Jeffrey Sokoloff

Measurements and simulations of micron size coplanar waveguides

S. Zhang et al. — Wed, 18 Apr 2012 13:37:36 PDT

Measurements and simulations have been done on a coplanar waveguide, which consists of a pair of slots which taper down to a separation and width of micron size. The purpose of this device is to permit one to concentrate microwaves or millimeter waves on magnetic samples of the order of a micron in order to do ferrimagnetic resonance (FMR) studies on such small samples. The transmission coefficient as a function of frequency found in the simulations agrees quite well with the measurements. The simulations show that the magnetic field at the pinch is about a factor of several thousand larger than the field of the incident wave. Results for the circuit parameters found from the simulations will be compared to the values for these parameters measured for this device, and the prospects for using the device for FMR studies on micron and submicron magnetic particles will be discussed.

Possible low-loss Faraday-rotation and phase-shifts using multilayer ferrite structures

J. B. Sokoloff et al. — Wed, 18 Apr 2012 13:37:34 PDT

It is suggested that it should be possible to make low insertion loss devices (e.g., Faraday rotators, phase shifters, and broad-band filters) to operate in the millimeter wave range using a multilayer array of thin films magnetized perpendicular to the films with waves of frequency below the magnon continuum normally incident.

Surface roughness and dry friction

J. B. Sokoloff — Wed, 18 Apr 2012 13:37:32 PDT

Persson's multiscale contact mechanics theory combined with a multiscale Brillouin-Prandtl-Tomlinson model is used to show that on the basis of these models “dry friction” [i.e., kinetic friction that remains at exceedingly small velocities (but still above the creep range) close to its value at higher velocities] should almost always occur for self-affine surfaces when the dominant interaction between two surfaces in contact is due to interatomic hard core repulsion, except for extremely smooth surfaces (i.e., surfaces with a Hurst index very close to 1).

Frequency-dependence of the ferromagnetic-resonance linewidth of barium ferrite

M. Elrayess et al. — Wed, 18 Apr 2012 13:37:30 PDT

Ferromagnetic resonance was measured in both a swept frequency mode of operation, in which the magnetic field was fixed, and a field swept mode, using field modulation techniques. Single-crystal spheres of 0.381, 0.305, and 0.483 mm in diameter were inserted in the waveguide and transmission was observed in the measurement. The g values for all the spheres averaged to 2.052 ± 0.011 and the uniaxial anisotropy field was 16.4 kOe. Our measurements show that the linewidth of barium ferrite is nearly independent of resonant frequency from 48 to 105 G Hz. This is in disagreement with the Kasuya-LeCraw two-magnon-one-phonon mechanism, which would predict a linewidth linear with the resonant frequency. Previous measurements [J. Magn. Magn. Mater. 54-57, 1141 (1986); IEEE Trans. Magn. MAG-22, 984 (1986)], however, show a strong temperature dependence, which rules out magnon scattering from static defects as the primary contributor to the intrinsic linewidth. These results are consistent with a single scattering process in which the uniform precession magnon is scattered into the continuum of magnons by time-dependent fluctuations of the trigonal symmetry ion between two equilibrium sites on either side of the mirror plane [J. Appl. Phys. 63, 3350 (1988) ]. This process differs from the usual two-magnon scattering in that the scattering mechanism is time dependent, and, therefore, the magnon frequency need not be conserved in the process.

Comparison of the kinetic friction of planar neutral and polyelectrolyte polymer brushes using molecular dynamics simulations

Yangpeng Ou et al. — Wed, 18 Apr 2012 13:37:28 PDT

We have simulated the relative shear motion of both neutral and polyelectrolyte end-grafted polymer brushes using molecular dynamics. The flexible neutral polymer brush is treated as a bead-spring model, and the polyelectrolyte brush is treated the same way except that each bead is charged and there are counterions present to neutralize the charge. We investigated the friction coefficient, monomer density, and brush penetration for both polyelectrolyte and neutral brushes with both equal grafting density and equal normal force under good solvent conditions. We found that polyelectrolyte brushes had a smaller friction coefficient and monomer penetration than neutral polymer brushes with the identical grafting density and chain length, and the polyelectrolyte brushes supported a much higher normal load than the neutral brushes for the same degree of compression. Charged and neutral brushes with their grafting densities chosen so that they support the same load exhibited approximately the same degree of interpenetration, but the polyelectrolyte brush exhibited a significantly lower friction coefficient. We present evidence that the reason for this is that the extra normal force contribution provided by the counterion osmotic pressure that exists for polyelectrolyte brushes permits them to support the same load as an identical neutral polymer brush of higher grafting density. Because of the resulting lower monomer density for the charged brushes, fewer monomer collisions take place per unit time, resulting in a lower friction coefficient.

Intrinsic ferromagnetic resonance linewidth of barium ferrite due to spin-wave scattering by trigonal site single-particle excitations

J. B. Sokoloff et al. — Wed, 18 Apr 2012 13:37:26 PDT

Time-dependent two-magnon scattering was previously proposed as a mechanism to explain the large magnitude of the ferrimagnetic resonance (FMR) linewidth of barium ferrite as a function of frequency. In the present work, it is shown that a quantum mechanical mechanism like the Kasuya-Le Craw process (KL) but with the phonon excitation replaced by a single-particle excitation of a trigonal site iron ion, which moves in an anharmonic potential well, gives a linewidth contribution of less than a tenth of an Oersted and proportional to the frequency, as in the KL mechanism. We conclude, based on this work and our previous work on the KL mechanism, that neither of these mechanisms can explain the observed FMR linewidths in barium ferrite at any frequency.

Ferrimagnetic resonance lineshape asymmetry due to Suhl instabilities

P. Dorsey et al. — Wed, 18 Apr 2012 13:37:24 PDT

We present calculations of the ferrimagnetic resonance lineshapes resulting from second order Suhl instabilities for thin films and spheres. We find that whereas a spherical sample has a lineshape which is symmetrical around the resonant frequency, a thin film has an asymmetrical lineshape. The calculations are in agreement with measurements that we have performed of the lineshape as a function of input power for thin film samples of both barium ferrite and yttrium iron garnet. When the magnetic field direction is changed from perpendicular to parallel to the film plane, the asymmetry of the lineshape at magnetic resonance changes in opposite sense relative to the resonant field. Theoretical estimated of the critical microwave field necessary for second order Suhl instabilities to occur are in agreement with measured critical fields.

Dry friction due to adsorbed molecules

C. Daly et al. — Fri, 11 Feb 2011 14:52:06 PST

Using an adiabatic approximation method, which searches for Tomlinson model-like instabilities for a simple but still realistic model for two crystalline surfaces in the extremely light contact limit, with mobile molecules present at the interface, sliding relative to each other, we are able to account for the virtually universal occurrence of "dry friction." The model makes important predictions for the dependence of friction on the strength of the interaction of each surface with the mobile molecules.

Effects of defects on friction for a Xe film sliding on Ag(111)

M. S. Tomassone et al. — Fri, 11 Feb 2011 14:52:05 PST

The effects of a step defect and a random array of point defects (such as vacancies or substitutional impurities) on the force of friction acting on a xenon monolayer film as it slides on a silver (111) substrate are studied by molecular dynamic simulations and compared with the results of lowest order perturbation theory in the substrate corrugation potential. For the case of a step, the magnitude and velocity dependence of the friction force are strongly dependent on the direction of sliding respect to the step and the corrugation strength. When the applied force F is perpendicular to the step, the film is pinned forF less than a critical force Fc. Motion of the film along the step, however, is not pinned. Fluctuations in the sliding velocity in time provide evidence of both stick-slip motion and thermally activated creep. Simulations done with a substrate containing a 5 percent concentration of random point defects for various directions of the applied force show that the film is pinned for the force below a critical value. The critical force, however, is still much lower than the effective inertial force exerted on the film by the oscillations of the substrate in experiments done with a quartz crystal microbalance (QCM). Lowest order perturbation theory in the substrate potential is shown to give results consistent with the simulations, and it is used to give a physical picture of what could be expected for real surfaces which contain many defects.

A mechanism for lubrication between surfaces with atomic level roughness

J. B. Sokoloff — Mon, 31 Jan 2011 09:39:00 PST

It is proposed that lubricant molecules adsorbed on an interface between two asperities in contact, which is rough on the atomic scale, can switch the interface from the strong to weak pinning regime, resulting in a large reduction in the static friction. This is proposed as a possible mechanism for boundary lubrication.