Common project IPCMS (Strasbourg)

"Magnetism of transition metal's surfaces and interfaces"

Summary

The project CNRS-RAS 1995-1997 was aimed to investigate the electronic structure of low -dimesional magnetic systems such as metallic surfaces, thin overlayers, films and superlattices.

Theoretical description of these systems was performed within the framework of model Hamiltonian approach. It includes self-consistent calculations of magnetic moments by real space recursion method in mean field approximation.

Such approaches based on Hubbard (HM) and Periodic Anderson Models (PAM) were developed and the influence of space defects - steps at the surface and interface, roughness and embedded clusters near interfaces, pinhole defects - on the magnetic structure of Low Dimensional Magnetic Systems (LDMS) was investigated.

The main results, which were obtained within the given project, are the following:

The theory for self-consistent determination of magnetic structure of spatially non-homogeneous systems is developed. Calculations of magnetic moments distribution of stepped Fe(Cr) surfaces and Cr(Fe) monolayers on these surfaces within HM and PAM shows that the main qualitative features of these distributions coincide for both models. It clarified the potentiality of the methods and the possibility of describing real experiments of FeCr low dimensional systems by these methods. Calculations performed allow rationalization of the apparently contradictory observations obtained by spin-resolved and angle-and-energy resolved photoemission.

The distribution of magnetic moments in Fe/Cr/Fe sandwiches with pinhole defects has been studied using PAM. Self-consistent calculations were performed in the Hartree-Fock approximation for ideal smooth and rough interfaces generated by special algorithm. It was shown that for a sandwich structure of Fe antiferromagnetically coupled across a Cr spacer the pinhole defects induce a non-zero magnetization at zero external magnetic field. Fe atoms inside pinholes show different magnetic moments in dependence of the number of Fe atoms among its nearest neighbors. The role of interface roughness and pinhole defects on the stability of magnetic states has been investigated through total-energy calculations.

A new method based on PAM for calculation of noncollinear magnetic ordering in 3d-systems is developed. Iteration process includes self-consistent determination of population numbers for the electrons with different direction of the magnetic moments at given atom relatively to the fixed axis. Analytical transformation based on the generalized "zeros and poles" methods was performed for the Green functions. It allows avoiding some time consuming numerical procedures and gives the possibility to develop efficient algorithm for the calculation of noncollinear magnetic structures of complex space non-homogeneous systems. Calculations performed for the parameters corresponding to Fe and Cr show qualitatively different dependencies of the magnetic moment magnitude and the energies of d-electrons on the angles, which define the direction of magnetic moments.

Magnetic structure of nonideal Fe/Cr interface was investigated within the framework of PAM. The distribution of magnetic moments obtained for two sets of interfaces (with different roughnesses) shows strong sensitivity of the magnetic structure on the interface roughness and interdiffusion. Application of the theory for interpretation of Mossbauer spectra of Fe/Cr superlattices was suggested.

In experimental field a set of superlattices ⁵⁷Fe/Cr with different thicknesses on Cr spacer were grown by using Molecular-Beam Epitaxy method. Investigations by means of Conversion Electron Mossbauer Spectroscopy allow to obtain important information about the distribution of hyperfine fields. This experiment gives some insight to the determination of the magnitude as well as directions of magnetic moments. Polarized neutron investigations performed at ILL (Grenoble, France) show a good quality of the superlattices structure.

Publications

1. C.Demangeat and V.M. Uzdin. Hubbard and Anderson periodic models for description of imperfect low-dimesional FeCr magnetic systems. J. Magn. Magn. Mater. 156 (1996) 202.
2. V.M. Uzdin and C Demangeat. Pinhole defects in Fe/Cr trilayers. J. Magn. Magn. Mater. 165 (1997) 458.
3. V.M. Uzdin, N.S Yartseva. Periodic Anderson Model for the discription of noncollinear magnetic structure in low-dimensional 3d-systems. Comp. Mat. Sci. 10 (1998) 211.
4. N.S. Yartseva, V.M. Uzdin, C. Demangeat. Magnetic structure of nonideal Fe/Cr interface, Comp. Mat. Sci. 10 (1998) 255.

Abstracts

1. N.S. Yartseva, V.M. Uzdin, and C. Demangeat. Magnetic structure of defects at the Fe/Cr interface. Role of noncollinearity. - Book of Abstracts of ICAM/E-MRS'97, Strasbourg (France), 16-20 June 1997, D-36.
2. V.M. Uzdin and N.S. Yartseva. Periodic Anderson Model for the description of noncollinear magnetic structure in low-dimensional 3d-systems.- Book of Abstracts of ICAM/E-MRS'97, Strasbourg (France), 16-20 June 1997, D-13.