Ferromagnetic nanostructures in the generalized Valenta model

Abstract

Ferromagnetic nanostructures are an area of great interest for modern physics. A comparison of experimental data and theoretical results shows that the use of the standard molecular field approximation is insufficient for the description of the nanostructure properties. Therefore in the present paper we use the reaction field approach in order to test the Valenta model generalized in this way. The agreement between experiment and theory is then excellent.

Ferromagnetic Nano-Structures in Valenta Model

“…It has been theoretically calculated and experimentally shown [14,19] that for two ferromagnetic Ni and Co films coupled by the indirect exchange interaction J inter (via the Cu film -Co/Cu/Ni/Cu(100) system) one obtains two different ordering temperatures and we can observe two susceptibility signals [22] one in T C,Co and a weaker one in T * C,N i . Then, we treat the T * C,N i as a quasi-critical phase transition temperature of the system.…”

“…By selecting the appropriate thicknesses of Co, Cu and Ni films, we can observe the lower ordering temperature of Co than the one of Ni. We present here the numerical calculations for these two different experimental situations performed in the frame of RFA [14]. In this case the internal energy U is given in the following form:…”

“…The Co/Cu interface is not differentiated because it is approximately compensated by the enhancement of the magnetic moment in the topmost layer facing the vacuum [25] and we take the same values of the exchange integral for all monoatomic layers forming the Co film. The interaction between the Co and Ni films in the Co/Cu/Ni/Cu(100) system takes place via the interlayer exchange coupling J inter , which depends on the nonmagnetic spacer thickness [14]. The anisotropy constant K N i including volume and the surface anisotropy term, can be distinguished as we did in the case of the exchange coupling, namely in each magnetic film we have in-plane anisotropy K N i and K Co while at the interface Ni/Cu we have…”

“…. Additionally, in order to take into account the boundary conditions connected with the surfaces state we consider also a perpendicular anisotropy κ S which is included in the parameter λ calculations [14]. The parameter λ appearing in equation ( 4) is the correlation parameter characteristic for RFA which is independent of (ν) due to symmetry conditions [26] and it is assumed to be homogeneous in the sample.…”

“…The part of the effective field arising from the reaction field does not favor one orientation over another while the molecular field is directed along the spontaneous magnetization axis. The Valenta model generalized in such a way has been applied for the description of ferromagnetic films separated by the nonmagnetic spacer forming the multilayer systems [14].…”