Monday, January 31, 2011 A few more instructive slides related to GMR and GMR sensors

Oscillating sign of Interlayer Exchange Coupling between two FM films separated by Ruthenium spacers of thickness varying from 0.3 nm to 3.3 nm (measured data). Ruthenium – an exotic metal from the Platinum group, with Z = 44. It had no major technological applications until it was discovered that it is particularly efficient in conveying interlayer exchange coupling between Cobalt-rich ferromagnetic films.

A schematic of a simplest GMR sensor. The thickness of the non-magnetic spacer is such that the coupling between the two FM films is antiferromagnetic. However, both FM layers are “free”, i.e., their magnetization vectors M 1 and M 2 are not “anchored” to anything. Hence, their mutual orientation can be changed by an external field exerted in any direction. Consequently, such devices are are sensitive only to the external field magnitude.

A schematic of a “spin valve” GMR structure. The top FM layer is a “free” layer – its magnetization direction can be changed by app- lying an external magnetic field B. The other FM layer is exchange- coupled, or “pinned” to a thick antiferromagnetic substrate, and therefore its magnetization does not react to B.

A spin valve does not react to fields exerted in certain directions: B This field does not change the magnetization direction in the top layer – no change in the resistance! B But field in this direction will change the magnetization di- rection in the top layer, and thus the sensor re- sistance will decrease. “Directional sensitivity” is often needed in technological applications!

However, such a design is still not perfect! The “pinned” layer is a source a field that produces an “offset” in the R vs. B characteristic…

Fortunately, the “offset problem” can be solved by a more sophisticated design, in which a single “pinned” FM layer is replaced by two FM layers separated by a thin Ru spacer that introduces a strong AFM coupling between them. Such a “trilayer” is usually referred to as an “artificial antiferromagnet”. The B fields produced by the two FM components cancel out one another. In such a spin valve design, the “pinning” AFM layer may even not be needed….

Spintronics The emergence of GMR devices marked the beginning of the Spintronics Era. What is spintronics? It is a novel branch of electronics. Conventional electronics is based on controlling the magnitude of electric currents. In contrast, in spintronics it is the spin state of the current (or its spin polarization, if you prefer) that is controlled.

What advantages may controlling of the current’s spin-state offer compared to conventional current’s magnitude controlling? Let’s take a short “brainstorming session”! I want to know your opinions…

GMR sensors are not EXACTLY spintronics devices… There are “spintronics- like elements” in GMR sensor operation, but the signal produced by the sensor is still a current signal. So, GMR sensors are still “spintronics and Conventional electronics HYBRIDES”… But there is nothing wrong with it, too radical Revolutions are not always good…

There are no “100% spintronics devices” yet, but things are certainly evolving in this direction…. A device that is “more spintronics” than a GMR sensor, is a Tunnel Magnetoresistance (TMR) junction. The design is similar to that of GMR sensors, except that instead of a metallic non-magnetic spacer there is an ultra- thin insulating layer. It acts as a barrier the electrons can pass through due to the quantum effect of tunneling. The probability of tunneling is different for the “parallel” and the “anti-parallel” configuration of the FM layers.

TMR sensors are even more efficient that the GMR sensors. They are now widely used in computer hard-drive reading heads. Another application of TMR sensors that seems to be “right Around the corner” is in Magnetic Random Access Memory (MRAM). Each TMR junction can store one bit of info.

In magnetic random access memory (MRAM) the magnetic moment of a magnetic material is used to store data. In this case, a magnetic moment pointing left can represent a "0", while a magnetic moment pointing right can represent a "1". (b) Data can be written to the material by sending an electric current down conductors that pass nearby. In this case, the magnetic field produced by current x puts the magne- tization into an intermediate state, and current y then triggers the magnetic moment to move to a particular orientation.

Next item: another class of magnetic materials that are highly Interesting from the viewpoint of spintronics are the so-called “half-metals”. In a half-metal, for one electron spin orientation (↑) the structure of the electronic bands is like that in a metal… But for the other spin orientation (↓) it is like in a typical semiconductor, with a distinct “energy gap”.

One interesting application of half-metals is in “spin-filters” that can be used for obtaining nearly 100% spin-polarized currents:

An ordinary electron current is a mixture of 50% spin-up electrons, and 50% spin-down ones – therefore, the net angular momentum it carries is exactly ZERO. In contrast, a spin-polarized current does carry angular momentum. MOREOVER, the angular momentum carried by the electrons can be transferred to other object. Adding angular momentum to an object may haeve the same effect as EXERTING TORQUE on the object! The torque-transfer effect, discovered independently by J. Slonczewski and L. Berger, can be used for changing the state of a TMR “memory cell”.