The term "Spin Hall Effect" was introduced by Hirsch in 1999. Indeed, it is somewhat similar to the classical Hall effect, where charges of opposite sign appear on the opposing lateral surfaces to compensate for the Lorentz force, acting on electrons in an applied magnetic field. However, no magnetic field is needed for SHE. On the contrary, if a strong enough magnetic field is applied in the direction perpendicular to the orientation of the spins at the surfaces, SHE will disappear because of the spin precession around the direction of the magnetic field.
Experimentally, the Spin Hall Effect was observed in semiconductors more than 30 years after the original prediction. The spin accumulation induces circular polarization of the emitted light, as well as the Faraday (or Kerr) polarization rotation of the transmitted (or reflected) light, which allows to monitor SHE by optical means.
The origin of SHE is in the spin-orbit interaction, which leads to the coupling of spin and charge currents: an electrical current induces a transverse spin current (a flow of spins) and vice versa. One can intuitively understand this effect by using the analogy between an electron and a spinning tennis ball, which deviates from its straight path in air in a direction depending on the sense of rotation (the Magnus effect).
The Inverse Spin Hall Effect, an electrical current induced by a spin flow due to a space dependent spin polarization, was first observed in 1984. More recently, the existence of both direct and inverse effects was demonstrated not only in semiconductors, but also in metals.
The SHE belongs to the same family as the anomalous Hall effect, known for a long time in ferromagnets, which also originates from spin-orbit interaction.
The SHE might be used to manipulate electron spins electrically. For example, in combination with the electric stirring effect, the SHE leads to spin polarization in a localized conducting region.
Direct Electronic Measurement of the Spin Hall Effect
The generation, manipulation and detection of spin-polarized electrons in nanostructures define the main challenges of spin-based electronics. Among the different approaches for spin generation and manipulation, spin-orbit coupling-which couples the spin of an electron to its momentum-is attracting considerable interest. In a spin-orbit-coupled system, a non-zero spin current is predicted in a direction perpendicular to the applied electric field, giving rise to a spin Hall effect. Consistent with this effect, electrically induced spin polarization was recently detected by optical techniques at the edges of a semiconductor channel and in two-dimensional electron gases in semiconductor heterostructures. Here we report electrical measurements of the spin Hall effect in a diffusive metallic conductor, using a ferromagnetic electrode in combination with a tunnel barrier to inject a spin-polarized current. In our devices, we observe an induced voltage that results exclusively from the conversion of the injected spin current into charge imbalance through the spin Hall effect. Such a voltage is proportional to the component of the injected spins that is perpendicular to the plane defined by the spin current direction and the voltage probes. These experiments reveal opportunities for efficient spin detection without the need for magnetic materials, which could lead to useful spintronics devices that integrate information processing and data storage.
Spin Hall Effect Detected at Room Temperature
Physicists in the US are the first to detect the spin Hall effect at room temperature, in what could be an important development in the quest for a practical source of spin-polarized electrons for spintronic devices.
Spin Separator
Observations were made in the 10 to 295K temperature range using Kerr rotation (KR) spectroscopy. The team, which also included researchers from Pennsylvania State University, will report their findings in an upcoming issue of the journal Physical Review Letters.
The spin Hall effect was first observed in GaAs at 20K by Awschalom and Yuichiro Kato in 2004. It consists of a spin current flowing in a transverse direction to the charge current in a non-magnetic material and in the absence of an applied magnetic field. The result is a measurable accumulation of "spin up" and "spin down" electrons at opposite edges of the conducting channel.
The effect could be of use in the growing field of spintronics, in which the intrinsic spin of the electron (in addition to its electrical charge) is exploited in the development of logic devices. The spin Hall effect could provide a source of spin-polarized electrons for injection into semiconductor devices. Such electrons could carry information based on the state (up or down) of their spin polarization.
According to Awschalom, the unique advantage of using the spin Hall effect is that it does not require a magnetic field or magnetic materials to generate and separate spins in the solid state.
In this experiment, the spin Hall effect was observed in thin films (1.5 μm thick) of the semiconductor material ZnSe. At higher temperatures the researchers noticed a reduction in the spin Hall effect, the spin coherence time (how long electron spin states remain coherent) and the spin polarization. The spin polarization at 20K was about ten times stronger than at room temperature and the spin diffusion length decreased from 1.9 to 1.2 μm over the same temperature range.
The researchers are now working on ways to boost the spin polarization to levels where nearly all electrons are polarized. In previous GaAs experiments performed at Santa Barbara, one electron in 10 000 were spin-polarized by the spin Hall effect.
Fuente: http://en.wikipedia.org/wiki/Spin_Hall_effect, http://www.nature.com/nature/journal/v442/n7099/abs/nature04937.html#top, http://physicsworld.com/cws/article/news/25862
Nombre: Rodriguez B. Joiver I.
Asignatura: CRF
Get news, entertainment and everything you care about at Live.com. Check it out!
No hay comentarios:
Publicar un comentario