We analyze a superconductor-ferromagnet (S/F) system with a spiral magnetic structure in the ferromagnet F for a weak and strong exchange field. The long-range triplet component (LRTC) penetrating into the ferromagnet over a long distance is calculated for both cases. In the dirty limit (or weak ferromagnetism) we study the LRTC for conical ferromagnets. Its spatial dependence undergoes a qualitative change as a function of the cone angle ϑ. At small angles ϑ the LRTC decays in the ferromagnet exponentially in a monotonic way. If the angle ϑ exceeds a certain value, the exponential decay of the LRTC is accompanied by oscillations with a period that depends on ϑ. This oscillatory behaviour leads to a similar dependence of the Josephson critical current in SFS junctions on the thickness of the F layer. In the case of a strong ferromagnet the LRTC decays over the length which is determined by the wave vector of the magnetic spiral and by the exchange field.
We analyze collective modes in two-band superconductors in the dirty limit. It is shown that these modes exist at all temperatures T below Tc provided the frequency of the modes is higher than the inelastic scattering rate and lower than the energy gaps ∆ a,b . At low temperatures these modes are related to counterphase oscillations of the condensate currents in each band. The spectrum of the collective oscillations is similar to the spectrum of the Josephson "plasma" modes in a tunnel Josephson junction but the velocity of the mode propagation in the case under consideration is much lower. At higher temperatures (∆ b < T < Tc) the spectrum consists of two branches. One of them is gapless (sound-like) and the second one has a threshhold that depends on coupling between the bands. We formulate the conditions under which both types of collective modes can exist. The spectrum of the collective modes can be determined by measuring the I-V characteristics of a Josephson junction in a way as it was done by Carlson and Goldman [12].
We consider a simple model of a multidomain superconductor-ferromagnet-superconductor (SFS) Josephson junction. Sign-alternating magnetization M in domains leads to a spatial modulation of the phase difference φ(x). Due to this modulation the Josephson critical current Ic may have a different sign depending on the ratio of the magnetic flux in a domain, 4πM a(2dF ), to the magnetic flux quantum. This phase modulation, but not a nonmonotonic dependence of the local critical current density jc, may be the reason for oscillations of the current Ic as a function of the F layer thickness 2dF or temperature, observed in experiments.
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