Suppression of noise in magnetic layers by a periodic spin torque.

The simultaneous effect of thermal noise and time-periodic spin torques on magnetic multilayers are treated in this work. Using two commonly studied magnetic systems with multiple stable states at zero temperature as examples, we show that periodic spin torques can enhance the stability of the system and suppress the noise due to interwell transitions. In the case of weak periodic spin torques, stochastic resonance, which is usually associated with ac magnetic fields, is also manifested for nonconservative torques. In more complex systems with a relatively low energy barrier, it is shown that high-frequency spin torques can inhibit interwell transitions and in effect suppress the telegraph noise due to the switching between neighboring states.

Ultrafast spin dynamics: the effect of colored noise.

Recent experimental results have pushed the limits of magnetization dynamics to pico- and femtosecond time scales. This ultrafast dynamics occurs in extreme conditions of strong and rapid fields and high temperatures. This situation requires a new description of magnetization dynamics, taking into account that the electron correlation time could be of the order of the inverse spin frequency. For this case we introduce a thermodynamically correct phenomenological Landau-Lifshitz-Miyasaki-Seki approach. We demonstrate the effect of the noise correlation time on the ultrafast demagnetization rate.

Origin of increase of damping in transition metals with rare-earth-metal impurities.

The damping due to rare-earth-metal impurities in transition metals is discussed in the low concentration limit. It is shown that all established damping mechanisms based on spin-orbit and/or spin-spin interactions cannot explain experimental observations even qualitatively. We introduce a different relaxation channel due to the coupling of the orbital moments of the rare-earth-metal impurities and the conduction p electrons that leads to good agreement with experiment. Using an itinerant picture for the host ions, i.e., write their magnetization in terms of the electronic degrees of freedom, is key to the success of our model.

Theory for phase transitions in diblock copolymers: the lamellar case.

A mean-field type theory is proposed to study order-disorder transitions (ODT's) in block copolymers. The theory applies to both the weak segregation and the strong segregation regimes. An energy functional is proposed without appealing to the random phase approximation (RPA). We find additional terms unaccounted for within the RPA. We work out in detail transitions to the lamellar state and compare the method to other existing theories of the ODT and numerical simulations. We find good agreement with recent experimental results and predict that the intermediate segregation regime may have more than one scaling behavior.