Crystallographic Structure of Cobalt Films on Cu (100)
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CRYSTALLOGRAPHIC STRUCTURE OF COBALT FILMS ON CU (100)
0. HECKMANN, H. MAGNAN *, P. LE FEVRE AND D. CHANDESRIS LURE, Universit6 de Paris Sud F-91405 ORSAY France * and SRSIM, CEA Saclay F-91191 GIF sur YVETTE France
ABSTRACT
The stable structure of cobalt is hexagonal closed packed (hcp), but cobalt can be stabilized in the face centered cubic structure (fcc) by epitaxy on Cu (100). These films are ferromagnetic with [110] in plane easy axis. The magnetic anisotropies of these films strongly depend on their structure, and in particular to the possible deviation from the isotropic fcc structure. We have studied these films by surface EXAFS. By recording the spectra both in normal incidence and in grazing incidence we have shown that the Co/Cu(100) films have aface centeredtetragonal structure : the mean nearest neighbour distance parallel to the surface is 2.55 A (same value as in bulk copper) and the interlayer bonds length is 2.50 A (same value as in bulk cobalt). We conclude that the films are in perfect epitaxy on copper (100) with a contraction of the lattice parameter perpendicular to the surface of 4%. A constant tetragonalization is observed for films of 2 to 15 monolayers. INTRODUCTION Thin metastable films of cobalt grown on Cu (100) are the subject of many magnetic and structural studies. MEED oscillations combined with in situ STM experiments1 show that cobalt grows layer by layer on Cu (100) except for the first two layers. The films are always ferromagnetic, independent of the film thickness, and the remanent magnetization is always parallel to the interface 2, with a strong in-plane anisotropy : the [110] direction is the easy axis 3,4 . The Curie temperature of films evaporated at 450 K is strongly increasing with the film 3 thickness from 130 K for 1.5 ML to 500 K for 2.5 ML . LEED results indicate that cobalt
epitaxially grows on Cu (100) with an fcc tetragonal structure2 ,5 . Surface EXAFS6 is a very attractive technique to measure the cristallographic structure of metastable thin films. It gives to a high precision the shape of the first neighbour shell 7 , including its possible asymmetry8 and its thermal broadening9 ,1 0 which is related to the elastic force constant between nearest neighbours (nn) in the film. Moreover, the linear polarization of the synchrotron radiation reveals information about the anisotropy of the crystallographic structure. EXAFS is a selective method: by measuring the EXAFS oscillations above the K edge of cobalt we are sure to be sensitive only to the local order in the cobalt film. Then, films of any thickness, coated films and multilayers can be characterized with the same precision. EXPERIMENTAL PROCEDURE The EXAFS experiments were performed at the Laboratoire pour l'Utilisation du
Rayonnement Electromagndtique (LURE) on the surface EXAFS set-up using a Si (311) double crystal monochromator installed on the wiggler beam line of DCI storage ring. The
samples Co/Cu (100) were prepared in a UHV chamber connected to the X-ray beam line. Cobalt is evaporated from a
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