Volume 30, Issue 4, 01 April 1959
Index of content:
30(1959); http://dx.doi.org/10.1063/1.2185960View Description Hide Description
If an anisotropic local atomic configuration is produced in some way or other, the magnetization is stabilized in a direction related to this anisotropy, causing an additional uniaxial magnetic anisotropy. This kind of an anisotropy in the atomic configuration is called a directional ordering. This may be an anisotropic distribution of constituent atoms in substitutional ferromagnetic alloys, of interstitial atoms in a body centered cubic lattice, or of different atoms or ions on the sites of mixed ferrites, etc. A brief explanation of the phenomena is given in each case. Also because of the coupling between the atomic arrangement and the direction of the magnetization, a relaxation phenomenon may be observed in the magnetization process which depends on the diffusion of atoms. The magnetic aftereffect arising from a small amount of carbon in α‐iron is explained in detail.
30(1959); http://dx.doi.org/10.1063/1.2185984View Description Hide Description
30(1959); http://dx.doi.org/10.1063/1.2185892View Description Hide Description
In a spinel, Sn4+ is known to occupy octahedral sites. Therefore manganese‐tin spinel, , would be antiferromagnetic because of the strong interaction between octahedral and tetrahedral ions characteristic of magnetic spinels. The magnetic exchange interaction in a (Mn2+)‐O2−‐[Mn2+] linkage is of particular interest because it is known to be much weaker than is the case for Fe3+ with which Mn2+ is isoelectronic; magnetic measurements on slightly oxidized Mn‐Sn spinel yield a Néel temperature of 58°K.
When the initial equality of the sublattice magnetizations of Mn2+ ions in octahedral and tetrahedral sites is upset by substitution of nonmagnetic, divalent ions, Mg2+ and Zn2+, for Mn2+, spontaneous magnetization appears. The Néel temperature is decreased by these substitutions largely as a consequence of the smaller number of interactions between the reduced number of Mn2+ ions in the two different sites. For equal substitutions for Mn2+ in , the moment changes more rapidly in the case of Mg2+ than Zn2+. The moment depends upon the size of the substituent ion because, in this case, the ions involved are spherical and of the same valence. A smaller size of these substituent ions leads to a greater predominance of occupation of the octahedral sites because substitution there increases the Madelung potential both by reduction of lattice constant and increase of oxygen parameter. The sequence, Mg2+, Zn2+, Mn2+, of increasing ionic sizes corresponds to that of increasing lattice constants, 8.60 8.67, 8.88 A, of the respective tin spinels. Substitution of Ge4+ for Sn4+ takes place principally in tetrahedral sites because of the very small size of Ge4+.
Similarly, spontaneous magnetization appears with oxidation of . The resultant oxidized manganese ions (presumably Mn3+) are of the same average valence, though of smaller size, than the octahedral ions and, therefore, also appear in octahedral sites. Because Sn4+ is rejected to maintain valence balance, the octahedral sublattice magnetization is increased by oxidation.
30(1959); http://dx.doi.org/10.1063/1.2185896View Description Hide Description
30(1959); http://dx.doi.org/10.1063/1.2185902View Description Hide Description
30(1959); http://dx.doi.org/10.1063/1.2185913View Description Hide Description
The concept of octahedral site preference energy in spinels has been extended to include Madelung and short‐ranged as well as crystal field terms. A set of site preference energies is formulated which can be used to predict the ionic distribution of spinels involving the nontransition as well as the transition ions. The agreement between predicted and experimentally determined ionic distributions is surprisingly good, and a number of heretofore puzzling distributions are explained.
30(1959); http://dx.doi.org/10.1063/1.2185924View Description Hide Description
The remagnetization time has been measured as a function of external field for a series of manganeseferrites,. The physical properties of these compounds which enter into the domain wall model of Menyuk and Goodenough and the rotational model of Gyorgy have also been measured. It is found that , the loss constant, is critical to the correlation of data with theory, and that neither theory can be unequivocally proved until more insight into the remagnetization process loss mechanism is acquired.
30(1959); http://dx.doi.org/10.1063/1.2185931View Description Hide Description
The most commonly offered interpretation of the low magnetic moments of chromium‐containing spinels has relied on the existence of the Yafet and Kittel ordering scheme. The experimental evidence for angular ordering schemes on the lattice of spinels is, however, inconclusive and contradictory. Prince, for instance, has obtained neutron diffraction data on which is consistent with, but does not uniquely determine, the Yafet‐Kittel scheme. Pickart, on the other hand, has found no evidence of the angular scheme in data on and , although he did establish that the moment of the lattice was lower than expected on a simple Néel picture. In this report, we present a new approach to this problem which appears to lead to a reasonable, adequate, and consistent treatment of the moment data.
It is usually supposed that the spin of the Cr3+ ion in the octahedral environment of the spinel site is giving a moment of three Bohr magnetons. A consideration of the crystal field theory pertinent to Cr3+ shows, however, that if the site is given a sufficient tetragonal or trigonal distortion, the doublet state, having a spin of and a moment of one Bohr magneton, becomes the ground state. It is known further that the sites can be distorted from regular octahedral symmetry by the presence of Jahn‐Teller distortions in neighboring sites. For example, Ni2+ and Cu2+ cause distortions when on sites, while Cu2+ and Mn3+ cause distortions when on the sites. The process by which the moment of an ion is reduced in this way is called “spin quenching.”
We therefore propose that the origin of the low moments in the chromites is the spin‐quenching effect imposed on the Cr3+ ions by the appropriate Jahn‐Teller distortions of their neighbors. We also emphasize that a macroscopically distorted phase is not required for this effect. It is only necessary that a distribution of distorting neighbors be present, sufficient in number to alter the symmetry about (and hence spin quench) at least a fraction of the chromium ions. With the aid of these hypotheses it has proved possible to explain the moment data for many chromites in a consistent way. Of particular interest are the recently studied systems, and .
30(1959); http://dx.doi.org/10.1063/1.2185947View Description Hide Description
According to Dunitz and Orgel, the large tetragonal distortions which occur in a number of transition metal oxides having the spinel structure are presumed to arise as a consequence of a Jahn‐Teller type distortion in the immediate environment of certain transition metal ions. All the observed large distortions can be correlated with the results of this crystal field treatment on the basis of the spatial ordering of the local distortions.
In this communication we investigate the detailed properties of the transformations from tetragonal to cubic phases which are observed at elevated temperatures. An approximate model has been constructed which explicitly takes into account the interactions between local Jahn‐Teller distortions about neighboring transition metal cations. By the use of the usual method of statistical mechanics it has proved possible to derive the thermodynamic behavior of this model, and hence to contribute to an understanding of the cooperative nature of these transformations. The principal result of importance is the demonstration that the transformations from tetragonal to cubic spinel phases are thermodynamically of the first order. That is, a latent heat, a volume discontinuity, lattice parameter discontinuities, and a lambda anomaly in the heat capacity are to be observed at the transformation temperature. The available evidence seems to support the conclusions drawn from the model.
30(1959); http://dx.doi.org/10.1063/1.2185956View Description Hide Description
For some time this laboratory has been interested in the rotational magnetic losses in ferrites at low frequencies. Apparatus has been developed and successfully operated in which these losses are measured at frequencies of the order of 0.1 cps in fields up to about 5000 gauss and over a reasonably wide temperature range. It consists of a quartz fiber torsion pendulum (the fiber perpendicular to the field) operating in a vacuum and in a reasonably constant temperature enclosure. The principle involved is that of observing the decay of free oscillations of the system, and determining the torques due to the damped oscillatory system. By a process developed in our laboratory the samples are ground into spheres to accuracies of the order of 0.1% which can be improved it desired.
In the absence of magnetic anisotropy of the sample, the method can, in principle, distinguish between Coulomb or constant torque losses (customarily assumed for rotational hysteresis loss torque) and loss torques proportional to the angular velocity (viscous). However, for all samples so far measured the anisotropy has been sufficiently large to prevent the disentangling of the Coulomb and viscous losses. (Qualitative indications of the anisotropy are obtained from the measurements and it is interesting that the losses and anisotropy seem to be proportional.) The procedure has been to assume the losses are of the constant torque type and determine them by making the best engineering fit with the decay curves.
Some Crystallographic and Magnetic Properties of Square‐Loop Materials in Ferrite Systems Containing Copper30(1959); http://dx.doi.org/10.1063/1.2185957View Description Hide Description
Rectangular hysteresis loops have been found in ferrite systems containing copper during an investigation based on the proposal by Baltzer that a zero or near‐zero value of the effective magnetocrystalline domain anisotropy is a necessary condition for loop squareness.
Data on hysteresis loop squareness of polycrystalline bodies as a function of composition, firing conditions and magnetostrictive effects are presented for the system: copper ferrite‐magnesium ferrite. This system is characterized in general by large grains and long switching times. Abrupt flux changes occur in the hysteresis loop at low temperatures. Simultaneously, a decrease in the coercive force with decreasing temperature and squareness values approaching unity are observed. No crystallographic transitions were detected at low temperatures.
Magnetostrictive measurements do not unequivocally show whether anisotropy goes through zero at compositional region of maximum squareness. The saturation magnetostriction and probably the go through a minimum in this region.
30(1959); http://dx.doi.org/10.1063/1.2185958View Description Hide Description
The magnetic anisotropy of a single crystal of nickel‐iron ferrite with composition is given between 450 and 4.2°K. At high temperatures the anistropy energy has cubic symmetry, and the absolute value of the first‐order anisotropy constant is a maximum at ca 200°K. Below 200°K, decreases until 10°K, at which temperature there is an abrupt transition in the anisotropy characteristics. Below 10°K the anisotropyenergy contains a uniaxial term of the form where and are the direction cosines of the magnetization at the measuring temperature and at the annealing temperature (10°K), respectively.
This low‐temperature behavior has been explained on the basis of existent theory of magnetic annealing. The model used to explain the uniaxial anisotropy term given above is shown to be consistent with the noncubic anisotropyenergy obtained upon cooling the sample through 10°K in the absence of an external field. The model is also shown to lead directly to a relaxation effect which contributes a term of the form to the anisotropyenergy above 10°K and explains the observed maximum in .
30(1959); http://dx.doi.org/10.1063/1.2185959View Description Hide Description
Major advances in computertechnology have been made by the use of magnetic devices. The speed of operation, storage capacity, and reliability of computers has been significantly increased by the development of the memory core. The present paper will describe some of the newer magnetic elements which may replace ferrite toroids in memory matrices. Ferrite materials are being used as sheets and multiaperture devices. The sheets show great promise in achieving a cost reduction. The multiaperture devices in memory applications offer the advantages of increased speeds, a wider operating temperature range, and nondestructive readout. The ferromagnetic materials are used in the form of thin films and wrapped wire. The films are capable of increased switching speeds at nominal current drives. The use of magnetic wire as a storage element offers new fabrication technology, a wider operating temperature range, and new functional structures. A comparison is made to highlight the device characteristics of the various structures.
30(1959); http://dx.doi.org/10.1063/1.2185961View Description Hide Description
A twistor shift register has been built and operated successfully. The design utilizes interaction effects which exist between magnetized regions on a magnetic wire. Only a single magnetic wire is required for a complete register. The information is stored as magnetically polarized zones which can be moved along the wire by means of a five phase pulse source.
No diodes are required. Therefore, drive powers can be greatly decreased since the only threshold consideration is the magnetic material itself. Bi‐directional operation is easily secured. The upper frequency limit has not been established; however, a several hundred kilocycle bit rate should be possible. Physically, the register could be made of no more than magnetic and copper wire. This should make fabrication considerably cheaper than conventional shift registers.
30(1959); http://dx.doi.org/10.1063/1.2185962View Description Hide Description
A magnetic rod suitable for performing the logical switching and storage functions required in a digital computer has been developed. The magnetic element consists of a silver‐coated glass rod upon which is electroplated a Fe‐Ni alloy several thousand angstroms thick.
A single element for a coincident‐current memory requiring two inputs, an inhibit winding, and a sense winding would consist of four separate single layer concentric solenoids wound over the magnetic rod. Memory matrices, each consisting of many solenoids, can be stacked and simultaneously threaded with the rod. The switching speed of the element operating in a coincident‐current mode is approximately 70 mμsec. The output voltage generated across a ten‐turn sense winding is 200 to 500 mv depending upon the alloy thickness.
The rod is also suitable as a multi‐input logical switch. Separate inhibiting windings wound over the rod perform the NOR function of its input literals. A total of thirty separate inhibiting windings have been demonstrated in the laboratory. Satisfactory operation in the 2‐ to 5‐Mc range using transistors in conjunction with the rod has been shown to be practical.
Continuous plating and testing in conjunction with automatic machine winding techniques make the rod appear economically attractive in digital computer applications.
30(1959); http://dx.doi.org/10.1063/1.2185963View Description Hide Description
An examination of the switching properties of square loop ferrites is presented. Switching times have been studied over the range 5 μsec to 10 mμsec. The switching parameters, threshold field and switching constant, have been studied as a function of temperature and of ceramic processing. The plot of inverse of switching time versus applied fields displays three nearly linear portions for which the slopes vary by a factor of from two to ten. The inverse slope known as the switching constant of the material has therefore three values; this is interpreted as indicating three mechanisms to be responsible for the process of flux reversal, each mechanism being dominant over a certain region of the switching curve. These mechanisms are proposed as being wall motion, incoherent rotation, and coherent rotation. A model allowing a coherent rotation process is proposed. Data are presented for several ferrites which have widely varying properties.
30(1959); http://dx.doi.org/10.1063/1.2185964View Description Hide Description
Present‐day computer speeds are limited by the magnetic components utilized in their construction. Such components have been fabricated from square loop ferrites of the system. Since such materials limit the speed at which the computers can operate, a new ferrite system, namely, the system, was studied and has been proven feasible for many new high‐speed applications. Some of the compositions of this new system exhibiting square hysteresis loops have switching constants as low as 0.200 oe μsec. Such materials in comparison with those of the system have a much lower coercive force, require lower driving currents, and have a flux reversal or switching time which is five times as fast. These materials have been used in fast switching multipath elements and matrix switch cores. The operation of such elements is described.