Two improved methods of achieving direct overwrite without a pulse magnetic field to write or erase domains are described. Both were demonstrated in special single-layer magnetooptical media with a compensation point of 125-30 degrees C and a Curie temperature of about 310 degrees C. One of these methods uses two laser beams at different power levels. The first beam is set at a low power level which is adequate for erasure but not for writing. This is possible because the laser energy required for erasure of written domains does not overlap with that for writing. The second beam is then fired at the higher pulse power required for writing to put in the new data. The second method uses only a single laser beam in which the pulse energy of the laser is modulated between a low level for erasure of previously written domains and a higher level for writing of new domains, directly overwriting the old data. Both methods have been experimentally demonstrated for variable-length domains so that the use of pulse-width modulation codes is possible. Scanning speeds as high as 15 m/s have been demonstrated. >

Exchange anisotropy was recently proposed as the origin of uniaxial perpendicular anisotropy in some RE-TM thin films. This magnetic anisotropy arises from the exchange interaction between the ferrimagnetic matrix and acicular shaped single domain ferromagnetic regions. Rotational hysteresis measurements on GdCo films show a non-vanishing value of the rotational magnetic hysteresis for magnetic fi...

The effect of temperature on the exchange coupling at the interface of three different bilayered materials was studied with the objective of developing temperature-stable single-domain materials for magnetoresistive readback heads. Exchange field (HE) was measured from room temperature to 245 °C for NiFe films coupled to FeMn, αFe2O3, and TbCo. In the permalloy-FeMn system, the exchange field decreases linearly, and reaches zero at about 150 °C, which is close to the Neel temperature of the antiferromagnet. These results agree well with previous work [C. Tsang and Kenneth Lee, J. Appl. Phys. 53, 2605 (1982)]. HE also decreases linearly in the αFe2O3 system, from 6.8 Oe at room temperature to 1.8 Oe at 245 °C. While the αFe2O3 system offers greater temperature stability, the exchange field produced is not very large, and the coercivity is somewhat high. Changes in HE with variations in the thickness of the αFe2O3 layer were also noted. Results for permalloy coupled to ferrimagnetic amorphous TbCo indicate a finite exchange field to at least 250 °C, with samples deposited at 90-V substrate bias having exchange fields of 12 Oe at 250 °C. Room-temperature coercivities of some 400-Å NiFe films coupled to TbCo have been measured to be as low as 1 Oe. These results suggest that films that have a low coercivity and an exchange field large enough to bias a magnetoresistive head can be produced with good temperature stability up to 250 °C.

Commercially available, reductively stabilized CrO2particles have a saturation moment of about 80 emu/g or about 80% of the theoretical maximum (100 emu/g) when extrapolated to infinite field. A reduction process used to stabilize the moment accounts for about 10 emu/g or 10% of the difference from the theoretical. The remaining 10 emu/g or 10% difference from the theoretical moment appears to be ...

Erasable magnetooptic (MO) recording is viewed as a possible successor to electromagnetic recording that is used today in high-performance disk drives for mainframe computers, hard disks, and floppy disks for personal computers and magnetic tape drives. A large number of rare earth-transition metal alloys have been investigated for use as the magnetic recording material. The properties of the films are very sensitive to the film deposition processing parameters. Accelerated life tests show that there are magnetooptic materials that do have all of the necessary properties and stability for use in computer digital storage.

The stability of reverse domains in thin films of magnetic materials having high coerciv-ity is investigated. It is shown that domains below a critical diameter are unstable and will collapse. To minimize this minimum domain diameter it is necessary to minimize domain wall energy while maximizing the product of magnetization times coercivity, M.Hc. Since M.Hc tends to be nearly a constant near the magnetic compensation temperature, it is concluded that domain wall energy a and the product M-Hcare critical parameters for assessing the use-fulness of a material for high density thermomagnetic recording applications.

A nonimpact magnetic printer is described that features quiet high-speed operation, uses plain paper and single component dry toner, and produces good copy at high humidity. The printer operates at 6000 lines/minute at a density of six lines per inch (lpi). A stationary printing head, 14-in long, records rows of dots at 120 dots per inch (dpi) on a flexible magnetic belt. The recorded dots are selected by a coded digital input to form alphanumeric characters. The system can also function as a plotter. The 1680 printing locations on the magnetic recording head are selected by a coincident current scheme that utilizes novel integrated drive circuitry. The magnetic image is developed with a single component dry magnetic toner and the toner image is transferred to paper and thermally fused thereto.

Thermomagnetic recording is the production of a remanent magnetization in a material by cooling it from a critical temperature in a small magnetic field. The most prominent applications for thermomagnetic recording presently under consideration are high density information storage and tape copying; other applications, e.g., display and magnetic printing, can also be envisioned. In this review of the field, we discuss the proposed applications of thermomagnetic recording, the physical principles of the various magnetic phenomena that can be used for this purpose, and the degree to which specific materials meet the requirements for thermomagnetic recording.

The technique of magnetic tagging using materials with different Curie temperatures was initially applied to the identification of oil spills. Continued development has demonstrated the feasibility of magnetic tagging of a very wide range of solids and liquids. This versatile technique offers the benefits of economy, sensitivity, stability, large vocabulary, easy recovery, and no environmental hazards. In addition, the detection system is simple, inexpensive and portable.

Magnetic printing is quite similar to electrostatic printing. The basic steps consist of recording a magnetic latent image, development of the recording with a magnetic ink and finally transferring the ink to a sheet of paper. Since papers on magnetic printing were first published in 1951, each step of this process has become highly developed. Impactless magnetic printers have been displayed at several conferences during the past few years. This review of the work on magnetic printers shows that printing speeds and print quality equal to electrostatic and ink jet printers can be achieved.

Exchange anisotropy describes a magnetic interaction across the interface between two magnetic materials. A shifted hysteresis loop, sinθ torque curve, and rotational hysteresis in magnetic fields greater than 2K/M s may result from this interaction if one of the materials is antiferromagnetic. This interaction has been found to exist between ferro-antiferromagnetic materials, ferri-antiferromagnetic materials, and ferri-ferromagnetic materials. The work of various people is discussed in terms of the expected behavior in these exchange coupled systems. Some interesting results of the exchange interaction are reviewed. These include improved properties of fine particle magnets, a memory effect in a mixed ferrimagnetic spinel, a rotatable anisotropy in thin films, and an explanation for the reverse magnetization of a deposit in the earth’s crust. The interfacial conditions necessary to obtain the interaction between the two magnetic systems are discussed, and it is shown that they are met in several cases. Models are presented which yield rotational hysteresis in magnetic fields greater than 2K/M s as has been found in all of the exchange coupled systems.

“Nonmagnetic” stainless steel is a paramagnetic material at room temperature with a face-centered cubic lattice structure. It has been shown by Kondorsky and Sedov that the susceptibility has an anomaly at 40°K which is quite characteristic of an antiferromagnetic transition. This result suggested to us that cold working such a material to transform part of the material to the ferromagnetic body-centered cubic structure might yield a shifted hysteresis loop, if the bcc structure is in exchange contact with the fcc material.A cold-worked (swaged) type 347 stainless steel did develop a shifted hysteresis loop when cooled in a magnetic field from room temperature to 4.2°K, while the hysteresis loop was symmetrical when the material was cooled in a zero magnetic field.We also measured the shift of the hysteresis loop as a function of temperature and found that the shift disappeared at 40°K, which is in agreement with the susceptibility measurements of Kondorsky and Sedov.

Some igneous rocks are magnetized in a direction opposite to that expected, if they had been cooled in the earth's magnetic field. These rocks are said to have a reverse thermo-remanent magnetization (reverse TRM). Uyeda has shown that the reverse TRM of the Haruna deposit in Japan is due to an ilmenite-hematite solid solution and has synthesized a solid solution that has a reverse TRM when cooled in fields as high as 16000 oe. He has put forth a theory which contains a mechanism similar to that found in the cobalt-cobaltous oxide system. Based upon Uyeda's explanation of the reverse TRM, we postulated that the material should have a shifted hysteresis loop and that it should be shifted in the opposite direction to that found in the Co-CoO system. We found that the solid solution—0.6 FeTiO3 0.4 Fe2O3—was shifted by 350 oe in the opposite direction to Co-CoO when cooled in a field. The loop was symmetrical when cooled in zero field. Moreover,we have shown that when the material is cooled in a magnetic field through the Morin transition the loop is shifted in the same direction as Co-CoO. These results confirm the general features of Uyeda'a model, although the detailed mechanism is still being studied. We believe that Uyeda's work definitely establishes the reverse TRM of the Haruna deposit as due to a magnetic phenomenon and not due to the reversal of the earth's magnetic field. We believe that the type of magnetic investigation reported in this paper may be applied to other deposits that have a reverse TRM to establish if they are also due to a magnetic phenomenon.

The increase in yield strength of solid mercury due to dispersed iron particles is experimentally determined to be proportional to d(Λ − d)(1) and therefore a function only of dΛ, where d is the particle diameter and Λ is the spacing between centers of the particles. Since d/Λ is proportional to ƒ13, where ƒ is the volume fraction of precipitate, these results show that the yield strength is a function only of the volume fraction of precipitate. In the case of coherent particles, the stressed region through which dislocations will not pass must be included. An investigation of the temperature dependence of the yield strength shows that the increase in yield strength at any absolute temperature T is proportional to d(Tm − T)(Λ − d), where Tm is the melting point of the mercury matrix.

Exchange anisotropy is the result of a magnetic interaction between a ferromagnetic and an antiferromagnetic system. This interaction was discovered in the Co–CoO system, wherein cobalt is the ferromagnetic and cobaltous oxide is the antiferromagnetic system. The material consisted of fine particles (∼200 A) of cobalt which had a coherent cobaltous oxide film. Above the Néel temperature where the CoO is the paramagnetic state the material had the expected behavior of a ferromagnetic. Below the Néel temperature the interaction between the ferromagnetic cobalt and the antiferromagnetic cobaltous oxide resulted in a displaced hysteresis loop and a nonvanishing value of the rotational hysteresis for applied magnetic fields greater than 2K/Is. Further investigation of the temperature dependence of this system has shown that the high field (H>2K/Is) rotational hysteresis vanishes precisely at the Néel temperature of cobaltous oxide. Work on the Fe–FeO system has shown a nonvanishing value of rotational hysteresis for temperatures below the Néel temperature of FeO and for magnetic fields greater than 2K/Is for iron. The temperature dependence of this high field rotational hysteresis shows that it vanishes at precisely the Néel temperature of FeO.

A new discovery has been made in the field of magnetic materials that manifests itself in the form of a displaced hysteresis loop. In addition, although from the same basic phenomena, the magnetic material has only one stable orientation in a magnetic field. In particular, it can be turned through 180/spl deg/ and it will still return to its original orientation. The material that exhibits this property is a compact of fine particles of cobalt (100-1000 /spl Aring/) that have a cobaltous oxide coating.