Vanadium as Overlayer on Ag(001)
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VANADIUM AS OVERLAYER ON Ag(001) H. NAIT-LAZIZ x, C. DEMANGEAT x and A. MOKRANI xx Universit6 Louis-Pasteur, 4, rue Blaise Pascal, 67070 Strasbourg, France xx Laboratoire de Physique du Solide, 2, rue de la Houssini~re, 44072 Nantes, France ABSTRACT The electronic and magnetic structure of small vanadium clusters adsorbed on Ag(001) is studied as a function of the exchange integral J of the Hubbard Hamiltonian and cluster size with bcc structure. For each topological arrangement, the transition from nonmagnetic to magnetic order is characterized. As in the case of films (I or 2 layers thick) epitaxially grown on Ag(001), antiferromagnetic coupling between V atoms is usually obtained. However, because the symmetry is lower in the present case, various magnetic configurations are displayed. INTRODUCTION Since a long time, vanadium has attracted interest for its specific magnetic properties: the isolated atom has a permanent moment of 3pB; however it is well known that bulk V is paramagnetic. Very recently, Bouarab et al [1 ] and DorantesDavila et al [ 2 ] have discussed respectively the onset of magnetic moments of vanadium free standing layers or as overlayer on Ag(001) and of free standing clusters of vanadium. The local magnetic moment, magnetic order and average magnetic moment are calculated for T=OK as a function of the intra-atomic exchange integral J. In the present communication we will consider clusters of
vanadium adsorbed on Ag(001). A one-layer cluster of 21 atoms and a two-layers cluster of 33 atoms are considered (figure 1). Free standing V clusters have been shown to become magnetic when the number of atoms is not too high [ 2, 3 ]. Two contradictory effects appear to be in work for these clusters: a supermagnetic effect [ 4 ] leading to a cancellation of the magnetic moment in the case of small size of the vanadium cluster and a coordination number effect which tends to cancel the moment of the cluster when the size is too great [ 2, 3 ] . Through Stern-Gerlach measurements, Douglas et al [ 4 1 have shown that the maximum true moment consistent with uncertainty is considerably below the calculated values of DorantesDavila et al [ 2 ] and Feng-Liu et al [ 3 ] . The small value may indicate that the lattice spacing is less than the bulk value. In order to stabilize the lattice parameter, experimentalist do perform growth of V on Ag(001). As an example, Moodera and Meservey [ 5 ] have recently obseved through surface impedance measurements, that small clusters of vanadium on Ag(001) do present interesting magnetic properties. Moreover, they have observed that these magnetic properties do depend drastically on the size of the adsorbed cluster. Recently, Dorantes-Davila and Dreyss6 [ 2 1have pointed out that a first-order transition appears for a critical value Jc , for all sizes of the cluster. Therefore, one have to be very cautious in the study of this type of systems. Christensen et al [ 6 ] have determined through Linear-Muffin-Tin-Orbital Mat. Res. Soc. Symp. Proc. Vol. 313. c1993 Materials Researc
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