Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session P27: Focus Session: Low-dimensional Magnetism |
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Sponsoring Units: GMAG Chair: Jian Shen, Oak Ridge National Laboratory Room: Morial Convention Center 219 |
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P27.00001: Magnetic Stripe Phase at the Spin Reorientation Transition of an Ultrathin Magnetic Film Invited Speaker: Magnetic long-range order can not exist in an isotropic 2D Heisenberg system at any finite temperature, but could be established by adding a magnetic anisotropy to the system. In experiment, this topic has been addressed by investigating the so-called spin reorientation transition (SRT) in magnetic ultra-thin films, where the perpendicular magneto-crystalline anisotropy cancels the in-plane magnetic shape anisotropy. In this talk, we present our study on the SRT of Fe/Ni/Cu(100) system using Photoemission Electron Microscopy (PEEM) chamber at beam line 7.3.1.1 of the Advanced Light Source. We show that (1) a homogenous magnetic state is energetically unstable against the formation of magnetic stripe domains, (2) the stripe domain width decreases exponentially towards the SRT point, and (3) the Curie temperature of the film is reduced to result in a paramagnetic phase within a narrow thickness gap in the SRT region. Using magnetic interlayer coupling to simulate the effect of an external magnetic field, we further studied the SRT in Co/Cu/(Fe/Ni)/Cu(100) system and revealed a universal behavior of the stripe domain width. Moreover a new metastable bubble domain phase was observed near the SRT point in Fe/Ni film, which enriches the magnetic phase diagram of a 2D magnetic system. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P27.00002: Induced V magnetic polarization effects in V/Fe/V trilayers Rosa A. Lukaszew, Cesar Clavero, Yongseong Choi, Daniel Haskel, Cecilia Sanchez-Hanke, Brian Kirby, Mike Fitzsimmons Understanding magnetic interactions at interfaces between nonmagnetic and magnetic elements and in some cases magnetic polarization of non-magnetic elements at these interfaces is a challenging topic. In addition these phenomena are interesting also due to their potential application in magneto-optical (MO) data storage devices and sensors. In the case of Fe/V interfaces, a fundamental question is the effect of interdiffusion on the magnetic moments of the intervening elements at the interface. Here, we present a structural, morphological and magnetic study of a V/Fe/V trilayer. While no significant diffusion of V into the Fe film was observed with x ray reflectometry (XRR) and polarized neutrons reflectometry (PNR), x ray magnetic circular dichroism (XMCD) and x ray resonant magnetic scattering (XRMS) measurements reveal V magnetic polarization at both Fe/V interfaces. The magnetization profile in the Fe and V films was obtained by XRMS and will be compared with that obtained with PNR. Our results show the complementarity of these techniques to further clarify V polarization phenomena in Fe/V interfaces. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P27.00003: Structure and magnetism of Ni$_{50}$Mn$_{50}$ monolayers on Cu$_{3}$Au(001) Waldemar Macedo, Reinaldo Oliveira Jr., Maximiliano Martins, Manoel Pires The growth, structure and magnetism of equiatomic NiMn ultrathin films on Cu$_{3}$Au(100) and the magnetism of Fe/FeMn bilayers on this substrate were investigated. NiMn alloys in a concentration range around 50-50{\%} have an L1$_{0}$ structure with lattice constants a and c of 3,74 e 3,52 {\AA}, respectively. This NiMn-phase is antiferromagnetic (AFM), with high N\'{e}el temperature ($\sim $ 1070 K), being very interesting for ferromagnetic / antiferromagnet systems with exchange bias effect and, therefore, for magneto-electronic devices. Cu$_{3}$Au is an ordered fcc phase with lattice parameter of 3,75 {\AA}, a substrate with very good epitaxial relationship with L1$_{0}$ NiMn. The NiMn monolayers were grown by coevaporation, under molecular beam epitaxy conditions, and characterized in situ by RHEED, LEED, XPS, AES and MOKE. Structural analysis revealed the epitaxy and layer-by layer growth at room temperature. MOKE measurements suggest that the Fe/Ni$_{50}$Mn$_{50}$ bilayers present exchange bias, indicating that equiatomic NiMn films, as grown on Cu$_{3}$Au(100) at room temperature, is AFM, as expected for the L1$_{0}$. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P27.00004: Ab-initio simulation of magnetic exchange force mycroscopy of the antiferromagnetic Fe monolayer on W(001) Cesar Lazo, Stefan Heinze, Vasile Caciuc, Hendrik Hoelscher Magnetic exchange force microscopy (MExFM) is a promising new technique to perform magnetic imaging with atomic resolution by measuring the magnetic exchange force between a magnetically coated tip and a magnetic sample [1]. Here, we apply density functional theory using the full-potential linearized augmented plane wave (FP- LAPW) method to investigate the exchange forces on the antiferromagnetic monolayer Fe on W(001) [2]. We use an Fe cluster as a tip model and include relaxations of the cluster and the surface. Surprisingly, relaxation effects of tip and sample depend sensitively on the local magnetic configuration. Therefore, relaxations play a crucial role for the magnetic signal. In particular, the onset of the exchange forces is shifted to larger distances, which facilitates their experimental observation. Based on the calculated force-distance curves we simulate MExFM images which display a competition of chemical and magnetic forces. Our simulations can explain the experimentally observed magnetic contrast [3]. [1] U. Kaiser \emph{et al}., Nature 446, 522 (2007). [2] A. Kubetzka \emph{et al}., Phys. Rev. Lett. 94, 087204 (2005). [3] R. Schmidt, C. Lazo, \emph{et al}., submitted (2007). [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:48AM |
P27.00005: First-principles calculation of low-dimensional magnetic structures Invited Speaker: Magnetism in low dimensions is of great interest for fundamental and industrial research. Density functional calculations are important to provide clear physical insights for search, design and optimization of magnetic nanostructures that are essential in new technologies. We have recently performed systematic studies for search of giant magnetic anisotropy energies in single atom such as 3d on CuN, monatomic wires encompassing 3d-5d atoms, magnetic thin films such as 3d on Cu and Au. We will review the physics that governs the magnetic anisotropy and other phenomena driven by spin-orbit coupling. We will also discuss our recent results of spin dynamics in nanoentities. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P27.00006: Shape Anisotropy and Magnetization Modulation in Hexagonal Cobalt Nanowires Zuwei Liu, Paichun Chang, Chia Chi Chang, Gerd Bergmann, Jia G. Lu Ferromagnetic Co nanowires with diameter around 90 nm are synthesized via low voltage electrodeposition method. High resolution transmission electron microscopy and x-ray diffraction results show that the nanowires are uniform in size, and consist predominantly \textit{hcp} structure with the magnetocrystalline easy axis ($c$-axis) perpendicular to the wire axis. SQUID measurement illustrates the dominance of shape anisotropy, manifested by the weak temperature dependence of the enhanced coercive field along the nanowire axis. The magnetic domain structures of individual, segmented or multiple nanowires are studied via magnetic force microscopy. It shows a strong dipole at the two ends of the nanowire, together with a spatial magnetization modulation along the nanowire with a period around 700 nm. Based on theoretical modeling, such intrinsic modulation originates from the competition between the magnetocrystalline polarization along the easy axis and the shape anisotropy along the nanowire axis. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P27.00007: Unique playground for complex magnetism: Fe monolayers on hexagonal transition-metal surfaces Stefan Heinze, Bjoern Hardrat, Paolo Ferriani, Ali Al-Zubi, Gustav Bihlmayer, Stefan Bluegel Recently, the complexity of magnetic order even in single monolayer (ML) thick magnetic films on non-magnetic substrates has been dramatically demonstrated by the discovery of a spin-spiral state for a Mn ML on W(110) [1] and a nanoscale magnetic structure for an Fe ML on Ir(111) [2]. Here, we use density functional theory calculations based on the full-potential linearized augmented plane wave method to systematically study the magnetic order of an Fe ML on hexagonal hcp (0001) and fcc (111) surfaces of $4d$- and $5d$-transition metals. We show that due to substrate $d$-band filling the exchange coupling changes gradually from antiferromagnetic (AFM) on Tc, Ru, Re, and Os to ferromagnetic (FM) on Rh, Ir, Pd, and Pt. On Ru and Re the AFM coupling leads to a non-collinear N\'eel ground state due to topological frustration of exchange interaction. On Ru, Rh and Ir, the nearest-neighbor exchange coupling is small and exchange beyond nearest-neighbors, higher order spin interactions, and anisotropic exchange interaction compete making these systems a playground for intriguing magnetic order. [1] M.~Bode {\sl et al.}, Nature {\bf 447}, 190 (2007). [2] K.~von Bergmann {\sl et al.}, PRL {\bf 96}, 167203 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P27.00008: Mean Field aspects of magnetic domain pattern evolution in ultrathin Fe/Cu(001) films Danilo Pescia, Alessandro Vindigni, Niculin Saratz, Oliver Portmann, Paolo Politi Ultrathin Fe/Cu(001) films are magnetized out of plane and represent an experimental counterpart of the 2D Dipolar Frustrated Ising Ferromagnet. Indeed, one of the most attractive feature of these films is the occurrence of a variety of magnetic domain patterns; the last few consist of superstructures of positive and negative magnetization which originate from the competition between long-ranged antiferromagnetic dipolar interaction and nearest-neighbor ferromagnetic exchange interaction. The experimental patterns are relatively free to change their characteristic length of modulation as well as their overall structure (striped, labyrinthine, etc...) when the temperature is varied. The Mean Field theory is able to reproduce the temperature dependence of some important experimental observables like the domain width and the inside-domain magnetization profile (obtained by SEMPA images). We report on some Mean Field predictions for the 2D Dipolar Frustrated Ising Ferromagnet in close relationship with quantitative analysis of experimental SEMPA images recorded on Fe/Cu(001) films. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P27.00009: Novel magnetism in gold and silver nanoclusters Weidong Luo, Stephen J. Pennycook, Sokrates T. Pantelides Ferromagnetic (FM) ordering in transition-metal systems (solids, surface layers, nanoparticles) arises from partially filled $d$ shells. Thus, recent observations of FM Au nanoclusters were unexpected, and an explanation has remained elusive. We report first-principles density-functional spin-polarized calculations for Au and Ag nanoclusters. We find that in highly symmetric Au nanoclusters, the highest-occupied molecular orbital (HOMO) is highly degenerate and partially filled by Au 6$s$ electrons with spins aligned according to Hund's rule. The nanoclusters behave like ``superatoms,'' with the spin-aligned electrons being itinerant on the outer shell of atoms. Similar results obtain for Ag nanoclusters. In contrast, the same kind of calculations for Pt nanoclusters find that FM ordering is controlled by the partially filled $d$ states in the usual way, and spin polarization generally occurs in many eigenstates of the Pt clusters. This research was sponsored in part by the DOE Office of Basic Energy Sciences, Division of Materials Sciences and Engineering and by the McMinn Endowment at Vanderbilt University. Computations were performed at the National Energy Research Scientific Computing Center. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P27.00010: Non-Arrhenius Ferromagnetism In 1D Systems Luca Spadafora, Fausto Borgonovi, Luca Celardo, Bruno Goncalves Topological phase space disconnection has been recently found to be a general phenomenon in isolated anisotropic spin systems. This phenomenon sets a general framework to understand the emergence of ferromagnetism in finite magnetic systems. Here we study its relevance for finite systems in contact with a heat bath. The existence of this threshold, inducing extremely large magnetic reversal time, is shown to be able to determine metastable ferromagnetic behavior in finite samples. Also, it acts as a real energy barrier. Under suitable conditions the law for average reversal times can be obtained analytically and confirmed numerically. Consistent differences from the expected Arrhenius law of reversal times are shown for short range interacting spin systems. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P27.00011: Spin-orbit induced spin-spin coupling between electrons in coupled quantum dots Jianmin Sun, Suhas Gangadharaiah, Oleg Starykh We investigate spin-spin interaction between electrons localized in spatially separated quantum dots. We show that in the presence of single electron spin-orbit interaction (of Rashba type) in the dots and Coulomb interactions between electrons, a new anisotropic coupling of the van der Waals type between spins emerges. Unlike the standard exchange this coupling does not require overlap of the wavefunctions, and as a result becomes dominant for large distance between the dots.This ferromagnetic interaction is important in the Wigner crystal state where the exchange processes are severely suppressed. [Preview Abstract] |
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