xxiii πανελληνιο ÏƒÏ Î½ÎµÎ´ÏÎ¹Î¿ Ï†Ï ÏƒÎ¹ÎºÎ·Ï‚ στερεας καταστασης & επιστημης ...

Linear relationship between Melting and Curie Temperatures in Ni-clusters
A. N. Andriotis 1* , Z.G.Fthenakis 1,2 , and M. Menon 3,4
1 Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas, P.O. Box 1385, Heraklio,
Crete, Greece 71110
2 Department of Physics, University of Crete, P.O.Box 2208, 71003 Heraklio, Crete, Greece
3 Center for Computational Sciences, University of Kentucky, Lexington, KY 40506-0045
4 Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506-0045
The temperature effects on the magnetic properties of transition metal clusters are investigated using a simulations
method which combines the classical potential approximation with the tight binding molecular dynamics (TBMD)
method. This is accomplished through a generalization of our recently proposed TBMD method which includes explicit
incorporation of spin-orbit and non-collinear spin interactions. The efficacy of this new method is demonstrated by
application to small and intermediate size Ni N clusters (N < 202) for which experimental data are available for
comparison.
Our computer simulations based on the Tight Binding (TB) and Classical potential (of Sutton-Chen)
Molecular Dynamics (MD), indicate that the melting and Curie temperatures of Ni clusters (consisting of N cl atoms)
are linearly related, (see Fig.1) that is :
T melt,N = A * T Curie,N + B ;
A, B = constants; A=0.54 ; B=443.82 o K
The Curie temperature, T Curie,N , is determined by locating the maximum of the specific heat, C v (T), obtained from the
derivative with respect to the temperature of the square of the average atomic magnetic moment. The latter is obtained
(i)
as the thermodynamic average of the average atomic magnetic moment obtained over N ran random cluster-spin
configurations of cluster atoms at each i-time-step as reaching the thermodynamic equilibrium. That is :
and
= ∫p T (E) μ i (E) dE
with p T (E) the canonical probability distribution function of total energy E.
Figure 1 Calculated melting and Curie temperatures for Ni clusters as a function of cluster size (left panel); their linear
relationship is indicated in the right panel. Experimental data from Refs. 4,5,6. The inset of the right panel indicates
the variation tith temperature of the Lindemann index for Ni 43 .
126