Volume 33, Issue 3, 01 March 1962
Index of content:
33(1962); http://dx.doi.org/10.1063/1.1728653View Description Hide Description
Studies of acoustic losses in single crystalgarnets and ferrites using magnetostrictively driven spherical resonators are described. Resonant modes have been observed in spheres of yttrium iron garnet (YIG) at ∼9 Mc and room temperature with Q′s of ∼107. This is approximately six times greater than any other known material at the same frequency and temperature. These high Q′s have now been observed in both shear and compressional modes. [R. C. LeCraw, E. G. Spencer, and E. I. Gordon, Phys. Rev. Letters 6, 620 (1961). In this reference only a certain compressional mode showed the unusually high Q.] It should be noted that the above Q in YIG may be only a lower bound. Contact losses have been eliminated by levitation and by using higher order modes. However, due to complicating factors such as the relatively large and variable dislocation density (revealed by etching) and surface defects, we are not yet certain whether the measured Q′s are characteristic of perfect YIG. The intrinsic Q could be higher than the above figure and still be consistent with the present results. The dependence of the internal friction Q −1 in pure YIG on frequency and temperature are given. The frequency dependence at 300°K is shown to be Q −1∼f in the megacycle range with possibly a slower dependence on f in the microwave range. Data are given on Q −1 at ∼20 Mc from 300° to 20.3°K. Q −1 is relatively independent of temperature down to ∼50°K. Below 50°K, Q −1 increases rapidly down to 20.3°K, indicating a large absorption maximum below 20.3°K. The origin of this maximum is at present unknown. Data are also presented on Co1.1Fei.9O4 at ∼20 Mc from 300° to 20.3°K. This material has a very large maximum (Q −1∼10−2) at 290°K. Below 50°K, however, it has quite remarkable properties, with Q −1 being lower than YIG. Instead of Q −1 increasing rapidly below 50°K, as in YIG, it decreases rapidly from 50° to 20.3°K, indicating very low losses in the helium range. Investigation of all the above effects is continuing in detail to obtain an understanding of the many questions raised by these results about acoustic losses in ferromagneticinsulators.
33(1962); http://dx.doi.org/10.1063/1.1728654View Description Hide Description
The oxygen parameter of CrO2, which has the rutile structure, has been determined to be u=0.301±0.004. Single‐crystal measurements of cell constants over the temperature range −200° to +200°C show the unusual decrease in the c axis with increasing temperature in agreement with x‐ray powder results reported by other workers. An estimate of the magnetocrystalline anisotropy has been obtained from measured magnetization curves of a small single‐crystal sphere of CrO2. The directions of easy magnetization lie in (100) planes at an angle of approximately 40° to the tetragonal axis.
33(1962); http://dx.doi.org/10.1063/1.1728655View Description Hide Description
Various amounts of the divalent ions of manganese,iron,cobalt, and nickel have been substituted for trivalent iron in yttriumirongarnet. The effect of substitution of Fe2+ or Mn2+ions in dodecahedral sites has also been studied. Electrical balance was accomplished by the simultaneous substitution of tetravalent silicon or germanium. Under these conditions, it is concluded that the divalent ions prefer octahedral sites.
The great affinity of silicon for the tetrahedral sites in the garnets results in the reduction of trivalent to divalent iron at high temperatures even in air atmosphere when appropriate amounts of finely divided silica are present. In , the maximum attainable value of x (under the conditions of our experiments) appears to be 0.45. In attempting to put Fe2+ions into dodecahedral sites, greatest success was attained when there was also some in the octahedral sites as in . The 0.1 was the maximum amount that could be put into the dodecahedral sites.
Although, it appears that the Co2+ ion is the easiest to substitute into yttriumirongarnet, it is perhaps the most difficult to understand, and only a few of the results on experiments with this ion are included.
In all substitutions of divalent magnetic ions not involving the dodecahedral sites, the 0°K moments are smaller than those calculated on the basis of the simplest interaction model. Several possible explanations are explored; a plausible one is the random canting of some tetrahedral ion moments resulting from weaker interaction with the octahedral divalent ions. The possibility that some of the divalent ions enter tetrahedral sites is not entirely excluded especially in the case of the Co2+ ion.
The divalent magnetic ions do not behave like the trivalent magnetic lanthanide ions when substituted in the dodecahedral sites. They appear not to contribute to the net spontaneous moment appreciably.
33(1962); http://dx.doi.org/10.1063/1.1728656View Description Hide Description
The stoichiometric compound FeS undergoes an electron‐ordering transition at T α≈140°C on heating, T α≈100°C on cooling. It is pointed out that the various magnetic, crystallographic, and electric properties associated with this transition are compatible with the formation of three‐electron Fe2+–Fe2+bonds within the basal planes. This rather unexpected effect brings to nine the number of different electron‐ordering transitions that can be identified. It is also noted that Mn2Mo3O8 probably represents a closely related type of electron ordering.
33(1962); http://dx.doi.org/10.1063/1.1728657View Description Hide Description
During an investigation of low‐loss ferrites, it was found that magnetically annealed nickel‐cadmium ferrites containing minor additions of cobalt and molybdenum oxide exhibited rectangular hysteresis loops with squareness ratios approaching unity at temperatures up to 400°C. Data showing the effect of formulation and temperature on squareness ratio and coercive force are presented.
33(1962); http://dx.doi.org/10.1063/1.1728658View Description Hide Description
The anisotropy of the composition system Mn x Fe3−x O4, 0.40≤x≤1.80, has been measured between 4.2° and 77°K by the torque method, supplementing previous measurements on this system by Penoyer and Shafer between 77° and 313°K. The anisotropy curves for 0.4≤x≤1.25 show a minimum below 110°K and appear to be the summation of a number of components of different sign and different temperature dependence—negative components which decrease in magnitude relatively slowly with increasing temperature and positive components which decrease more rapidly. Over most of the composition range studied K 1 is negative at 4.2°K, but at x=0.80 the magnitude of the positive, low temperature contribution is sufficient to change the sign of K 1 from negative to positive at 34°K. In the region x≤1.0, this contribution can be attributed to the presence of Fe2+ ions, but such an explanation is not applicable to the positive contributions observed in compositions for which x≥1.25 and in which there is no Fe2+. For 1.25≤x≤1.80, negative values of K 1 at 4.2°K of unexpectedly large magnitude (<−3.7×105ergs/cm3) are observed. Unlike values at 77°K in this composition region, these values of K 1 are of too large a magnitude to be attributed to the splitting of the 3d levels of Fe3+ ions in a cubic crystalline field.
33(1962); http://dx.doi.org/10.1063/1.1728659View Description Hide Description
Various samples of δFeOOH have been prepared all with substantially the same particle shape and size but with differing low‐temperature saturation magnetizations. The particles were found to be hexagonal platelets with a mean diameter of 200 A and a thickness of 30 A. The magnetization of the samples was varied by varying the preparation conditions and also by partially decomposing the material to αFeOOH and αFe2O3. It has been shown that at a little above room temperature the powders were all superparamagnetic and that the variations of magnetization with preparation conditions qualitatively support a model proposed by Francombe and Rooksby. The variation of the Blocking temperature of a particular preparation as its magnetization was reduced by successive decomposition is also discussed. The results are tentatively interpreted in terms of a constant term corresponding to the normal anisotropy energy and a variable one due to the Lorentz internal field.
33(1962); http://dx.doi.org/10.1063/1.1728661View Description Hide Description
A series of fine‐grained single‐phase ferrites was prepared according to the formula Ni1−x Zn x Fe2O4, where x took the values 0, 0.10, 0.33, 0.50, and 0.67. A previously‐described process combining the so‐called flame‐spraying and hot‐pressing techniques was used to obtain grain sizes of approximately 0.1 micron in the densified bodies. μ′ and μ″ were measured to 1000 Mc, with grain size and composition as parameters. Measurements were also made to 3800 Mc on a ferriteannealed through the critical size for multi‐domains, confirming previously‐reported theory that magnetic poles on the domain walls are the source of the microwave peak at about 2000 Mc. Wall displacement appears to contribute to the static μ0 in large‐grained ferrites when x>0.
In addition, the temperature dependence of initial permeability μ0 was studied. The temperature coefficient of μ0 was found to increase with grain size. While the μ0 vs temperature curves were of various shapes generally, still a set of ferrites made up of differing compositions and grain sizes were obtained having linear temperature dependences. Of practical interest are those with slopes of −220 ppm/°C, 0 ppm/°C (NPO), and 220 ppm/°C.
Microwave properties were studied also. Because zero‐field (H dc=0) measurements showed that fine‐grains (i.e., below the critical size for multidomains) eliminated the so‐called microwave loss peak in the rf dispersion, a decrease in the low‐field loss follows. Also, microwave measurements of the main resonance loss susceptibility vs rf power were made at X band using a cavity technique. Although normal critical fieldshc were generally found, one sample with x=0.67 and ΔH=655 oe showed an anomalous rise in χ″/χ0″ at high powers, significantly expanding its power‐handling capability.
33(1962); http://dx.doi.org/10.1063/1.1728662View Description Hide Description
An investigation in the ternary system FeVO has been made of the chemical, crystallographic, and magnetic properties of the corundum FeVO3, and the spinels FeV2O4 and Fe2VO4· FeVO3 has been reported in the literature as Fe2+V4+O3 and antiferromagnetic at room temperature. However, the study reported in this paper shows that these results are incorrect. Oxidation of the spinel FeV2O4 results in the formation of some FeVO3. Conversely, reduction of FeVO3 ultimately forms FeV2O4+Fe. Therefore both the iron and vanadium are trivalent in the compound Fe3+V3+O3. Furthermore, FeVO3 has a small magnetic moment at room temperature which persists up to approximately 445°K as well as an anomalous rise in the magnetic moment below 150°K. A change in the relative intensity of several lines in the x‐ray pattern of the material, which may indicate a shift in the position of the metal atoms was observed down to liquid nitrogen temperatures. However, no change in the symmetry was observed. The magnetic properties of FeV2O4 and Fe2VO4 are also studied. The magnetization curves of both these materials indicate the existence of a magnetic transition at 275°K for Fe2VO4 and 70°K for FeV2O4.
33(1962); http://dx.doi.org/10.1063/1.1728663View Description Hide Description
Nickel‐iron spinels with compositions between NiFe2O4 and Ni2FeO4 have been prepared under oxygen pressures up to 2000 psi and at temperatures between 250° and 1000°C. Good agreement was found between the measured lattice parameters of the resulting spinel phases and the values obtained by extrapolating from the previously determined spinel series between Fe3O4 and NiFe2O4. Spinel compositions with lattice parameters as low as 8.3005 A were obtained; this compares with 8.3885 for Fe3O4 and 8.3394 for NiFe2O4. As is the case of the spinel solid‐solution series between Fe3O4 and NiFe2O4, the composition of these nickel‐rich spinels was found to depend strongly on both the oxygen pressure and the temperature. These relationships have been determined and are briefly discussed. Since it had been shown that there is essentially no measurable deviation from Végard's law between Fe3O4 and Ni2FeO4, lattice parameter measurements were a convenient method of determining the compositions of the spinel phase. The fact that single phase nickel‐iron spinels can be prepared with nickel concentrations greater than NiFe2O4 indicates that Ni3+ replaces Fe3+ in the spinel lattice. Of the several possible configurations for a composition such as Ni2FeO4, there is strong evidence from magnetic moment measurements that on octahedrally ligated spinel sites the preferred arrangement is the [Ni2+Ni3+] ion pair. The corresponding decrease in moment as Ni3+ substitutes for Fe3+ between NiFe2O4 and Ni2FeO4 agrees very well with that predicted by the Néel antiparallel array theory. A lowering of the Curie temperature was also observed in this compositional range.