Fermi surface which is easily disrupted by perturbation. Thus, the properties of Cr are sensitive to small amounts of dopant atoms, application of pressure, etc. which has been well studied in bulk. We have taken advantage of thin film growth techniques to study the effects of nanoscale structure on the properties of Cr and Cr1-xAlx alloys. The first part of my talk will discuss our research on polycrystalline Cr thin films, where variables such as strain and disorder crucially affect the SDW. We find that Cr thin films can be ISDW like in bulk Cr, or transition to commensurate SDW (CSDW) or mixed depending on deposition conditions and the resulting thin film microstructure. The transport properties are also strongly affected, as quasilocal defect states inside the SDW gap cause resonant scattering. This results in anomalous features such as residual resistivity ranging between 3 and 400 $\mu$O -cm and significant resistivity minima at low temperature. Further evidence of quasilocal states inside the SDW gap is seen in the enhanced electronic density of states (DOS) from specific heat measurements of Cr thin films. The second part of my talk will discuss Cr1-xAlx alloys. The addition of Al to Cr causes the ISDW to transition to CSDW for x = 0.03. Cr1-xAlx also exhibits previously unexplained semiconducting behavior for x = 0.15-0.30. I will discuss our ongoing theoretical and experimental research which suggests that a chemically ordered, rhombohedrally distorted Cr3Al structure occurs in nanosized domains and causes a hybridization gap on part of the Fermi surface. The CSDW causes a gap on another part of the Fermi surface, so that the semiconducting behavior can be explained by a combination of structural and magnetic affects. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D19.00002: Suppression of Macrospin Tunneling by Nanomechanical Interference Lorien Hayden, Alexey Kovalev, Gerrit Bauer, Yaroslav Tserkovnyak This research considers the quantum dynamics of a nanomechanical resonator coupled to a macrospin of a magnetic nanoparticle. Suppression of macrospin tunneling by nanomechanical interference is demonstrated. By approximating the macrospin molecule as a two level system, the results are extended to the magnetopolariton splitting between resonantly coupled Fock states in which are observed similar interference patterns. The mentioned interference effects should be observable in a single molecule magnet bridged between two leads. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D19.00003: Spin Tunneling in a Rotating Nanomagnet Michael O'Keeffe, Eugene Chudnovsky We study spin tunneling in a magnetic nanoparticle with biaxial anisotropy that is free to rotate about its anisotropy axis. Exact instanton of the coupled equations of motion is found that connects degenerate classical energy minima. We show that mechanical freedom of the particle renormalizes magnetic anisotropy and increases the tunnel splitting. \\[4pt] M. F. O'Keeffe and E. M. Chudnovsky, cond-mat, arXiv:1011.3134. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D19.00004: Theory of Raman Scattering in One-Dimensional Quantum Magnets Masahiro Sato, Hosho Katsura, Naoto Nagaosa Raman scattering is one of the powerful tools to study the quantum dynamics of the spin systems, and has been studied for a long term. Conventionally, Raman scattering spectra have been interpreted in terms of the two-magnon processes, from which the exchange coupling can be estimated. However, it is known that the magnon is not a good elementary excitation in low- dimensional quantum spin systems, especially in 1D, and the theoretical studies on the Raman processes in 1D have not been developed compared with those for other electromagnetic processes such as NMR and ESR. Here we have developed a theory for Raman scattering of 1D quantum magnets. We show that Raman spectrum can detect some interesting excitations such as spinon pairs, solitons, etc, depending on the additional interactions to the conventional Heisenberg one and polarization direction of external electromagnetic wave. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D19.00005: Surface ferri-magnetism in some antiferromagnetic materials Yongbin Lee, Bruce Harmon In an antiferromagnetic material with inversion symmetry the electronic energy bands are spin degenerate because of time reversal symmetry. However, at the surface the inversion symmetry is broken, which opens the possibility of breaking the spin degeneracy and inducing a significant net moment on the surface. As an example of this spin degeneracy breaking, we discuss the electronic structure of antiferromagnetically ordered BaMn$_{2}$As$_{2}$. Unlike the bulk bands, its surface bands can individually possess a net spin polarization. Also the bulk bands in this material have a gap, however the calculated spin polarized surface states cross the Fermi level. Our calculations show that an applied field perpendicular to the surface and along the spin axis induces a significant net surface magnetization, which does not extend significantly into the bulk. - Work at the Ames Laboratory was supported by the US DOE, Basic Energy Science, under contract No. DE-AC02-07CH11358. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D19.00006: Spin-orbit interactionsand magnetic field in antiferromagnetic triangular molecular magnets J.F. Nossa, M.F. Islam, C.M. Canali, M.R. Pederson Frustrated triangular molecular magnets such as Cu$_3$ are characterized by a doubly generate S=1/2 ground-state (GS) with opposite chirality. Recently it has been proposed theoretically [1] and verified by ab-initio calculations [2] that an external electric field can efficiently couple these two chiral spin states, even in the absence of spin-orbit interaction (SOI). SOIs are nevertheless important, since they introduce a splitting in the GS manifold. In this talk we will discuss different schemes on how to evaluate within spin density functional theory the effect of the SOIs on the chiral states. The connection between SOI and the Dzyalozhinsky-Moriya interaction will be discussed. We will also present calculations of the energy dependence on an external magnetic field, whose presence is important to achieve full control of the spin-electric coupling within the manifold of the GS chiral doublets. \\[4pt] [1] M. Trif et. al. Phys. Rev. B 82, 045429 (2010) and M. Trif et. al. Phys. Rev. Lett. 101, 217201 (2008) \\[0pt] [2] F. Islam et. al. Phys. Rev. B 82, 155446 (2010) [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D19.00007: Real-space imaging of Kondo screening in a two-dimensional Kondo lattice Ying Jiang, Yanning Zhang, Juexian Cao, Ruqian Wu, Wilson Ho Kondo lattice systems exhibit nonuniversal many-body behaviors, mainly resulting from the competition and interplay between onsite Kondo screening and intersite coupling. In reduced spatial dimensions, the many-body correlation effects are expected be to more relevant. We report the realization of a two-dimensional (2D) Kondo lattice formed by self-assembled triplet oxygen molecules on the Au (110)-1$\times $2 reconstructed surface. By mapping the Kondo resonance in the 2D O$_{2}$ lattice with a scanning tunneling microscope, the interplay between the intermolecule coupling and the onsite Kondo effect was manifested as the unexpected coexistence of both local and nonlocal Kondo screening at the atomic level. While the latter provides evidence of the collective deconfinement of magnetization in Au, the former shows local hybridization between the Kondo clouds of nearest-neighbor O$_{2}$ molecules, as revealed by density functional calculations. These findings may assist in our understanding of the unusual electronic properties in various strongly correlated electron systems, such as heavy fermion compounds and Kondo insulators. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D19.00008: Single oxygen and CO molecules on Au(110): what can we learn? Yanning Zhang, Ying Jiang, Wilson Ho, Ruqian Wu Studies of O$_2$ and CO molecules on the reconstructed Au (110) surface are crucial for the understanding of unusually high activity of Au nanostructures that are useful in heterogeneous catalysis. Moreover, the O$_2$/Au system is ideal to probe the Kondo effect using surface science techniques, due to the persisting magnetization of triplet O$_2$ in physisorption systems. In this work, scanning tunneling microscope (STM) measurements and density functional calculations were performed to investigate the adsorption geometries and physical properties of single O$_2$ and CO molecules on Au(110). The calculated atomic structures and vibration frequencies are comparable favorably with our STM experimental results at low temperature, allowing efficient establishment of structural models. Interestingly, the O$_2$ molecule takes a defect site over the Au row, with a tilted geometry. The magnetic moment of O$_2$ is still as large as ~1.9 $\mu_B$, which furthermore induces a pronounced Kondo resonance in a large spatial region. The cloud of Kondo enhancement was found to closely follow the distribution of the calculated spin density at the fermi level, a correlation which is important for the understanding of Kondo effect in molecular systems. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D19.00009: Universal chiral magnetism in low-density 2D Kondo lattice model Dmitry Solenov, Dmitry Mozyrsky, Ivar Martin We demonstrate that (quasi-)two-dimensional systems comprised of localized moments and itinerant electrons form non-coplanar magnetic crystal states when the Kondo coupling energy is smaller than the chemical potential. These states arise for fully isotropic local exchange coupling between electrons and magnetic ions and do not require a spin-orbit (Dzyaloshinskii-Moriya) interaction or magnetic field. We give an analytical argument on instability of simple co-planar states and show that the states with non-zero chirality (degree of non-coplanarity) are energetically favorable. Numerical modeling is performed to estimate the ground state configurations. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D19.00010: Low temperature magnetic dynamics in one-dimensional Co(II) molecular chains A. Amjad, G.M. Espallargas, J.M. Clemente-Juan, R. Klemm, E. del Barco, E. Coronado, M. Evangelisti We present a low-temperature study of one-dimensional Co-based molecular chains, trans-[CoCl$_{2}$(3,5-Br$_{2}$py)$_{2}$]. Ac and dc susceptibility experiments show that the cobalt ions tend to form anisotropic ferromagnetic chains, whose properties are dictated by the thermal excitations of 1D domain walls. The observation of anomalies in the hysteresis loops of the sample on increasing the magnetic field sweep rate reveals interesting dynamical effects at the individual chain level. On decreasing the temperature, no evidence for 3D ordering was observed in specific heat measurements, although the magnetization was strongly irreversible below $\sim $450 mK. Possible sources for this absence of a magnetic phase transition could be the weakness of the interchain interactions, the presence of single-ion anisotropy at skew angles, disordered domains, and lattice defects, etc. These possibilities will be studied both experimentally and theoretically. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D19.00011: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 5:06PM - 5:18PM |
D19.00012: Spatial confinement effect on TbMn$_{2}$O$_{5}$ nanorods Chun Chuen Yang, Jing Huei Wang, Wei Luen Huang, Chang Yu Weng, Cih Lian Hong, Yang Yuan Chen Series of TbMn$_{2}$O$_{5}$ nanorods were fabricated by hydrothermal method with different annealing temperatures. Three samples which width by length equal to 10(4) nm $\times$ 38(14) nm, 25(6) nm $\times$ 64(18) nm, and 101(25) nm $\times$ 216(54) nm are identified by TEM images, x-ray diffraction, and SAED schemes. Furthermore discovery show that the preferred growth direction is along $c$ axis (length). Ac magnetic susceptibility and specific heat measurements revealed incommensurate ($\sim $41 K) and commensurate ($\sim $38 K) Mn antiferromagnetic ordering peaks are only appeared in 101(25) nm $\times$ 216(54) nm sample. In this case, the small size effect resulted entropy difference of Mn magnetic ordering is 27 {\%} less than bulk one. At 5 K, a small hysteresis loop was also observed in the identical sample and indicated the FM domains occurred. No such magnetic and thermal behaviors were found in another two samples. We believe this is attributing to spatial limitation and distortion caused by low surface-volume ratio. The estimated magnetic correlation length of Mn is in between 25 and 64 nm. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D19.00013: Tetrairon(III) Single Molecule Magnet Studied by Scanning Tunneling Microscopy and Spectroscopy Youngtek Oh, Hogyun Jeong, Minjun Lee, Jeonghoon Kwon, Jaejun Yu, Shariful Islam Mamun, Gajendra Gupta, Jinkwon Kim, Young Kuk Tetrairon(III) single-molecule magnet (SMM) on a clean Au(111) has studied using scanning tunneling microscopy (STM) and spectroscopy (STS) to understand quantum mechanical tunneling of magnetization and hysteresis of pure molecular origin. Before the STM studies, elemental analysis, proton nuclear magnetic resonance (NMR) measurement and Energy Dispersive X- ray Spectroscopy (EDS) were carried out to check the robustness of the sample. The STM image of this molecule shows a hexagonal shape, with a phenyl ring at the center and surrounding six dipivaloylmethane ligands. Two peaks are observed at 0.5 eV, 1.5 eV in the STS results, agreeing well with the first principles calculations. Spin-polarized scanning tunneling microscopy (SPSTM) measurements have been performed with a magnetic tip to get the magnetization image of the SMM. We could observe the antiferromagnetic coupling and a centered- triangular topology with six alkoxo bridges inside the molecule while applying external magnetic fields. [Preview Abstract] |
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