We investigate the influence of the exchange field on the Andreev bound states in a ferromagnetic (F) film backed on one side by a superconductor (S). Our model accounts for diffusive reflection at the outer surface and possible backscattering at the FS interface. Phase shifting of the Andreev level by the exchange field results in an oscillatory behavior of the density of states of F as a function of the layer thickness. We show that our results agree quantitatively with recent experiments. DOI: 10.1103/PhysRevLett.86.308 PACS numbers: 74.50.+r, 74.80.-g, 75.70.-i Probing the proximity effect by tunneling spectroscopy of induced superconducting correlations has a long history. Early experiments on the proximity density of states (DOS) [1] could be understood in the tunneling model of McMillan [2]. Recently the spatial dependence of the proximity density of states in normal metals has been measured [3] and successfully explained in terms of the quasiclassical theory [4]. The influence of spin splitting by a parallel magnetic field was measured in [5] and found to coincide with a Zeeman-split density of states. A new experimental and theoretical challenge is to extend these studies to the superconducting proximity effect in ferromagnets.Previous experimental investigations have concentrated on thermodynamic properties of FS multilayers. Here oscillations of the superconducting critical temperature T c as a function of the thickness of the F layers have been predicted [6] and found experimentally [7]. However, the interpretation of the experimental results depends on many fitting parameters. For example, little is known about sample parameters like the FS-interface quality [8]. In our opinion, it is also questionable if the theoretical approach using the diffusive quasiclassical formalism [9] is applicable for all F layers of typical thicknesses 10-50 Å.The most recent experiments have concentrated on other properties of FS layers. Ryazanov et al. [10] studied the supercurrent through a thin ferromagnetic layer and found a nonmonotonic temperature dependence, which can be interpreted in terms of a p-phase shift due to the exchange splitting. Kontos et al. [11], on the other hand, have studied the DOS in a thin ferromagnetic layer in contact to a superconductor. An oscillatory behavior of the induced superconducting correlation was observed for layers of different thicknesses and attributed to influence of the exchange field. It is this experiment that motivated our present study.Bearing this in mind, we study the superconducting proximity effect in a thin ferromagnetic layer. The F film is characterized by an homogenous exchange splitting h. We model the film as a ballistic layer with rough boundaries. Band mismatch and disorder at the interface may lead to enhanced backscattering at the FS boundary. We will derive a general formula for the subgap density of states depending only on the length distribution of classical trajectories in the F layer. The resulting density of states shows as a signature of the ex...

We consider the non-local quantum transport properties of a graphene superconducting spin-valve. It is shown that one may create a spin-switch effect between perfect elastic co-tunneling (CT) and perfect crossed Andreevreflection (CAR) for all bias voltages in the low-energy regime by reversing the magnetization direction in one of the ferromagnetic layers. This opportunity arises due the possibility of tuning the local Fermi-level in graphene to values equivalent to a weak, magnetic exchange splitting, thus reducing the Fermi surface for minority spins to a single point and rendering graphene to be half-metallic. Such an effect is not attainable in a conventional metallic spin-valve setup, where the contributions from CT and CAR tend to cancel each other and noise-measurements are necessary to distinguish these processes.

We theoretically demonstrate the capability of a ferromagnetic-normal interface in graphene to focus an electron wave with a certain spin direction. The essential feature is the negative refraction Klein tunneling, which is spin resolved when the exchange energy of ferromagnetic graphene exceeds its Fermi energy. Exploiting this property, we propose a graphene normal-ferromagnetic-normal electronic spin lens through which an unpolarized electronic beam can be collimated with a finite spin polarization. Our study reveals that magnetic graphene has the potential to be the electronic counterpart of the recently discovered photonic chiral metamaterials that exhibit a negative refractive index for only one direction of the circular polarization of the photon wave.

Monolayer graphene with an energy gap presents a pseudospin symmetry broken ferromagnet with a perpendicular pseudomagnetization whose direction is switched by altering the type of doping between n and p. We demonstrate an electrical current switching effect in pseudospin version of a spin valve in which two pseudoferromagnetic regions are contacted through a normal graphene region. The proposed structure exhibits a pseudomagnetoresistance, defined as the relative difference of resistances of parallel and antiparallel alignments of the pseudomagnetizations, which can be tuned to unity. This perfect pseudomagnetic switching is found to show a strong robustness with respect to increasing of the contact length, the effect which we explain in terms of an unusually long range penetration of an equilibrium pseudospin polarization into the normal region by proximity to a pseudoferromagnet. Our results reveals the potential of gapped graphene for realization of pseudospin-based nanoelectronics.Comment: 6 pages, 5 figure