H. Suhl

Quantum theory of atom-surface scattering: Debye-Waller factor☆

Abstract The Debye-Waller factor, introduced historically for X-rays, was used later for electrons, neutrons, and atoms as well. In this process of extension, however, the assumptions on which the Debye-Waller theory rested became more and more questionable until in the case of atoms (whose scattering from surfaces is both strong and slow) serious modifications are necessary. In the present article four models are discussed in order. In Model 1 a fast atom impinges on a surface whose atoms all vibrate deviating from their equilibrium positions by the same vector displacement ϱ . In Model 2 again the impinging atom is fast, but the atoms in the surface vibrate incoherently rather than coherently. It is shown that both Models 1 and 2 yield the conventional Debye-Waller result in the infinite crystal atom mass limit (for Model 2 Einstein oscillators have also to be assumed) and it is also shown how corrections to this result can be built. Turning then to slow impinging atoms, in Model 3 a slow atom impinges on a hard crystal surface, interacting with the rapidly varying potential of the vibrating solid. Model 3 is discussed in detail and it is shown that the Debye-Waller exponent can be written in terms of a time integral of the product of two correlations: the force correlation and the displacement correlation. The result is a dramatic increase of diffraction of relatively heavy atoms (with respect to the conventional theory). Finally, in Model 4 the impinging atom is again slow but the crystal is soft rather than hard. This case is more difficult to treat but a preliminary analysis again indicates a dramatic increase of diffraction since the soft solid adjusts itself to the instantaneous atom position leading to elastic scattering. The experimental implications of the present theory, especially for neon scattering from surfaces, are discussed.

Theory of the magnetic damping constant

The aim of this paper is to express the effects of basic dissipative mechanisms involved in the dynamics of the magnetization field in terms of the one most commonly observed quantity: the spatial average of that field. The mechanisms may be roughly divided into direct relaxation to the lattice, and indirect relaxation via excitation of many magnetic modes. Two illustrative examples of these categories are treated; direct relaxation via magnetostriction into a lattice of known elastic constant, and relaxation into synchronous spin waves brought about by imperfections. Finally, a somewhat speculative account is presented of time constants to be expected in magnetization reversal.

Theory of oscillatory oxidation of carbon monoxide over platinum

It is shown that the finite rate of decay for temperature fluctuations of the catalytic surface in the Langmuir-Hinshelwood reaction: 2 CO + O2 → 2 CO2 over Pt can give rise to a self-sustained oscillatory regime. The waveforms and periods are in good qualitative agreement with experiments by Dauchot and Van Cakenberghe.

Coercivity enhancement above the Néel temperature of an antiferromagnet/ ferromagnet bilayer

Single-crystal thin films of the antiferromagnet FeF2 have been used to exchange bias overlayers of Fe. An unexpected coercivity enhancement is observed at temperatures above the Neel temperature of the FeF2. This coercivity reaches a peak value of over 600 Oe close to the Neel temperature and persists to above 300 K. The coercivity is correlated with the growth of an anisotropy in the ferromagnet, the increase of the antiferromagnetic susceptibility and the increase of the ferromagnetic resonance linewidth. We argue that the growth of spin fluctuations in the antiferromagnet leads to an enhanced ferromagnetic anisotropy, and therefore coercivity, above the Neel temperature.

Thin‐film magnetic patterns in an external field

The behavior of ideally soft ferromagnetic films in the presence of a weak coplanar magnetic field is explored by the method of characteristics. Solutions are found which have no internal field or in some cases have field‐free zones. Results are specifically given for circular and elliptic disks, the infinite strip, and the semi‐infinite plane. The disk solutions have internal domain walls.

Ginsburg-Landau theory of two antagonistic order parameters: Magnetism and superconductivity

Abstract An attempt is made to construct a Ginsburg-Landau theory of socalled magnetic superconductors. Two order parameters, the magnetization field and the gap function, are introduced in such a way as to inhibit each others growth. It is found that the non-local character of the superconducting order parameter must be taken into account in any evaluation of effects of the critical magnetic fluctuations. Some predictions are made within the limits of Ornstein-Zoernicke-like fluctuation theory and some comparison is made with available data.

Two-oxidation-state theory of catalyzed carbon dioxide generation☆

Abstract This paper analyzes oscillatory carbon monoxide oxidation on the basis of a mechanism involving two different adsorption sites for the oxygen. The mechanism is quite different from that previously proposed and in a certain temperature range at least, agrees much better with some recent experiments.

Dynamic relaxation in thin films

The dynamic magnetization process in thin film platelets is studied by numerical micromagnetics. Simulations are conducted without the addition of the phenomenological damping term in the Landau–Lifshitz equation. Excess Zeeman energy is transferred to magnetostatic-exchange coupled spin waves. This allows the average magnetization to relax towards the equilibrium configuration. The strength of the applied field and the size of the platelet both affect how much energy can be transferred to the spin wave modes. For large enough platelets and fields that are not too large, nearly complete reversal can be obtained without a mechanism to dissipate magnetic energy.

Measurement of the vortex core in sub-100 nm Fe dots using polarized neutron scattering

We use polarized neutron scattering to obtain quantitative information about the magnetic state of sub-100 nm circular magnetic dots. Evidence for the transition from a single domain to a vortex state, as a function of the dot diameter and magnetic field, is found from magnetization curves and confirmed by micromagnetic and Monte-Carlo simulations. For 20 nm-thick Fe dots with diameters close to 60 nm, the vortex is the ground state. The magnetization of the vortex core (140±50 emu/cm3) and its diameter (19±4 nm) obtained from polarized neutron scattering are in agreement with simulations.

Chaotic motion of domain walls in soft magnetic materials

We have studied numerically one‐dimensional rf driven motion of domain structures. The equation of motion solved has Landau–Lifshitz damping and includes all the basic phenomenological magnetic interactions: demagnetizing field, anisotropy field, and exchange field. We have found that for a large range of parameters, the spatial average of the magnetization is chaotic in time, and the spatial pattern at fixed time itself is likewise chaotic. The power spectrum of the chaotic time series has a 1/f shape. The phase boundary between chaotic and nonchaotic motion is described, and a limited analytical insight into this problem is discussed.