Science | Sarnjeet Dhesi

Sarnjeet Dhesi Nanoscience

Prof Sarnjeet S Dhesi is principal beamline scientist on the Nanoscience beamline, using PhotoEmissionElectron Microscopy (PEEM), X-Ray Magnetic Circular and Linear Dichroism and Soft X-ray Resonant Diffraction to understand magnetism and electronic structure in ultrathin films, nanostructures and transition metal oxides. The Nanoscience beamline (I06) uses a PEEM combined with a sample preparation chamber housing a scanning tunnelling microscope to understand low-dimensional effects in solid state physics. The instruments on I06 are designed to study nanomagnetism and surface physics using spatially resolved x-ray photoemission and x-ray absorption spectroscopy.

Email: Sarnjeet Dhesi
Tel: +44 (0) 1235 778056
Beamline I06: Nanoscience

Key Research Areas

Low dimensional effects on the magnetocrystalline anisotropy in ultrathin films, orbital ordering in transition metal oxides, PhotoEmission Electron Microscopy (PEEM), nanotechnology.

Current Research Interests

(a) Charge, orbital and spin ordering in the MnO2 planes of La0:5Sr1:MnO4

Orbital ordering (OO), which involves a spatial redistribution of valence states, has attracted renewed interest in recent years even though it has been studied since the predictions of John Goodenough over 50 years ago. In the original model Goodenough proposed that the Mn ions in manganites are found in two distinct charge states, which allows the possibility of OO influencing the magnetic ordering. For La_0.5Sr_1.5MnO₄ the Mn sites can be considered to have an average valence of +3.5 at room temperature, but below the charge ordering (CO) temperature (T_CO) ~217K two inequivalent sites have been proposed. The degeneracy in the valence states of one site can then be lifted by a Jahn-Teller distortion (JTD) of the oxygen octahedra or by antiferromagnetic spin ordering leading to orbital ordering (OO).

In the case of OO the temperature (T_OO) is equivalent to T_CO. On further cooling below the Néel temperature (T_N) of ~120K a long-range CE-type antiferromagnetic structure develops. The two charge-separated Mn sites, which are denoted Mn³⁺ and Mn⁴⁺ (although the charge separation is fractional) display a checker board pattern (see figure a). This pattern is made up of ferromagnetic zig-zag chains, where the Mn⁴⁺ sites form the corners and the Mn³⁺ sites are in the middle of the straight segments. Adjacent zig-zag chains are antiferromagnetically aligned with respect to each other. There is a JTD of the O atoms consisting of an elongation of the Mn⁴⁺ -O bonds along the zig-zag segments. Soft X-ray Resonant Diffraction (SXRD) using 2p ® 3d transitions is a direct probe of the states involved in OO. By comparing the energy dependence of the OO SXRD intensity with ligand-field calculations a distinction between the JTD and OO contributions can be made. If hybridisation is taken into account then comparison of the SXRD energy dependence with cluster calculations reveals that spin ordering for T_OO < T < T_N is important for the OO.

Selected Publications

1. Isolating the interface magnetocrystalline anisotropy contributions in magnetic multilayers, S.S. Dhesi, H.A. Dürr, M. Münzenberg, and W. Felsch, Phys. Rev. Lett. 90, 117204 (2003)
2. Giant magnetic anisotropy of single cobalt atoms and nanoparticles, P. Gambardella, S. Rusponi, M. Veronese S. S. Dhesi, C. Grazioli, A. Dallmeyer, I. Cabria, R. Zeller, P. H. Dederichs, K. Kern, C. Carbone, H. Brune, Science 300, 1130 (2003)
3. Unravelling orbital ordering in La0.5Sr1.5MnO4, S. S. Dhesi, A. Mirone, C. De Nadai, P. Ohresser, P. Bencok, N. B. Brookes, P. Reutler, A. Revcolevschi, A. Tagliaferri, O. Toulemonde, G. van der Laan, Phys. Rev. Lett. 92, 056403 (2004)
4. Surface diffusion of Cr adatoms on Au(111) by quantum tunneling, P. Ohresser, H. Bulou, S. S. Dhesi, C. Boeglin, B. Lazarovits, E. Gaudry, I. Chado, J. Faerber,F. Scheurer, Phys. Rev. Lett. 95, 195901 (2005)