S. Krompiewski

Itenerant spin glass states and asperomagnetism of amorphous Fe and iron-rich Fe/Zr alloys

Abstract A realistic tight-binding approach for the self-consistent calculation of the itinerant spin configurations of disordered metallic systems has been developed and is used to study numerically the problem of the magnetic states at T = 0 K of iron-rich amorphous alloys of the type Fe1−xZrxHy, and of fictitious amorphous Fe of different densities. In our approach, the local spin polarization is not restricted to the z-direction, i.e. the polarization magnitude as well as its direction can vary from site to site. The calculations show that the spin configurations depend strongly on the preparation, and that changes of the relative spin orientations can lead to drastic changes of the itinerant atomic moments. Metastable spin configurations with essentially isotropic distribution can be prepared by relaxing the system in gradually vanishing external fields with isotropically distributed randomness and zero spatial average, whereas with a non-vanishing average the so-called asperomagnetic configurations are obtained, i.e. ferromagnetic states with randomly frozen transverse components. For iron-rich Fe1−xZrx alloys with x = 0.07, according to our calculation, the isotropic spin glass configuration would have slightly lower energy than the asperomagnetic state, by amounts corresponding to ≈ 0.003 eV per electron, while for the hydrogenated system Fe1-xZrxHy with y ≈ 2x, the asperomagnetic state would be favoured by ≈ 0.004 eV. For two computer models of (fictitious) amorphous Fe with densities of 7.39 and 9.19 g cm−3, the spin-glass and the asperomagnetic states have roughly the same energy, although for the low-density sample the magnitudes of the moments are quite different in both states.

Itinerant magnetism and spin glass states of iron rich amorphous alloys

Abstract We generalize our self-consistent treatment of the itinerant magnetism of disordered or amorphous transition metal alloys, given in a series of recent papers, in such a way that now also itinerant spin glasses can be treated; i.e. not only the local magnitude, as before, also the local polarization direction can now differ from site, due to a subtle interplay between the isotropic intra-atomic Coulomb interaction and the anisotropic hopping terms in the Hamiltonian. Using a realistic approach with all relevant orbitals, this theory is then applied to a detailed numerical study of the magnetism of iron-rich amorphous Fe-Zr alloys, including hydrogenated samples, and of fictitious amorphous Fe at various densities. As a result we find that in the non-hydrogenated Fe-Zr alloys and in amorphous Fe the transverse components, although summing up to zero, can locally be almost comparable to the longitudinal polarization per atom.

Magnetic properties of Fe/Ag multilayers with interface roughness by a first-principles tight-binding LMTO method

Abstract A first principles tight-binding linear muffin-tin orbital method has been applied to describe magnetic properties of epitaxially grown bcc-Fe(001)/fcc-Ag(001) and bcc-Fe(110)/fcc-Ag(111) multilayers. The atomic sphere approximation has been adopted and the Hamiltonian to first order in E  E v has been constructed in terms of self-consistently calculated potential parameters and screened structure constants. It is shown by real-space calculations that although in the absence of disorder the interface states possess higher magnetic moments than the bulk ones, the interface roughness accompanied by chemical disorder increases the average magnetic moment per Fe, reducing simultaneously rather drastically the difference in magnetizations between the interface and the central atomic layers.

Magneto-volume- and hydrogenation effects in Fe-based amorphous alloys

Abstract We have studied the volume dependence of the local magnetic moment in (fictitious) amorphous Fe and (real) amorphous Fe 1− x M x (M = Zr or B) with particular emphasis on the effect of hydrogeneration for Fe 1− x Zr x H y , with x ≈0.1 and y = 2 x , by means of self-consistent numerical calculations. Fora-Fe we find a broad distribution of the local magnetic moments, with both positive and negative contributions and a mean value of ≈1μ B . However, for smaller densities than that of bcc-Fe, μ increases rather drastically to ≈2μ B . For Fe 1− x B x , our calculated moment distributions agree with experimental hyperfine-field distributions; also for the Zr-alloys, our calculation reproduces the concentration dependence of the magnetization. Furthermore, we find that the drastic increase of the magnetization by hydrogenation in the range around x = 0.1 is a consequence of the volume increase.

First-principles studies of modulated Co/Cu superlattices with strongly and weakly exchange-biased Co-monolayers

Abstract First-principles calculations have been performed in order to determine effective exchange integrals between strongly and weakly exchange-coupled Co monolayers in certain novel periodic CoCu 2 /CoCu n -superlattices and related systems with three or more non-equivalent Co planes. For 3 ≤ n ≤ 6 we find that there are two non-equivalent exchange integrals j and J , which have opposite signs, i.e. the strong coupling J is antiferromagnetic and the weak coupling j ferromagnetic, and differ for n ≠ 4 from each other by one order of magnitude. It is shown that the results depend on the system as a whole and cannot be obtained from separate parts. From our results we suggest that ‘spin valve’ systems with similar modulated structures should be considered when trying to obtain good magneto-resistance together with low switching-fields.

Oscillatory thickness dependence of magnetic moments and interface-induced changes of the exchange coupling in Co/Cu and CoNi/Cu multilayers

Abstract We perform first-principles calculations for the three multilayer systems (100)-Co 1 /Cu n , NiCo 2 Ni/Cu n and Co 4 Cu n , and find from a comparison of the results for system 2 and 3 that amplitude and phase of the exchange coupling are sensitive to the magnetic-slab/nonmagnetic-spacer interface. Moreover, we observe that for the system 1 the averaged magnetic moment of the magnetic slab oscillates with the spacer thickness similarly as the exchange coupling.

Oscillatory exchange coupling and induced Cu moments in fct-(001) -n1-Co / n2-Cu multilayers

Abstract By ab initio LMTO calculations in the atomic sphere approximation we study the oscillatory thickness dependence of the exchange coupling between ferromagnetic Co slabs grown epitaxially on the (001) surface of a fcc Cu spacer in a n 1 -Co/ n 2 -Cu multilayer structure and find not only the well-known oscillations with the spacer thickness n 2 , but — as a new result — also a pronounced oscillation with the magnetic thickness n 1 . We also find a small, but significant Cu spin polarization of different sign in the first two layers near the interface.

On the resistivity of amorphous Fe-Zr alloys in the magnetic state: measurements and numerical simulations

Abstract We have performed (i) measurements at T = 4 K and (ii) numerical simulations of the resistivity at T = 0 K for the itinerant amorphous magnetic alloys Fe 1− x Zr x over a large concentration region. The resistivities are measured with sputtered thin films and calculated numerically by direct evaluation of the Kubo formula for small, but realistic computer models, using a self-consistent semi-empirical approach for the description of the itinerant magnetism of the system. The numerical results for the total resistivity ϱ agree with the experiments, showing values around 170 μΩ cm, except for concentrations below x ≈ 0.15, where resistivities around ≈ 120 μΩ cm are found. Outside this region, in the magnetic state the contribution of carriers with minority spin polarization to the conductivity is significantly larger than that of electrons with majority spin, according to the calculations. Finally, the calculations suggest that this difference would also appear in the region of the small x -values for hydrogenated samples, e.g. for Fe 0.93 Zr 0.07 H 0.14 .

Magnetic excitations around the first peak of the structure factor in amorphous ferromagnets, within the framework of the hubbard model☆

Abstract A band theoretical approach has been applied to finite computer-simulated random systems of hard spheres. The scattering law has been computed and a dependence of local magnetic moments on their environments has been tested. Magnetic excitations around the scattering vector corresponding to the first peak of the structure factor have been shown to be gapless. As an alternative approach the so-called polycrystalline approximation has been used.

Conductance of atomic-scale metallic contacts with controllable interface hopping integrals

We study the conductance of metallic point contacts between two external leads. Landauer conductance has been computed under constraint of atomic charge neutrality. For Cu and Ni the conductance increases nonlinearly with the number of atoms within the contact layer (unlike the Sharvin conductance). Moreover, the current polarization of nickel turns out to be sensitive to interface conditions.