User talk:Stt 2009

Understandable by all

"When an electric current passes from one ferromagnetic layer via a non-magnetic layer into another ferromagnetic layer, the spin polarization and subsequent rotation of this current can induce a transfer of angular momentum that exerts a torque on the second ferromagnetic layer." This torque transfer is called spin torque transfer.

If WKB-approximation is assumed, a transfer of vectorial spin accompanies an electric current perpendicular to two parallel magnetic films connected by a normal metallic spacer. Such a junction with two ferromagnetic electrodes is shown below: [Fig.: 1 vom Bild-Paper]

It is build of several layers and each has its own function. The bottom layer consists for example of tantal in order to fix the Cu-seed-layer on the Si-Waver. Without this layer, the Cu-layer would not persist. The FeMn layer above is used to pin the ferromagnetic layer (NiFe). The Spacer consists of Cu, followed by the NiFe-non-pinned ferromagnetic. On top of this, the MTJ(magnetic tunnel junction) consist of a back with variable width, fixed on a last tantal-layer.

The material of the spacer and the ferromagnets is widly spread, many combinations like (Co/Cu/Co) or (CoFeB/MgO/CoFeB) are used.

If a voltage is applied between the two electrodes, the electrons tunnel through the spacer, reaching the second magnetic electrode, transfering a part of their angular momentum onto the electrons in the second electrode. The effect of switching the orientation of the second ferromagnet by passing through electrons with a known angular momentum and let them transfer a bit of it is called spin torque transfer. The whole process is also called current induced magnetic switching (CIMS). Every system which shows the Giant_magnetoresistance or Tunnel_magnetoresistance effect will also show the CIMS because the CIMS is a consequence of the additional torque exerted on the magnetization, which itself is a result of the spin transfer from the conduction electrodes to the locatized magnetic moments.

\begin "Nerd content" We now look at the spin and charge transport in a biased planar tunnel junction with two ferromagnetic electrodes assuming the free-electron-like model. A tunnel junction consisting of two ferromagnets, which are seperated by a non-magnetic layer is assumed for the following calculations and shown in the following figure [Fig: 1, Bild 2] The left electrode is semi-infinite with a fixed magnetic orientation S_l (in-plane). The right electrode is relativly thin with a free magnetic orientation..

Two coordinate systems are given, each for one ferromagnet. The z-axis of each coordinate system points along the netto magnetic moment of its ferromagnet. If the electron (spin) is between the two electrodes, it is decribed by the left system. The x-axis are orientated along the plane in which the netto magnetic moments are. As an result, the y-axis are perpendicular to both (orthonormal-system) an point right into the ferromagnets. To calculate the in-plane(x') and out-of-plane(y'=y) components of the spin torque in the right film a positive definition of the charge current is presumed when the currents flows from the thin to the thick layer. For this positive (bias-)voltage the electrons(and their spins) flow from the left ferromagnet to the right one. As prooved in [ Bild 2], the spin-torque T can be written as (1) (2) (3) (4) where J(J~) denotes the uth component of the spin current density. calculated in the barrier near to the left (right) ferromagnet. These components represent the spin current density which is not absorbed in the left FM and tunnels through the barrier and reaches the right FM. When this layer is thick(or semi-infinity) the x' and y componets are absorbed, J~ vanishes and the in-plane and out-of-plane exerted torques on both layers are the same. ==> (1)=(3) and (2)=(4) [Bild 2]#

To determine the spin torque needed for further calculation, the spin current componets arouse interest. After a long and painful calculation and serveral assumptions (free-electron-like, zero temperature limit) the uth component of the the right ferromagnetic-induced spin current density is supposed to be (16). E_F is the fermi-energy in the left(source) electrode. The integration is performed over the energy associated with the motion perpendicular to the layer planes and the summation is over the two spin-subbands. \end "Nerd content"

Applied science

The STT is also used as writing technology where data is written by re-orienting the magnetisation of a thin magnetic layer in a tunnel magnetoresistance (TMR) element using a spin-polarised current. An electrical current is generally unpolarised (consisting of 50% spin-up and 50% spin-down electrons), a spin polarised current is one with more electrons of either spin. By passing a current through a thick magnetic layer one can produce a spin polarised current.

At very small device scales it is possible that a spin polarised current can transfer its spin angular momentum to a small magnetic element. Spin torque transfer magnetic RAM (STT-MRAM) has the advantages of lower power-consumption and better scalability over conventional MRAM. Spin torque transfer technology has the potential to make possible MRAM devices combining low current requirements and reduced cost, however the amount of current needed to re-orient the magnetisation is, at present, too high for commercial applications and the reduction of this current density alone is the basis for a lot of current academic research in spin-electronics.

Hynix Semiconductor and Grandis formed a partnership in April 2008 to explore commercial development of STT-RAM technology.

See also

• Magnetoresistive Random Access Memory (MRAM)
• Spin (physics)

External links

• Spin torque applet
• Solicitation Number: BAA08-16 from the Defense Advanced Research Projects Agency
• J.C. Slonczewski:"Current-driven excitation of magnetic multilayers(1996)", Journal of Magnetism and Magnetic Materials Volume 159, Issues 1-2, June 1996, Pages L1-L7 [1]
• D. C. Ralph, M. D. Stiles:"Spin Transfer Torques", Journal of Magnetism and Magnetic Materials 320(7), 1190-1216 (2008)[2]

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