Showing papers on "Coercivity published in 1990"

Grain size dependence of coercivity and permeability in nanocrystalline ferromagnets

Abstract: Amorphous ribbons of composition Fe/sub 74.5-x/Cu/sub x/Nb/sub 3/Si/sub 13.5/B/sub 9/ (x=0, 1 at.%) have been annealed between about 500 degrees C and 900 degrees C. This produced a series of crystallized samples with grain sizes between about 10 nm and 300 nm and with coercivities H/sub c/ and initial permeabilities mu /sub i/ varying over several orders of magnitude. The best soft magnetic properties (H/sub c/ approximately=0.01 A/cm and mu /sub i/ approximately=80*10/sup 3/) were observed for the smallest grain sized of about 10 nm. With increasing grain size D, coercivity steeply increases following a D/sup 6/-power law (up to D approximately=50 nm). H/sub c/ then runs through a maximum of H/sub c/ approximately=30 A/cm and decreases again for grain sizes above 150 nm according to the well-known 1/D law for polycrystalline magnets. The initial permeability was found to vary in a similar manner, essentially being inversely proportional to coercivity. The variation of the soft magnetic properties with the average grain size is discussed and compared with the predictions of the random anisotropy model and other theories for the magnetization reversal. >

Identification of ferromagnetic minerals in a rock by coercivity and unblocking temperature properties

Abstract: The common ferromagnetic minerals have distinctive, characteristic coercivities and thermomagnetic properties. The analysis of the acquisition curve of isothermal remanent magnetization (IRM) is a useful but often ambiguous diagnostic technique. For a more conclusive interpretation, IRM acquisition must be combined with subsequent thermal demagnetization of the IRM. A modification of this method is proposed as a more powerful analytical technique. Different coercivity fractions of IRM are remagnetized in successively smaller fields along three orthogonal directions. The thermal demagnetization of each orthogonal component of the composite IRM is then plotted separately. This method often gives a clearer interpretation of the ferromagnetic mineral content of a rock. Examples are described for limestone and sandstone samples.

Time-dependent magnetic phenomena and particle-size effects in recording media

Abstract: Particle size is very important in determining the behavior of magnetic recording media. Particles must be large enough to provide adequate stability against time-dependent magnetic effects; these effects are seen as a dependence of coercivity on the duration of field application, so that the coercivity relevant to long-term storage stability can be significantly less than that relevant to high-frequency writing. On the other hand, the need for high signal-to-noise ratios, narrow transitions, and smooth tape or disk surfaces dictates that particles be as small as practical. Optimum particles for high-density recording will represent a compromise between these conflicting requirements. >

High coercivity in Sm2Fe17Nx magnets

Abstract: Using mechanical alloying and a subsequent two‐step heat treatment we produced magnetically isotropic microcrystalline Sm2Fe17Nx samples with room‐temperature coercivities up to 24 kA/cm (30 kOe). The remanence and the energy product are equivalent to similarly prepared Nd‐Fe‐B samples, but the properties at elevated temperatures are superior because of the high Curie temperature of 470 °C and the large anisotropy field of 14 T at room temperature. From differential scanning calorimetry it is concluded that the 2:17 nitride is metastable. It decomposes into Sm nitride and α‐Fe above 600 °C.

Hysteresis properties of ultrathin ferromagnetic films

Abstract: We have investigated the hysteresis properties of Au/Co/Au films with ultralow Co thicknesses and perpendicular easy axis. At low temperature we observe a very strong thickness dependence of the coercivity, whereas striking dynamical effects are present at room temperature. We propose a model of wall motion which provides a consistent explanation of the overall observations. This interpretation emphasizes the crucial role played by the roughness with respect to the hysteresis properties of ferromagnetic ultrathin films.We have investigated the hysteresis properties of Au/Co/Au films with ultralow Co thicknesses and perpendicular easy axis. At low temperature we observe a very strong thickness dependence of the coercivity, whereas striking dynamical effects are present at room temperature. We propose a model of wall motion which provides a consistent explanation of the overall observations. This interpretation emphasizes the crucial role played by the roughness with respect to the hysteresis properties of ferromagnetic ultrathin films.

Longitudinal media for 1 Gb/in/sup 2/ areal density

Abstract: Media with low noise at high transition density that demonstrate satisfactory recording performance at an areal density, of 1 Gb/in/sup 2/ when combined with dual-element (magnetoresistive read/inductive write) heads have been fabricated. A media structure of C/CoPtCr/Cr was utilized over a range of magnetic parameters: coercivity approximately=1600-1800 Oe, remanence-thickness product approximately=0.7*10/sup -3/ emu/cm/sup 2/, and coercive squareness approximately=0.7-0.8. Media noise reduction was accomplished by optimizing the film-growth characteristics to reduce intergranular exchange coupling in the magnetic layer. The low-noise characteristics of the media are manifested in their low transition jitter values, 5 nm for 3- mu m track width, and the absence of supralinear increase in media noise power with linear density up to 3000-3500 fc/mm. The -6-dB rolloff densities are in the range 4000-5000 fc/mm. Overwrite values are typically better than 40 dB. Microstructural analysis indicates that the reduced transition noise of the present media is due to physical separation of the grains in the magnetic films, which reduces the exchange coupling between the magnetic grains. The reduced coercive squareness of the low-noise media degrades the overwrite performance and is also expected to decrease the linear density resolution of the media. >

Gigabit density recording using dual-element MR/inductive heads on thin-film disks

Abstract: Inductive-write and magnetoresistive (MR)-read dual-element heads of very narrow trackwidths and narrow gaps have been designed, fabricated, and recording-tested on thin-film media of high coercivity and squareness. Results show excellent writability at modest write currents of 50 mA, large readback amplitudes with good signal-to-noise performance, very high linear-density resolution up to 5000 fc/mm (-6-dB density), and trackwidths close to the design dimensions. These results show that such head-disk systems are capable of recording operations at a target areal density of 1 Gb/in/sup 2/. Another result is that novel techniques for measuring nonlinear transition shifts as well as offtrack and squeeze performance have been developed. >

A Molecular Ferromagnet with a Curie Temperature of 6.2 Kelvin: [Mn(C5(CH3)5)2]+[TCNQ]-.

Abstract: The study of magnetic phase transitions in insulating molecular solids provides new insights into mechanisms of magnetic coupling in the solid state and into critical phenomena associated with these transitions. Only a few such materials are known to display cooperative magnetic properties. The use of high-spin molecular components would enhance intermolecular spin-spin interactions and thus a series of chargetransfer (CT) salts have been synthesized that utilize the spin S = 1 molecular cation, [Mn(C5(CH3)5)2]+ (decamethylmanganocenium). The structure and cooperative magnetic behavior of [Mn(C5(CH3)5)2]+[TCNQ- (decamethylmanganocenium 7,7,8,8-tetracyano-p-quinodimethanide) are reported. This salt is a bulk molecular ferromagnet with the highest critical (Curie) temperature (Tc = 6.2 K) and coercive field (3.6 x 103 gauss), yet reported for such a material.

High speed magneto-resistive random access memory

Abstract: A high speed read MRAM memory element is configured from a sandwich of magnetizable, ferromagnetic films surrounding a magento-resistive film which may be ferromagnetic or not. One outer ferromagnetic film has a higher coercive force than the other and therefore remains magnetized in one sense while the other may be switched in sense by a switching magnetic field. The magneto-resistive film is therefore sensitive to the amplitude of the resultant field between the outer ferromagnetic films and may be constructed of a high resistivity, high magneto-resistive material capable of higher sensing currents. This permits higher read voltages and therefore faster read operations. Alternate embodiments with perpendicular anisotropy, and in-plane anisotropy are shown, including an embodiment which uses high permeability guides to direct the closing flux path through the magneto-resistive material. High density, high speed, radiation hard, memory matrices may be constructed from these memory elements.

Effect of energetic bombardment on the magnetic coercivity of sputtered Pt/Co thin‐film multilayers

Abstract: Pt/Co multilayers are an attractive candidate for a magneto‐optical recording medium. However, films sputter deposited in Ar have coercivities too small (100–350 Oe) to be practical in recording. By sputter depositing multilayers in Kr or Xe instead of Ar, we achieved coercivities ∼1000 Oe, suitable for recording. We attribute the lower coercivity of Ar‐sputtered films to interfacial mixing of Pt and Co layers by energetic bombardment from Ar gas atoms that recoil from the Pt target.

Very large magnetoresistance effects induced by antiparallel magnetization in two ultrathin cobalt films

Abstract: Magnetoresistance (MR) and magnetization measurements by SQUID have been performed on Au/Co.Au/Co/Au multilayers in which the two ultrathin cobalt films have perpendicular magnetizations and different coercive fields Hc1 and Hc2 due to their different thicknesses. The two Co layers are magnetically uncoupled, as shown by their well separated square hysteresis loops. The MR exhibits a plateau for magnetic fields H such as Hc1

Granular Fe in a metallic matrix

Abstract: Ultrafine Fe particles with coercivities in excess of 500 Oe have been obtained by rapid annealing of sputtered metastable FexCu1−x alloys. The enhanced magnetic properties of these phase‐separated materials are typical of single‐domain Fe grains, and can be controlled by the annealing temperature. The Cu matrix, however, is highly conducting, unlike conventional granular materials where the matrix is generally insulating. The evolution of the magnetic properties and microstructure during annealing are described.

Batch fabricated sensors for magnetic force microscopy

Abstract: In this letter we describe force sensors suitable for magnetic force microscopy (MFM) made by coating silicon microcantilevers with various thin magnetic films. These MFM force sensors can be batch fabricated and their magnetic properties tailored by choosing appropriate coatings. This is demonstrated by analyzing the MFM response for magnetization patterns written on a longitudinal recording test sample. Selectable components of the sample stray field can be measured by coating the tips with high coercivity films. Tips coated with low coercivity films such as Permalloy respond to the total stray field. Sample perturbations due to the tip stray field are small for thin‐film tips, allowing the imaging of low coercivity samples such as Permalloy.

rf‐diode‐sputtered iron nitride films for thin‐film recording head materials

Abstract: We have obtained iron nitride films with a saturation magnetization of 21 kG, coercivity less than 1 Oe, and hard axis permeability of 4000. The saturation magnetostriction decreased as input power increased, or as the N2/Ar flow rate ratio decreased. X‐ray analysis revealed that an increase in input power or a decrease in the flow rate ratio resulted in less γ’‐Fe4N phase in the films deposited. The coercivity was also very sensitive to these variations. The iron nitride films did not show structural changes under vacuum annealing from 150 to 300 °C, but their coercivity decreased slightly. The coercivity increased rapidly after annealing above 350 °C, however.

High coercivity in mechanically alloyed Sm‐Fe‐V magnets with a ThMn12 crystal structure

Abstract: Microcrystalline Sm‐Fe‐V magnets with a ThMn12 crystal structure were prepared by mechanical alloying and a subsequent reaction heat treatment. In Sm15Fe70V15 a coercivity of 11.7 kOe is obtained. This is the best value reported so far for 1:12 magnets. As expected from their high anisotropy fields, the 1:12 magnets, therefore, show coercivities which are common in the Nd‐Fe‐B system.

Improved corrosion and temperature behaviour of modified Nd-Fe-B magnets

Abstract: The characteristics of the high energy product NdFeB magnets obtained by the powder metallurgy route have been improved by appropriate vanadium additions. Specific combinations of V and Co lead to an intrinsic coercive force of about 1350 kA/m (17 kOe) in Dy-free magnets, without any large decrease of remanence. If 1.5-at.% Dy is added, load line down to -B/ mu H=0.5 can be achieved at 180 degrees C without irreversible losses. Corrosion resistance is improved by blocking the selective oxidation of the Nd-rich phase. As a consequence, organic-coated magnets are able to withstand 15 days at 115 degrees C-1.75 bars in a H/sub 2/O saturated atmosphere without any degradation. It is concluded that this set of new properties will enlarge the application fields of Nd-F-B permanent magnets. >

Role of atomic mobility in the transition noise of longitudinal media

Abstract: The relationship between grain growth morphology and the transition noise of high-density CoPtCr media on Cr underlayers is examined. The growth morphology is shown to depend on the sputtering pressure, substrate bias, and substrate temperature. Development of isolated magnetic grains is promoted when the mobility of the sputtered atoms is reduced, i.e. at high sputtering pressure, low substrate temperature, and no substrate bias. As the magnetic grains become more isolated, the media transition noise is significantly reduced, accompanied by a simultaneous reduction in the coercive squareness. This indicates that the transition noise is primarily governed by the intergranular exchange coupling among the magnetic grains, which can be optimized by controlling the grain growth morphology. The microstructural features of the decoupled media correspond to the zone I structure in J.A. Thornton's (1986) microstructure zone diagram. >

Magnetic recording medium

Abstract: PURPOSE:To control a peak shift to be small by employing an alloy magnetic film having a prescribed chemical formula. CONSTITUTION:The magnetic recording medium having a construction wherein a nonmagnetic ground film 2, an alloy magnetic film 3 and a protective film 4 are formed in this sequence on a nonmagnetic substrate 1. This alloy magnetic film 3 has a composition expressed by a formula Co100-X-Y-ZCrXPtYTaZ, wherein X is 7 to 12, Y 4 to 10 and Z 0.5 to 1.8 (atom %). For the nonmagnetic substrate 1, a float glass plate reinforced chemically is used. The substrate is covered with a gas occluding metal film 6 so that an impurity gas released from the surface of glass may not mix in the ground film 2 at the time of covering with the film 2, and an unevenness forming material 5 is provided on this metal film 6. Moreover, the nonmagnetic ground film 2 having a laminated structure of a first ground film 22 and a second ground film 21 is provided for covering, and in some cases, the alloy magnetic film 3 and the protective film 4 are provided thereon for covering. In this way, a peak shift in a high frequency area can be controlled to be small while an excellent coercive force and record reproduction noise of the magnetic film 3 of the alloy of three elements of CoCrPt are retained as they are.

Hard magnetic phase in rapidly quenched Zr-Co-B alloys

Abstract: In order to identify a hard magnetic phase in rapidly quenched Zr-Co-B alloys and clarify its magnetic properties, Zr-Co-B ribbons, Zr-Co ribbons, and Zr-Co ingots were studied. The hard magnetic phase is interpreted as a Zr/sub 2/Co/sub 11/ intermetallic compound. This compound has a Curie temperature of 500 degrees C and uniaxial magnetic anisotropy with an anisotropy field of 34 kOe. The magnetization of this compound was estimated to be 67 emu/g at 15 kOe. In addition to the hard magnetic phase, the low and high T/sub c/ phases appear in both binary and ternary alloys. The low T/sub c/ phase is FCC Zr/sub 6/Co/sub 23/ with T/sub c/=180 degrees C. The magnetization of Zr/sub 6/Co/sub 23/ was estimated to be 44 emu/g at 15 kOe. The high T/sub c/ phase is cobalt including a small amount of zirconium. In Zr-Co-B alloys, suitable boron addition is shown to enhance the coercive force. On the other hand, the addition increases the magnetization. While the boron addition produces cobalt, it reduces Zr/sub 6/Co/sub 23/ or quenches its ferromagnetism. >

Magnetic hysteresis and Mössbauer studies in ultrafine iron particles

Abstract: The magnetic hysteresis behavior of ultrafine Fe particles has been examined by SQUID magnetometry and Mossbauer spectroscopy. Ultrafine Fe particles have been prepared by vapor deposition under argon atmosphere in pressures from 1 to 6 Torr and passivated by exposure to oxygen at low pressure. Particles with size below 10 nm showed a small coercivity (∼100 Oe) at room temperature, which increased drastically upon cooling to cryogenic temperatures (1000–1500 Oe). The larger particles had the highest coercivity at room temperature (∼1000 Oe) which increased by 100% at 10 K. Mossbauer studies revealed that the Fe particles are surrounded by an Fe oxide layer composed of ultrafine Fe3O4 or γ‐Fe2O3 grains. At room temperature the oxide component gives a very broad absorption superimposed on an α‐Fe spectrum. The enhanced coercivity at room temperature may be attributed to anisotropy induced by exchange interaction between the ferromagnetic core and the ferri‐ or antiferromagnetic oxide layer.

Fabrication and magnetic properties of granular alloys

Abstract: Granular alloys of iron with other transition metals have been fabricated in both Al2O3 and SiO2 matrices. Both stable (Fe‐Ni, Fe‐Co) and metastable (Fe‐Cu) alloys have been achieved. Under appropriate deposition conditions, the grains are single‐phase alloys ranging in size from 15 to 50 A. Superparamagnetism and enhanced coercivity have been observed, and the effect of grain size on these phenomena has been investigated. We have also studied the effects of the different insulating matrices.

Phase relations and magnetic properties of new phases in the Fe-Nd-Al and Fe-Nd-C systems and their influence on magnets

Abstract: The drastic increase in coercivity observed after adding Al in Fe-Nd-B-based magnet alloys seems to originate partially from the occurrence of new Al-stabilized phases in the grain boundary region and at the surface of the Fe/sub 14/Nd/sub 2/B grains. To describe these phases, they were synthesized and their properties were determined. One of these phases is antiferromagnetic with a high susceptibility of 0.05, while a second one is ferromagnetic with an anisotropy field higher than that of the Fe/sub 14/Nd/sub 2/B ( Phi ) phase. Magnetization is only half of the Phi phase because of a higher Nd content. The phase relations of these phases in the Fe-Nd-Al system are reported. It is pointed out that it is impossible so far to produce hard magnets from Fe-Nd-C by a sintering process due to phase relations and an inhibition of the formation of Fe/sub 14/Nd/sub 2/C. The low nucleation and growth rate of Fe/sub 14/Nd/sub 2/C can be accelerated by the addition of small amounts of a fourth element to the Fe-Nd-C material. The stable phase relations reveal the occurrence of new magnetically hard phases as well as very corrosive phases, which make this part of the system unsuitable for hard magnet production. >

Structure and magnetic properties of rapidly-solidified iron-platinum and iron-palladium alloys

Abstract: Rapid solidification techniques and systematic heat treatment have been employed to vary the microstructure of equiatomic Fe-Pt and Fe-Pd alloys to gain a better understanding of the fundamental processes controlling the magnetic properties in this genre of alloy. Preliminary results on the Fe-Pd alloy indicate that the magnetically hardened material is fully ordered and exhibits a well-developed polytwinned state. Melt-spinning resulted in an increase in coercivity in the Fe-Pd alloy over the bulk material by a factor of about two. It is suggested that this enhanced magnetic hardness stems from microstructural refinement within the polytwinned state. Prolonged aging of both melt-spun and bulk Fe-Pd alloys leads to very little change of coercivity. This is suggested to be related to the behavior of the polytwinned state during prolonged aging in which the twin cluster size essentially remains constant. The initial results on the Fe-Pt alloy indicate that the details of the mechanism of hardening in this alloy may be somewhat different from those in the Fe-Pd system, although both develop the polytwinned state after prolonged aging. >

Low‐temperature and high‐temperature hysteresis of small multidomain magnetites (215–540 nm)

Abstract: Approximately equidimensional magnetite crystals, with mean sizes of 215, 390, and 540 nm, respectively, have been produced by reducing hematite crystals. Isothermal magnetic hysteresis properties show a clear progression toward multidomain-like behavior as the mean grain size increases. Saturation remanences Mrs are only 5–10% of saturation magnetization Ms, coercive forces HC are low (5.5–8 mT), and both Mrs and HC have grain-size dependences compatible with those previously established for smaller and larger hydrothermally produced magnetites. Coercivities during remanence acquisition are greater than those measured during demagnetization. The difference between acquisition and destructive fields increases in the larger grains as a result of the increasing importance of the internal demagnetizing field. The low-temperature transition is well expressed in the Mrs and HC data of the 540-nm sample but is more subdued for smaller grains. Magnetostrictive, magnetocrystalline, and magnetostatic mechanisms in turn govern coercivity as the temperature rises. Remanence and coercivity ratios, Mrs/Ms and HR/HC, are almost temperature independent up to 500°C, indicating that domain wall configurations resulting from saturating fields are about the same at any temperature. A thermal fluctuation analysis of high-temperature coercive force data suggests that regions 200–250 nm in size are thermally activated as a unit in grains of all sizes; these are likely domain walls. Apparent demagnetizing factors calculated from both low- and high-temperature data are consistent with a mixture of two-domain (2D) and three-domain (3D) grains in all samples. However, theories of remanence in conventional 2D and 3D grains or in mixtures of 2D, 3D, and metastable single-domain grains do not explain the data in a satisfying way.

Sm‐Fe‐Ti magnets with room‐temperature coercivities above 50 kOe

Abstract: Using mechanical alloying and an additional annealing, we prepared bulk material of a new Sm20 Fe70 Ti10 phase observed before only in sputtered films deposited in the amorphous state and then crystallized. Crystallization of rapidly quenched and partially amorphous ribbons leads to a two‐phase material with a considerable amount of this phase but with the Fe2 Sm phase as a majority phase. This new phase (named 20:70:10 phase) has a Curie temperature of 380 °C and an estimated saturation magnetization of 6–7 kG. The magnetically isotropic, mechanically alloyed samples show room‐temperature coercivities of up to 50.3 kOe.