Bogolyubov Institute for Theoretical Physics, 14-b Metrolohichna str., Kiev 03680 Ukraine Conductivity of the defectless, perfect crystal graphene is found at the neutrality point at zero temperature and in the limit of large dielectric constant of the substrate. The steady state of the graphene with weak current is assumed to be an ideal, rare plasma of particle and hole excitations governed by the Boltzmann kinetic equation.

In magnetic memory devices, logical bits are recorded by selectively setting the magnetization vector of individual magnetic domains either 'up' or 'down'. In such devices, the fastest and most efficient recording method involves precessional switching: when a magnetic field B(p) is applied as a write pulse over a period tau, the magnetization vector precesses about the field until B(p)tau reaches the threshold value at which switching occurs. Increasing the amplitude of the write pulse B(p) might therefore substantially shorten the required switching time tau and allow for faster magnetic recording. Here we use very short pulses of a very high magnetic field to show that under these extreme conditions, precessional switching in magnetic media supporting high bit densities no longer takes place at well-defined field strengths; instead, switching occurs randomly within a wide range of magnetic fields. We attribute this behaviour to a momentary collapse of the ferromagnetic order of the spins under the load of the short and high-field pulse, thus establishing an ultimate limit to the speed of deterministic switching and magnetic recording.

We present a theory of the stripe domain structure in a thin ferromagnetic 61m with single-ion easy-axis magnetic anisotropy and long-range dipole interactions, for a wide range of temperatures and applied magnetic field. The domains exist at temperatures below the reorientational phase transition from out-of-plane to in-plane magnetization. The system of stripes can be described as a liquid crystal with a preferred domain-wall orientation. The positional order is destroyed by both thermodynamical meandering of domain walls and by the proliferation of dislocations. Spatial anisotropy generated by the fourth-order exchange energy stabilizes the stripe domain structure and pins its orientation. For any temperature below the reorientational phase transition there exists a critical perpendicular-to-plane magnetic field, which separates multidomain and monodomain states of the film. The theory explains recent experimental observations.

We present a theory of the stripe domain structure in a thin ferromagnetic 61m with single-ion easy-axis magnetic anisotropy and long-range dipole interactions, for a wide range of temperatures and applied magnetic field. The domains exist at temperatures below the reorientational phase transition from out-of-plane to in-plane magnetization.The system of stripes can be described as a liquid crystal with a preferred domain-wall orientation. The positional order is destroyed by both thermodynamical meandering of domain walls and by the proliferation of dislocations. Spatial anisotropy generated by the fourth-order exchange energy stabilizes the stripe domain structure and pins its orientation. For any temperature below the reorientational phase transition there exists a critical perpendicular-to-plane magnetic field, which separates multidomain and monodomain states of the film. The theory explains recent experimental observations.