Showing papers on "Coercivity published in 1977"

Electrical after-effects in Pb(Ti, Zr)O3 ceramics

Abstract: Pb(Ti, Zr)O3 ceramics doped with Al, Mn or Fe exhibit constricted or asymmetrically displaced hysteresis loops. This distortion is due to an internal bias field Ei which can exceed the coercive field strength. Ei depends on the Ti/Zr ratio and on the kind of dope, and it increases with increasing doping concentration.After repeated cycling of the hysteresis loop the distortion disappears. This process, called “hysteresis relaxation,” obeys a time law of the form Ei(t) ∼exp(-t/τ)and is found to behave both like a field and thermally activated process. The activation energy of all investigated specimens doped with transition metal ions (Mn, Fe) lies between 0.6 and 0.7 eV, that of Al-doped samples is about 0.8 eV.Comparison of the behaviour of the dielectric and mechanical losses with that of the internal bias shows that these effects are closely correlated. From this correlation some conclusions can be drawn concerning the nature of Ei, the physical causes of the observed after-effects, and the influence o...

Effects of Porosity and Grain Size on the Magnetic Properties of NiZn Ferrite

Abstract: The effects of porosity and grain size on the magnetic properties of NiZn ferrites were examined on groups of specimens with controlled porosity and grain size; both the permeability and theB-H loop were measured. The demagnetizing factor, which is proportional to porosity, was estimated on the basis of the permeability-porosity relation determined. The discussion of the permeability-grain size relation takes into consideration the dependence of domain wall spacing on grain size. The experimental results for the B-H loops indicate that the remanent magnetic flux density is independent of grain size and the coercive field is independent of porosity. Both the remanent magnetic flux density-porosity and the coercive field-grain size relations are explained via the formulas proposed.

Microstructure of aged (Co,Cu,Fe)7Sm magnets

Abstract: The microstructure of (Co,Cu,Fe)7Sm magnets has been characterized at various states of aging by transmission electron microscopy. At peak magnet properties, the magnet consists of cells of 17 : 2 phase about 500 A in diameter, separated by a thin continuous fully coherent 5 : 1 boundary phase. This structure evolved by precipitation of ordered 17 : 2 particles from a disordered 5 : 1 matrix, and coarsens with aging until full coherence is lost and the cellular structure breaks up. The overaged alloy consists of semicoherent 5 : 1 particles in a 17 : 2 matrix, with a single set of interface dislocations accommodating the lattice mismatch. The effect of these various microstructural changes on domain‐wall pinning and coercivity is discussed.

Initial magnetization, remanence, and coercivity of the random anisotropy amorphous ferromagnet

Abstract: We analyze the coercivity, remanence, and initial magnetization of a single-domain, single-component amorphous material. We assume a uniform ferromagnetic exchange and concentrate on the effect of single-ion anisotropy. We find a threshold anisotropy below which the coercivity is zero. At large anisotropy, the coercivity is proportional to the anisotropy energy. Exchange pulling increases the remanence of amorphous materials. We suggest that exchange pulling between crystallites may also be operative in small-grain metallic polycrystals.

Magnetic properties of a new type of rare-earth cobalt magnets Sm 2 (Co, Cu, Fe, M) 17

Abstract: The effects of various additive elements on the magnetic properties of Sm-Co-Cu-Fe quaternary alloys were studied. It was found that zirconium was the best additive element for improvement of their magnetic properties. The best properties were so far obtained for an alloy of Co, 25.5wt% Sm, 8wt% Cu, 15wt% Fe, 1.5wt% Zr. The energy product of this alloy attains 30MGOe by a step tempering following the solution treatment. In addition to excellent magnetic properties, Sm 2 (Co, Cu, Fe, Zr) 17 type permanent magnets exhibited the desirable temperature coefficient of residual magnetization of 0.03%/°C and a temperature coefficient of intrinsic coercive force of 0.16%/°C.

Hysteresis curves and magnetization processes in a model for an amorphous magnet with random uniaxial anisotropy

Abstract: We consider the magnetic field behavior of a simple model for an amorphous ferromagnet with random single‐ion anisotropy. The spins are taken as classical unit vectors located at sites of a dense random packing model and coupled via nearest‐neighbor exchange interactions. An easy‐axis direction is chosen at random for each site. For large anisotropy, the coercivity is very large and the hysteresis curves resemble those found for amorphous TbFe2 at low temperature. As the anisotropy is reduced, there is a fairly abrupt change in the behavior of the model indicating that sufficiently weak random anisotropy is effectively averaged away. We have examined the energy of metastable states. The results indicate that the canted aligned state is always more stable than low‐moment spin‐glass‐like states. We also consider the magnetization process in the presence of imposed domain walls and find that wall motion is preceeded by magnetization rotation in the high‐coercivity regime.

The influence of planar defects on the coercive field of hard magnetic materials

Abstract: Planar defects are found to act as strong pinning centres in hard magnetic materials. The interaction of the domain wall with planar defects is assumed to originate in a local perturbation of exchange coupling and crystalline anisotropy. The coercive field is calculated within the framework of micromagnetic continuum theory as well as with a discrete lattice theory which accounts for the interaction between the individual atomic spins. The theory has been applied to discuss the influence of grain boundaries and antiphase boundaries in the intermetallic compound Co5Sm.

Intensity and direction of magnetization in oceanic basalts

Abstract: The magnetic properties of oceanic basalt samples from 55 DSDP sites (301 samples) and 112 dredge haul sites (311 samples) show wide variations, indicative of heterogeneity in the basaltic layer. Remanent intensity and Konigsberger ratio are log-normally distributed and range over three orders of magnitude for each collection. For DSDP samples between-site variation is greater than within-site variation, suggesting horizontal variation of magnetization within the source layer of magnetic anomalies. Crustal magnetization estimates from inversion of short-wavelength oceanic magnetic anomalies may therefore be in error. By using arithmetic means rather than the geometric means appropriate for log-normal distributions, estimates of the magnetization of Layer 2 of around 4 × 10 −3 Gauss are obtained for the Indian, Pacific and north Atlantic oceans and Caribbean Sea, while lower averages are found in the south Atlantic and Antarctic oceans. Using this as a representative value for abyssal areas, the thickness of the magnetic source layer is greater than commonly assumed, and probably more than a kilometre. This interpretation is supported by DSDP data from some individual sites, notably the deep holes of Leg 37. The DSDP median destructive fields average only 123 Oe. Although many sites have much higher stabilities than this, there is evidence for unstable NRM at many sites where samples exhibit a strong tendency to acquire viscous remanence components. The low coercivity secondary magnetizations confuse identification of the stable direction during magnetic cleaning. Acquisition of high coercivity secondary remanence (CRM) as a result of maghemitization, which is extensive in the DSDP collection, may also have altered the original NRM direction. Remanent inclination data from four Tertiary DSDP sites (ages 38–40 Ma) have been combined to give a tentative Eocene VGP for east Antarctica at 68°S 64°E. It has not been possible to compare DSDP data in this way at other sites. Palaeolatitudes deduced from DSDP basalt remanent inclinations show large discrepancies from values expected from plate reconstructions at many sites. Incompletely removed secondary magnetizations, tectonic effects and palaeo-reconstruction errors, alone or in combinations, may be responsible for these discrepancies. A major source of deviation from the expected direction may be the rapid cooling of the basalt which does not allow adequate averaging of secular variation.

The coercive force spectrum of magnetite at high temperatures: Evidence for thermal activation below the blocking temperature

Abstract: Thermal activation of magnetization is the mechanism proposed by Neel to explain TRM (thermoremanent magnetization) and VRM (viscous remanent magnetization). Neel's theory is difficult to test because in a small constant field, magnetization changes occur only at or near particle blocking temperatures. By increasing the field, the blocking temperature can be lowered to any desired temperature, including room temperature. Isothermal hysteresis or demagnetization data, particularly coercive forces, therefore provide information about thermal fluctuations at temperatures well below the usual blocking temperatures of low-field TRM or VRM. We apply thermal fluctuation analysis (Dunlop) to high-temperature measurements of average coercive force and of the coercive force spectrum (from alternating-field demagnetization). The samples incorporate representative fractions (average grain sizes from 0.04 to 0.22 μm) of the lower pseudo-single-domain (PSD) range of magnetite and one fraction (1–5μm size spread) from the upper PSD range. The ordinary coercive force HC has the same temperature dependence as the softest 20–30 per cent of the coercive force spectrum, whereas the remanent coercive force HR has the same temperature dependence as the median demagnetizing field. HR is, therefore, a more meaningful average over the coercive force spectrum than HC. The predicted dependence of Neel's ‘fluctuation field’Hq on temperature, on grain size and on microscopic coercive force HK are followed quite closely by the smaller PSD grains, but not by the 1–5 μm grains. By extrapolation, the critical size for (room temperature) superparamagnetic behaviour in magnetite is found to be (250+25–10) A, in good agreement with previous experimental and theoretical estimates. On the basis of fluctuation analysis, domain walls with a minimum width ˜0.1μm carry the permanent magnetic moment of small multidomain magnetite grains. This wall moment, thermally activated as a unit, rotates in singledomain-like fashion. Wall displacement plays a secondary role in two-domain particles, at least in those<0.22 pm in size: fluctuation fields at the TRM blocking temperature, as inferred from the Neel wall displacement model of TRM, are inconsistent with observed (and predicted) magnitudes and size variation of Hq. However, in 1–5μm grains, although PSD moments are generally accepted as the explanation of their enhanced low-field TRM intensity, hysteresis and alternating-field demagnetization results do not give unequivocal evidence of the existence of PSD moments. High-field isothermal magnetic processes in these larger ‘PSD’ grains are equally well accounted for by wall displacements alone.

Magnetic Properties of Rare Earth Gallium Intermetallic Compounds

Abstract: Magnetic measurements on the single crystal RGa intermetallic compounds (R=Pr, Nd, Sm, Tb, Dy, Ho and Er) have been carried out in the 4.2 K to 350 K temperature range. All the compounds, except ErGa, are ferromagnetism with high coercive force along the easy direction. ErGa compound is antiferromagnetism. The magnetization curves for the heavy rare earth compounds were fairly well explained on the basis of the molecular field model, including the crystal field. But, for light rare earth compounds, the agreements between the experimental and the calculated values were not so good. Crystal field parameters B 2 0 and B 2 2 were determined from the anisotropy of paramagnetic Curie temperatures on the basis of the molecular field theory. These parameters were compared with those of the point charge model.

Permanent magnet alloy

Abstract: PURPOSE: To increase the Curie point and coercive force (Hc) of a permanent magnet and to improve the thermal stability by substituting Si for part of B in the magnet made of an intermetallic compound consisting of a rare earth metal, Fe, Co and B. CONSTITUTION: An alloy having a composition represented by formula 1 (where R is one or more kinds of rare earth elements, 0≤x≤0.5, 0.02≤y≤0.3, 0.002≤z ≤0.15, and 4≤A≤7.5) is melted in vacuum or a gaseous Ar atmosphere, and the molten alloy is cast into an ingot. This ingot is pulverized in a gaseous N 2 atmosphere, and the resulting powder is press-molded in a magnetic field of 15kOe. The molded body is sintered in a gaseous Ar atmosphere, and the sintered body is rapidly cooled and heat treated at 400W800°C. Thus, an intermetallic compound having a composition contg. Si substituted for part of B is formed, and a permanent magnet made of the compound consisting of rare earth metals, Fe, Co, B and Si is obtd. This permanent magnet has an increased Curie point, increased coercive force (Hc) and superior thermal stability. COPYRIGHT: (C)1985,JPO&Japio

The effect of the B s of recording head cores on the magnetization of high coercivity media

Abstract: Two types of recording heads, the cores of which had different values of saturation flux density B s and almost the same values of effective permeability, were prepared and the recording characteristics with Ni-Co plated discs of varying coercivity H c (410-900 Oe) were examined. The materials of the head cores were a sputtered alloy (Fe-Si-Al) film ( B_{s} = 11 000 G) and a single crystalline Mn-Zn ferrite ( B_{s} = 3800 G). Results showed that the B s of the head core had to be about five times the H c of the media to produce the beneficial effects of high H c on short wavelength recording. From this point of view, the alloy film head has the advantage in high linear density recording because of its large B s . By using an experimental laminated alloy film head of narrow-track width (60 μm), the core of which had a large effective permeability (such as 115 at 40 MHz), sine-wave signals of short wavelength (smaller than 1.5 μm) and of high frequency (such as 37 MHz) were recorded on a high H c (900 Oe) plated disc and reproduced with the same head successfully.

An electron microscope study of Sm‐Co‐Cu–based magnetic materials with the Sm2Co17 structure

Abstract: In an electron microscope study of aged Sm‐Co‐Cu–based magnetic materials with the Sm2Co17 structure, it is shown that the previously reported coercivities of these materials are a consequence of the presence of very fine precipitates of the Cu‐containing SmCo5 phase which act as pinning centers. In comparing the microstructures of alloys with different Sm/Co,Cu ratios, and with some Co replaced by Fe, a marked difference is observed both in the precipitate morphology and the fault density of the matrix phase. Possible reasons for this are discussed, and it is shown that the lattice misfit has a marked effect on the morphology.

Preparation by sputtering of thick sendust film suited for recording head core

Abstract: A 12 μm thick film with large saturation flux density, 11 000 G, and high effective permeability, such as 240 at 20 MHz, was prepared by using tetrode sputtering equipment from a ternary alloy that consisted of Si:10.5 wt.%, Al:5.6 wt.%, and Fe:bal. (Si rich Sendust alloy) at a target voltage of 1000 V DC . These were the best conditions for obtaining the minimum value of coercivity with the glass substrate in our experiments. The electric resistivity and Vickers hardness of the film became greater than the same qualities in bulk alloys. The increase in effective permeability at high frequencies, and the higher resistivity and hardness are believed to originate from the granular structure of the film. In this study, it was proved that the magnetic properties, especially the coercivity and the initial permeability of the film were due, not only to the effect of magnetostriction and crystalline anisotropy, but also to the effect of film structure.

Anomalous increase of coercivity in iron oxide powder coated with sodium polyphosphate

Abstract: An anomalous increase of coercivity in a non-stoichiometric acicular iron oxide powder which was treated with a chain like sodium metaphosphate was investigated. The coercivity increased by a factor of about 2 in the acicular iron oxides and about 3 in the granular one, respectively. The magnetization decreased in inverse proportion to the increased coercivity. The coercivity of the treated iron oxides which was washed with water to remove the sodium metaphosphate returned to the value or to less than the value of the untreated iron oxides. The large magnetic anisotropy was estimated from the magnetic hysteresis loop. A constricted hysteresis loop was observed in treated iron oxides. The experiment suggests that an origin of the increased coercivity is due to the surface magnetic anisotropy which is based on the electrostatic coupling, participating in a absorbed compound.

Scaling relations in magnetic, mechanical and superconducting pinning theory

Abstract: The coercive field in ferromagnetic materials, the critical shear stress in alloys, and the critical current density in type-II superconductors are represented in terms of powers of the defect density, the strength and the range of the defect interaction, and of the rigidity of the domain wall, dislocation line, or flux line lattice, for the three types of material respectively. Between the exponents of these parameters general relations are valid which have to be satisfied by each detailed theory. These scaling relations hold for both, attractive and repulsive obstacles. A general rule is given on how to combine the pinning forces of different kind of obstacles.

Sintered Ce-Co-Cu-Fe-Ti magnets

Abstract: Ce(Co‐Cu‐Fe‐Ti)z alloys were prepared with 5.6⩽z⩽7.4. They were ground into powders and sintered in Ar atmosphere. It was found that Ti enhanced coercivity and rectangularity. A permanent magnet with Br=7400 G, IHc=6100 Oe, (BH)max=13 MG Oe, and rectangular hysteresis loop was achieved on sintered samples having the composition Ce(Co0.727Fe0.12Cu0.14 Ti0.013)6.5.

Low‐Field Magnetic Properties of Amorphous Alloys

Abstract: The transition-metal-based ferromagnetic amorphous alloys made by the continuous splat-quenching method are excellent soft magnetic materials with low coercivity. Furthermore, their low-field magnetic properties are sensitive to heat treatments and can be greatly improved by such treatments. A review of low-field properties of these alloys such as anisotropy, remanence, and coercivity (as-received and after heat treatments) is presented together with data on the Fe/sub 80/B/sub 20/ amorphous alloy and the effect of fast-neutron irradiation.

Evaporated amorphous GdCo-films with perpendicular anisotrophy by controlled oxygen contamination

Abstract: Vacuum deposited amorphous GdCo-films with strong perpendicular magnetic anisotropy were prepared by thermal evaporation of the constituents at an oxygen pressure during film fabrication between 2.5 × 10-8Torr and 2 × 10-7Torr. K u -values above +2.3×105erg/cm3at 4 ±M s = 1000 Gauss were obtained in this pressure range. The coercivity of films with 4 ±M s = 1000 Gauss increases from 15 Oe at 2.5 × 10-8Torr to 150 Oe at 1.5 × 10-7Torr O 2 . In this paper the fabrication technique of these films is described and the measured magnetic and physical properties are reported and discussed. A model to explain the origin of the perpendicular anisotropy is proposed.

Magnetic properties and magnetic reversal peculiarities of single crystals of R(Co 1-x Ni x ) 5 compounds

Abstract: The basic properties (saturation moments, Curie temperatures, anisotropy constants) and magnetic reversal processes have been studied systematically on the bulk single crystals of quasibinary solid solutions R(Co 1-x Ni x ) 5 , where R = Y, La, Ce, Sm, and Gd. The magnetic reversals have been studied also on the single crystals of similar compounds with R = Nd, Dy, and Er. The general property of all studied systems is a very high coercive force H c connected with domain wall pinning in compounds of quasibinary region. The H c (x) dependenses for all systems are similar qualitatively. There exists a maximum of H c in region x = 0.6 - 0.7 for compounds with trivalent R, and in region x = 0.3 - 0.4 for compounds with tetravalent R. The single exception is the Gd system, where H c has rather low values for all x. The explanation is suggested for H c (x) dependence in which the compositional heterogeneities are considered as the main reason of domain wall pinning in the quasibinary compounds. These heterogeneities are always present as a result of statistical nature of Ni substitutions for Co.

The Preparation, Characterization and Magnetic Properties of Synthetic Analogues of Some Carriers of the Palaeomagnetic Record

Abstract: Synthetic analogues of two magnetic mineral systems have been prepared: (i) a titanomagnetite system Fe2.4-δAlδTi0.6O4 (0<δ<0.45). (ii) a titanomaghemite system prepared by oxidation of Fe2.4-δMgδTi0.6O4(0<δ<0.35). The following properties were determined: unit cell edge, Curie temperature, thermomagnetic behaviour, saturation magnetization, isothermal remanence and coercive force. The introduction of Al, Mg and vacancies into titanomagnetites reduces saturation magnetization and coercive force. Curie temperature is lowered by the presence of Al and Mg ions but rises with increasing concentration of vacancies.

Dependence on metalloid content for magnetic properties of Fe-Si-B metallic glasses

Abstract: On the splat-cooled metallic (Fe .79 Si .21 ) 1-x B x . glasses the concentration dependence of the magnetic properties has been investigated. For representative specimens, magnetic domain structures have been observed. The crystallization temperature increases and the Curie temperature monotonously decreases from 713 K to 671 K contrary to the result for reported Fe-P-C or Fe-P-B glasses as the metalloid content is increased. The highest value of crystallization temperature (797 K) is about 150 K higher than those for iron-rich glassy alloys reported up to now. The coercive force also depends on the boron content and the lowest value (0.04 Oe) has been obtained at about X=0.10. The magnetization process is discussed in the light of domain patterns observed.

An investigation of saw-tooth domain walls in Ni/Fe/Co films

Abstract: Head-on domains having saw-tooth wall configurations in Ni/Fe/Co films are discussed in relation to the existing theoretical models The existence of Neel tails within such wall structures as previously proposed is supported and discussed with respect to: (a) direct magneto-optic observations and measurements, and (b) a comparison of the predicted saw-tooch amplitude and angles with the observed values A large number of films were investigated and the wall angles were found to be dependent upon the ratio of the coercivity to anisotropy field, Hc/Hk, as predicted in the Neel tail model, whilst the saw-tooth amplitude was determined predominantly by the coercivity Effective wall widths of the order of 100 mu m have been observed in 014 mu m thick 80/17/3 Ni/Fe/Co films

Magnetic recording and photo reproducing system

Abstract: PURPOSE: To achieve higher density of recording and perform writing and reading with good efficiency by using a composite medium having a write layer of high coercive force permitting vertical magnetization as a magnetic disc and a soft-magnetic read layer of high magneto-optic effect. COPYRIGHT: (C)1979,JPO&Japio

Process for producing ferromagnetic metal powder

Abstract: A ferromagnetic metal powder having a narrow distribution of coercive force produced by reducing a salt of a metal capable of forming a ferromagnetic material as an aqueous solution thereof, where the reducing reaction is carried out while maintaining the temperature of the aqueous solution during and/or after the reduction reaction at about 20° C higher than the temperature or less of the aqueous solution at the immediate beginning of the reduction reaction.

Bubble materials characterization using spatial filtering techniques

Abstract: A technique for utilizing the spatial coding of information in an optical system is applied to the problem of bubble materials characterization. A simple system for static characterization of characteristic length and magnetization is described and the results of measurements presented. This concept is then extended to the measurement of coercivity.