Volume 30, Issue 4, 01 April 1959
Index of content:
30(1959); http://dx.doi.org/10.1063/1.2185841View Description Hide Description
The saturation magnetization of nickel particles in the range 10–100 A has been determined by extrapolating from magnetizations measured at 4.2°K in fields up to 104 oersteds. Measurements have been made prior to, and after, the admission of hydrogen to the nickel at room temperature. The nickel is supported on silica in the manner familiar in heterogeneous catalysis. The number of electrons paired per hydrogen atom adsorbed has been calculated from the fractional change in saturation magnetization assuming the magnetic moment per atom of nickel to be 0.606 Bohr magneton. The ratio of electrons paired to hydrogen atoms adsorbed has been shown to be less than one. On sintered samples in which the average nickel particle diameter is 40–80 A the ratio is about 0.7. On smaller nickel particles the ratio may be less. This ratio does not appear to be sensitive to surface coverage. The fact that the ratio of electrons paired to hydrogen atoms adsorbed is less than one is considered evidence for a localization of the electronic interaction between adsorbent and adsorbate.
30(1959); http://dx.doi.org/10.1063/1.2185842View Description Hide Description
alloys were studied for magnetic properties, crystal structure, and metallographic appearance. A superlattice having the stoichiometric composition was found to be markedly magnetic. The structure of this alloy is pseudo‐f.c.c. with the C atom in the central position, the Mn atoms in the 000 and 0 sites, and the Co atoms in the 0 and 0 sites. A magnetic structure was deduced from a consideration of the effect of atomic separation on magnetic vector alignment and an indication that electrons transfer from the central C atom to its nearest neighbors, the face atoms.
30(1959); http://dx.doi.org/10.1063/1.2185843View Description Hide Description
The effect of temperature on the magnetic properties of six commercially available nickel‐iron alloys is reported for the temperature range −60°C to +250°C. The trade names of the alloys are: Hipernik, Deltamax, Hipernik V, Supermalloy, 4‐79 Mo‐Permalloy, and Hymu 80.
The temperature dependence of both ac and dc magnetic properties was determined experimentally over the temperature range. Dc properties were measured by the standard ballistictesting method. At each test temperature sufficient data was taken to plot a normal magnetization curve. Remanent induction and coercive force were also determined at each temperature, these properties being measured relative to magnetizing forces in excess of 100 oersteds. Ac properties were measured by means of a modified Hay bridge, the tests being conducted at frequencies of 60, 400, and 1000 cps. At each test temperature, and for each frequency, total core loss was measured at various inductions in the range characteristic of the alloy being tested. The data presented are the average of the results of tests on six samples of each alloy.
30(1959); http://dx.doi.org/10.1063/1.2185844View Description Hide Description
An experimental study was made of the effects of temperature on the dc and 60 cps magnetic properties of the following alloys: Orthonol, 4‐79 Mo Permalloy, AEM 4750, L‐ and Z‐Silectron, Transformer A, Audio Transformer A, 11.7 Alfenol, 15.5 Alfenol, Thermenol, 7‐70 Perminvar, and Supermendur.
The results indicate that, in general, an increase in temperature decreases the coercive force and the maximum and residual inductions. Maximum and initial permeabilities increase with increasing temperature until the Curie temperature is approached, then they decrease. The initial permeability generally reaches a maximum at a lower temperature than does the maximum permeability.
30(1959); http://dx.doi.org/10.1063/1.2185845View Description Hide Description
Polycrystalline silicon‐iron strip with (110)  texture was heat‐treated in a magnetic field. Both ac and dc properties were investigated before and after magnetic annealing with the field parallel to the 〈100〉, 〈110〉, and 〈111〉 directions. Distinct improvement in properties was obtained, particularly in the 〈110〉 and 〈111〉 directions. This improvement is the result of the realignment of domain boundaries to 〈100〉 directions that more nearly coincide with the direction of the applied field. The anisotropy energy that arises from the magnetic annealing is about 900 ergs/cm3, which is but a small fraction of the magnetocrystalline anisotropy energy of this material.
30(1959); http://dx.doi.org/10.1063/1.2185846View Description Hide Description
Single crystal samples of beta‐brass containing 0.1% iron were annealed at 300°C and 400°C in order to develop the iron precipitate particles. At intervals during the anneals, magnetic data were obtained and analyzed in terms of the size, shape, orientation, and general magnetic behavior of the precipitate particles.
During the earliest annealing stages, superparamagnetic behavior was observed, and particle diameters were calculated from the Langevin relation. On continued annealing the magnetic properties indicated the presence of mixtures of superparamagnetic and single domain particles, and subsequently mixtures of single domain and multidomain particles.
Torque curves showed that the particles were single crystals, with iron‐like magnetocrystallineanisotropy, oriented with the same crystal habit as the matrix. Single domain behavior was first observed when the particle diameters were of the order of 200 A. Elongation of the particles in the 〈111〉 directions and rotational hysteresis were noted when single domain behavior appeared. The magnetocrystallineanisotropy coefficients calculated from the torque data were reasonably close to those of iron. Maximum remanence was reached before maximum coercive force due to the much larger initial susceptibility of the superparamagnetic as compared to the multidomain particles. This was confirmed by the shapes of the hysteresis loops for the various cases.
30(1959); http://dx.doi.org/10.1063/1.2185847View Description Hide Description
The dynamic magnetostrictive properties of a number of magnetic materials were determined as a function of composition and processing. The materials investigated include nickel, 2V‐Permendur, a cobalt‐nickel alloy, several Ni‐Fe and Al‐Fe alloys, and a group of ferrites represented by the formula with and . The parameters measured were: electromechanical coupling coefficient , reversible permeability , dynamic magnetostrictive constant , Young's modulus, and stress sensitivity .
The stress sensitivity of both the Al‐Fe alloys and the cobalt‐substituted nickel ferrous‐ferrites is sensitive to cooling rate. Ordered Al‐Fe samples containing approximately 12.0% aluminum were more stress sensitive by a factor of six than were disordered samples of the same compositions. Disordering was accomplished by quenching from above the ordering temperature (approximately 600°C). When 6% of the nickel in cobalt‐substituted nickelferrite was replaced by divalent iron, the value of increased by approximately 40% for quenched samples but remained relatively constant for slow‐cooled samples.
The largest values of observed in the Ni‐Fe alloys (in excess of 106 dynes/cm2) occur in the range of 40% to 52.5% nickel. Various processing techniques were employed on the Ni‐Fe alloys in this range: slow cooling, quenching,magnetic annealing, and variation in annealing temperature between 900°C and 1220°C. Such variations in processing failed to produce more than a 10% change in the value of .
30(1959); http://dx.doi.org/10.1063/1.2185848View Description Hide Description
Alnico V magnets were exposed to temperatures between 350 and 550°C for as long as 1000 hours. Remanence was determined at room temperature intermittently during the exposures. The changes in remanence appeared to be caused by metallurgical as well as magnetic processes. The two effects were separated and it was found that the Alnico V material responds to temperatures as low as 350°C.
30(1959); http://dx.doi.org/10.1063/1.2185849View Description Hide Description
30(1959); http://dx.doi.org/10.1063/1.2185850View Description Hide Description
Single domain ferromagnetic particles at elevated temperatures can behave magnetically in a manner analogous to the Langevin paramagnetism of moment‐bearing atoms. The main distinction is that the moment of the particle may be 105 times the atomic moment. The experimental and theoretical foundations of this effect are reviewed and indication is made of the applications of this phenomenon. Lastly, attention is drawn to some current problems in this area.
30(1959); http://dx.doi.org/10.1063/1.2185851View Description Hide Description
Weil and others have used the temperature variation of remanence and a formula of Néel's to determine volume‐distribution curves for a powder of single‐domain particles. The basic principle is that the time constant for spontaneous reversal of the magnetization, through thermal agitation, is effectively infinite for particle volume and zero for , where varies with absolute temperature . Néel's derivation is open to criticism, in that the gyromagnetic properties of the particles are taken into account only up to a certain point in the argument and are thereafter ignored. A new formula has been derived by adaptation of a method of Kramers to angular coordinates and to a gyroscopic equation of motion. Like Néel's theory, this gives for the mean rate of transition between orientations a formula of the form ; is the same in both theories, but in the new theory . Here , , , , in Gilbert's equation of motion. The new values of vs time constant do not differ seriously from Néel's.
30(1959); http://dx.doi.org/10.1063/1.2185852View Description Hide Description
The experimental physical and magnetic properties of elongated single‐domain permanent magnets have been previously reported and are briefly reviewed with respect to their means of preparation. A successful process is described for making ESD magnets available commercially at a limited initial rate. The material produced is described and contains elongated single‐domain particles of iron or iron‐cobalt embedded in a metallic matrix to protect them from environmental attack. Maximum magnetic energies of 2.2 megagauss‐oersteds for iron and 3.6 megagauss‐oersteds for iron‐cobalt are available.
30(1959); http://dx.doi.org/10.1063/1.2185853View Description Hide Description
Micropowders of stoichiometriccopper,nickel, and cobaltferrites were prepared by low temperature (<800°C) treatment of the coprecipitated metal oxalates. The magnetic properties of each ferrite were determined as functions of particle or crystallite size in the range from 70 to >2000 A. Particle sizes were determined from x‐ray line broadening and from electron micrographs.
The micropowders exhibited superparamagnetic, single domain, or multidomain behavior depending on the particle size. The critical size for single domain behavior, depending on composition, was between 300 and 700 A. In this size range, high coercive force and remanence were observed in spite of appreciable sintering of the particles in some cases. The remanence values and the magnitude and temperature dependence of the coercive force indicated that magnetocrystalline anisotropy determined the properties of the single domain particles. The magnetic properties of the very small particle size samples showed the large temperature dependence characteristic of superparamagnetism.
Quenched and slowly cooled copperferrite micropowders were prepared with similar particle sizes. The remanence and coercive force of the quenched samples were much lower than those of the slowly cooled samples. This indicated different critical sizes in these two types of copperferrite. This could be due to the difference in either the moments or the magnetocrystalline anisotropy.
30(1959); http://dx.doi.org/10.1063/1.2185854View Description Hide Description
The shape distribution curves of single‐domain magnetite powders calculated from magnetic data do not agree with those observed in the electron microscope. The Stoner‐Wohlfarth theory, from which the magnetic unaveraging process was derived, ignores magnetocrystalline anisotropy. This may be one of the contributory causes to the lack of agreement. To establish this, the shape curves were measured magnetically on both acicular and equant magnetite powders at various temperatures. The magnetocrystalline anisotropy of magnetite is temperature dependent, changing from about at room temperature to at −165°C.
The magnetically calculated shape curves show some change with temperature; this indicates a contribution from the magnetocrystalline anisotropy.
30(1959); http://dx.doi.org/10.1063/1.2185855View Description Hide Description
The magnetic fields around submicroscopical iron particles were tested with iron oxide colloid in an electron microscope. The colloid particles usually spread evenly by diffusion.Magnetic fields of several hundred oersteds are required to concentrate the colloid particles visibly at certain spots. We found colloid around iron particles concentrated to various degrees. The colloid was attracted less by smaller particles. The smallest particles did not attract any colloid.
The particles were measured and grouped into three following categories: (a) Particles with strong, all around colloid attraction, indicating many domains. Stem volume of the dendritic particles much bigger than 109 A3, (b) Particles with colloid attractions at spots, indicating one or a few domains. (c) Particles without colloid attraction, indicating no well‐developed domains. Stem volume much less than 109 A3.
30(1959); http://dx.doi.org/10.1063/1.2185856View Description Hide Description
The theoretical interpretation of the size‐dependence of the coercive force of one ferromagneticpowders and thin films is still an unsolved problem. Kittel and Néel's treatments are discussed, and some recent work of the author along similar lines is presented. An alternative heuristic approach is suggested, leading to an expression and is compared to some of the recent experimental data.
30(1959); http://dx.doi.org/10.1063/1.2185857View Description Hide Description
A novel, light‐weight, moldable permanent‐magnet material is described. Elongated‐single‐domain (ESD) particles of iron‐cobalt are coated with a thermosetting plastic which serves as matrix. The particles are aligned while the plastic is in a liquid phase. The matrix is then solidified, and the agglomerate ground to a −30 mesh freeflowing powder. Each of the particles of this powder consists of submicroscopic elongated aligned particles. The freeflowing powder can then be directionally pressed in conventional presses to produce magnets of complex shape held to precision dimensions. Energies of gauss‐oersteds have been achieved in magnets whose density is 4 g/cm3 and whose temperature coefficient is 0.008%/°C. The material's properties make it well suited to applications where a light‐weight, high energy, precision‐dimensioned easily fabricatedmagnet, capable of withstanding adverse environmental conditions is required. An excellent application of this material is the moving‐magnet‐instrument rotor.
30(1959); http://dx.doi.org/10.1063/1.2185858View Description Hide Description
The magnetic and physical stability of permanent magnets compacted from elongated single domain iron particles in metallic and organic matrices under various environmental conditions are reported. Changes resulting from two years exposure to 95% relative cycling humidity and 100°C were measured. The influence of powder compacting temperatures, pressures, powder size, and additives are reported. Compacts with lead matrices exposed to 100°C and 95% relative humidity showed gradual magnetic and dimensional changes from insignificant variations after one month to maximums of 0.5% over a two‐year period. These results compared favorably with results obtained for Alnico V magnets subjected to identical conditions. Temperatures of 200° and 250°C produced magnetic changes in the order of 1% after one month with physical degradation occurring after two months at 250°C. Magnetic changes of 1% and weight and volume changes in the order of 2% occurred in organic matrix magnets after one month at 100°C. The magnetic changes resulted from a slow oxidation of the iron particles which caused a small decrease of intrinsic saturation induction and an increase in coercive force.
30(1959); http://dx.doi.org/10.1063/1.2185859View Description Hide Description
Microwave and low frequency oscillations have been observed in single crystalyttriumirongarnet disks placed in a microwave cavity with an applied dc field normal to the disk. These oscillations occur when the microwave power incident on the cavity exceeds a certain critical value in the milliwatt range. The oscillations occur as sidebands above and below the incident frequency and can also be observed as a modulation of the microwave output of the cavity.
The above phenomenon is associated with the ferromagnetic resonance instability due to the fact that the resonant frequency of a disk depends on and thus on the rf magnetic field used to drive it. The combination of the disk and the resonant cavity with high power, provides the conditions necessary for relaxation oscillations to be set up.
30(1959); http://dx.doi.org/10.1063/1.2185860View Description Hide Description
A consideration of the fundamental TE modes in a wave guide containing a low‐loss ferrite slab leads to a mode with a group velocity corresponding to power transfer in a single direction only. A completely reactive isolator, however, is prohibited by general energy conservation principles. We have analyzed a rectangular wave guide with a ferrite slab against one wall. We have found that there are higher order propagating modes wherever the unidirectional TE mode is cut off. Furthermore, it can be experimentally demonstrated that these modes can be excited from a simple boundary with no variations along the magnetic field. A possible mechanism for this excitation will be described. It is suggested that these modes may carry the power when the unidirectional TE mode is cut off. An experimental situation in which the proposed reactive isolator is tried will be described. Where the reactive isolator should be cut off and perfectly reflecting, it is highly absorptive. This can be explained by the lossiness of the higher order gyromagnetic modes.