Day by day, the capacity is growing for the storage of information on hard disks, which constitute one of the computer’s types of memory. It is the result of the development of new materials, used in the devices called read/write heads, which bring the texts and images from the memory. The hard disk read/write heads are based on a principle of physics that may not sound very attractive to those who understand very little of the innards of the computer: it is the so-called magnetoresistance – the variation of the electrical resistance of a material submitted to a magnetic field.
This process for recovering information is also used in magnetic sensors that control automobile brakes and clutches, in landmine detectors and in pacemakers. But for them to be able to contain more information in the same space, based on this same principle, a group of physicists from the University of São Paulo has arrived at results that are fostering the prospect of a family of ceramics known as manganites replacing other fundamental devices of the computer, which reduce the resistance to the passage of electricity and so increase the precision and speed of the reading of the data: these are the magnetic multilayers adopted in computers following the discoveries by a physicist from the Federal University of Rio Grande do Sul (UFRGS), Mario Norberto Baibich.
In 1988, Baibich identified the potential of magnetic multilayers when using 40 tiny stacked plates of iron, a magnetic material, and chrome, which is nonmagnetic. He then noticed that the material chosen and its distribution in parallel and alternating the layers, when submitted to a magnetic field, reduced by up to 100% the resistance to electric currents – the reduction hitherto achieved was less than 5%. This effect, considered fantastic for the time, became known as giant magnetoresistance and attracted followers. “Giant magnetoresistance made possible the construction of magnetic sensors of very small sizes, with a greater capacity for reading and sensitivity”, explains Baibich, whose article communicating these results, published in November 1988, is still one of the Physical Review Letters’ most cited. “It was from then that the information technology industry began to produce the hard disks that we find nowadays in our computers.”
The manganites go beyond what Baibich discovered. “There are situations in which the variation of the electrical resistance of the manganites to the application of a magnetic field is much greater than those observed in magnetic multilayers, which could make it possible to speed up the process of reading and transmission of information in the same physical space”, comments Renato de Figueiredo Jardim, a professor from the Physics Institute. “We would have a quicker reading, with greater sensitivity and precision.” The studies coordinated by Jardim, which are running parallel to those carried out by the teams from the universities of Tokyo, in Japan, and California, in the United States, revealed new properties of this material that are contributing towards explaining how a normally insulating ceramic is transformed to the point of conducting electricity as well as some metal alloys. They have now shown, for example, that it is in a continuous and gradual way, without undergoing any brusque alterations, that the manganites acquire their most noteworthy properties, ceasing to be an insulating material to become a conductor of electricity and being transformed from a nonmagnetic material to begin to behave like a magnet. Getting to know the nature of these transformations, which the physicists call phase transitions, is essential for using this material.
Made up in the majority by manganese, to which oxygen is added, a chemical element from the family of lanthanides, in particular lanthanum, and another from the group of earth alkalines, like calcium and barium, the manganites are not conductors of electricity at room temperature. For them to become conductors, the lanthanum has to be partly replaced by calcium and the material submitted to very low temperatures, in the order of minus 120 degrees Celsius. “At that temperature”, Jardim observes, “the manganites lose their properties of an insulating material and are transformed into compounds with metallic characteristics, good electrical conductors”. Submitted to this temperature, they are also turned into a ferromagnetic material, endowed with magnetic properties similar to those of a magnet.
In a material that behaves like a magnet, the electrons line up, spinning around their own axis always in the same direction – a magnetic property of atomic particles known as spin. In these conditions, the application of a magnetic field creates an enormous variation of electrical resistance, which constitutes a type of magnetoresistance – not the giant kind, like the one discovered by Baibich, but even greater, called colossal, identified in the manganites in 1993 by German physicians. “With the alignment of spins, a preferential path emerges, through which the electric current may pass without many obstacles”, says Fábio Coral Fonseca, a physicist from the Institute of Nuclear Energy and Research (Ipen), who also takes part in the research. As a result, depending on the magnetic field applied and on the temperature, the resistance to the passage of the electric current may fall by as much as 10,000%.
Jardim and his doctoral pupil José Antonio Souza, with physicists from the University of Montana, United States, demonstrated in an article published in May 2005 in Physical Review Letters that this transformation, called second-order phase transition, occurs in a continuous and gradual way, without the ceramic undergoing brusque alterations. “Continuous phase transition used to be accepted only for some families of manganites, but we found it in many of them, regardless of the elements that make them up”, Jardim comments. As a consequence, some properties of this ceramic, like the phase transition from insulator to metal and from a nonmagnetic material to a magnetic one, must be reviewed.
In another study, published in Physical Review B, Jardim and Souza demonstrated another important detail: the effect of magnetoresistance can be much amplified when the lanthanum is partly replaced by yttrium, increasing the potential for the technological use of the manganites. This replacement of one chemical element by another results, in actual fact, in a new material, with different properties. This is a hybrid, made up of small islands of magnetic material, soaked in a matrix of insulating material, and endowed with metallic characteristics.
Another discovery refers to the stimuli that can be used for the manganites to pass through these transformations – from an insulator to a conductor, and from nonmetallic to metallic. In an article from January this year, also in Physical Review B, Jardim, Fonseca and doctoral student Alessandro de Souza Carneiro observed that great variations in the electrical resistance of the manganites can be obtained not only in the presence of a magnetic field, but also with the application of an electric current through the material.
When the manganites are in their initial insulating stage, the electric current finds difficulties in crossing the material, but looks for alternative paths, which offer less resistance. But, in these conditions, a transition from the conductor stage to the insulator stage may also take place.
Should the applications become effective, the manganites will represent a third stage in the recent history of magnetic sensors and readers. The first was the so-called inductive system, and the second, the magnetoresistant system, based on the magnetic multilayers.
The multilayers replaced the previous system, the inductive one, consisting of a coil made of thin copper wire, which detects the recorded magnetic field, generating a current. It is the reading of this current that makes it possible, in turn, to read the recorded magnetic fields. As this coil is not very sensitive, the magnetic field to activate it must be very intense. “To comply with this requirement”, Baibich says, “the sensor unit, made up of a recording coil and a detection coil, also needs to be very large and cannot be fitted into such tight spaces as the inside of a hard disk”.
Despite its limitations, the inductive system is still used in magnetic cards, like those of the banks, and in magnetic tapes for recorders. “After the revolution that the magnetic multilayers brought about in information technology”, Jardim comments, “if we manage to use the potential of the manganites and of giant magnetoresistance, we will be able to construct a new generation of devices for computers”.
Study of intergranular phenomena in ceramic oxides (nº 99/10798-0); Modality Thematic Project; Coordinator Reginaldo Muccillo – Ipen;
Investment R$ 696,105.59