Spin current - Page 7

Spin Ratchets - a new electronic structure for generating spin current

Researchers from the Institut Català de Nanotecnologia (ICN), in Barcelona have demonstrated a new device that induces electron spin motion without net electric current. They call this device a 'ratchet', in analogy to a ratchet wrench which provides uniform rotation from oscillatory motion. The Spin Ratchets achieve directed spin transport in one direction, in the presence of an oscillating signal. Most important, this signal could be an oscillatory current that results from environmental charge noise; thus future devices based on this concept could function by gathering energy from the environment.

The ratchet efficiency can be very high - reported results show electron polarizations of the order of 50%, but they could easily exceed 90% with device design improvements. The spin ratchet, which relies on a single electron transistor with a superconducting island and normal metal leads, is able to discriminate the electron spin, one electron at a time. The devices can also function in a “diode” regime that resolves spin with nearly 100% efficacy and, given that they work at the single-electron level, they could be utilized to address fundamental questions of quantum mechanics in the solid state or to help prepare the path for ultrapowerful quantum or spin computers.

Read the full story Posted: Dec 17,2010

New theory generalizes spin transfer torques, total angular momentum current, and mechanical torques

Paul Haney and Mark Stiles from the NIST Center for Nanoscale Science and Technology (CNST) developed a new theory of current-induced torques that generalizes the relationship between spin transfer torques, total angular momentum current, and mechanical torques. This new theory is also applicable to more materials than previous theories.

The basic idea is that there are two types of current-induced torques: a mechanical torque acting on the lattice, and a spin transfer torque (STT) acting on the magnetization. STT is a known phenomenon that is the basic of several technologies such as STT-MRAM and nanoscale microwave oscillators.

Read the full story Posted: Dec 09,2010

Switching Orientation of Magnetization in Thin Metallic Film can be Achieved using Diffusion of Electron Spins

In magnetic memory devices, information is stored in magnetic elements and typically retrieved by applying a small, external magnetic field. More convenient, however, is the use of a spin-polarized current, in which moving electrons exert a torque on a magnetic element and can switch the direction of its magnetization.

Unfortunately, moving electrons can give rise to electrical noise, which reduces the efficiency of the magnetization control. Now, Yoshichika Otani from the RIKEN Advanced Science Institute in Wako and colleagues have overcome this problem by using a pure spin current*, that is, a diffusion of electron spins without charge motion.

By examining the electronic transport properties of their device, the researchers were able to demonstrate that when the current injected into the first junction is high enough, it creates a spin current high enough to reverse the magnetization at the second junction. Most importantly, the magnetization can be reversed back by applying the same amount of current in the opposite direction.

Via  AZONano

Read the full story Posted: Feb 13,2009

Pure Spin Currents In Silicon Generated, Modulated, And Electrically Detected

Scientists at the Naval Research Laboratory (NRL) have generated, modulated and electrically detected a pure spin current in silicon, the semiconductor used most widely in the electronic device industry. Magnetic contacts on the surface of an n-type silicon layer enable generation of a spin current which flows separately from a charge current. The spin orientation is electrically detected as a voltage at a second magnetic contact. The relative magnetizations of these contacts allow full control over the orientation of the spin in the silicon channel. This was accomplished in a lateral transport geometry using lithographic techniques compatible with existing device geometries and fabrication methods.

In this very recent work, NRL scientists first inject a spin polarized electrical current from a ferromagnetic iron / aluminum oxide tunnel barrier contact into silicon, which generates a pure spin current flowing in the opposite direction (see figure). This spin current produces shifts in the spin-dependent electrochemical potential, which can be electrically detected outside of the charge path at a second magnetic contact as a voltage.

The NRL team showed that this voltage is sensitive to the relative orientation of the spin in the silicon and the magnetization of the detecting contact . They further showed that the orientation of the spin in the silicon could be uniformly rotated by an applied magnetic field, a process referred to as coherent precession, demonstrating that information could be successfully imprinted into the spin system and read out as a voltage. The generation of spin currents, coherent spin precession and electrical detection using magnetic tunnel barrier contacts and a simple lateral device geometry compatible with "back-end" silicon processing will greatly facilitate development of silicon-based spintronic devices.

Read the full story Posted: Dec 04,2007

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.

David Awschalom and colleagues at the Center for Spintronics and Computation at the University of California, Santa Barbara observed the current-induced spin-polarization of electrons and the spin Hall effect in thin surface layers of ZnSe.

The 'spin Hall' is 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. 

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Read the full story Posted: Sep 12,2006

Taking Microwaves for a Spin

Most lasers operate at infrared and visible wavelengths, but the first laser, developed in 1954, was actually a maser--it used microwaves. In an upcoming issue of PRL, researchers in the Netherlands propose a new design for a maser that would work by harnessing the spins of electrons.

Watts and his Groningen colleague Bart van Wees propose that a maser could be created with a three-layer sandwich, with a ferromagnetic material like iron on top, a paramagnetic material like aluminum on the bottom, and a thin layer of electrical insulator separating the two. Applying a magnetic field pointing downward would create two electron energy states: a ground state for spins pointing down and an excited state for spins pointing up. Electrons could be excited to the higher-energy, spin-up state by sending in microwave radiation.

But the incoming microwaves would not generate enough excited electrons to get the population inversion needed for a maser. So the researchers propose supplementing the excited electron population with a "spin current" from the ferromagnet into the paramagnet--that is, a current whose electrons have a preferred spin-up orientation. The team calculates that with this boost, the paramagnet would contain enough excited electrons for maser action to start spontaneously, and the sandwich would emit amplified, coherent, microwave radiation.

Read more here 

Read the full story Posted: Aug 27,2006