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Volume 34, Issue 4, 01 April 1963
34(1963); http://dx.doi.org/10.1063/1.1729457View Description Hide Description
Microwave resonance measurements are reported near 16.8 Gc in the temperature range 1.5° to 77°K for a crystal of composition Co0.005Mn0.995Fe1.99O4. Sharp peaks were observed in the field for resonance and linewidth as a function of orientation below 10°K. The applied field directions for which these peaks occur lie on cones with axes along  directions and semivertical angle 34°. At higher temperatures the peaks in ΔH split into two and this splitting increases with increasing temperature. The general resemblance of the behavior below 10°K to that of YIG doped with certain rare‐earth ions is pointed out and by analogy, the peaks are attributed to near crossovers in the low lying energy levels of the cobalt ion. A model for relaxation which was useful for nickelferrite containing Fe2+ions and for rare‐earth doped YIG is applied qualitatively and on this basis the minimum energy level splitting at the near crossover is tentatively de‐deduced to be 11 cm−1. The occurrence of crossovers at the angles concerned is not predicted by models for the Co2+ ion which have been used previously in theories of the magnetocrystalline anisotropy of spinel ferrites containing cobalt.
34(1963); http://dx.doi.org/10.1063/1.1729458View Description Hide Description
The specific heats of a series of ferrites Mn x Fe y O4 with x+y≈3, have been measured in the range 1.5°K to 6°K. The specific heats are anomalously large in this temperature range, and the anomaly increases with increasing manganese concentration. If the relatively small estimated lattice contribution is subtracted out, the resulting Cv vs T curves are concave downward, suggesting a Schottky‐type curve with a maximum in the range 7–10°K. Assuming that a valence transition, presumably involving manganese and iron atoms, is responsible for the specific heat anomaly, then the specific heat curves indicate that the temperature corresponding to the transition energy is in the range 18–25°K. An estimate of the amount of entropy involved indicates that the number of ion pairs involved in the transition is less than one‐tenth the number of MnFe2O4 molecules. This suggests that transitions involving manganese ions on octahedral sites alone could account for the specific heat anomaly.
34(1963); http://dx.doi.org/10.1063/1.1729459View Description Hide Description
Pure lithiumferrite can be obtained as a very good square‐loop material if certain precautionary steps are taken in material preparation, firing, and cooling. Due to its simple composition and high Curie temperature, this material is of practical as well as of theoretical interest.
In a series of memory cores prepared under various conditions, grain size was found to be the determining factor for switching time and threshold field. The series covered a range of 2 to 30 microns in average grain size. Below 2 microns good squareness was not obtained even in samples with uniform grain size. Cores with 2.3‐μ average grain size proved to have as little as 120‐nsec switching times under coincident current test conditions. Results indicate that there is little magnetic interaction between radially adjacent grains. Therefore, the best guarantee to obtain a square hysteresis loop requires a relatively narrow distribution of grain sizes. Cores fired under conditions which resulted in a wide distribution of grain sizes exhibited poor squareness. The change of threshold field with temperature of lithiumferrite is only 0.15% per °C compared with 1.5% per °C in some extreme cases of commonly used MgO–MnO ferrites with high ZnO content. The procedures used for obtaining grains of uniform size are discussed. Adjusting the powder particle size to a certain fraction of the grain size desired in the finished core was found to be the best physical method of controlling grain growth without introducing complications through use of oxide additions acting as fluxes or grain growth inhibitors. Powder particle size was measured by use of x‐ray diffraction line broadening techniques. As a very practical relative measure of the powder particle size in the submicron range, the intrinsic coercive force can be used.
34(1963); http://dx.doi.org/10.1063/1.1729460View Description Hide Description
Susceptibilitymeasurements on single crystals of KFe11O17 show an anomalous antiferromagnetic behavior, which we propose to call antiferrimagnetism. The behavior follows from Weiss field theory on a simplified (i.e., two‐sublattice) model.
The magnetic structure of K2NiF4 has been determined by neutron diffraction by Legrand and Plumier at C.E.N., Mol (Belgium). Measurements on their single crystal show that χ∥ and χ⊥ become nearly equal at ≈100°K, i.e., far below the temperature where χ is maximum. This is believed to be due to a gradual breaking up of the long‐range order in the basal layers. Results obtained on powders of La0.5Sr1.5MnO4 and La1.5Sr0.5CoO4 are also given.
Pure polycrystalline samples of EuS, EuSe, and EuTe with rocksalt structure show θ values of +16, +6, and −7°K; EuTe has a Néel temperature at +11°K. The ferromagnetism of EuO, EuS, and EuSe is due to a predominant direct positive exchange interaction Eu‐Eu, the antiferromagnetism of EuTe to a predominant next‐nearest neighbor superexchange interaction Eu‐X‐Eu. Neither interaction is believed to change sign in the series. Measurements of M vs T at various field strengths are in very good agreement with Weiss field theory.
Oxidic spinels with diamagnetic ions in A sites have so far shown negative θ values. The substances Ge[Fe2]O4, Ge[Co2]O4, and Ge[Ni2]O4 have θ values of −15, +90, and 0°K, respectively. The materials Ge[Ni2−2δLiδFeδ III]O4 have positive θ values, increasing linearly with δ to 180°K for δ=0.5. Here the Ni2+–Fe3+ interaction must be positive and this interaction is thought to be a direct one, as is discussed.
34(1963); http://dx.doi.org/10.1063/1.1729461View Description Hide Description
Samples in the series BaCo x Fe12−x O19−x F x were prepared by firing appropriate mixtures of oxides and fluorides in a carefully dried oxygen atmosphere at 1350°C, with values of x ranging from 0 to 2.0. Single crystals of mixtures having x values of 0.3, 0.4, 0.5, 1.0, and 2.0 were grown from a flux composed of equimolar quantities of Na2CO3 and Fe2O3. The crystals had the BaFe12O19structure for x values below 0.5 and the BaFe18O27 (W) structure for higher values of x. The mixture with x=0.5 yielded both kinds of crystals. Previous measurements of saturation magnetization in the easy direction had indicated that the Co2+ ions were substituting for Fe3+ in the tetrahedral sites. The positive anisotropy of the crystals decreased with increasing cobalt substitution, reaching a minimum at about x=0.47. A crystal having approximately this composition appeared to have very little anisotropy by qualitative tests. Torque measurements indicated that this crystal has a cone of easy magnetization. Comparisons of the magnetization of powders at a constant field (6000 Oe) with the calculated saturation value indicated that the powders approached saturation most closely at x=0.47, again suggesting a minimum in anisotropy at that concentration. The strong effect of tetrahedral Co2+ on the anisotropy is believed to be related to the presence of a trigonal component in the local crystalline field, owing to the presence of one fluoride ion and three oxygen ions in the immediate vicinity.
34(1963); http://dx.doi.org/10.1063/1.1729462View Description Hide Description
The magnetic moments and susceptibilities of the piezoelectric‐ferromagnetic compounds Ga2−x Fe x O3 have been measured over a temperature range from 4.2° to 350°K. The saturation moment at 0°K and the Curie temperature follow a linear compositional relationship. The magnetic properties of these compounds are sensitive to heat treatment. In the vicinity of the Curie temperature, the magnetization follows a (Tc−T)2 law. At low temperatures, the deviation in the magnetization follows an exponential of the form exp (−98.8°K/T). In the paramagnetic region, the Curie constants for these substances differ radically from values expected for an ensemble of S state ferric ions. From the experiments reported here, it seems that one must assume in these compounds that the ferric ions have an exceedingly strong local crystalline interaction.
34(1963); http://dx.doi.org/10.1063/1.1729463View Description Hide Description
34(1963); http://dx.doi.org/10.1063/1.1729464View Description Hide Description
Ir4+, substituted in ferroxdure (BaFe12O19), has been observed to couple ferrimagnetically. Replacement of Fe3+ by Ir4+Zn2+, according to the formula BaFe12−2x 3+Ir x 4+Zn x 2+O19, reduces the intrinsic axial anisotropy. The room temperature anisotropy becomes planar at x=0.3–0.4 and increases to 22 000 Oe at x=0.6. Magnetization and Curie temperature are decreased by the substitution of Ir4+Zn2+; ferrimagneticresonancelinewidth is increased. The nonaxial anisotropy is believed to derive from spin‐orbital coupling of Ir4+.
Single crystals up to ¼ in. across were grown from a Bi2O3 flux in platinum crucibles. Specimens are hexagonal tabular, black, lustrous, and well‐developed. Single‐crystal x‐ray diffraction patterns showed that a 0 increases with increasing Ir4+Zn2+ substitution while c 0 remains unchanged.
34(1963); http://dx.doi.org/10.1063/1.1729465View Description Hide Description
The influence of milling on the intrinsic coercive force (Hci ) of different ferritepowders has been studied recently by several investigators. These have ascribed changes of coercive force to dislocations which are introduced through milling and are removed through subsequent annealing.
In our own studies of bariumferritepowders we relate the changes of Hci to changes in particle size. This interpretation is supported by experimental data and electron micrographs. The large particles of bariumferritematerial (calcined at 1300°C) change during ball milling from predominantly multidomain size (several microns) to nearly superparamagnetic size (0.02–0.5 microns). Accordingly, coercive force increases at first and then decreases as milling is extended.
Annealing of ball milled material between 800° and 1100°C results in slight sintering. Since the sintering reduces the number of nearly superparamagnetic particles, considerable increases in coercive force occur.
These interpretations are further supported by the temperature dependence of the coercive force. After short milling, when multidomain particles are still predominant, Hci decreases as temperature of powder is lowered. After extended milling, however, this effect changes sign which is attributed to thermal fluctuations of the magnetization in nearly superparamagnetic particles.
34(1963); http://dx.doi.org/10.1063/1.1729466View Description Hide Description
The linewidths in ferrimagnetic resonance ΔH and the frequency dependence of the anisotropy field for iron garnets containing rare‐earth ions are explained on the basis of a model for relaxation which was used by Clogston for nickel ferrous ferrite. It is assumed that the population of the rare‐earth ion energy levels relaxes towards equilibrium with a finite relaxation time τ. Consequently for ωτ≳1, where ω is the measurement frequency, significant departures from equilibrium occur and give rise to the effects noted above.
If the static torque, linewidth, and the resonant field can be measured in a temperature range where only the lowest two rare‐earth ion energy levels are appreciably populated, τ can be deduced for the magnetization along  and .
The consequences of the model are described; they differ quite extensively from those of the fast relaxing ion model. A quantitative comparison with the theory for iron garnets containing ytterbium supported the theory and yielded values for τ. In the case of erbium, however, the static torque was found to be strongly field dependent and hence only a qualitative comparison is possible. It is nevertheless concluded that the effects observed support the model. At 55°K τ for the ground doublet is about 10−11 sec.
34(1963); http://dx.doi.org/10.1063/1.1729467View Description Hide Description
Bariumferrite platelet‐shaped particles have been prepared with a diameter of 0.1 microns, which is an order of magnitude smaller than those obtained by conventional sintering and regrinding techniques. The preparation technique used has been optimized to produce the complete ferrite with σ s =68 emu/g. Due to the small particle size, extremely high intrinsic coercive forces have been obtained, (Hc =5350 Oe), in good agreement with the value calculated from the Stoner‐Wohlfarth (SW) coherent rotating model for single‐domain noninteracting particles when shape and crystal anisotropy are both taken into account. Furthermore, the measured hysteresis loop for an unoriented sample agrees well with the theoretical SW loop. Similar hysteretic magnetization properties have been obtained in strontiumferrite. The single‐domain nature of these powders is also demonstrated by their temperature dependence of coercive force where little change is obtained over the range −200° to +150°C. An estimate of the relative interaction field magnitude for the small particles of bariumferrite has been obtained by comparing its initial anhysteretic susceptibility with that of an acicular iron oxide powder.
34(1963); http://dx.doi.org/10.1063/1.1729468View Description Hide Description
A review of the magnetoplumbite magnet development is given from the early magnets of Takei to the recent high energy barium ferrite magnets of Tomholt. A method for rapidly testing a large number of ferrite permanent magnet specimens is described which was used on several thousand specimens. The results are summarized for portions of the ternary systems M1O·M2O·Fe2O, where M1 and M2 stand for Ba, Sr, Pb, or Ca. A modified strontium ferrite magnet could be developed which appears to have a higher magnetic energy than any other oxide magnet material previously known. Typical values are: Br =4100 G, IHc =3000 Oe, (BH)max=4.0 mgo. At lower coercive forces and under careful laboratory conditions, modified strontium ferrite magnets can be prepared with a (BH)max approaching 5 mgo. A number of permanent magnet applications are mentioned for which the new material is most suitable.