High Anisotropy Field Research Articles

Magnetic properties of R2(Fe, Co, Al)14B compounds where R = Pr and Nd

The results of magnetic measurements performed on Pr2Fe14−x−yCoxAlyB and Nd2Fe14−x−yCoxAlyB alloys having x⩽7 and y ⩽0.7 are reported. In addition to cobalt, the small aluminium content leads to materials having high Curie temperatures and anisotropy fields. The magnetic properties of the above systems are analysed in correlation with the crystal structure.

Distinctive characteristics of barium ferrite media

Barium ferrite magnetic recording media have demonstrated the capability of achieving much higher recording densities than other particulate media. Even though the best performance is obtained with perpendicularly oriented media, longitudinally oriented and non-oriented Ba-ferrite media also exhibit exceptional recording characteristics. These media have very distinctive magnetic and switching field (SFD) characteristics, exhibiting very small SFD values, almost rectangular remanence loops (indicating no irreversible losses over a wide range of reverse fields), and minor loops very close to the major loop until the driving field becomes almost equal to the coercivity. These unique magnetic and switching field characteristics can explain the very low or negligible demagnetization losses (including recording demagnetization) which we observe in Ba-ferrite media and which result in their outstanding recording performance. The fundamental reasons for these unique magnetic and switching field properties are the predominant high uniaxial crystalline anisotropy of the platelets and their single domain character. On the negative side, the high anisotropy field is directly implicated as the main reason behind the marginal overwrite modulation which characterizes these media.

Magnetic properties of Nd 2Fe 14− x− yCo xAl yB alloys

The magnetic properties of Nd 2Fe 14− x− y Co x Al y B alloys having x⩽3 and y⩽1 are reported. For the composition range x=(2÷3) and y⋍0.5, materials with high Curie temperatures and anisotropy fields are obtained. The saturation magnetizations of these compounds are somewhat decreased as compared to the value evidenced in Nd 2Fe 14B. The effect of aluminium on the anisotropy field and coercive force of Nd-Fe-Co-Al-B magnets is analysed.

The importance of gyromagnetic remanence in alternating field demagnetization. Some new data and experiments on G.R.M. and R.R.M.

During alternating field demagnetization with the three-axis stationary method, the acquisition of a spurious magnetization, the intensity of which increases with increasing peak field, is observed in a wide range of samples, mainly of volcanic rock type. We show that this magnetization is clearly linked to high field anisotropy of the sample and to the applied AF direction, supporting Stephenson's theory on gyroremanent magnetization. GRM linked to the sample anisotropy can be of the same order of magnitude as RRM at low speeds of rotation. Another kind of gyromagnetic remanence can also be created when stationary alternating field treatments are applied to successive non orthogonal axes. Such results put constraints on AF demagnetization procedures: (1) The multiaxis stationary procedure proposed by McFadden to improve the efficiency of the three orthogonal axis method is not suitable. (2) If GRM is observed during a three orthogonal axis procedure, the method described by Dankers & Zijderveld must be used (3) Provided that the noise level introduced by the RRM is reduced by an alternating sense of rotation, the tumbler method should be the most appropriate.

Microstructure of high-energy product Fe-Nd-B magnequench ribbons

Melt-spun Fe-Nd-B MAGNEQUENCH® ribbons have been produced by Croat et al. with energy products in excess of 10 MG.Oe using a relatively narrow window of composition and quenching speed. The hard magnetic phase has subsequently been identified as the Nd2Fe14B compound which has a very high anisotropy field. The microstructure of the MAGNEQUENCH® ribbon which has a maximum energy product of 14.1 MG•0e was found to consist of equiaxed Nd2Fe14B grains surrounded by a very thin intergranular film. This paper presents the results of some of our earlv work on the microstructural characterization of high energy product MAGNEQUENCH® ribbons having nominal compositions of Nd13Fe82.6B4.4 and Nd15Fe79.9B5.1. The purpose of this investigation was to characterize the microstructures of various MAGNEQUENCH® ribbons for correlation with their magnetic properties.

The 59Co nuclear magnetic resonances in the intermetallic compounds

The anisotropic hyperfine fields of the intermetallic compounds have been studied by the spin echo method and were obtained from the NMR spectra of 59Co nuclei in ThCo 5 and YCo 5 which have the high anisotropy fields resulting from the orbital moments. The intermetallic compounds, ThCo 5.3 and YCo 5.3, have been also investigated.

Domain-wall dynamics in garnet films with orthorhombic anisotropy

The dynamics of domain walls in (110) oriented manganese containing garnet films have been studied. Bubble films and special films in which a single straight domain wall can be stabilized have been prepared. The magnetic anisotropy in these films is orthorhombic and the influence of the in-plane anisotropy field on the maximum domain-wall velocity has been studied. The observed maximum velocities, varying between 40 and 1500 m s−1, are usually considerably lower than the maximum velocities predicted by theory. However, for very high in-plane anisotropy fields the experimental maximum velocities approximate the theoretical ones. From the comparison of different samples it has been concluded that the maximum velocity of the bubbles having no Bloch lines is not very different from the maximum velocity of straight domain walls. Furthermore, it has been shown that the maximum velocity is independent of the film thickness. The influence of external in-plane fields on the maximum velocity also has been studied. In-plane fields in the hard direction of magnetization have no influence on the maximum velocity. In-plane fields small compared with the in-plane anisotropy field increase the maximum velocity considerably if these in-plane fields are applied in the direction perpendicular to the hard axis of magnetization. The influence of stray fields on the maximum velocity is discussed.

Properties of magnetic fluid particles

Very fine particles of magnetite, nickel ferrite, and cobalt ferrite were produced by grinding coarser powders in a ball mill with a carrier fluid and a surfactant. The particles were examined by means of chemical analysis, electron microscopy, x-ray diffraction, magnetic measurements, and Mossbauer spectroscopy. Properties were determined before and after removing the surfactant coating. The most significant observation was that in some systems a large fraction of the spins was pinned in extremely high anisotropy fields as a result of bonding to the surfactant molecules. Anomalous magnetic hysteresis behavior was also noted when the surfactant coating was present.

A.C. susceptibility measurements in the presence of D.C. magnetic fields in CoGa alloys — A cluster spin glass system

Low field a.c. susceptibility measurements in the presence of large d.c. magnetic fields are reported on Co x Ga 1− x alloys ( x = 0.585, 0.60 and 0.63), a cluster spin glass system, in the temperature range of 77 to 500 K. A peak in the a.c. susceptibility is observed in each case. The peak position shifts towards lower temperatures with increasing d.c. magnetic fields. In conjunction with the zero field NMR measurements reported earlier, these results may be taken to imply the existence of a continuous spectrum of freezing temperatures — cluster with highest anisotropy field having the lowest freezing temperature.

Magnetically tunable millimetre-wave filter with single-crystal barium ferrite

Barium-ferrite (BaFe12O19) single crystals with a high anisotropy field and low millimetre-wave (mm-wave) losses have been grown from high-temperature solutions. The high internal field (17/4π MA/m) considerably reduces the external magnetic field required for spin resonance at mm-wave frequencies. Due to the progress in single-crystal barium-ferrite technology, it is possible to extend the tuning range of filters up to 90 GHz. A compact tunable filter, similar to y.i.g.-filters, has been built and the first results are reported.

Magnetic anisotropy of granitic rocks of the Bergell massif (Switzerland)

Magnetite as well as ilmenohematite are contained by the Bergell granitic rocks, but the fabric of low field susceptibility is due to the magnetite only and is equivalent to the macroscopic biotite fabric. Microscopic observations and high field anisotropy measurements show that the ilmenohematite trigonal axes are aligned parallel to the directions of minimum low field susceptibility anisotropy. The deflection of the stable direction of natural remanent magnetization caused by the alignment of ilmenohematite is corrected using the susceptibility anisotropy data. The resulting virtual palaeopole position deviates from other Oligocene to Miocene pole positions of stable Europe suggesting that since the time of intrusion some 25 my ago the Bergell massif has been rotated anti-clockwise by an angle of ∼30° about a vertical axis.

Magnetic behavior of USe at low temperatures

The magnetization versus temperature behavior of USe has been investigated in the temperature range from 2.8 to 300°K and in various applied fields up to 21 kOe. A sharp decrease in magnetization is found at low temperatures. This behavior seems to be associated with the rhombohedral distortion in crystal structure, giving rise to a high anisotropy field, below the Curie temperature. Large magnetic hysteresis effects at low temperatures confirm these arguments.

The temperature dependence of the domain structure of uniaxial single crystals with high-anisotropy field

The temperature dependence of the domain structure of single crystals of uniaxial ferrimagnetic oxides with high-anisotropy field in the SrO( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6 - x</tex> )Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</tex> Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> compositional series, with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x = 1.8</tex> and 2.4, was investigated. The domain configurations of thermally and/or ac field demagnetized states and the changes in dc and ac fields were observed by means of the colloid technique using a suspension in paraffine oil. The temperature dependence of the wall energy density between room temperature and the Curie point (270°C) of the compound with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x = 1.8</tex> was computed using the temperature dependence of the saturation magnetization and the domain width for the Kittel-like domain structure. It has been found that the room temperature domain structure of the compound with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x = 2.4</tex> depends upon the mode of demagnetization, i.e., thermally or in the ac field. The ac demagnetized states are more stable than the thermally demagnetized ones. The samples exhibit a room temperature memory of the previous magnetization, which decreases with rising temperature. This memory is completely lost after heating at 400°C. The peculiarities of the temperature dependence of the domain structure and magnetic behavior on thermal cycling are explained by considering the existence of magnetic inhomogeneities within the crystal.

Stability, infrared spectrum and magnetic properties of FeBO 3

The stability of the compound FeBO 3 with the calcite structure and Fe 3BO 6 with norbergite structure has been determined. Magnetic measurements on polycrystalline FeBO 3 revealed the compound to be a weak ferromagnet with a high anisotropy field below 115°C. Infrared spectra were used to determine characteristic absorption bands of the independent (BO 3) and (BO 4) groups.

Magnetization Processes and Reversal in Ferrimagnetic Oxides with High-Anisotropy Field

The magnetization process of SrO·4Fe2O3·2A12O3 single crystals with 2–4 domains was investigated. The total area of the Bloch walls diminished during their displacement and the saturation was reached at 75 Oe. The magnetization reversal occurred by nucleation and growth of a reverse domain and the nucleation field shifted towards higher negative values for increased values of the magnetizing field. For nucleation fields higher than 75 Oe the reversal of the magnetization occurred abruptly and the hysteresis loop became ideally rectangular.

The Influence of the Magnetic Field on the Domain Structure of Ferrimagnetic Oxides with High Anisotropy Field

AbstractThe magnetization process of SrO × (6 − x) Fe2O3 × x Al2O3 single crystals (with 0 ≦ × ≦ 2) is investigated by recording the modifications of the domain structure under the influence of an external magnetic field. It is found that this process occurs in two stages: in the first stage, at lower fields, the magnetization occurs mainly by Bloch wall displacements; in the second stage (at higher fields) regions of the inverse magnetized domains, or entire domains of this type suddenly collapse. For samples with x = 0 and 1 honeycomb domain structures can be obtained by applying a magnetic field along the hard direction.

Magnetization reversal in ferrimagnetic oxides with high anisotropy field

Abstract The curve of initial magnetization, the critical field for wall displacement, and the dependence of the nucleation field on the magnetizing field for single crystals of SrO·4Fe2O3·2Al2O3 were determined, by investigating the modifications of the domain structure under the influence of a magnetic field.

Domain Structure of Hexagonal Ferrimagnetic Oxides with High Anisotropy Field

The changes in domain structure as a function of the thickness L of the single crystals of the SrO·(6−x)Fe2O3·xAl2O3 compositional series were investigated. On the strontium ferrite single crystals, the same four models of Bitter patterns as in the case of magnetoplumbite and barium ferrite were observed. For the other compositions the limits of appearance of these four models shift towards higher values of the thickness as x increases as a result of the increased anisotropy field/saturation magnetization ratio. The thickness dependence of the domain width D changes from a D∼L⅔ dependence to a D∼L½ one for x≥1.8. In the case of crystals with x=2, the Bloch walls are parallel to the hexagonal axis everywhere. The finite dimensions of the samples may considerably influence the domain structure, which makes it possible for structures with two or even one domain only to be obtained, regarding relatively large samples.

A 5-MM Resonance Isolator (Correspondence)

The rectangular waveguide resonance isolator operating at frequencies from 3000 to 24,000 mc is a simple and compact device since the dc magnetic field requirements are relatively low. In the 5-mm range, however, resonance isolators are not practical if conventional ferrites are used because very high magnetic fields of about 20,000 oersteds are required to obtain resonance at these high frequencies. By using highly oriented Ferroxdure, resonance isolators in the millimeter range become feasible because of the high internal anisotropy field of 17,000 oersteds exhibited by this material. Thus, with Ferroxdure a magnetic fieId of a few thousand oersteds is sufficient for resonance in the 5-mm region.