Showing papers on "Coercivity published in 1981"

Kinetic effects in coercivity measurements

Abstract: The coercivity of a magnetic material is a crucial parameter in determining its use in recording. The apparent value of this quantity is found to depend upon the time scale of interest. A method of measurement that observes changes of magnetization on a short time scale will yield a larger coercivity than one that measures changes over a longer time scale. The cause of this phenomenon is thermally assisted reversal of magnetic domains. Thus, for a magnetic recording material, the coercivity relevant to a high-speed writing process is greater than that relevant to transition broadening and long-term storage stability. The difference between the short-and long-term coercivities becomes more pronounced as the size of the magnetic switching unit becomes smaller. Therefore, these kinetic effects will become increasingly important in determining the ultimate limitations of future high-density media. Data are presented that illustrate the variation of coercivity with time scale in presently used and experimental recording materials.

A review of coercivity mechanisms (invited)

Abstract: Mechanisms of magnetization reversal and factors determining coercivity are reviewed for various permanent‐magnet materials.

Microstructure and properties of step aged rare earth alloy magnets

Abstract: Alloys with compositions Co‐25.5 wt % Sm‐8 wt % Cu‐15 wt % Fe‐3 wt % Zr and Co‐Sm‐Cu‐Fe‐1.5 wt % Zr have been step aged to produce magnets with coercive force (iHc) in the range of 10–25kOe, much higher than those reported so far in the literature for the Zr alloys. The high coercive force magnets are typically aged at 800–850 °C for 10–30 hours following the solution treatment at 1150 °C. Subsequently, these are step aged to produce materials with high coercivity. The microstructure in all these alloys has a two phase cellular morphology with 2:17 phase surrounded by a 1:5 boundary phase. The long aging treatments at 800–850 °C lead to coarsening of the two phase structure. The subsequent step aging does not change the morphology, but only changes the chemical composition of the two phases. Best properties are obtained in materials with a coherent microstructure of optimum boundary phase thickness and optimum chemical composition. The highest values of iHc obtained so far are ∼26kOe and ∼16kOe for the 3% Zr and 1.5% Zr alloys respectively. The best hard magnetic properties of (BH) max = 33 MGOe and iHc = 13kOe are for a 25% Sm‐20%Fe‐4 Cu‐2% Zr alloy.

Magnetic properties of amorphous and crystallized (Fe0.82B0.18)0.9Tb0.05La0.05

Abstract: The magnetic properties of (Fe0.82B0.18)0.9Tb0.05La0.05 in both the amorphous and crystallized states are presented. When amorphous, the alloy is magnetically soft and has a low coercive force, comparable to the corresponding alloy without rare earths. Upon annealing near the crystallization temperature of 900 K, however, the intrinsic coercive force at 300 K rises to 9 kOe, with a remanent magnetization of slightly less than 5 kG. In the amorphous state this and related alloys appear to have potential for use in devices requiring large, isotropic magnetostriction, while in the crystallized state they appear potentially useful as low cobalt permanent magnets.

Domains, domain walls and the coercive field of amorphous ferromagnets

Abstract: Magnetic properties of the hysteresis loop of magnetostrictive amorphous ferromagnetic alloys are predominantly determined by the magnetoelastic coupling energy between the spontaneous magnetization and internal stresses. Domain structures as well as the pinning of domain walls are governed by long-range and short-range stresses, respectively. As sources of elastic stresses we have detected quasidislocation dipoles exerting stress fields which vary as 1/r2. The coercive field is shown to be determined by four types of interactions : 1) volume pinning by stress sources; 2) intrinsic fluctuations of material properties; 3) surface irregularities; 4) relaxation phenomena.

Magnetic and structural properties of Rh substituted Co‐Cr alloy films with perpendicular magnetic anisotropy

Abstract: RF sputtered Co0.78−xCr0.22Rhx (where x is 0–0.06) alloy films for high density perpendicular recording were studied. First, Co‐M (M = Cr, Ti, V, Mo, Pd, and Rh) alloy films were studied. They exhibit hcp crystallization and have a high orientation of the c‐axis normal to the film plane. However, they have a smaller coercive force (less than 300 Oe), and with the exception of Co‐Cr, they do not have a columnar structure or satisfy the relation Hk ≳4πMs. Next, Co‐Cr‐Rh alloy films were studied. They also exhibit hcp crystallization and their structure is columnar. They show perpendicular magnetic anisotropy similar to that of Co‐Cr, and in particular, a higher c‐axis orientation was obtained. The ΔΘ50, half the width of a rocking curve of a (002) peak, is about 20–40% smaller than that of Co‐Cr. By measuring the recording characteristics of Co‐Cr‐Rh disks (on an anodized aluminum substrate), a maximum recording density of 23,000FRPI was obtained.

Preparation of barium ferrite films with perpendicular magnetic anisotropy by DC sputtering

Abstract: Barium ferrite thin films with perpendicular magnetic anisotropy have been prepared by DC diode sputtering. When a sintered target with appropriate composition was used, c-axis oriented Ba ferrite film was obtained whose easy direction of magnetization was normal to the film plane. Its saturation magnetization Ms, coercive force Hc\perp and magnetic anisotropy constant Ku were 400 emu/cc, 680 Oe and 1.67 × 106erg/cc, respectively. Ba ferrite films may be usable as are Co-Cr films as a perpendicular magnetic recording medium.

The micromagnetic propertis of high-coercivity metallic thin films and their effects on the limit of packing density in digital recording

Abstract: The micromagnetic structures of the high-coercivity, isotropic, and high-squareness thin films of sputtered Co-Re have been investigated using transmission electron microscope (TEM) Lorentz imaging and electron deflection methods. From the behavior of the magnetic ripple structure under applied field and the configuration of the local surface fields observed in these experiments, the existence of magnetic clusters in these films was verified. Based on the interpretation of the field dependence of the ripple formation and the hysteretic properties of the film, it is concluded that the formation of the magnetic clusters is a spontaneous process resulting from intercrystalline interactions and local inhomogeneities in the anisotropy. The effects of such cluster formation on longitudinal magnetic recording were investigated. The results show that the reduction of dipole energy at the transition region between two oppositely magnetized regions can be achieved by a stepwise rotation of the magnetization vector of an individual cluster in the form of a vortex. This type of rotation creates a finite transition length which is limited by the size of the magnetic cluster of the film. Consequently, it is concluded that the maximum packing density for saturation recording in these types of films would be less than that predicted by the phenomenological equation, which was derived based solely on considerations of the demagnetization field and the coercivity of the film.

Combined radio and magnetic energy responsive surveillance marker and system

Abstract: A surveillance marker has a semiconductor connected between arms of a dipole antenna, one arm being of non-ferrous conductive material and the other of high permeability low coercivity material such as permalloy. Higher coercivity magnetizable pieces are disposed adjacent the low coercivity arm, and all is sandwiched between paper or other insulating layers. The marker is detected by a combination of RF field and low frequency magnetic field generating and receiving units that energize a master alarm only when both units simultaneously detect the marker signals. Magnetizing the higher coercivity pieces suppresses a signal that would otherwise be reradiated by the marker from the high permeability arm thereby rendering the marker undetectable.

Co-Cr perpendicular recording medium by vacuum deposition

Abstract: Magnetic and structural properties, crystal orientation, resistance to atmospheric corrosion, and depth profile of vacuum deposited Co-Cr perpendicular anisotropy films are presented. In contrast to the RF sputtered Co-Cr films having a rather small deposition rate, the present study confirms the applicability of vacuum deposition to the preparation of Co-Cr perpendicular anisotropy films at a deposition rate up to 6000 A/sec. It was found that films with relatively high magnetization and coercive force could be made which at the same time exhibited good corrosion resistance.

Crystallization of amorphous Fe81B13.5Si3.5C2

Abstract: Crystallization of 25‐mm wide ribbon of metallic glass Fe81 B13.5Si3.5 C2 (Metglas 2605SC) has been studied by differential scanning calorimetry, measurement of magnetic coercivity, x‐ray diffraction, and transmission electron microscopy. Samples were annealed isothermally for 5–180 min between 345 and 450 °C. Dendritic precipitates of alpha iron are detected after anneals ?375 °C and cause the first exothermic peak. A second exothermic peak corresponds to crystallization of the balance of the matrix primarily into body‐centered‐tetragonal (bct) grains. The results are collated in a time‐temperature‐coercivity diagram. The minimum coercivity after field annealing is 0.03 Oe.

Coercive force and 90° domain wall motion in ferroelectric PLZT ceramics with square hysteresis loops

Abstract: The polarization reversal of some tetragonal-phase PLZT ceramics is studied. The results are explained in terms of 90° domain wall motions. A reported theory originally derived for rhombohedral-phase PLZT 6/65/35 ceramics is shown to be applicable to tetragonal-phase PLZT ceramics also by introducing the remanent degree of 90° domain alignment γ90. As a result, the calculation of coercive force is possible for various PLZT compositions considering the interaction of 90° domain wall motions and reacting forces established by the internal stress state of ferroelectric ceramics. The frequency dependence of coercive force and measurements of switching strain, including depolarization data obtained from either mechanically or electrically produced external forces, confirm the interpretation of predominantly occuring 90° reorientations and are consistent with the extended theory.

Observation of large room‐temperature coercivity in melt‐spun Nd0.4Fe0.6

Abstract: An intrinsic room‐temperature coercivity of 7.45 kOe has been found in melt‐spun Nd0.4Fe0.6; this value is the largest ever reported for a rare‐earth–iron alloy. A significant correlation was found between quench rate and coercivitiy, suggesting that the quenched alloys consist of fine crystallites or clusters, whose average particle size can be matched to the single‐domain optimum by the appropriate quench rate.

Magnetic properties of individual acicular particles

Abstract: A review is made of the work on the measurement and interpretation of the properties of individual particles, of the type incorporated into recording tape. It was shown that the remanent coercive force H r (O) of particles of any of the materials measured covered a range of several hundred Oersted. A high proportion of the particies were multiple, that is they comprised several particles lying side by side: it is known that most of the particles in recording tape are of this type. An investigation was made of the nature of the reversal process, and of the origin of the range in H r (O), for particles of γ-Fe 2 O 3 , CrO 2 and metal powder (iron). The variation of H r (0) with angle between the field and particle axis was determined for γ-Fe 2 O 3 and CrO 2 . The results of the above work were used in an attempt to synthesize the remanent loop of a tape from the knowledge of the properties of its constituent particles.

High coercivity rare earth-iron magnets

Abstract: Ferromagnetic compositions having intrinsic magnetic coercivities at room temperature of at least 1,000 Oersteds are formed by the controlled quench of molten rare earth-transition metal alloys. Hard magnets may be inexpensively formed from the lower atomic weight lanthanide elements and iron.

On the coercivity of cobalt-ferrite epitaxial iron oxides

Abstract: Acicular γ-Fe 2 O 3 particles were treated in alkaline solution containing cobaltous and ferrous ions. When cobalt content is increased, the surface area of the oxides decreases, and cobalt-ferrite is considered to be crystallized on the surface of γ-Fe 2 O 3 . The coercivity of the oxides remarkably increases with increasing the cobalt content, and the increase of coercivity is considered to be due to the crystalline magnetic anisotropy of the cobalt-ferrite. The coercivity of the oxides is explained by considering the shape anisotropy of acicular particles and the crystalline magnetic anisotropy of cobalt-ferrite.

Recording tapes using iron nitride fine powder

Abstract: Magnetic recording pigment of Fe 4 N particles was prepared by nitrogenizing acicular metal iron powder. The treatment to obtain stoichiometric Fe 4 N was carried out at 400°C for 3 hours in NH 3 -H 2 (3:1) mixed gas. The saturation magnetization and coercive force were 120 emu/g and 700 Oe respectively. The Curie point of the powder coincided well with that of bulk material. Test tapes were prepared with this Fe 4 N powder. The remanent magnetic flux density (Br) of the tape was 2200 G and the coercive force was 670 Oe. From test immersion in saline solution, it is confirmed that the chemical stability of the Fe 4 N tape is superior to the Fe 4 N tape is superior to the metal tape.

Co-Cr films with perpendicular magnetic anisotropy

Abstract: The magnetic properties of RF sputtered Co-Cr films have been studied over a range of preparation conditions extending previous work. Reproducible well-developed perpendicular anisotropy, as would be required for perpendicular recording, was found when the deposition conditions were maintained at an optimum of 170°C substrate temperature, 1.5 KV RF voltage and 5 microns argon pressure. Film thickness was nominally 1 micron. The optimum deposition rate in our system was 120 angstroms/minute due to the differential sputtering rates of Co and Cr. The resulting films had coercivities up to 1500 oersteds and (00.2) hcp X-ray peak width ΔΘ 50 of 5 degrees, with secondary X-ray peaks observed as well. Columnar growth was observed with grains less than 800 angstrom diameter. A sublayer of poor orientation was observed. Magnetic moment was observed to be about 35% higher than that reported by Iwasaki. Initial background pressure less than 6 × 10-7torr was essential for formation of well-oriented films. The presence of nitrogen caused the appearance of the fcc phase and the loss of perpendicular coercivity. Loss of coercivity also occurred when film composition was nonuniform.

SEM studies of magnetic domains in amorphous ribbons

Abstract: Static and dynamic observations of magnetic domains in amorphous Fe-B-Si and Fe-B-Si-C ribbons have been made employing a 200 kV scanning electron microscope (SEM) and a special stage allowing in situ application of mechanical stresses and dc and ac magnatic fields. Average domain width in the demagnetized state is found to decrease with increasing demagnetizing frequency, and to be relatively insensitive to tensile stress. Under ac conditions, a great range in the mobility of individual walls is often observed. Some domain walls are strongly pinned, in many cases by obvious surface defects. The implications of these and other observations on hysteresis and eddy-current losses, coercivity, and permeability are discussed.

Ferromagnetism in metallic intercalated compounds FexTaS2 (0.20⩽x⩽0.34)

Abstract: Magnetic susceptibility, electrical resistivity, Mossbauer effect (ME) and transmission electron microscopy measurements in single crystals of the metal intercalated layer compounds FexTaS2 (0.20⩽x⩽0.34) are reported. The compounds are metallic, ferromagnetic and have large coercive force. ME measurements show the iron to be high‐spin Fe2+, with a distribution of hyperfine fields at low temperatures. Transmission electron microscopy suggests the existence of clusters. These results may explain the diversity of magnetic preperties on nonstoichiometric intercalated compounds such as FexTaS2.

Effects of surface treatment and thinning on magnetic properties of rapidly quenched amorphous alloy ribbons

Abstract: The magnetization curves and hysteresis loops of rapidly quenched transition‐metal–glass‐former amorphous ribbons of various compositions were measured as they were chemically thinned, down to about 10% of their original thickness. The specific saturation moment did not change showing that the ribbons are chemically homogeneous through their thickness. In general, the approach to saturation of as‐cast ribbons remained remarkably constant. Although annealing experiments indicate that the approach to saturation is controlled by stresses, they are not distributed in such a way that they are affected by thinning of the sample. The coercive force initially changed rapidly, either up or down, with decreasing thickness, then increased linearly with reciprocal thickness. The results can be simply interpreted in terms of surface and volume pinning.

Mn‐Al‐C for permanent magnets (invited)

Abstract: Mn‐Al‐C alloy for permanent magnets has merit for several reasons: a) carbon stabilizes the metallurgical phases, deformation temperature is high enough for plastic transformation to produce anisotropic τ‐phase efficiently; b) useful magnetic energy exceeds the best hexaferrites; the alloy can be more efficient than Alnico 8 and some other high‐cobalt materials; c) Mn sources are more widely distributed and the metal is much less costly to obtain than cobalt. Carbon control and tooling life are key factors to obtain a commercial product. Micrographs illustrate the metallurgical phases. Although Mn‐Al‐C has a lower temperature coefficient for magnetic flux changes than the hexaferrites, its maximum use will be limited to 100–125 °C. Certain energy‐efficient electric motors could be an important application.

Recent developments in magnetic recording materials (invited)

Abstract: Modern recording surfaces call for small, uniform, highly dispersible and orientable particles with high coercivity and an appropriate moment density or, alternatively, thin films having similar properties. Greatly improved particles of all kinds have been introduced in the last few years. The list includes iron oxides with compositions in the range Fe2O3–Fe3O4 (Hc = 250–450 Oe), cobalt‐modified iron oxides and chromium dioxide (Hc = 450–700 Oe), and particles of iron, cobalt and their alloys (Hc = 700–1100 Oe). Increasing coercivity corresponds at present to lesser commercial importance and unfortunately, the more attractive magnetic properties seem almost invariably to be associated with thermal, mechanical or chemical instability. Metallic films continue to be attractive because of their thinness, magnetic material undiluted by non‐magnetic material, flexibility of magnetic properties and low noise, but their disadvantages include poor reproducibility, poor resistance to wear and propensity to high lev...

Manufacture of amorphous thin film

Abstract: PURPOSE: To obtain an amorphous thin film with low coercive force and high initial permeability by sputtering or vapor-depositing an alloy consisting of a nonmetallic element and a rare earth element replaced partially by a metallic element such as Ti, Zr or Hf and the balance essentially transition metallic element. CONSTITUTION: An alloy expressed by a formula M a T b X c Z d is prepared. In the formula M is ≥1 kind of element selected from Fe, Ni and Co, T is ≥1 kind of element selected from Mn, Cr, W, etc., X is ≥1 kind of element selected from Zr, Ti, Y, etc., Z is ≥1 kind of element selected from P, B, C, etc., a+b+c+d= 100, 0≤b≤95, 30≤a+b≤95, 0≤c≤70and 0≤d≤30. The alloy is sputtered or vapor-deposited to manufacture an amorphous thin film. This film is different from a conventional amorphous thin film in alloy composition and has high thermal stability. when the film is magnetized, it shows superior magnetic characteristics, that is, low coercive force and high permeability. COPYRIGHT: (C)1983,JPO&Japio

Coercivity and unit particle size of metal pigment

Abstract: Iron based metal pigments exhibiting coercivity as high as 1000-1400 Oe, and iron nitride (Fe 4 N) pigments exhibiting coercivity as high as 500- 800 Oe for magnetic recording media was prepared from acicular particles of iron-cobalt oxalate, iron oxalate, iron oxides or iron oxyhydrates by reduction in hydrogen or following nitridation in ammonia. The particles of the pigment maintained the characteristic original shape of the oxalates, oxides or oxyhydrates. In addition, the particles contained a number of pores in them, and the particles consisted of small unit particles linked together to form a stereo-network structure. The coercivity of the pigment was related to the pore size, the specific surface area and the crystallite size. A study of the unit particle which is essentially related to the single domain particle was made. The size of the unit particle was determined by an electron microscope (D1), calculated from the pore size (D2), calculated from the specific surface area (D3) and determined by X-ray diffractometry (Lc). At the higher coercivity level of the metal pigments, D2, D3 and Lc are all in good agreement, that is, they all have a nearly spherical single crystallite as large as 200-300 A which plays the role of a single unit particle. In the case of iron nitride (Fe 4 N) pigment, D2 and Lc are in good agreement, at the value of 300-400 A.