Influence of topography and Co domain walls on the magnetization reversal of the FeNi layer in FeNi/Al_2\_2O_3\_3/Co magnetic tunnel junctions

We have studied the magnetization reversal dynamics of FeNi/Al$\_2$O$\_3$/Co magnetic tunnel junctions deposited on step-bunched Si substrates using magneto-optical Kerr effect and time-resolved x-ray photoelectron emission microscopy combined with x-ray magnetic circular dichroism (XMCD-PEEM). Different reversal mechanisms have been found depending on the substrate miscut angle. Larger terraces (smaller miscut angles) lead to a higher nucleation density and stronger domain wall pinning. The width of domain walls with respect to the size of the terraces seems to play an important role in the reversal. We used the element selectivity of XMCD-PEEM to reveal the strong influence of the stray field of domain walls in the hard magnetic layer on the magnetic switching of the soft magnetic layer.Comment: 8 Pages, 7 Figure

Dynamics of magnetic domain wall motion after nucleation: Dependence on the wall energy

The dynamics of magnetic domain wall motion in the FeNi layer of a FeNi/Al2O3/Co trilayer has been investigated by a combination of x-ray magnetic circular dichroism, photoelectron emission microscopy, and a stroboscopic pump-probe technique. The nucleation of domains and subsequent expansion by domain wall motion in the FeNi layer during nanosecond-long magnetic field pulses was observed in the viscous regime up to the Walker limit field. We attribute an observed delay of domain expansion to the influence of the domain wall energy that acts against the domain expansion and that plays an important role when domains are small.Comment: Accepted for publication in Physical Review Letter

Weakly Coupled Motion of Individual Layers in Ferromagnetic Resonance

We demonstrate a layer- and time-resolved measurement of ferromagnetic resonance (FMR) in a Ni81Fe19 / Cu / Co93Zr7 trilayer structure. Time-resolved x-ray magnetic circular dichroism has been developed in transmission, with resonant field excitation at a FMR frequency of 2.3 GHz. Small-angle (to 0.2 degree), time-domain magnetization precession could be observed directly, and resolved to individual layers through elemental contrast at Ni, Fe, and Co edges. The phase sensitivity allowed direct measurement of relative phase lags in the precession oscillations of individual elements and layers. A weak ferromagnetic coupling, difficult to ascertain in conventional FMR measurements, is revealed in the phase and amplitude response of individual layers across resonance.Comment: 22 pages, 6 figures submitted to Physical Review

Interplay between magnetic anisotropy and interlayer coupling in nanosecond magnetization reversal of spin-valve trilayers

The influence of magnetic anisotropy on nanosecond magnetization reversal in coupled FeNi/Cu/Co trilayers was studied using a photoelectron emission microscope combined with x-ray magnetic circular dicroism. In quasi-isotropic samples the reversal of the soft FeNi layer is determined by domain wall pinning that leads to the formation of small and irregular domains. In samples with uniaxial magnetic anisotropy, the domains are larger and the influence of local interlayer coupling dominates the domain structure and the reversal of the FeNi layer

Effects of surfaces on resistor percolation

We study the effects of surfaces on resistor percolation at the instance of a semi-infinite geometry. Particularly we are interested in the average resistance between two connected ports located on the surface. Based on general grounds as symmetries and relevance we introduce a field theoretic Hamiltonian for semi-infinite random resistor networks. We show that the surface contributes to the average resistance only in terms of corrections to scaling. These corrections are governed by surface resistance exponents. We carry out renormalization group improved perturbation calculations for the special and the ordinary transition. We calculate the surface resistance exponents \phi_{\mathcal S \mathnormal} and \phi_{\mathcal S \mathnormal}^\infty for the special and the ordinary transition, respectively, to one-loop order.Comment: 19 pages, 3 figure

Spin Diffusion in Classical Heisenberg Magnets with Uniform, Alternating, and Random Exchange

We have carried out an extensive simulation study for the spin autocorrelation function at T=∞ of the one‐dimensional classical Heisenberg model with four different types of isotropic bilinear nearest‐neighbor coupling: uniform exchange, alternating exchange, and two kinds of random exchange. For the long‐time tails of all but one case, the simulation data seem incompatible with the simple ∼t −1/2 leading term predicted by spin diffusion phenomenology

On the relevance of percolation theory to the vulcanization transition

The relationship between vulcanization and percolation is explored from the perspective of renormalized local field theory. We show rigorously that the vulcanization and percolation correlation functions are governed by the same Gell--Mann-Low renormalization group equation. Hence, all scaling aspects of the vulcanization transition are reigned by the critical exponents of the percolation universality class.Comment: 9 pages, 2 figure

Interfacial adsorption phenomena of the three-dimensional three-state Potts model

We study the interfacial adsorption phenomena of the three-state ferromagnetic Potts model on the simple cubic lattice by the Monte Carlo method. Finite-size scaling analyses of the net-adsorption yield the evidence of the phase transition being of first-order and $k_{\rm B} T_{\rm C} / J = 1.8166 (2)$.Comment: 14 page