Up to now, electronic devices have relied on the precise control of electronic charge. Yet in all microelectronic devices used today, we neglect the fact that nature has given electrons a spin. On the other hand, ferromagnetic phenomena are, of course, ultimately the result of just electron spin. For a long time, spin-dependent electronic properties of ferromagnetic materials and micromagnetic phenomena have been considered in a completely independent way. However, today scientists are confronted with the fact that the electron spin as the relevant physical phenomena enters industrial applications.
The new area in magnetic research and in industrial application of magnetic materials started with discovery of exchange coupling in periodic arrangement of thin sheets of ferromagnetic material with spacers made of normal metal. Under suitable circumstance, these exchange-coupled multilayers exhibit a dramatic change in their electrical resistance under the influence of varying external fields. Since this is quite surprising for a metallic system, the effect was named the giant magnetoresistance (GMR) effect.
The following provide a over view over interlayer exchange coupling, giant magnetoresistance and finally its applications.
Interlayer exchange coupling
The possibility of an exchange interaction between ferromagnetic films across nonmagnetic metallic interlayers has been considered for a long time. In 1986, “Layered Magnetic Structures: Evidence for Antiferromagnetic Coupling of Fe Layers across Cr Interlayers” showed that “interlayer exchange coupling” is identified and characterized in Fe/Cr structures. They investigated that exchange coupling of Fe layers across Au and Cr interlayers by means of light scattering from spin waves.
Further more, in 1990, unexpected, long-range, damped oscillations in the variation of the coupling strength with the thickness of the spacer layers were discovered. They reported the discovery of antiferromagnetic interlayer exchange coupling and enhanced saturation magnetoresistance in two new metallic superlattice system, Co/Cr and Co/Ru.
We can see antiferromagnetic(AF) coupled and ferromagnetic(FM) coupled at the upper peak and lower peak. The period is around 1.3nm and 1.5 nm for Ru and Cr.
Meanwhile, the coupling and its oscillations have been established on a large number of layered magnetic structures.
At the same time theories were developed which explain interlayer exchange coupling in terms of an indirect coupling of magnetic films via the metal electrons in the interlayer. There is a strong similarity to the famous Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, which describes the interaction of isolated magnetic moments in metallic host lattices and in rare earth metals.
Giant Magnetoresistance
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From the follow figures we can see that in multilayers the effect is much stronger than in double layers.
Multilayers
Double layers
GMR maybe be observed when the relative orientation of the magnetizations in adjacent layers changes. The biggest effect occurs when an antiferromagnetic arrangement is changed into a ferromagnetic one, for example, by an applied field. The antiferromagnetic arrangement can be provided by antiferromagnetic interlayer exchange coupling. In this case, oscillations of the interlayer exchange as a function of the thickness of the nonmagnetic layers give rise to the oscillations of the GMR effect.
Applications
Apart from the ever-existing basic interest the present and potential future applications of magnetic multilayers are a very strong pushing force to the current research in this area. Among all fields of possible application the magnetic recording industry certainly extended the strongest momentum.
In analog as well as digital magnetic recording there is a strong and continuous effort to permanently decrease the characteristic size of a magnetic bit, i.e., the size of the magnetized area of the recording medium that represents the information unit. In standard tape systems as well as in hard-disk drives the inductive write head magnetizes the medium within the surface plane such that adjacent domains have a head-to-head or a tail-to-tail configuration. In such a longitudinal recording set-up the recording medium consists of a multilayer configuration which involves, however, in general only one magnetically active thin film sandwiched between carrier and protective layers. Much effort is presently concentrated towards optimizing the magnetic thin alms to be capable of carrying increased bit densities. Since, however, longitudinal recording media are not composed of multilayer as discussed here, their future development will not be discussed further in the present context.
However, another issue concerning longitudinal recording is particularly important: The areal bit density which can be achieved in longitudinal, and vertical recording as well, obviously depends on the bit size produced by the inductive write head as well as on the ability of the read head to detect those tiny bits. Actually it turns out that the latter aspect is the crucial one. Earlier read heads were based on magnetic induction as are the write heads. The introduction of AMR read heads, since 1980 in tape systems and in the beginning of the nineties also in hard-disk systems, has considerably increased the yearly growth of the areal bit density in longitudinal recording: For hard-disk systems from 30%per year for inductive thin film heads to 60% per year for AMR heads. GMR recording heads which are presently introduced into the marked for hard disk drives are expected to sustain or even further increase this annual growth. It is evident that the extreme competition in magnetic-recording technology exerts considerable pressure on further developments in GMR-based magnetic multilayer systems.
Apart from applications in magnetic data storage other present applications of thin film and multitilayer devices mainly cover the sensitive detection of magnetic fileds in position and angle detection systems in magnetometers, in compass systems, and for measuring electrical currents. So far these applications are predominantly based on AMR sensors, however, the future use of GMR-based multiltilayer devices is considered as very promising.
Trends in magnetic storage
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The major industrial driving force behind the study of the GMR effect is its suitability for application in read heads for digital magnetic recording. As may be seen from figure, in hard-disk recording the introduction of read heads making use of the AMR effect, at the beginning of the 1990s, has in-creased the yearly growth of the areal bit density from about 30% per year to about 60% per year. The application of GMR materials is expected to help sustain this annual growth after its projected introduction in commercial systems in the second half of this decade. In commercial tape systems, AMR read heads were first introduced in1980by Philips: as an additional read head in compact cassette systems, they were used to retrieve auxiliary low-frequency information from the tape, written superimposed on the two audio channels. An early application (1985) of multichannel AMR heads was the professional 32-track Philips Communication Logging System. Another application is the Digital Compact Cassette (DCC) consumer audio system, which has been introduced in the beginning of the nineties. Thin film MR heads are produced by photolithographic processing. whereas hard disk systems combine a very high areal density with a very short access time, data tapes have the advantage of being removable, having a very high volume bit density, and having a very low price per bit stored. For certain applications magnetic tape systems are therefore expected to remain or become of interest. Examples of an existing and a potential future application are backup of computer data and digital video recording, respectively. As in the case of hard-disk recording, the introduction of GMR-based read heads could lead to a further increase of the bit density. For portable applications, the higher sensitivity of GMR-based heads is also of interest as a means of reducing the power consumption of the head.
