Magnetic Thin Films
The earliest recorded work with magnetic thin films took place in the 1880s and was carried out by German physicist August Kundt. Well known for his research on sound and optics, Kundt’s pioneering efforts in thin film production were spurred by his desire to observe the dispersion of light not only by gases and liquids, but also metals. The lengthy and difficult process Kundt employed to create the first magnetic thin films involved the electrodeposition of a metal on platinized glass.
Y-123 Superconductor on Lanthanum Aluminate
The electrodeposition, or electroplating, process was first discovered in the early nineteenth century, soon after the invention of the battery by Alessandro Volta. Up until Kundt’s innovative use for the practice, electrodeposition was primarily utilized to produce gold or silver coated costume jewelry, eating utensils, and other small personal items. The coating was achieved by placing an item in a metal salt solution and linking it in an electrical circuit, in which it served as the cathode. An electrode (often consisting of the same metal that was to be plated on the item) placed in the solution would be employed as the anode in the circuit. By passing electricity through the circuit, metal ions present in the salt solution could be forced to form a layer on the cathode due to attractive forces. If the cathode was rotated during the process, a complete coating of the item could be achieved.
Achieving a very smooth, even coating via electroplating is a very difficult undertaking and requires a significant amount of skill. Yet Kundt successfully produced well formed thin films of nickel, iron, and cobalt. Utilizing them he was able to monitor the extent of the rotation in the polarization of light as it was passed through the films. As had been described by Michael Faraday some years before, the rotation was in a parallel direction to the magnetization of the film employed. The phenomenon, known as the Faraday effect, is a clear indication that light and magnetism are somehow related.
Gadolinium-123 Superconductor on Lanthanum Aluminate
Kundt’s studies with magnetic thin films established the direction for the vast majority of thin film research undertaken prior to the mid twentieth century. At that time, interest in magneto-optics and thin films waned somewhat as the introduction of computers spawned a search for practical means of information storage. Magnetic materials were suddenly at center stage of a large number of studies due to the considerable potential market for magnetic memory storage. Around 1955, permalloy emerged as the most promising material for any type of computer memory based on magnetic thin films for several reasons. Most notably, permalloy exhibits soft-magnetic properties in thin film form, allowing its magnetization direction to be reversed with little effort, and a magnetization axis can be generated in the material through a procedure known as field cooling.
Despite the significant interest in thin film-based computer memory, ferrite cores were employed in most early computer memory systems. However, thin film memory, despite certain difficulties with its production and use, appeared commercially in the UNIVAC 1107 in the early 1960s. In the machine, thin film magnetic memory only comprised a very small amount of the total memory available, largely due to the substantial expense of the new technology. Thus, the UNIVAC 1107 employed thin film memory for its 128-word general register stack, but depended upon the much less pricey and more primitive core memory technology for the rest of its storage. In applications in which expense was of little concern, such as for government or military computer systems, thin film memory comprised a greater proportion of total memory and was introduced earlier.
Nickel Oxide on Sodium Chloride
The earliest thin film memory utilized was formed from the iron-nickel alloy permalloy. Thin film memory was developed by the Sperry Rand Corporation, which received financial aid for the project from the United States government. The process to produce the thin films entailed the deposition of permalloy onto small plates of glass through the use of a mask and vacuum evaporation methods. Once the permalloy was in place, multilayer circuits were placed over them in order to establish connections.
Methods of producing magnetic thin films have since evolved considerably and a variety of techniques are in use today. Two of the most widely employed methods of thin film production are molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). With MBE, one or more metals or other pure materials are deposited onto a crystal wafer a layer of atoms at a time under extremely-high vacuum conditions to form a single crystal. When MOCVD (also known as metalorganic vapour phase epitaxy, organometallic vapour phase epitaxy, or organometallic chemical vapour deposition) is employed, the growth of crystals takes place at moderate pressures rather than in a vacuum. The development of both of these techniques has its origins in the work of American researcher John Arthur, who in the late 1960s found that gallium arsenide could be produced by directing a beam of gallium and arsenic toward the flat surface of a crystal.
Nickel Oxide on Indium Phosphide
Similar to Kundt’s difficulty in producing smooth magnetic thin films utilizing the process of electroplating, scientists continue to have difficulty in generating flawless thin films when using more modern techniques. A key problem that must be overcome to produce usable thin films is compatibility between the magnetic material and substrate to which it is applied. The best films are made when the surface of the substrate has a perfect crystalline structure that resembles the one that the metal will form over it. For example, a substrate with a hexagonal crystalline structure would not be able to produce a thin film with a square-based structure because the overlying crystal usually adopts the ordering of the substrate upon which it is built during deposition. Crystallization problems during thin film production can result in striations, crumpling, clumping, wrinkling, or other surface deformations.
When magnetic thin films are not smooth, they cannot be used for most applications, though they can reveal new information about thin films in general and lattice matching. A thin film with the proper texture for practical employment has a surface that is so flawless it resembles a mirror when examined with an optical microscope. Though substrate and metal compatibility is very important in achieving such a surface, it is not the only factor that must be considered. Production conditions, such as temperature and pressure, can have a profound effect on the viability of magnetic thin films, and fine-tuning processes is very important for producing high quality and affordable specimens. Improvements are continually being sought though magnetic thin films are already used in various applications because new advances may open up additional avenues for their employment and lead to greater thin film reliability, versatility, and affordability.
One arena in which magnetic thin films already have a substantial history is magneto-optical recording. The films first employed for this application in 1958 were manganesebismuth films. Other types of films have been developed for magneto-optical storage since that time. The films best suited for this use are act as hard magnets at room temperature but at moderately elevated temperatures adopt the characteristics of a soft magnet so that they can be remagnetized with relative ease. This facilitates the storage of information by heating a small area of film with a laser and then cooling it back to room temperature while it is under the influence of a magnetic field. The laser in the system is also able to be used to read the stored data due to the Kerr magneto-optical effect.
Nickel Oxide on Magnesium Oxide
Since the 1970s, there has also been significant interest in developing useful rare earth transition metal (RETM) thin films. Due to the properties of the transition metals, thinner films can be produced from them than from many other materials. This in addition to other favorable qualities exhibited by RETM thin films make eventually lead to their adoption for use in magneto-optical minidisks or other emerging technological equipment. Some problems, such as their tendency to corrode under certain conditions, still may need to be surmounted before their employment becomes common.
Magnetic thin films have been, and are expected to continue as, an important factor in the miniaturization and increased speed of many technologies. As demands have increased, magnetic thin film systems have increased in complexity. Rather than being used alone, magnetic thin films today often comprise only part of a multilayered structure (often referred to as a superlattice). Other components in such structures may include non-magnetic as well as additional magnetic materials. Over the last few decades, studies of multilayered magnetic thin film systems have resulted in the discovery of previously unknown physical processes.
Nickel Oxide on Sodium Chloride
In 1986, a process that came to be called interlayer exchange coupling (IEC) was first observed. IEC occurs when a pair of ferromagnetic layers are separated by a non-magnetic metal layer, or spacer, and is manifest by the either the ferromagnetic or antiferromagnetic alignment of the strata under the influence of a magnetic field. The extent of this alignment, or coupling, varies with the thickness of the spacer layer.
A few years after the discovery of IEC, the phenomenon of giant magnetoresistance (GMR) was discovered. The GMR effect, in which considerable changes in resistance occur, can be observed in multilayer systems exposed to a magnetic field. Together the phenomena of IEC and GMR have cause quite a stir in the scientific world, prompting advances in various technologies and an array of studies into such areas as spin electronics and magnetoelectronics.
GMR has been especially fundamental in helping magnetic thin films achieve broader usage. In 1994, IBM reported that they had successfully built the first thin film based equipment that took advantage of the GMR effect. The innovative devices introduced by the company were extremely sensitive sensors designed to detect information on magnetic hard disks. Less than three years after the company’s announcement, the sensors made possible by GMR were released in IBM personal computers. The success of the sensors, which not only increased sensitivity but decreased noise, was so great that GMR-based drives quickly became a dominant technology in the industry. Furthermore, other devices that employ the GMR effect are likely to become central in other markets. Some applications that seem particularly promising for multilayered films displaying the GMR effect include metal detectors, traffic monitors, braking systems, alarms systems, and electrical safety devices.