AVS2004 Session NS-MoA: Magnetic Imaging and Spectroscopy

Monday, November 15, 2004 2:00 PM in Room 213D

Monday Afternoon

Start	Invited?	Item
2:00 PM	Invited	NS-MoA-1 Spin-Polarized Scanning Tunneling Microscopy: Achievements and Perspectives M. Bode, K. Von Bergmann, O. Pietzsch, A. Kubetzka, R. Wiesendanger (University of Hamburg, Germany) Within the past decade spin-polarized scanning tunneling microscopy (SP-STM) was developed to a mature technique which not only allows for ultra-high spatial resolution studies of magnetic nanostructures, but also enables the direct correation with the sample`s topography and spin-resolved electronic structure. By reviewing the main achievements of SP-STM, which include the observation of size-dependent reorientation transitions ¹, the impact of strong external fields on magnetic nanowires ², atomic resolution of antiferromagnetic monolayers ³, and the direct observation of thermal switching events of indivitual superparamagnetic entities ⁴, we will discuss the strength and limitations of the technique. Possible future developments will be sketched and evaluated. ¹ A. Kubetzka et al., Phys. Rev. B 63, 140407 (2001). ² O. Pietzsch et al., Science 292, 2053 (2001). ³ S. Heinze et al., Science 288, 1805 (2000). ⁴ M. Bode et al., Phys. Rev. Lett. 067201 (2004).
2:40 PM	Invited	NS-MoA-3 Atomic-Scale Spin-Polarized Scanning Tunneling Microscopy of Magnetic Transition Metal Nitride Surfaces A.R. Smith, R. Yang (Ohio University); H.Q. Yang (Texas A&M University); W.R.L. Lambrecht (Case Western Reserve University); A. Dick, J. Neugebauer (Fritz-Haber-Institut der MPG, Germany) Spin-polarized scanning tunneling microscopy (SP-STM) can achieve ultimate magnetic resolution on surfaces, even down to the atomic scale.¹^,²^,³ In earlier work, we have shown the resolution of the spin structure of a novel antiferromagnetic (aFM) surface, Mn₃N₂ (010), with a model row-wise aFM structure.³ The surface is prepared using molecular beam epitaxy with a Mn effusion cell and radio frequency N plasma. The magnetic information appears as an additional component which is added to the non-magnetic component in the STM line scan. Furthermore, the magnetic information is bias-dependent; both the amplitude and polarity of the magnetic profile vary with the STM bias. The bias-dependence is understood as energy-dependent variations of the spin density of states of tip and sample. Both the magnetic and non-magnetic information can be extracted from the total STM image and compared with simulations based on theoretical calculations. Using first principles density functional theory, the local density of states for the surface is calculated for Mn₃N₂ (010). Two methods of simulation have been investigated for SP-STM. First, we have applied the atom superposition method (ASM). Second, we have simulated the images using the full Tersoff-Hamann (T-H) approach.⁴ We find that the T-H method is, in general, necessary for a correct simulation of the data due to the spin-dependent orbital lobes of the surface atoms. We furthermore apply the full T-H theory with different numbers of tip atoms to best model the STM data with good success. ¹ ¹S. Heinze {\it et al.}, Science {\bf 288}, 1805 (2000). ²D. Wortmann {\it et al.}, Phys. Rev. Lett. {\bf 86}, 4132 (2001). ³H. Yang {\it et al.}, Phys. Rev. Lett. {\bf 89}, 226101 (2002). ⁴J. Tersoff et al., Phys. Rev. Lett 50(25), 1998 (1983).
3:20 PM		NS-MoA-5 Spin-Polarized Scanning Tunneling Microscopy Study of Single-Crystallized Nanodot Arrays T.-H. Kim, J.H. Choi, J. Seo, Y. Kuk (Seoul National University, South Korea) We have developed a method to grow regularly patterned Fe nanodot arrays with in situ deposition. A self-sustained porous alumina mask was fabricated with an aluminum-coated Si substrate. Using the alumina shadow mask with perfectly ordered pores, we fabricated well-ordered Fe nanodot arrays.¹ Fe nanodot arrays with 0.2-10 nm thicknesses, 50-120 nm diameters, and 100-200 nm periods were successfully grown on a W(110) substrate in ultra-high vacuum. Fe nanodots were single-crystallized by mild annealing. Our shadow mask technique can be the simple and fast method to obtain high-density arrays over a macroscopic area. Spin-polarized scanning tunneling microscopy (SP-STM), one of the most powerful techniques to study magnetic nanostructures, can image surface domain structures with a lateral resolution reaching the atomic scale.² Well-ordered Fe nanodot arrays showed a stronger magnetic interaction between dots than randomly distributed Fe islands. We have performed micromagnetic simulation³ to study interaction between dots. ¹ J. H. Choi, T.-H. Kim, J. Seo, and Y. Kuk (submitted to APL). ² S. Heinze, M. Bode, A. Kubetzka, O. Pietzsch, X. Nie, S. Blugel, and R. Wiesendanger, Science 288, 1805 (2000). ³ We used the OOMMF program (http://math.nist.gov/oommf/). .
3:40 PM		NS-MoA-6 Spin-Resolved Core Level Photoemission of Ferromagnetic Thin Films G.D. Waddill, T. Komesu (University of Missouri-Rolla); S.A. Morton (Lawrence Berkeley National Laboratory); J.G. Tobin (Lawrence Livermore National Laboratory) We present spin-resolved 2p core level photoemission results for thin films of Fe, Co, and Ni. The films are bcc Fe on a Ag(100) substrate, fcc Co on Cu(001), and fcc Ni/Co/Cu(001). All films have an in-plane magnetic easy axis. We observe spin polarization in the main photoemission peaks consistent with trends in the bulk magnetic moments of the transition metals. In addition, Ni and Co have satellite peaks due to electron correlation effects and we see spin polarization in the Ni 6 eV satellite peak and much weaker spin polarization in the 4 eV Co peak. The existence of a satellite peak in the Co 2p spectrum is somewhat controversial and this data marks the first observation of spin-polarization in that peak. In addition, in the Ni/Co/Cu(001) system we have preliminary results for very thin films of Ni where charge transfer from the Co to the Ni will effect the electronic and magnetic properties of both films. For a 3 monolayer Ni film on Co we see differences in the photoelectron spin polarization of both Co and Ni compared to results for thicker Ni and Co films. These results emphasize the importance of spin-resolved photoemission in understanding the combined Coulomb, spin-orbit, and exchange interactions in the presence of interatomic electron correlation effects and configuration mixing that effect the photoemission process.
4:00 PM		NS-MoA-7 Spin Polarized Electron Energy Loss Spectroscopy on Ferromagnetic Thin Films T. Komesu, G.D. Waddill (University of Missouri-Rolla); J.G. Tobin (Lawrence Livermore National Laboratory) Spin-polarized electron energy loss spectroscopy (SPEELS) developed in the 1980s and has become a valuable technique for probing Stoner excitations and spin waves. SPEELS is sensitive to the occupied and unoccupied parts of the spin-split electronic structure of materials, and consequently SPEELS is a complementary technique to spin-resolved photoemission and inverse photoemission that more directly probe the occupied and unoccupied spin-split band structure respectively. Our results, using an unpolarized electron source with spin analysis shows sharp spin-dependent energy loss features in electron scattering from ferromagnetic thin films of Fe, Ni, and Co grown on Ag(100) and Cu(001). This is in contrast to most previous SPEELS studies (primarily using spin-polarized sources and spin analysis) where very broad featureless spectra are observed. We attribute the majority spin peaks we observe to spin-flip exchange scattering from the magnetic films, with the lowest energy feature corresponding to the exchange splitting for the films. The observed minority spin peaks are attributed non-flip exchange scattering.
4:20 PM	Invited	NS-MoA-8 High-Resolution Imaging of Magnetization Dynamics Using X-PEEM A. Scholl (Lawrence Berkeley National Laboratory) Ultrafast x-ray pulses (p-sec to f-sec) promise to be an ideal tool to probe the dynamics of magnetic materials. X-rays are sensitive to both ferromagnetic and antiferromagnetic order. Sum rules allow us to quantify spin moment, orbital moment and magnetic anisotropy specific for each element in a sample. High spatial resolution on the order of nanometers can be obtained using x-ray microscopy techniques using zone-plates or electron microscopes. As an example, a study of the precessional dynamics of magnetic vortices, 3-dimensional magnetic curls, will be presented [1]. The dynamics is probed at 100 nm spatial resolution and 70 ps temporal resolution using the PEEM-2 Photoemission Electron Microscope at the Advanced Light Source. It will be demonstrated that the vortex chirality or handedness, which is determined by the out-of-plane magnetization of the vortex core, governs the sub-ns dynamics of the structure, leading to a precessional motion of the vortex center. The dynamics is initiated by a sub-ns field pulse triggered by a laser, which is synchronized to the x-ray source. In contrast, on longer time scales it is known that damping dominates and the dynamics is governed by the in-plane domain structure. The measured vortex speed and the internal magnetic field at the core will be compared with the result of micromagnetic simulations and with the static susceptibility of the magnetic structure. The potential of studying processes beyond the Landau-Lifshitz-Gilbert dynamics using currently developed ultrafast x-ray techniques will also be discussed. [1] S.B. Choe et al., Science 304, 420 (2004).
5:00 PM		NS-MoA-10 Magnetic Interaction between a Ferromagnetic Substrate and Adsorbed Manganese Porphyrin Molecules A. Scheybal, T. Ramsvik (Paul Scherrer Institute, Switzerland); R. Bertschinger (Paul Scherrer Institut, Switzerland); M. Putero-Vuaroqueaux (L2MP-CNRS, France); T.A. Jung (Paul Scherrer Institute, Switzerland) The magnetic interaction between a magnetized thin film cobalt substrate and adsorbed manganese(III)-tetraphenylporphyrin chloride (MnTPPCl) molecules has been studied using X-ray magnetic circular dichroism (XMCD). In the regime of submonolayer coverage a clear circular dichroism is observed at the Mn L₃₂-edge, verifying that a net magnetization is set up by the manganese ions in the adsorbed molecules. An element specific hysteresis study shows that the magnetic properties of the molecules mirror those of the cobalt substrate. From this and from temperature dependent studies it is concluded that exchange interaction between the cobalt film and the molecules is the dominant cause for this induced magnetism. To our knowledge, this is the first time that an exchange coupling between adsorbed organic molecules and a ferromagnetic substrate has been demonstrated by XMCD at 3d-transition metal L₃₂-edges, thereby allowing direct information about both orbital and spin magnetic moments¹. As MnTPPCl is the parent compound of the [Mn(III)-porphyrin][TCNE] family of molecular magnets (TCNE = tetracyanoethylene)², the here presented molecular system and experiment provides a model system for the study of the magnetic interaction at the interface between a conventional ferromagnet and a molecular magnet. Furthermore, an organic semiconductor like tris(8-hydroxyquinoline) aluminium (Alq₃) can be used to prepare an organic spin-valve exhibiting giant magnetoresistance, as it has been demonstrated recently.³ Thus, the understanding of local magnetic coupling in molecular materials is of utmost importance for the application of magnetic materials in ever smaller dimensions. ¹ G. Schütz et al., Phys. Rev. Lett. 58 (1987) 737 ² J.S. Miller et al., Adv. Mat. 4 (1992) 498 ³ Z.H. Xiong et al., Nature 427 (2004) 821.