Index of content:
Volume 34, Issue 4, 01 April 1963
34(1963); http://dx.doi.org/10.1063/1.1729391View Description Hide Description
This paper describes a magneto‐optical method for obtaining visual display and electronic readout of magnetic information recorded on a nonspecular oxide surface. A recorded oxide surface is placed in contact with a specular ferromagneticsurface, an external magnetic field is applied normal to the two surfaces in contact; and the recorded information is read out by means of the Kerr effect through the back of the specular surface. The method is a useful tool for studying defects in the writing process independently of the reading process in investigations of magnetic recording on oxide surfaces.
It was observed that when the external field was applied, the magnetic pattern was either sharpened or caused to spread, depending on the polarity of the field.
34(1963); http://dx.doi.org/10.1063/1.1729392View Description Hide Description
The high‐field switching mechanism of polycrystallineferrite material is investigated. Direct experimental evidence for rotational switching is obtained by observing the charges in the longitudinal and transverse components of the magnetization, for a long (L=1.33 cm), thin‐walled (o.d.=0.05 cm, i.d.=0.028 cm) ferrite tube, during the reversal process. For a 10‐Oe circumferential pulse field in the presence of a 4.3‐Oe longitudinal bias field, it is found that at least 30% of the total magnetization reverses by a rotational process.
34(1963); http://dx.doi.org/10.1063/1.1729393View Description Hide Description
Results of an experimental investigation to obtain square‐loop materials for use in multiaperture logic cores at elevated temperatures are described. Compositions in the lithium‐nickel‐zinc ferrite system were found to exhibit well‐developed hysteresis loops and squareness ratios of 0.96 at temperatures in excess of 300°C. For certain processing conditions, the squareness ratio of lithiumferrite increases when substitutions of Ni2+ are made for Li1+ and Fe3+ in the system Li0.5−x/2Ni x Fe2.5−x/2O4. A maximum in the 25°C squareness ratio of 0.90 is observed for x=0.04. Further improvement of technical hysteresis loop properties are obtained by introducing Zn2+ according to the formula Li0.48−y/2Zn y Ni0.04 Fe2.48−y/2O4. A maximum in the 25°C squareness ratio of 0.957 is observed for y=0.05. The substitution of Zn2+ decreases the coercive force, and increases the switchable magnetic induction.
34(1963); http://dx.doi.org/10.1063/1.1729394View Description Hide Description
Certain samples of polycrystallineyttriumirongarnet and lutetiumirongarnet have previously been found to acquire a square 60‐cps hysteresis loop when cooled to 78°K in a magnetic field. It is found that these are not normal square‐loop materials, for at sufficiently slow excitation (ca. 0.5 cps for yttriumirongarnet, one cycle per hour for lutetiumirongarnet) they revert to normal, but strongly frequency‐dependent loops. At 78°K, for proper amplitude step excitation, the response of these cores is a delayed pulse. For lutetiumirongarnet this delay can be several hundred μsec in duration. The behavior of these materials can be qualitatively explained by domain wall instability produced by a diffusion after‐effect mechanism.
34(1963); http://dx.doi.org/10.1063/1.1729395View Description Hide Description
Electrically alterable switching characteristics have been observed in ferrite memory cores. Abnormal switching characteristics are caused by previous time‐limited switching and are not removed by subsequent ``hard'' saturation. The peaking time, peak amplitude, and magnitude of the flux change are all modified. It is proposed that the changes in switching behavior are due to formation of 360° Bloch walls during the combination of time‐limited switching and remagnetization to saturation.
34(1963); http://dx.doi.org/10.1063/1.1729396View Description Hide Description
The response of 90° and 180° (001)‐plane domain walls, acted upon by a damping pressure due to diffusion after‐effect, is investigated for steps and ramps of applied field. The resultant motion, using the Néel energy‐level scheme for diffusants, is shown to be dynamically stable in the case of the 90° wall, in the sense that the incremental damping coefficient is never negative. Dynamic instability is shown to be possible for the 180° wall, and appears in the step response as a delay between the time of application of the field and the time of an observable response. The ramp response is unstable in a finite sample only when a minimum ramp slope has been exceeded. The ramp response is used to discuss the motion of the 180° wall under triangular‐wave drive, and results similar to certain experimental observations are obtained. More publication is planned at a future date.
34(1963); http://dx.doi.org/10.1063/1.1729397View Description Hide Description
Flux‐switching phenomena characteristic to multipath cores result from the dependence of switching in one leg (a physical branch between the core junctions) on switching in the others. Reversible Δφ in a leg driven far into saturation may cause appreciable irreversible Δφ in other legs. This effect is analyzed by tracing the variations of φ and F of each leg of a three‐leg core under two conditions: a positive drive on an inside leg, and a negative drive on an outside leg. Due to variations in air‐flux linkage, a tight winding on the driven leg minimizes this effect. Upon termination of the drive pulse, the legs are shown to be left in a ``stressed'' state; i.e., a residual H field, sustained by magnetic poles near the leg junctions, remains in each leg.
34(1963); http://dx.doi.org/10.1063/1.1729398View Description Hide Description
A study has been made which determines the ultimate one‐to‐zero ratio which can be realized in the permeability sensing technique of nondestructive readout of binary information from ferrite memory cores. The study is limited to the calculation and measurement of the ratios of incremental permeabilities of various magnetic states for ferrites in which the permeabilities can be attributed to a rotational process. Calculations are based upon a polycrystalline, randomly oriented grain ferrite with a negative anisotropy constant. Each individual magnetic moment is considered to be constrained to an easy axis only by the anisotropy field. Measurements of possible ratios between the permeabilities for the following three conditions are shown to agree within experimental error to those predicted by the calculations.
I. Completely demagnetized sample.
II. Measuring field parallel to the direction of the remanence existing after the application of a saturating field.
III. Measuring field orthogonal to the direction of the remanence existing after the application of a saturating field.
The experimental verifications were arrived at by measurements performed on a long, thin, extruded ferrite tube. It is concluded that the best one‐to‐zero ratio can be obtained with materials having as high an initial permeability as possible and which still depend upon the rotational mode of operation. The highest‐one to‐zero ratio then possible is equal to 3.26. Furthermore, this ratio is obtained by using two orthogonal remanence states with the measuring field applied parallel to the remanence which is to represent the binary zero.
34(1963); http://dx.doi.org/10.1063/1.1729399View Description Hide Description
A plot of the inverse switching time vs the applied field yields three regions of different slope for square‐loop ferrites. The low‐field region has the lowest slope and is commonly attributed to domain wall motion. As the field is increased, a break in the curve occurs and the slope increases. This intermediate‐field portion of the curve has been explained by a nonuniform rotational model, and its projection to the field axis is termed the threshold field H 0 for nonuniform rotation. In coincident‐current memory operation, twice the coercive force is the maximum drive which can be employed for switching from one remanent state to the other. Therefore, it is desirable to minimize the extent of the domain wall region so that the coercive force will approximate H 0 as closely as possible. Then 2Hc will correspond to a higher value of the inverse switching time on the nonuniform rotational portion of the curve, and very fast switching can be attained. Prevalent theories describing domain wall motion in the vicinity of nonmagnetic inclusions indicate that domain wall motion can be impeded by the addition of nonmagnetic second phases of proper size and distribution in a magnetic matrix, resulting in an increase in Hc without affecting H 0. A similar approach was used by Nesbitt and Gyorgy on gold‐doped Permalloy.
To some extent, the desired effect can be obtained by underfiring the ferrite. However, such firing treatments are difficult to control. A method is described whereby the coercive force is increased while employing a noncritical firing treatment. This has been achieved by the inclusion of palladium as a second phase in a magnesium—manganese ferrite. Particles of the order of 1500 Å were dispersed in the ferrite, and toroidal samples were fired at temperatures between 1250° and 1400°C for firing times up to 12½ h. At 1400°C, within the range of 7½ to 12½ h, variations in Hc were very small. Moreover, in samples containing 10% palladium,Hc increased from 0.9 to 2.7 Oe without changing the H c of 3.5 Oe. In coincident current operation, the 0, 10, and 20% samples switched in 0.70 0.16, and 0.10 μsec, respectively.
34(1963); http://dx.doi.org/10.1063/1.1729400View Description Hide Description
Switching signals by pulsed magnetic fields up to 40 Oe were observed on a single‐crystal YIG window frame with legs along <100> directions. A clear magnetic oscillation (∼175 Mc) was found for fields less than about 3 Oe. The amplitude and the damping of the oscillation are enlarged by increasing the drive further, and finally the total remagnetization signal coincides with the initial pulse of the oscillation. Two critical fieldsHc 2 (7.7 Oe) and Hc 3 (20 Oe) were found which correspond to the theoretical minimum critical field for rotational switching for this geometry and to the observed onset of the single‐pulse remagnetization, respectively. Switching coefficients of the order of 0.2 μsec Oe and 0.1 μsec Oe are obtained in the regions below and above Hc 2, respectively. It is suggested that the nucleation and/or growth of small reversed domains in the early stages of flux reversal may create magnetostatic fields which play an important role in determining the characteristics of these two regions and that it might be better called a noncoherent oscillatory rotational mode.
34(1963); http://dx.doi.org/10.1063/1.1729401View Description Hide Description
A single crystal of nickelferrite was cut to form a toroidal core whose major face was a (111) plane and given a thermal and magnetic anneal to induce an effective easy axis with circumferential symmetry. This paper gives the results of an experimental study of the switching properties of the specimen. The crystal could be switched in a ``single wall mode'' which resulted in a rectangular sense signal when the drive was low, and for increased drive it exhibited the double peak, sense signal characteristic of multiple wall switching. Unlike polycrystalline materials, the sense signal did not show a smooth envelope, but was the superposition of rather reproducible gross features and a random fine structure. No two switching waveforms were alike in detail. Data are presented on the effect of drive and reset upon the sense waveform, the switching coefficient Sw , the differential sense signal observed on opposite sides of the core, and the irregular envelope of the elastic flux. The wall velocity was found to be linear with drive for the single wall mode. The data are interpreted in terms of a wall motion model (stressing the statistical nature of the growth process), and its relation to polycrystalline materials is discussed.
34(1963); http://dx.doi.org/10.1063/1.1729402View Description Hide Description
The magnetization of the circularly anisotropic magnetic wire developed for cylindrical film memories can be disturbed by a combination of two fields parallel to the hard and easy anisotropy axes, respectively. The measured threshold levels of the fields needed to disturb the wire are smaller than predicted by the rotational switching model. Under the specific conditions encountered in the memory, the reduction in the threshold levels is considerable and must be taken into account in the design of the memory. The threshold levels are very dependent on the way that the disturbing fields are applied. A pulsed field applied many times in a direction normal to the magnetization is considerably more effective in reducing the threshold levels than is a dc field of the same amplitude. The threshold relationships for different combinations of pulsed and dc fields in both easy and hard anisotropy directions are presented. The effect of reduced threshold levels on the design of a memory is discussed. They can result in inter‐bit interaction. The amplitude of the readout signal may also change following particular writing sequences.