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magnetic bubble[mag′ned·ik ′bəb·əl]
a tiny cylindrical magnetic domain in a ferromagnetic or ferrimagnetic material. Magnetic bubbles are isolated uniformly magnetized domains having the shape of circular cylinders and a direction of magnetization that is opposite to the direction of magnetization of the rest of the material (Figure 1). They were discovered in orthoferrites and hexaferrites in the late 1950’s. The proposal that magnetic bubbles be used in computer technology was first made in 1967.
In practice, magnetic bubbles are obtained in thin (1–100 micrometers [μm]) plane-parallel platelets or films of single-crystal ferrimagnetic materials, for example, ferrimagnetic garnets, or of amorphous ferromagnetic materials, such as alloys of transition d- or f-elements with a single direction of easy magnetization that is perpendicular to the surface of the platelet. The magnetic field that generates magnetic bubbles is known as a bias field and is applied in the direction of easy magnetization. In the absence of an applied bias field, the domain structure of a platelet exhibits a disordered labyrinthine pattern. When a bias field is applied, domains that do not extend to the edges of the platelet shrink into magnetic bubbles. The magnetization vector J of the bubbles is oriented in the direction of easy magnetization.
Isolated magnetic bubbles exist over a specific range of bias fields; this range extends to several percent of the material’s saturation magnetization. The lower stability limit corresponds to the transformation of magnetic bubbles into domains with a different shape, while the upper limit corresponds to the collapse of the bubbles. The stability of magnetic bubbles results from a balance between the following three forces: the force of the interaction between the magnetization of the bubbles and the bias field; the force associated with the existence of bubble-domain walls, a
force that is analogous to surface tension; and the force of the interaction between the magnetization of the bubbles and the demagnetizing field of the rest of the magnetic material. The first two forces tend to shrink the magnetic bubbles, while the third tends to stretch them. At the moment of formation, the radius of a magnetic bubble is maximum. As the bias field is increased, the radius of the bubble decreases; at a certain critical field strength Hc, the compressive forces begin to exceed the tensile forces, and the bubble collapses (Figure 2). The actual dimensions of magnetic bubbles depend on the physical parameters of the material and the thickness of the film, in addition to the bias field. At the center of the region of stability, the diameter of a magnetic bubble is roughly equal to the thickness of the film.
In a uniform bias field, magnetic bubbles are stationary. In a spatially nonuniform field, the bubbles propagate into a region of lower field strength. The maximum velocity of magnetic bubbles ranges from 10 to 1,000 m/sec for various materials. The velocity is limited by energy transfer from moving bubbles to, for example, the crystal lattice or spin waves and by the interaction of the bubbles with crystal defects (the velocity increases as the number of defects decreases). Magnetic bubbles may be observed visually through a microscope in polarized light by means of the Faraday effect.
Thin epitaxial rare-earth ferrimagnetic-garnet films and amorphous films made from d- or f -metal alloys are now being used in digital-computer memories for the recording, storage, and reading of information in the binary number system. The digits 0 and 1 of the binary code are represented by the presence or absence, respectively, of magnetic bubbles at a given location in the film. Magnetic films exist in which the diameter of a magnetic bubble is less than 0.5 μm. In principle, such films make it possible to record information with a density of greater than 107bits/cm2.
A read/write system that has actually been implemented is based on the propagation of magnetic bubbles in magnetic films with the aid of thin (0.3–1 μ.m) T-and l-shaped, Y-and l-shaped, and chevron-shaped elements made of a soft-magnetic material, such as a Permalloy, and deposited directly on the film containing the bubbles. The elements are magnetized by a rotating in-plane magnetic field Hrot (Figure 3) in such a way that the field gradient that causes the bubbles to propagate is produced in the required direction. Devices for controlling the propagation of magnetic bubbles with the aid of Permalloy elements operate at rotating-field frequencies of approximately 1 megahertz, which corresponds to a recording or reading rate of about 1 megabit/sec. Information is recorded by means of magnetic-bubble generators, which operate on the basis of local magnetic reversal in the material by the pulsed magnetic field of a current passed through a pin-shaped conductor. A possible arrangement for the generation and propagation of magnetic bubbles is shown in Figure 4.
Detectors that operate on the basis of magnetoresistance are used to read information in bubble memories. A magnetoresistive bubble detector is a specially shaped deposited patch of conducting material, such as a Permalloy, whose resistance depends on the magnetic field to which it is exposed. When a magnetic bubble propagates past such a detector, the magnetic field of the bubble changes the resistance of the detector. The change in resistance may be detected on the basis of a change in the voltage drop across the detector.
Bubble memories are highly reliable and are characterized by a low cost per bit. The use of magnetic bubbles is a possible route for the further development of electronic computers.
REFERENCESBobeck, A. H. “Properties and Device Applications of Magnetic Domains in Orthoferrites.” The Bell System Technical Journal, 1967, vol. 46, no. 8.
“Tsilindricheskie magnitnye domeny v magnitoodnoosnykh materialakh: Fizicheskie svoistva i osnovy tekhnicheskikh primenenii.” Mikroelektronika, 1972, vol. 1, issues 1 and 2.
O’Dell, T. H. Magnetic Bubbles. New York, 1974.
Bobeck, A. H., and E. Della Torre. Magnetic Bubbles. Amsterdam, 1975.
Bobeck, A. H., P. I. Bonyhard, and J. E. Geusic. “Magnetic Bubbles—an Emerging New Memory Technology.” Proceedings of the Institute of Electrical and Electronics Engineers, 1975, vol. 63, no. 8.
Boiarchenkov, M. A. Magnitnye elementy avtomatiki i vychislitel’noi tekhniki. Moscow, 1976.
Lisovskii, F. V. Fizika tsilindricheskikh magnitnykh domenov. Moscow, 1976.
F. V. LISOVSKII