Showing papers on "Ferrimagnetism published in 1981"

Diffraction of X-rays by magnetic materials. I. General formulae and measurements on ferro- and ferrimagnetic compounds

Abstract: The calculation of the amplitude of X-rays scattered by a magnetically ordered substance, carried out in the relativistic quantum theory (i.e. taking the spin into account), is detailed. The effect of the orbital momentum is described in an appendix. The practical formulae dealing with the polarization of the beams are given both in a simple form for the usual experiments and in a complete form, using the Stokes vectors, for the most general case. The experiments show a change in the intensity of the X-rays diffracted by a ferromagnetic (pure iron) or a ferrimagnetic (zinc-substituted magnetite) powder when the magnetization, perpendicular to the diffraction plane, is reversed. The relative values of these intensity changes range from 10-4 to 5 x 10-3 and agree in sign and magnitude with the predictions. They are proportional to the spin-density structure factor multiplied by the imaginary part of the charge- density structure factor; the large anomalous scattering of the Cu Kα radiation in the iron-containing samples is used in the present experiments.

Magnetization Process of Exchange-Coupled Ferrimagnetic Double-Layered Films

Abstract: Magnetization curves are theoretically calculated for exchange-coupled ferrimagnetic double-layered (ECDL) films, each layer of which has a uniaxial anisotropy with the easy axis perpendicular to the film plane and exhibits a rectangular hysteresis loop. With this model are obtained the conditions for the appearance of anomalous and inverted hysteresis loops observed by the Kerr magneto-optic method. Experiments are made on Gd-Fe/Tb-Fe double-layered films deposited successively in a vacuum better than 10-6 Torr. By comparing the experimental switching field for the Gd-Fe layer in ECDL films with the theory, the energy density of the interface wall is estimated to be 1–2 erg/cm2.

Co energy and magnetization anisotropies in RCo5 intermetallics between 4.2 K and 300 K

Abstract: The magnetocrystalline anisotropy has been known for a long time to be very large in the RCo5 intermetallics. The recent availability of high D.C. magnetic fields up to 200 kOe has enabled us to carry out an extensive experimental study of the energy and magnetization anisotropies in YCo5 from 4.2 K to 300 K. The magnetization anisotropy reaches 4% at 4.2 K. From a polarized neutron study on NdCo5 we find that this anisotropy of magnetization originates mainly from the 2c site. Previous measurements on YCo5 had shown that the orbital contribution to the 2c moment is large. Hence, we conclude that the Co magnetization anisotropy in RCo5 comes from a large orbital contribution to the magnetic moment. Finally, we account for the observed discontinuity in the magnetization at its reorientation in RCo5 ferrimagnets, and for the absence of a similar discontinuity in NdCo5 which is a ferromagnet.

Electrical conductivity of lithium-nickel ferrites

Abstract: The electrical conductivity of Li-Ni ferrites was studied as a function of composition and of temperature. X-ray analysis and Seebeck coefficient studies were also carried out so that the conductivity of these mixed ferrites could be discussed. The dependence of the electrical conductivity of Li-Ni ferrites on composition is explained on the basis of the cation distribution. All the ferrites exhibit a transition near the Curie temperature with a change in slope of the conductivity (log(σT)) versus temperature (103T) curve. It is found that the activation energy in the paramagnetic region is higher than that in the ferrimagnetic region for all the ferrites.

Magnetizations and Mössbauer Spectra of YFe2O4-x

Abstract: Effects of oxygen deficiency on magnetic properties of YFe 2 O 4- x ( x =0.00, 0.031, 0.040, 0.055 and 0.095) have been investigated by magnetization and Mossbauer effect measurements. When x ≧0.055, the compounds show so-called parasitic ferrimagnetism below 210 ∼220 K. When x ≦0.031, on the other hand, they undergo two successive first-order phase transitions at around 230 K and 190 K, without any thermoremanence at any temperature. The compound with x =0.040 exhibits intermediate characteristics. Lattice defects introduced by the oxygen deficiency are considered to play an important role in the disappearance of the first-order transitions and the appearance of the parasitic ferrimagnetism.

Magnetooptical polar kerr effect in ferrimagnetic garnets and spinels

Abstract: The complex polar Kerr spectra of ferrimagnetic garnets and spinels have been studied in the spectral region from 2 to 6 eV. The influence of Ga, Al, In, Sc, Pr, Sm, Tb and Ho on garnet spectra is presented and discussed. The reflectivity and dielectric tensor element spectra were also obtained for selected samples and the data is then used in the explanation of the spectra in terms of charge transfer transitions. In particular the bands near 3.4, 3.6, 4.3 and 4.7 eV are respectively assigned to the transitions to t 2g , e, t 2 and e g orbitals. The effect of polishing on reflectivity spectra of lithium ferrite is briefly discussed.

Magnetic properties and resonance studies of Gd(CoxAl1−x)2 compounds

Abstract: Results of magnetic measurements, ferromagnetic resonance and electron paramagnetic resonance (EPR) studies of the Gd(CoxAl1−x)2 system are presented. These compounds are ferrimagnetic in the concentration range x > 0.55. The cobalt moment is markedly dependent on composition. The reciprocal susceptibility has a non-linear temperature dependence. The magnetic behaviour of these compounds is analysed using three models, namely the molecular field approximation, the model which assumes that cobalt has an exchange-enhanced paramagnetic susceptibility and the local environment model. All these describe the experimental data rather well. For x < 0.55 the compounds are ferromagnetic. In this case the reciprocal susceptibility follows a Curie-Weiss law. For ferrimagnetic compounds the resonance data at 78 K are described using the Wangsness relation. The EPR measurements for x < 0.55 reveal that the slope of the linewidth b and the g values are not temperature dependent. For ferrimagnetic compounds both b and the g values are functions of temperature. These data are discussed in terms of a model based on the molecular field approximation. The composition dependences of gCo and gGd values are analysed. Finally, the magnetic behaviour of cobalt atoms in pseudo-binary Laves phase compounds is discussed.

On the magnetic behaviour of (GdxY1-x)Fe3 compounds

Abstract: The results of magnetic measurements performed on (GdxY1-x)Fe3 compounds, in the temperature range 4.2K-1300K, are reported. The YFe3 compound is ferromagnetic. Replacing Y by Gd, the iron magnetisation is oriented antiparallel to that of the gadolinium. The two magnetisations compensate at the composition x approximately=0.76. The thermal variation of reciprocal susceptibility for YFe3 shows Curie-Weiss behaviour, while in the case of ferrimagnetic compounds this shows a non-linear dependence of the Neel type. The effective iron moments, as well as the exchange interactions inside and between sublattices, are determined. The composition dependence of the iron moments is due to the variations of the exchange field acting on these atoms.

The Curie-Weiss-Néel model of a ferrimagnet

Abstract: An analogue, for ferrimagnetism, of the Curie–Weiss model ferromagnet is introduced. The resulting structure, the Curie–Weiss–Neel model, is based on a two sublattice description in which spins of ...

Glass-encapsulated magnetic materials and methods for making them

Abstract: Magnetic materials exhibiting ferrimagnetic and/or superparamagnetic behavior, produced by impregnating porous glasses with iron and, optionally, manganese, cobalt and nickel compounds, reacting the compounds with the porous glass to form bound complexes, heating the glass to expel unreacted organometallics, and further heating to convert the bound complexes to ferrimagnetic and/or superparamagnetic oxide crystallites, are described. Ferrimagnetic iron oxide crystallites exhibiting coercivities above 10,000 Oe can be made.

A magnetic resonance study of the Fe2O3/Al2O3 catalyst in a CO-N2O reaction.

Abstract: A magnetic resonance study was performed in order to determine the phase of iron oxide supported on Al2O3 and to measure in situ the surface conditions during the reaction. The irreversible transition of α-Fe2O3 into Fe3O4 by the reduction of the supported catalyst and the reversible transition between Fe3O4 and γ-Fe2O3 were confirmed at 280 °C. The spectrum appearing at g=2.03 and having the line width of about 1000 gauss clearly showed ferrimagnetic behavior; it could be attributed to the ferrimagnetic resonance (FMR) of Fe3O4 and/or γ-Fe2O3. Kinetic study suggested that the CO–N2O reaction took place in the oxidation-reduction mechanism. The concentration of γ-Fe2O3 was calculated from not only the kinetic equation, but also the in situ measurement of the FMR signal; these values were consistent. The γ-Fe2O3/Al2O3 had nearly the same activity as the α-Fe2O3/Al2O3, whereas the unsupported γ-Fe2O3 had a lower activity than the α-Fe2O3. Such a difference in catalytic activity was discussed in comparison w...

The temperature dependence of magnetization of amorphous Fe5Co70Si15B10 ribbons

Abstract: We have measured the temperature dependence of magnetization and coercive force of amorphous Fe5Co70Si15B10 ribbons in the intermediate temperature region below the Curie point. The M(T) curves manifest a plateau in the temperature region between 220 and 300 C in relatively low fields of technical saturation, i.e., M(T) is nearly constant. The plateau of M(T) curves from d.c. measurements disappears in higher fields. The M(T) curves have been analyzed by the use of a modified molecular field theory. They suggest the coexistence of a ferrimagnetic phase with a ferromagnetic phase. The ferrimagnetic phase has a TN at 513K, and TC at 640K, which is experimentally indistinguishable from TC of the ferromagnetic phase. On the Hc(T) curves, the coercivity decreases slightly with increasing temperature at first, then increases sharply up to a maximum value near 563K.

Efficient HGMS for highly magnetic materials

Abstract: The range of applicability of high gradient magnetic separation has been extended to recovery of ferrimagnetic material from fluids containing an equal weight of nonmagnetic particles at solids concentrations of 20 to 35%. A newly designed matrix permits unimpeded passage of nonmagnetic particles as large as 3 mm and at the same time, recovers the ferrimagnetic particles in thin sheets. In this manner entrainment of nonmagnetic materials within flocs of ferrimagnetic particles is minimal. It is demonstrated that efficient separation of magnetite having a mean particle diameter of 12 μm from fine coal having a mean particle size of 850 μm can be attained at a throughput of 30 tons of magnetite/hr/ft2(300 metric ton/hr/m2).

Quasi-optics of the coupling of guided magnetic waves for the normal magnetization

Abstract: The coupled mode equations governing the interaction of the guided magnetic waves propagating in opposite directions along a ferrimagnetic film having a periodic array of shallow rectangular grooves on one of its surfaces are deduced using quasi‐optical techniques for the case in which the magnetization is normal to the surfaces of the film. Effects caused by the deviation of the wave frequency from the Bragg frequency and by the presence of small magnetic losses are also taken into account. For a surface corrugation extending only for a finite length along the film, the longitudinal boundary value problem is solved to obtain the reflection, the transmission, and the absorption coefficients. The coupled mode equations are found to yield correctly the power conservation relation.

New observations on natural pyrrhotites: magnetic transition in hexagonal pyrrhotite

Abstract: Quenching hexagonal pyrrhotite, FeeSls, from 225'C retains the high temperature ferrimagnetic phase. The rate of loss of magnetism when the sample is annealed at temperatures below the ferrimagnetic stability range allows the determination of an activation energy for vacancy diffusion. The measured value of 50.2 kJ/mol agrees closely with that of,Fryt et al. (1979). In the course of a study of hexagonal pyrrhotites Nairne in South Australia. The sample consisted of by means of a magnetic thermobalance (Bennett about 50 mg of pyrrhotite sealed in vacuo in a silica and Graham, 1980), it was observed that the anti- capsule. A series of curveswasobtainedfortherate ferro-magnetic ferri-magnetic transition which usu- of loss of magnetism at different temperatures' of ally occurs at about 210'C can be delayed by which six are shown in Figure 1, corrected and quenching the ferri-magnetic phase from 225'C to scaled to show the main trends. There is a maxiroom temperature in a current of air. If the sample mum reaction rate at about 200"C. Below this is then annealed at temperatures between about temperature the reaction slows because of lower 70"C and the transition temperature of 210"C, it diffusion rates, and at higher temperatures the rate loses its magnetism at a rate which depends on slows because the free energy for the change to the temperature. This work is related to that of Maru- low temperature structure decreases as the transisak and Mulay (1979, 1980), but because it is tion temperature is approached. following the demagnetization reaction, our work There are several problems in accurately quantiextends over a much wider temperature range. fying the kinetic data. Because the sample was We believe that the high temperature ferri-mag- sealed in silica to avoid oxidation, it reached annetic structure is stabilized by a vacancy superlat- nealing temperatures slowly, and a zero of time was tice which develops above the transition tempera- indeterminate. The initial decrease in magnetic susture, and that the transition to the low temperature ceptibility in a raw demagnetization curve (Fig. 2) is antiferro-magnetic structure depends on a vacancy due to thermal demagnetization as the sample heats reordering process; this view is reinforced by the to ambient furnace temperature. The magnitude of results described below. However, following Na- this decrease can be predicted for a given temperakazawaand Morimoto (1971) and Kissin (1974),we ture by reference to the demagnetization curve of understand the high temperature phase to have an monoclinic pyrrhotite, the Curie temperature of "nA" ordered structure, and the low temperature which is only -50"C above that of the hexagonal phase the "mC" structure. As suggested by Mori- phase. The rate of temperature rise of the sample moto,NakazawaandWantanabe(L974)thesestruc- can be inferred from the curve in Figure 2; it is tures are probably partly ordered and/or intimately easier to determine accurately at the lower temperaintergrown rational superlattices. The temperature tures, where demagnetization due to vacancy at which the naturally occurring "5C" structure movement does not interfere because it is too slow. forms is still not well known, if indeed it can be There is an initial rapid fall in magnetization which formed in the laboratory after heat treatment. is accomplished in about I minute, but it is clear The results in the present paper were all derived from Figure 2 and from runs above the transition from a single sample of natural pyrrhotite from temperature (Fig. 3) that the sample does not reach

The magnetism of the NiAs-type solid solution Fe-Se-Te

Abstract: The magnetism of the nickel arsenide type solid solution Fe1−xSe1−yTey was examined in the ferrimagnetic and the paramagnetic range using a Faraday balance capable of susceptibility measurements from 100 up to 1100 K. The ferrimagnetism of the iron selenide extends far into the ternary range with gradually decreasing Curie temperatures. The composition dependence of the saturation moments is in good agreement with a simple ionic concept indicating a notable ionic contribution to the chemical bonding. The observed discontinuity of the unit cell volume reflects the importance of exchange magnetostriction effects. The strictive forces act along the c-axis only, since the parameter a of the hexagonal cell is not affected. The experimental results cast further doubt upon the speculation about antiferromagnetic exchange couplings in hexagonal iron tellurides; the paramagnetic state is more likely.

Structure and magnetic properties of Gd(Co1−xNix)5 compounds

Abstract: The lattice constants, Curie temperature and magnetization of the pseudo-binary system Gd(Co1−xNix)5 were determined as functions of composition. The microstructure of the as-cast samples is duplex for x ⩽ 0.5; the annealed samples are single phase with a CaCu5-type hexagonal structure. The composition dependence of the lattice constants of Gd(Co1−xNix)5 compounds does not obey Vegard's law. Compensation points were observed in the magnetization versus temperature curves for compounds with x ⩽ 0.6; however, they vanish at x = 0.8. These findings are elucidated in the light of the currently accepted ferrimagnetic coupling scheme.

Fabrication of nonreciprocal microwave components using thick film ferrimagnetic pastes

Abstract: This paper reports results of investigations on edge guided mode circulators and junction circulators using thick film ferrimagnetic pastes in the frequency range from 2 to 12 GHz. Performance of the circulators realized with the thick film ferrimagnetic pastes will be compared with those made on YIG substrates in respect of insertion loss, isolation and bandwidth. The study shows that while good isolation bandwidth is obtainable with edge guided mode circulators, the insertion loss remains high.

Displaced hysteresis loop of Cu‐Mn‐Al alloy

Abstract: The 44.7Cu‐20.6Mn‐34.7Al (at. %) alloy, which exhibits a very high coercive force after aging at 300 °C, has been found to show a displaced hysteresis loop when it is cooled to 77 K in a magnetic field. The hysteresis loop of the sample which aged for more than 60 min at 300 °C is shifted along the magnetization axis instead of along the field axis. The magnetization measured at 300 K for a maximum applied field of 15 kOe becomes larger than that at 77 K after aging for 45 min at 300 °C. In the magnetization versus temperature curves of the sample aged for more than 45 min at 300 °C, the magnetization decreases monotonically with decreasing temperature. The increase of coercive force, which is obtained in the sample aged for more than 45 min at 300 °C, corresponds to the increase of the displacement of the hysteresis loop. These magnetic behaviors are considered to be caused by the exchange anisotropy interaction between the regions of ferromagnetic Heusler or Mn‐poor phase and ferrimagnetic or antiferrom...

Magnetization and Magnetocrystalline Anisotropy in Gd–Sm Alloys

Abstract: Measurements of magnetization at 4.2 K and paramagnetic susceptibility up to room temperature have been done on Gd-Sm single crystals and polycrystals less than 30 at.% Sm concentration. The magnetic structure is a ferrimagnetic configuration in the ordered regions for the measured samples. The magneto-crystalline anisotropy constants K 2 0 and K 4 0 were determined from the analysis of magnetization curves on the basis of the molecular field theory. The values of K 2 0 and K 4 0 are negative and positive, respectively, in all concentration ranges, and the magnitude increase quadratically with increasing Sm concentration. From these results, the values of the anisotropic exchange interaction between two magnetic ions in the Gd-Sm alloys, K 2 0 (AEI) Gd-Sm and K 2 0 (AEI) Sm-Sm , were estimated to be 42 and -24 cm -1 , respectively.