Ultrafast all-optical spin injection in silicon revealed at FERMI

A revolutionary and energy-efficient information technology encoding digital data in electron spin (spintronics) by combining semiconductors and ferromagnets is being developed worldwide. Merging of memory and logic computing of magnetic based storage devices and silicon-based logic transistors is expected to ultimately lead to new computing paradigms and novel spin-based multifunctional devices. The advantages of this new technology would be non-volatility, increased data processing speed, reduced electric power consumption. All of them are essential steps towards next generation quantum computers.

To create spin-based electronics with potential to revolutionize information technology, silicon, the predominant semiconductor, needs to be integrated with spin functionality. Although silicon is non-magnetic at equilibrium, spin polarized currents can be established in Si by a variety of approaches such as the use of polarized light, hot electrons spin injection, tunnel spin injection, Seebeck spin tunneling and dynamical spin pumping methods, as had been demonstrated recently. In general, spin polarized currents refer to the preferential alignment of the spin angular momentum of the electrons in a particular direction.

Nowadays, considerable effort is dedicated on achieving spin-polarized currents in silicon using injection of super-diffusive spin currents from a ferromagnetic contact triggered by ultrafast optical pulses. This concept is schematically illustrated in Fig. 1a. First, laser-excited electrons in the ferromagnetic metal, nickel for the case of our experiment, undergo strongly asymmetric spin diffusion, which in turn leads to a spin diffusion (j_s) away from the irradiated region. Such spin diffusion removes angular momentum from the irradiated area, which rapidly demagnetizes, namely in less than 150 fs (gray to red arrows). As the super-diffusive current tunnels the Schottky potential energy barrier at the Ni/Si interface, a spin-polarized current is then injected in the semiconductor and, in turn, transient magnetization (M_js) is induced.

Figure 1: a) the optical generation of spin polarized superdiffusive currents across a ferromagnetic/semiconductor interface is illustrated. b) the principles of TR-MOKE experiment are illustrated  together with a cross-section TEM image describing the quality of the Ni/Si interface.

In our work, we have investigated the spin injection in silicon by means of a pump-probe experiment, in which the optical pump pulse triggers the super-diffusive spin current and the probe FEL pulse, whose photon energy is tuned to either Ni or Si core resonances, follows the ultrafast evolution of the magnetization in nickel and silicon. Sensitivity to magnetization is obtained by measuring the angular Kerr rotation of the photon polarization resulting from the interaction of the photon pulse with the magnetic sample. The dynamic variation of the Kerr rotation, or time-resolved magneto-optical Kerr effect (TR-MOKE), has been measured using the extreme ultra violet Wollaston-like polarimeter (TONIX) in operation at the MagneDyn beamline for magneto-dynamics studies at the FERMI FEL.

The schematic of the experiment is sketched in Fig. 1b. The sample consists of a 10 nm nickel film deposited on a Si(111) substrate by means of laser pulsed deposition. The nickel film has been capped with a silver layer for preventing oxidation. To control the smoothness of the interface and limit the formation of nickel silicides at the ferromagnet/silicon interface, the Si surface was passivated by deposition of ultrathin Si3N4 layer. Cross sectional TEM analysis confirmed the good quality of the interface. The optical pump (yellow pulse) was an 80 fs, 800 nm pulse, whose intensity is almost completely absorbed in the nickel layer. The FEL pulses tuned at the Ni (red pulse) and Si (blue pulse) edges follows the variation in time of the Kerr rotation of the photon polarization induced by the dynamics of the magnetization in the Ni and Si layers, respectively.

In conclusion, the importance of our work is twofold: on the one hand, we have directly accessed the ultrafast transient spin diffusion from a metallic ferromagnet to the silicon semiconductor. On the other hand, this investigation was made possible by extending the MOKE spectroscopy to the time domain and to the EUV photon energy range. This demonstrates the robustness of time-resolved MOKE spectroscopy as a tool for tackling open problems in the field of magneto-dynamics. In perspective, by injecting and detecting spin-polarized currents in semiconducting materials, one could combine magnetic storage with electronic readout in a single semiconductor device, resulting in a number of obvious benefits.

This research was conducted by the following research team:

Simone Laterza^1,2, Antonio Caretta¹, Richa Bhardwaj¹, Roberto Flammini³, Paolo Moras⁴, Matteo Jugovac⁴, Piu Rajak⁵, Mahabul Islam⁵, Regina Ciancio⁵, Valentina Bonanni¹, Barbara Casarin², Alberto Simoncig¹, Marco Zangrando^1,5, Primoz R. Ribic¹, Giuseppe Penco¹, Giovanni De Ninno¹, Luca Giannessi¹, Alexander Demidovich¹, Miltcho Danailov¹, Fulvio Parmigiani^1,6, and Marco Malvestuto^1,5

¹ Elettra - Sincrotrone Trieste S.C.p.A., Basovizza, Trieste, Italy
² Department of Physics, University of Trieste, Trieste, Italy
³ Istituto di Struttura della Materia-CNR (ISM-CNR), Roma, Italy
⁴ Istituto di Struttura della Materia-CNR (ISM-CNR), Basovizza, Trieste, Italy
⁵ Istituto Officina dei Materiali (CNR-IOM), Basovizza, Trieste, Italy
⁶ International Faculty, University of Cologne, Cologne, Germany

Contact person: ,

Local contact person: