The spin-transfer torque oscillator is based on the interaction of a spin-polarized current with a magnetic film. This interaction results in two effects: magneto-resistance and spin-transfer torque. Magneto-resistance is the dependence of the electrical resistance on the relative orientation of the magnetization and the spin of the incident electrons. Spin-transfer torque is the torque exerted by the spin-polarized electrons on the magnetic film. In the spin-transfer torque oscillator, the spin-transfer torque is used to drive a GHz oscillation of the magnetization direction of the magnetic film. This oscillation is then transformed into an oscillating electrical signal by the magneto-resistance effect. The electrical signal can be used as a reference oscillator in wireless equipment.
The basis of magneto-electronics is the manipulation of the electron spin.
Electron, the particles that carry electrical current, do not only possess charge,
but also spin. This spin makes every single electron act as a tiny magnet.
Although all materials contain a large number of electrons, most of them are not magnetic. This is because the spin-orientation differs from electron to electron, thereby averaging out the magnetization to zero. Only in magnetic materials the electron spins add up to generate a net magnetic field.
Fig. 1 Top: non-magnetic material. The electron-spins, represented by the arrows and red-and-white color, are randomly oriented. Bottom: magnetic material. The electron-spins are aligned and the material behaves as a magnet.
For magneto-electronic applications, one would like to create and manipulate electrical currents that have a net spin moment. A convenient way to create such a spin-polarized current is by sending it through a magnetic material. Within the magnetic material, which naturally exhibits a net spin moment, the current will quickly get spin-polarized. When the current leaves the magnetic material, it will retain its spin-polarization for a certain distance in the non-magnetic material.
-polarized current in, e.g., copper or, with This way it is possible to inject a spin
somewhat more difficulty, in semiconductors. The distance over which a considerable spin-polarization is retained depends on the material and varies from tens of nanometers to tens of micrometers.
Fig. 2 Spin-polarized current injected in a normal metal. When a current is send from a magnet (left, red box) to a normal metal (right, blue box), its spin-polarization will be retained over a certain distance.
In the previous section it was shown how one can inject a spin-polarized current into a non-magnetic metal. However, the most interesting phenomena occur when a spin-polarized current is injected into a magnetic material. For example, the resistance that the spin-polarized current experiences will depend on the direction of the spin-polarization with respect to the magnetization of the magnetic material. This so-called magneto-resistance effect will be addressed in the next section. Here we will the discuss spin-torque excerted by such a current on the magnetic material.
Spin-torque is the torque exerted by a spin-polarized current on a magnetic film. When the spin of the incident electrons and the magnetization of the film are not parallel, a torque will be exerted on the magnetic film. The process is illustrated schematically in Fig. 3. The electrons entering the magnetic material (right) from the non-magnetic material (left) have their spin initially pointing in the top-right direction. However, due to scattering and dephasing processes the spin of the incident electrons will quickly align with that of the magnetization. Looking at the spin-direction on the left- and right-side of the interface, we see that the spin-component transverse to the magnetization direction has been absorbed by the magnetic material. This absorption will cause a torque on the magnetic film that tries to rotate the magnetization clockwise.
Fig. 3 Spin-torque. A spin-polarized current is injected from a non-magnetic material (left) into a magnetic material (right). The current is polarized in the top-right direction, while the magnetic north of the magnetic material is pointing upwards. The spin of the injected current is quickly absorbed by the magnetic material. For sufficiently high currents this absorption-process will results in a change of the magnetization direction of the magnetic material.
Using the spin-torque induced by high currents one can actually switch the magnetization direction of a magnetic film. This concept is of great interest for application in magnetic memories; it can be used to directly switch the state of a magnetic bit by a current. Besides switching the magnetic film, the spin-toruqe can also drive a continuing precession of the magnetization direction. In this case the magnetization direction will continually change, tracing out a complex trajectory at GHz frequencies. This GHz precession will be exploited in this project to create a novel type of oscillator.
More information on the details of spin-torque can be found in, e.g., the original articles of Slonzcewski and Berger .
Magneto-resistance is the dependence of the resistance of material on a magnetic field. For the spin-torque oscillator, we will be using the Giant
Magneto-Resistance (GMR) effect. The GMR effect is depicted in Fig. 4. In the figure a three-layer structure is shown, consisting of a magnetic, a non-magnetic and a magnetic layer.
The left magnetic layer is merely used to create a spin-polarized current in the middle layer, and was for clarity left out from Fig. 3. The spin-polarized electrons in the middle layer, carying the electric current, will have to enter the right magnetic layer. When their spin is polarized parallel to the magnetization of the right layer this will go easy, as depicted in the top part. Consequently, the resistance of the trilayer is low. However, when the right magnetic layer has its magnetization anti-parallel to the spin-polarization, the electrons will enter with more difficulty and the resistance will be high. Note that the non-magnetic middle layer merely serves to decouple the magnetic layers; without the non-magnetic layer it would not be possible to control the
magnetization-orientation of the left and right magnetic layers separately.
Fig. 4 GMR layer structure consisting of a magnetic (left), non-magnetic (middle) and magnetic (right) layer. The left magnetic layer serves to polarizes the current in the 'up' direction. The spin-polarized current then travels through the normal metal before entering the right magnet. If the right layer has its magnetization parallel to the left magnet, the electrons can enter more easily (low resistance) than when the magnetizations are anti-parallel (high resistance). The structure shown in Fig. 4 can be used as a magnetic field sensor. The left magnetic layer can be made such that its magnetization direction remains fixed, while the (free) right magnetic layer will follow small changes in an external field. The resistance of the trilayer will now depend on the magnitude and direction of the magnetic field. This principle is used, e.g., in read heads for magnetic harddisks.
In the spin-torque oscillator, the GMR effect is used to convert the current-induced motion of the free magnetic layer in an electrical signal.