U. Krey

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.

Amorphous magnetism: Theoretical aspects

Abstract The different classes of disordered magnetic systems (spin glass systems, crystalline magnetic alloys, and amorphous ferro- and ferrimagnets) are characterized and delimited with respect to each other. The changes of the magnetic moment per atom and of the Curie temperature observed for the amorphous systems are described shortly and possible origins suggested. Finally, the special features of the magnetic excitation spectrum and of the critical phenomena of amorphous systems are pointed out. It is stressed that typical spin glass phenomena might also appear in amorphous systems.

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.

On the critical behaviour of disordered systems

Abstract It is shown by general renormalization group analysis that in disordered systems there is no quasi-homogeneous critical behaviour with positive specific-heat exponent α.

Itenerant magnetic properties of amorphous metallic systems: Numerical studies of the Fe-B alloys

Abstract A self-consistent method to compute local magnetic moments and spin polarized electron densities of states and to test the stability of the ferromagnetic vs. the paramagnetic state is developed within the framework of the itinerant electron theory. The tight-binding Hamiltonian, with Coulomb correlation among d electrons, is parametrized according to the Slater-Koster interpolation scheme. The method goes beyond the rigid band splitting approximation. We apply it to the Fe 1− x B x amorphous system for 0 ⩽ x ⩽0.6 and obtain the magnetization for T =0 as a function of x in agreement with experimental data in the experimentally-accessible concentration region.

UNUSUAL BEATS OF THE PERPENDICULAR-CURRENT GIANT MAGNETORESISTANCE AND THERMOPOWER OF MAGNETIC TRILAYERS

(received July 1996) Oscillations of the giant magnetoresistance (GMR) and thermo-electric power (TEP) vs. both the thickness of the non-magnetic spacer and also that of the ferromagnetic slabs are studied in the current-perpendicular-to-plane (CPP) geometry of magnetic trilayer systems, in terms of a singleband tight-binding model without impurities. The spin-dependent conductance has been calculated from the Kubo formula by means of a recursion Green’s function method and the semi-infinite ideal-lead wires trick. Additionally the TEP is obtained directly from Mott’s formula. In general, the thickness oscillations of the GMR and the TEP may have just one or two (short and long) oscillations. The long period, related to spectacular beats, is apparently of non-RKKY type. The TEP oscillations are strongly enhanced with respect to those of the GMR, have the same periods, but different phases and a negative bias. The phenomenon of giant magnetoresistance (GMR) in magnetic multilayers has been intensively studied both experimentally [1,2] and theoretically [3–7] for more than five years now. After the paper [5] was published it has become clear that one can expect a large magnetoresistance even in systems which have no impurities and no structural defects. As pointed out in [7], the GMRoscillations have not only RKKY- type components, but may additionally reveal large particular non-RKKY oscillations (see below). For the case of semi-infinite ferromagnetic slabs, a period of those extra oscillations with the spacer thickness in the CPP (current perpendicularto-plane) geometry has been shown in [7] to originate from values of the in-plane wavenumber ~ kk at which at least one spectral density vanishes at the ferromagnetic interface. In the present letter we study the CPP-GMR (currentperpendicular-to-plane) behaviour of layered systems of the type W/F1/S/F2/W, where W stands for a semiinfite ideal lead wire, F1 and F2 for different ferromagnets, generally with different thicknesses, and S for the non-magnetic spacer. Thus, apart from the leads, we are dealing with the usual magnetic trilayer systems. Our aim is to show that very pronounced beats may also, under some circumstances, appear as a function of the thicknesses of the ferromagnetic slabs. Besides the giant magneto-resistance effect, we also calculate the corresponding effect for the thermo-electric power (TEP).

Itinerant magnetic properties of amorphous metallic systems II: Magnetic moment distribution in the Fe-B alloys and the magnetism of amorphous Fe

Abstract For realistic structural models of amorphous Fe1-xBx with 0.2⩽x⩽0.6 and for the fictitious case of x = 0 (amorphous iron, a-Fe), we calculate the distribution function p(ml) of the local magnetic moments ml by means of a self-consistent itinerant electron approach. Generally, p(ml) is rather broad and agrees with experimental hyperfine field distributions. For x→0 and x > 0.5, even negative moments appear, although m remains positive. Additionally we find that for a-Fe a rather drastic transition from paramagnetic behaviour at densities ρ ρc would happen, if the magnetization could be forced to remain homogeneous. However, if the ml are allowed to relax to their local values, no such transition happens. Finally, we study the question whether the spread of the local moments is important for the density of states and find that this is the case except for 0.2⩽x⩽0.35.

Study of Anderson localization by a continued-fraction method

Abstract The electron localization is studied for Andersons tight-binding model of a disordered two-dimensional square lattice. For a large system of 104 sites the averaged squared modulus | G 00 | 2 of the Greens function is evaluated by a continued-fraction method. From this quantity, following Andersons criterion, the energy of the mobility edge is found as a function of the degree of disorder. Also the Anderson transition is recognized.

Dynamics of diluted attractor neural networks with delays

We present an analysis of the attractors of a deterministic dynamics in formal neural networks characterized by binary threshold units and a nonsymmetric connectivity. It is shown that in these networks a stored pattern or a pattern sequence is represented by a cloud of attractors rather than by a single attractor. Dilution, which we describe by a power-law scaling, and delayed couplings are shown to equip this type of network with a dynamic behaviour that is interesting enough for simplified models of biological motor systems.

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.