Spin doctor

Nobel prizewinner Peter A. Grunberg on GMR and its spin-off, spintronics

Today, compact and fast hard discs are a useful and indispensable tool for the information society. We are only a few mouse clicks away from our data, which grow at a rapid rate.

This convenient access has been possible due to a discovery which was made at Forschungszentrum Jülich and Paris-Sud University at the same time in the late 1980s: the so-called GMR effect. This effect is the latest example of how findings in basic research may shake up our daily lives.

Giant magnetic resistance (GMR) describes the fact that the electrical resistance of a thin magnetic stack can be greatly changed through external magnetic fields. The effect is based on the influence of the magnetisation on the motion of the electrons in the stack.

The discovery of GMR in 1988 led to very rapidly growing research activities, which established a unique and cutting-edge area in research today: spintronics.

The findings in basic research on magnetic materials led to novel layer structures that can be used for ultrasensitive sensors, thus enabling extremely high densities of magnetically stored data to be read precisely.

Within only 10 years, the GMR effect was transferred from the laboratory to industrial application and is used now many million times a day to read magnetic bits and bytes: since the mid-1990s, read heads in all common hard discs have been based on GMR sensors. These sensors made it possible to increase the data density to the extent known today and to shrink the size of hard discs at the same time.

Today, all of us benefit from the developments that followed this success of basic research: with dimensions smaller than a matchbox, these hard discs are currently used in transportable MP3 players, camcorders or digital TV devices.

Moreover, GMR sensors are used in many other technology areas, for example, as steering angle sensors in motor vehicles, as sensors in bio-arrays to detect functionalised biomolecules, or in many other areas of  engineering.

However, spintronics is also developing a whole range of alternative lines which deal with a number of different spin transport processes. At Forschungszentrum Jülich, there are about 50 scientists who are currently working in this promising field on a cross-institute basis.

A prominent example is tunnelling magnetoresistance (TMR). In 1995, this effect was observed for the first time at ambient temperature and reaches even higher values than GMR. From a technological point of view, the TMR effect is highly interesting, since magnetic tunnel junctions can be fabricated to a size of less than 100 nanometres.

They are ideal storage cells for non-volatile main memories, so-called magnetic random access memories (MRAM). MRAMs could be an alternative to the semiconductor memory chips currently in use, since they promise many advantages when applied in computers, for example, lower energy consumption, no need for moving parts and stability of data even without current flow.

The discovery of the spin-current switching phenomena a few years ago even has a good chance of repeating the technological success story of GMR.

With a sufficiently strong spin-polarised current, it is possible to switch the magnetisation of a thin layer. This makes it possible to switch, in particular, very small magnetic elements and therefore enables the solution to a severe scaling problem in MRAM development.

The smaller the individual storage cells in the MRAM, the larger the magnetic fields, which are necessary to switch these cells, must be.

At the same time, however, the risk that adjacent cells are also unintentionally influenced by the magnetic field increases and thus their information content may be lost.

This problem can be avoided with the aid of the spin-current switch. The first MRAMs capable of exploiting this property are already on the market, but as yet only for special purposes. But the devil is in the detail: the structures which are involved are extremely small.

The production of these objects is an effort that requires both instrumental and personnel excellence. The current density which is required for the switching is close to the destruction limit for these structures. This is one of the challenges that still have to be solved.