Magnetic Thin Film

a polycrystalline or monocrystal-line layer of a ferromagnetic metal, alloy, or magnetic oxide (such as a ferrite) 0.01 to 10 microns (μ) thick. Such films are used as memory elements in computer technology and as indicators in physics research. The metallic films are produced by vacuum or electrolytic deposition of the metal on a substrate (as a solid layer or separate “spots”); oxide films are produced by means of chemical reactions and other methods. The thickness of magnetic thin films is comparable with the equilibrium dimensions of magnetic domains. Their small thickness prevents the appearance in them of appreciable induced currents (eddy currents) during magnetization reversal. These and other peculiarities of magnetic thin films cause their physical properties to differ from the properties of massive specimens of magnetic materials.

In a metallic magnetic thin film of the order of 0.1 μ thick the magnetization is uniform with respect to the thickness and is oriented in its plane. Such films, which are prepared in a magnetic field, have substantial magnetic anisotropy; the direction of easy magnetization is along the field and has a rectangular hysteresis loop.

The value of the coercive force H_c (the threshold field for magnetization reversal) of a film made of Permalloy (80-82 per-cent Ni; remainder Fe) 0.1 to 10 μ thick is 0.2 to 2 amperes per cm (A/cm).

An important property of the magnetic thin films used in computer technology is the speed of their magnetization reversal. Permalloy films are capable of magnetization reversal in about 10^-9 sec (faster than other magnetic materials) when pulsed by fields of the order of 10 A/cm; in this case the speed of the magnetization reversal is partly limited by the inertial properties of the elementary carriers of magnetic moment (spins).

Particular features of ferromagnetic resonance and galvano-magnetic properties are observed in magnetic thin films; upon magnetization reversal of a film, a population inversion of the magnetic nuclear levels occurs in it in 10^-9 sec, and a maser effect becomes possible.

A peculiar periodic distribution of magnetization, which partially emerges from the plane of the film (a band domain structure), is produced in metallic magnetic films of the order of 10 JLL thick. The field needed to rearrange the structure is 10-100 A/cm for Permalloy films and decreases upon heating, especially by a light beam. Films made of a Mn-Bi alloy are magnetized at right angles to the surface, and the diameter of the independently magnetized areas can be reduced to 1 JLI. The films and thicker layers of oxides of the rare earths are transparent to visible light, an important consideration when studying their magnetization processes and technical applications.

Memory and logic devices based on the direction of rotation of the magnetization of the film’s elements or of parts of the film, on the shifting of domain boundaries, and on changes in the parameters of band domain structures are produced using magnetic thin films. Information can be recorded and read out nondestructively both by electric signals fed along wires and by a light beam. In widespread memory devices of the matrix type, thin magnetic films are used that have a rectangular hysteresis loop with two stable antiparallel directions of magnetization corresponding to the recording of “0” or “1” in the binary number system (1 bit of information). The direction of magnetization established by the recording signal determines the polarity of the signal during readout and, as a result, the nature of the recorded information (“0” or “1”). In addition to single-layer and multilayer plane Permalloy magnetic thin films, cylindrical films deposited directly on a wire are also used in such devices. The density of recorded information reaches 100 bits per sq mm. Low-coercivity magnetic thin films are also used in combination with layers of rare-earth magnetic oxides, ferrite garnets, and others up to 100 μ thick in which cylindrical magnetic domains whose magnetization is normal to the surface of the layer may be created. Up to 600 domains can be arranged on 1 sq mm of such a film: this is promising for further miniaturization and for increasing the speed of response of computers. Films with a band domain structure are used for the optical recording of images, particularly for holography.