Vortex ratchet reversal: The role of interstitial vortices

Triangular arrays of Ni nanotriangles embedded in superconducting Nb films exhibit unexpected dynamical vortex effects. Collective pinning with a vortex lattice configuration different from the expected fundamental triangular "Abrikosov state" is found. The vortex motion which prevails against the triangular periodic potential is produced by channelling effects between triangles. Interstitial vortices coexisting with pinned vortices in this asymmetric potential, lead to ratchet reversal, i.e. a DC output voltage which changes sign with the amplitude of an applied alternating drive current. In this landscape, ratchet reversal is always observed at all magnetic fields (all numbers of vortices) and at different temperatures. The ratchet reversal is unambiguously connected to the presence of two locations for the vortices: interstitial and above the artificial pinning sites.Comment: 21 pages, 4 figures, 1 Tabl

Control of spin injection by direct current in lateral spin valves

The spin injection and accumulation in metallic lateral spin valves with transparent interfaces is studied using d.c. injection current. Unlike a.c.-based techniques, this allows investigating the effects of the direction and magnitude of the injected current. We find that the spin accumulation is reversed by changing the direction of the injected current, whereas its magnitude does not change. The injection mechanism for both current directions is thus perfectly symmetric, leading to the same spin injection efficiency for both spin types. This result is accounted for by a spin-dependent diffusion model. Joule heating increases considerably the local temperature in the spin valves when high current densities are injected ($\sim$80--105 K for 1--2$\times10^{7}$A cm$^{-2}$), strongly affecting the spin accumulation.Comment: 6 pages, 5 figure

Exchange Bias Induced by the Fe3O4 Verwey transition

We present a study of the exchange bias in different configurations of V2O3 thin films with ferromagnetic layers. The exchange bias is accompanied by a large vertical shift in the magnetization. These effects are only observed when V2O3 is grown on top of Ni80Fe20 permalloy. The magnitude of the vertical shift is as large as 60% of the total magnetization which has never been reported in any system. X-Ray diffraction studies show that the growth conditions promote the formation of a ferrimagnetic Fe3O4 interlayer. The change in the easy magnetization axis of Fe3O4 across the Verwey transition at 120 K is correlated with the appearance of exchange bias and vertical shift in magnetization. Both phenomena disappear above 120 K, indicating for the first time a direct relationship between the magnetic signature of the Verwey transition and exchange bias.Comment: Accepted for publication Physical Review

Thermoelectric spin voltage in graphene

In recent years, new spin-dependent thermal effects have been discovered in ferromagnets, stimulating a growing interest in spin caloritronics, a field that exploits the interaction between spin and heat currents. Amongst the most intriguing phenomena is the spin Seebeck effect, in which a thermal gradient gives rise to spin currents that are detected through the inverse spin Hall effect. Non-magnetic materials such as graphene are also relevant for spin caloritronics, thanks to efficient spin transport, energy-dependent carrier mobility and unique density of states. Here, we propose and demonstrate that a carrier thermal gradient in a graphene lateral spin valve can lead to a large increase of the spin voltage near to the graphene charge neutrality point. Such an increase results from a thermoelectric spin voltage, which is analogous to the voltage in a thermocouple and that can be enhanced by the presence of hot carriers generated by an applied current. These results could prove crucial to drive graphene spintronic devices and, in particular, to sustain pure spin signals with thermal gradients and to tune the remote spin accumulation by varying the spin-injection bias

Spin transport in lateral spin valves and across a metal-insulator transition in V₂O₃

Spin valves is a class of spintronic devices that use spin degree of freedom. They have been used to study various condensed matter phenomena. Applications of the spin valves proved to be very useful in every-day electronics. Modern nano-fabrication techniques allowed further development of lateral spin valves. It was shown before that they separate spin and charge currents. These devices can provide a better insight into the spin transport and serve as a basis for the next generation of applications. In this thesis spin injection and detection in metallic lateral spin valves with transparent interfaces were studied using dc current. This allowed observation of two types of backgrounds present in the non-local spin valve signal. One of them originates from the inhomogeneous current distribution in the device. The other arises from the Joule heating. This heating was found to increase average temperature of the device by ̃100 K for the 10¹² A /m² current density. The magnitude of the non-local spin valve signal is symmetric with the current direction for current densities up to approximately 4x10¹¹ A/m². This indicates that the type of spin accumulation in the non- magnetic metal can be effectively controlled by the current direction. For higher current densities the signal becomes smaller when the current is injected from ferromagnetic electrode into the non-magnetic metal. This is explained by the spin-dependent Seebeck effect - an additional spin current is induced by a temperature gradient inside the ferromagnetic electrode. Non-local signal in copper/permalloy lateral spin-valves was measured as a function of distance between ferromagnetic electrodes, and Cu thickness. Extracted Cu spin diffusion length and permalloy spin-polarization decrease for smaller Cu thickness. This is explained by an additional spin-flip scattering at surfaces and interfaces of devices. Finally, Ni/V₂O₃/permalloy current-perpendicular- to-plane spin-valve devices were measured as a function of temperature. Unique geometry of the device allows observation of the metal-insulator transition in V₂O₃ at ̃160 K. Spin-valve effect was found in the device below the transition temperature. However the effect disappears at higher temperatures. Only anisotropic magnetoresistance of Ni was measured above ̃160 K. The observed magnitude of the spin valve effect, and its disappearance above the transition temperature, cannot be explained by simple 2- channel model for a device with transparent interfac