The training effect for exchange bias in field-cooled Co/CoO bilayers films is investigated. Previous experiments on the same system have shown that, starting from the ascending branch of the first hysteresis loop, coherent magnetization rotation is the dominant reversal mechanism. This is confirmed by the performed numerical simulations, which also indicate that the training is predominantly caused by changes of the rotatable anisotropy parameters of uncompensated spins at the Co/CoO interface. Moreover, in contrast with what is commonly assumed, the exchange coupling between the rotatable spins and the ferromagnetic layer is stronger than the coupling between the ferromagnet and the spins responsible for the bias. Thus, uncompensated spins strongly coupled to the ferromagnet contribute to the coercivity rather than to the bias, whatever the strength of their magnetic anisotropy.

Author(s): Anoosheh Yazdi, Alexei Ivlev, Sergey Khrapak, Hubertus Thomas, Gregor E. Morfill, Hartmut Löwen, Adam Wysocki, and Matthias Sperl

The glass transition is investigated in three dimensions for single and double Yukawa potentials for the full range of control parameters. For vanishing screening parameter, the limit of the one-component plasma is obtained; for large screening parameters and high coupling strengths, the glass-trans...

[Phys. Rev. E 89, 063105] Published Mon Jun 30, 2014

In the present work, the influence of the magnetic field and dust polarization force on the Jeans
instability of self-gravitating strongly coupled inhomogeneous dusty plasma has been investigated.
The dusty plasma containing strongly correlated negatively charged dust grains and weakly correlated
Maxwellian electrons and ions which are embedded in a uniform magnetic field is considered. The
construction of the equations is done by employing the generalized hydrodynamic (GH) model for
magnetized strongly coupled dusty plasma. In deriving a dispersion relation, the plane-wave
solutions are used on the linearized perturbation equations. The analysis is done by normal mode
analysis theory. The dispersion relation is analyzed to obtain the Jeans criterion of instability.
Numerical results are presented to show the effect of the polarization parameter, the magnetic field
and the strong correlation effect of dust. The growth rates are further compared in the kinetic and
hydrodynamic reg...

The diffusion of projectiles drifting through a target of strongly coupled dusty plasma is investigated in a simulation. A projectile's drift is driven by a constant force F. We characterize the random walk of the projectiles in the direction perpendicular to their drift. The perpendicular diffusion coefficient Dp ⊥ is obtained from the simulation data. The force dependence of Dp ⊥ is found to be a power law in a high force regime, but a constant at low forces. A mean kinetic energy Wp for perpendicular motion is also obtained. The diffusion coefficient is found to increase with Wp with a linear trend at higher energies, but an exponential trend at lower energies.

We investigate the structural properties of a small binary system of dusty plasma subjected to an anisotropic external confinement. We have found that the ground state can form various symmetrical configurations, which generally correspond to variations of a multiple-ring structure. The presence of ...

[Phys. Rev. E 89, 062318] Published Thu Jun 26, 2014

The results of a numerical study on the influence of topological defects on mass transfer processes
are presented for two-dimensional nonideal systems. Calculations have been performed for Yukawa and
inverse-power potentials in a wide range of parameters, corresponding to the experimental conditions
in the laboratory dusty plasmas. The relations between a number of topological defects and diffusion
coefficients for interacting particles are obtained for the first time.

The normal modes for three to seven particle two-dimensional (2D) dust
clusters in a complex plasma are investigated using an N-body simulation. The
ion wakefield downstream of each particle is shown to induce coupling between
horizontal and vertical modes. The rules of mode coupling are investigated by
classifying the mode eigenvectors employing the Bessel and trigonometric
functions indexed by order integers (m, n). It is shown that coupling only
occurs between two modes with the same m and that horizontal modes having a
higher shear contribution exhibit weaker coupling. Three types of resonances
are shown to occur when two coupled modes have the same frequency. Discrete
instabilities caused by both the first and third type of resonances are
verified and instabilities caused by the third type of resonance are found to
induce melting. The melting procedure is observed to go through a two-step
process with the solid-liquid transition closely obeying the Lindemann
criterion.

Small quasi-two-dimensional (2D) dust clusters consisting of three to eleven
particles are formed in an argon plasma under varying rf power. Their normal
modes are investigated through their mode spectra obtained from tracking
thermal motion of the particles. Detailed coupling patterns between their
horizontal and vertical modes are detected for particle numbers up to seven and
discrete instabilities are found for dust clusters with particle number greater
than nine, as predicted in previous theory on ion-flow induced mode coupling in
small clusters. The instabilities are proven to be induced by resonance between
coupled horizontal and vertical normal modes.

In this paper, the growth rate of the Weibel instability is calculated for an energetic relativistic electron beam penetrated into a strongly coupled plasma, where the collision effects of background electron-ion scattering play an important role in equations. In order to calculate the growth rate of the Weibel instability, two different models of anisotropic distribution function are used. First, the distribution of the plasma and beam electrons considered as similar forms of bi-Maxwellian distribution. Second, the distribution functions of the plasma electrons and the beam electrons follows bi-Maxwellian and delta-like distributions, respectively. The obtained results show that the collision effect decreases the growth rate in two models. When the distribution function of electrons beam is in bi-Maxwellian form, the instability growth rate is greater than where the distribution function of beam electrons is in delta-like form, because, the anisotropic temperature for bi-Maxwellian distribution function in velocity space is greater than the delta-like distribution function.

Ion acoustic solitons and double layers are studied in a collisionless plasma consisting of cold heavier ion species, a warm lighter ion species, and hot electrons having Boltzmann distributions by Sagdeev pseudo-potential technique. In contrast to the previous results, no double layers and super-solitons are found when both the heavy and lighter ion species are treated as cold. Only the positive potential solitons are found in this case. When the thermal effects of the lighter ion species are included, in addition to the usual ion-acoustic solitons occurring at M > 1 (where the Mach number, M, is defined as the ratio of the speed of the solitary wave and the ion-acoustic speed considering temperature of hot electrons and mass of the heavier ion species), slow ion-acoustic solitons/double layers are found to occur at low Mach number (M number density of lighter ion species tends to 1 (i.e., no heavier species). An interesting property of the new slow ion-acoustic mode is that at low number density of the lighter ion species, only negative potential solitons/double layers are found whereas for increasing densities there is a transition first to positive solitons/double layers, and then only positive solitons. The model can be easily applicable to the dusty plasmas having positively charged dust grains by replacing the heavier ion species by the dust mass and doing a simple normalization to take account of the dust charge.