Showing papers on "Coercivity published in 1997"

Magnetic properties of graphitically encapsulated nickel nanocrystals

Abstract: Graphitically encapsulated ferromagnetic Ni nanocrystals have been synthesized via a modified tungsten arc-discharge method. By virtue of the protective graphitic coating, these nanocrystals are stable against environmental degradation, including extended exposure to strong acids. The magnetic properties of the encapsulated particles are characterized with regard to the nanoscale nature of the particles and the influence of the graphitic coating which is believed to be benign insofar as the intrinsic magnetic properties of the encapsulated nanocrystals are concerned. The Curie temperature of graphitically encapsulated Ni nanocrystals is the same as that of microcrystalline Ni. However, saturation magnetization, remanent magnetization, and coercivity of these particles are reduced, for a range of temperatures. The unique features are compared with those of unencapsulated nanocrystalline and coarse microcrystalline nickel particles.

High resistive nanocrystalline Fe-M-O (M=Hf, Zr, rare-earth metals) soft magnetic films for high-frequency applications (invited)

Abstract: Microstructure, soft magnetic properties, and applications of high resistive Fe-M-O (M=Hf, Zr, rare-earth metals) were studied. The Fe-M-O films are composed of bcc nanograins and amorphous phases with larger amounts of M and O elements which chemically combine each other. Consequently, the amorphous phases have high electrical resistivity. The compositional dependence of magnetic properties, electrical resistivity, and structure have been almost clarified. For example, the high magnetization of 1.3 T, high permeability of 1400 at 100 MHz and the high electrical resistivity of 4.1 μΩ m are simultaneously obtained for as-deposited Fe62Hf11O27 nanostructured film fabricated by rf reactive sputtering in a static magnetic field. Furthermore, Co addition to Fe-M-O films improves the frequency characteristics mainly by the increase in the crystalline anisotropy of the nanograins. The Co44.3Fe19.1Hf14.5O22.1 film exhibits the quality factor (Q=μ′/μ′′) of 61 and the μ′ of 170 at 100 MHz as well as the high Is of ...

Angular dependence of exchange coupling in ferromagnet/antiferromagnet bilayers

Abstract: Angular dependence of the exchange coupling in a NiFe/CoO bilayer film has been measured. Different angular dependence and symmetry have been observed for the exchange field ${H}_{E}$ and the coercivity ${H}_{C}.$ The values of ${H}_{E}$ and ${H}_{C}$ are due to unidirectional and uniaxial components of the magnetic anisotropy energy, in which the odd and the even terms lead to ${H}_{E}$ and ${H}_{C},$ respectively. The proposed free energy accounts for the experimental results.

Thickness dependence of the switching voltage in all-oxide ferroelectric thin-film capacitors prepared by pulsed laser deposition

Abstract: Thin-film ferroelectric capacitors consisting of PbZr0.53Ti0.47O3 sandwiched between La0.5Sr0.5CoO3 electrodes have been deposited using pulsed laser deposition. The combination of oxidic perovskite-type materials results in capacitors with a coercive field (Ec) which is comparable with values for bulk ceramics. Textured thin-film capacitors with a columnar microstructure show lower switching voltages than epitaxial films. No thickness dependence of Ec and a good endurance up to 1011 cycles have been observed for epitaxial as well as textured capacitors with oxidic electrodes. In contrast, capacitors with a metallic top electrode show an increase of Ec with decreasing thickness of the ferroelectric layer. We show that charge injection can explain the experimentally observed increase of Ec with decreasing ferroelectric layer thickness. An overview is given of the growth conditions needed for PbZr0.53Ti0.47O3 films, because the precise stoichiometry is of the utmost importance for the capacitor quality.

Angular dependence of nucleation by curling in a prolate spheroid

Abstract: The nucleation field is calculated for a small ferromagnetic prolate spheroid as a function of its size, its elongation, and the angle between the applied field and the spheroid axis. If there is no other switching mode except for magnetization curling and coherent rotation, these results probably represent the coercivity. The calculation is rigorous only for an isotropic particle, but a first order approximation is also given for a uniaxial or cubic anisotropy whose easy axis is parallel to the long axis of the spheroid.

Magnetic and mechanical properties of micromachined strontium ferrite/polyimide composites

Abstract: In this work, strontium ferrite/polyimide composite thin films are fabricated and characterized for micromachining applications. The application of these materials in microelectronics and micromachining dictates the use of different polymers than those previously used for conventional plastic magnets due to fabrication compatibility constraints. The material investigated here consists of magnetically anisotropic strontium ferrite particles suspended in a benzophenone tetracarboxylic dianhydride-oxydianiline/metaphenylene diamine polyimide matrix. Magnetic mechanical, and processability properties of these composites are investigated for a strontium ferrite loading range of 55%-80% by volume. Intrinsic coercivity H/sub ci/ residual magnetic flux density B/sub r/ and maximum energy product (BH)/sub max/ have been determined. For an 80% by-volume concentration loading of ferrite, H/sub ci/ of 318 kA/m B/sub r/, approaching 0.3 T, and (BH)/sub max/ of 11900 T/spl middot/A/m have been achieved. Biaxial Young's modulus and residual stress are determined using a slightly modified in situ load/deflection technique. The biaxial Young's modulus increases with increasing the magnetic powder loading. The materials have been deposited and patterned using two techniques: (1) screen-printing and (2) spin-casting, followed by photolithography. Finally, a simple magnetic microactuator made with those materials has been fabricated and tested, which demonstrates the usefulness of those materials to micromachining.

Magnetic tunnel junction magnetoresistive read head with sensing layer as flux guide

Abstract: A magnetic tunnel junction (MTJ) magnetoresistive read head for a magnetic recording system has the MTJ sensing or free ferromagnetic layer also functioning as a flux guide to direct magnetic flux from the magnetic recording medium to the tunnel junction. The MTJ fixed ferromagnetic layer and the MTJ tunnel barrier layer have their front edges substantially coplanar with the sensing surface of the head. Both the fixed and free ferromagnetic layers are in contact with opposite surfaces of the MTJ tunnel barrier layer but the free ferromagnetic layer extends beyond the back edge of either the tunnel barrier layer or the fixed ferromagnetic layer, whichever back edge is closer to the sensing surface. This assures that the magnetic flux is non-zero in the tunnel junction region. The magnetization direction of the fixed ferromagnetic layer is fixed in a direction generally perpendicular to the sensing surface and thus to the magnetic recording medium, preferably by interfacial exchange coupling with an antiferromagnetic layer. The magnetization direction of the free ferromagnetic layer is aligned in a direction generally parallel to the surface of the medium in the absence of an applied magnetic field and is free to rotate in the presence of applied magnetic fields from the medium. A layer of high coercivity hard magnetic material adjacent the sides of the free ferromagnetic layer longitudinally biases the magnetization of the free ferromagnetic layer in the preferred direction.

Influence of a 36.8° grain boundary on the magnetoresistance of La0.8Sr0.2MnO3−δ single crystal films

Abstract: Epitaxial ferromagnetic manganite films have been sputtered on bicrystal substrates. Their magnetoresistance was measured as a function of magnetic field and temperature. The grain boundary magnetoresistance at low temperature is separated from the intrinsic magnetoresistance near the Curie temperature. The grain boundary magnetoresistance peaks at about 100 Oe and saturates at about 2 kOe. For a La0.8Sr0.2MnO3 film with a grain boundary angle θ=36.8° a field independent component r0=4.1×10−6 Ω cm2 was separated from a field-dependent component which has its maximum rH=2.3×10−6 Ω cm2 for H of order the coercive field.

Magnetoresistive sensor for high temperature environment using iridium manganese

Abstract: A magnetoresistive read sensor fabricated on a substrate includes a ferromagnetic layer that is exchange coupled with an antiferromagnetic layer made of a defined composition of iridium manganese. A tantalum layer is used so that the exchange field and coercivity do not change with variations in annealing temperature. The antiferromagnetic layer is formed with a material composition of Ir x Mn 100-x , wherein x is in the range of 15 23. In an embodiment of a spin valve structure, the tantalum layer is disposed over the substrate and the antiferromagnetic layer is in direct contact with a pinned ferromagnetic layer. In another embodiment, the IrMn layer is formed over a soft active layer. In a third embodiment using exchange pinning, spaced IrMn regions are formed over the active magnetoresistive layer to define the sensor track width.

Magnetic sensor using tunnel resistance to detect an external magnetic field

Abstract: A magnetic sensor including a first magnetic layer having an axis of easy magnetization in a first direction; a second magnetic layer having an axis of easy magnetization in a second direction different from the first direction; a third magnetic layer positioned between the first magnetic layer and the second magnetic layer, and having a smaller coercive force than the first magnetic layer and the second magnetic layer; a first insulating layer interposed between the first magnetic layer and the third magnetic layer; and a second insulating layer interposed between the second magnetic layer and the third magnetic layer. An external magnetic field is detected by the use of tunnel resistance between the first magnetic layer and the third magnetic layer and tunnel resistance between the second magnetic layer and the third magnetic layer.

Structure and magnetism of hcp-Co fine particles

Abstract: We have synthesized hcp-Co fine particles with the average diameter of less than 50 nm by sputtering Co in a somewhat high inert gas pressure. A nearly perfect hcp phase can be stabilized by increasing sputtering gas pressure and decreasing sputtering power in contrast to conventional preparation conditions. Magnetic measurements have revealed the clear relation of coercivity with the volume ratio of hcp phase. That is, the increase in the hcp phase considerably enhances the coercivity. Appearance of the hcp phase seems to be related to the promotion of allotropic phase transformation in Co initiated by lattice defects within each particle.

High coercivity, epitaxial Sm–Co films with uniaxial in-plane anisotropy

Abstract: Epitaxial Sm–Co(1100) and (1120) films have been grown by magnetron sputtering onto Cr(211) and (100) buffer layers, respectively. The Sm–Co(1100) films exhibit uniaxial in-plane anisotropies of ≈20–25 T and room-temperature coercive fields that increase to 4.1 T as the film thickness decreases to 75 A. The 3 T coercivities of the (1120) films are independent of thickness.

CoPt–C nanogranular magnetic thin films

Abstract: Cobalt–platinum–carbon thin film was deposited with a chemical composition of Co50Pt15C35. The film had a nanogranular morphology with a grain size ranging from 5 to 15 nm. It consisted of cobalt–platinum grains which had a faulted hexagonal close-packed phase and were separated by graphitelike carbon boundaries. The film in-plane coercivity was 1500 Oe, compared to a few hundreds oersteds in the case of cobalt–carbon. This result establishes a way of fabricating high coercivity cobalt–carbon based materials, which have potential applications as high density magnetic recording media.

Thin film magnetic alloy having low noise, high coercivity, and high squareness

Abstract: A Co--Pt based magnetic alloy which has been doped with a relatively high amount of nitrogen, e.g., at or above 1 at. % is obtained having high coercivity, for example in the range of 1400 Oe or above, and an increased signal-to-noise ratio as compared to the same Co--Pt based alloy which has not been doped with nitrogen. The alloy is vacuum deposited, for example, by sputtering, and the nitrogen may be introduced from the sputtering gas or from the sputtering target. Other low-solubility elements providing the grain uniformity and isolation include: B, P, S, C, Si, As, Se and Te.

90? Domain Reorientation and Electric-Field-Induced Strain of Tetragonal Lead Zirconate Titanate Ceramics

Abstract: The effect of the reorientation of 90° domains on the electric-field-induced strains was studied for tetragonal lead zirconate titanate (PZT) ceramics. An in situ X-ray diffraction (XRD) method was used to evaluate the 90° domain reorientation under electric fields. The strains caused by the reorientation were calculated and compared with the electric-field-induced longitudinal strains measured with a laser displacement meter and the strains expected from piezoelectric d-constants. It was experimentally confirmed that the electric-field-induced strains of PZT ceramics were composed of strains due to the piezoelectric effect and the 90° reorientation. After poling treatment, a small portion of the 90° domains relaxed and reoriented, giving rise to a reversible reorientation of the 90° domains by the electric field. These reversible reorientations contributed to the electric-field-induced strains. In "soft" PZT ceramics, the degrees of 90° reorientation caused by poling and by the subsequent application of an electric field had a close correlation with the tetragonality of the crystal lattice rather than with the coercive field.

Magnetic properties of arrays of “holes” in Ni80Fe20films

Abstract: We have studied the magnetization reversal and magnetoresistance behavior in lithographically defined structures based on a 400 A Ni80Fe20 film The structures consist of square arrays of holes with size d in the range from 05 to 15 μm fabricated using electron beam lithography and an optimized pattern transfer process For the field applied along the intrinsic easy axis, a marked increase in the coercive field is observed as the hole size is decreased This has been attributed to the pinning of the domain walls in the vicinity of the holes However, for the field applied along the intrinsic hard axis direction, there is a marked increase in the remanence as the hole size is reduced due to the competition between the intrinsic uniaxial anisotropy field and the shape induced magnetic anisotropy field Unusual magnetoresistance effects are observed as a function of orientation of applied field in submicron structures

Magnetic recording medium

Abstract: PROBLEM TO BE SOLVED: To provide a thin film magnetic recording medium comprising a CoSm alloy magnetic layer and having high corrosion resistance, high coercive force and high saturation magnetic flux density by specifying the compsn. of the material for the magnetic layer. SOLUTION: This magnetic recording medium is produced by forming a nonmagnetic metal base layer 2 and a magnetic layer 3 on a base body 1 formed by forming a nonmagnetic metal layer 12 on a nonmagnetic substrate 11, and further forming a protective layer 4 and a lubricating layer 5 on the magnetic layer 3. The magnetic layer 3 consists of a CoSm alloy having a compsn. containing <=5at.% metal M selected from Mo, Cr, Ni, Ta or alloy of these, <=5at.% Pd, 15-25at.% Sm and the balance Co. Thereby, since the metal M renders the CoSm alloy in a passivation state, corrosion resistance of the alloy can be improved. The simultaneously included Pa suppresses crystallization which is easily caused by the metal M and promotes to change the magnetic layer 3 into an amorphous state. Thus, the intrinsic magnetic characteristics of the CoSm alloy which can develop its magnetic characteristics in an amorphous state are not impaired.

Magnetic properties of exchange-coupled α-Fe/Nd–Fe–B multilayer thin-film magnets

Abstract: The multilayer thin films having the form of Ti(30 nm)/Fe(dFe)/[Nd–Fe–B(dNd–Fe–B)/Fe(dFe)] ×5/Ti(30 nm)/glass were fabricated on glass substrates by means of radio frequency (rf) sputtering, with dFe varied from 0 to 50 nm and dNd–Fe–B from 0 to 100 nm. Magnetization measurements have revealed that the minor loops of the films are reversible in a certain range of demagnetization fields except for the films with dFe∼0 nm or dNd–Fe–B<10 nm. Micromagnetic calculation of magnetization curves has been performed, which has well reproduced the observed hysteresis loops including spring-back behavior and the dependence of the coercive field HcJ on dFe and dNd–Fe–B. From the comparison between experiment and calculation, we can infer that the interlayer exchange-coupling strength is about 10% of the intralayer couplings, which are found to be almost independent of dFe and dNd–Fe–B.

A composite of polyaniline with both conducting and ferromagnetic functions

Abstract: A composite of polyaniline (PANI) with both conducting and ferromagnetic functions was synthesized by a chemical method proposed by the authors. For the electrical properties, its room-temperature conductivity was measured to be about 10−1 S/cm when doped with 1.0M HCl, and it is independent of the preparation conditions, such as reaction temperature and concentration of FeSO4 solution. Temperature dependence of the conductivity of the composites at temperature between 77 and 450 K is controlled by thermal activation and dedoping processes, which result in the decrease of conductivity with increase of temperature as T > 320 K. For their magnetic properties, unusual ferromagnetic properties with high saturated magnetization (M2) and lower coercive force (Hc = 0) were observed. An effect of the preparation conditions on the ferromagnetic properties of composites was observed. The higher the reaction temperature and the concentration of FeSO4 solution, the higher the saturated magnetization was observed. No hysteresis feature (i.e. Hc = 0) for any PANI composites synthesized in this paper was observed, and this is independent of the preparation conditions. This may be attributed to the nanometer size of the magnetic particles existing in composites. Thus, it suggests that the doping of PANI leads to electrical properties of composites, whereas the nanocrystalline magnetic particles (Fe3O4) are responsible for the observed ferromagnetic properties of PANI composites. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 2129–2136, 1997

High exchange anisotropy and high blocking temperature in strongly textured NiFe(111)/FeMn(111) films

Abstract: Strongly textured NiFe(111) underlayers, deposited by dc magnetron sputtering with applied substrate bias, are utilized to grow large, epitaxial grains of FeMn(111). These perfectly oriented (111) NiFe/FeMn bilayers exhibit the highest interfacial exchange anisotropy energy (0.17 erg/cm2), lowest coercivity (∼2 Oe) of the pinned layer, and highest blocking temperature (205 °C) ever reported, to the best of our knowledge. The relationship between the blocking temperature and the x-ray diffraction intensity of FeMn(111) indicates that the exchange interaction across the NiFe and FeMn interface is enhanced by the well-oriented, large, defect-free FeMn(111) crystals.

Properties of epitaxial ferroelectric PbZr0.56Ti0.44O3 heterostructures with La0.5Sr0.5CoO3 metallic oxide electrodes

Abstract: PbZr056Ti044O3 (PZT) epitaxial ferroelectric thin films on a LaAlO3 (100) substrate, covered by a metallic oxide electrode La05Sr05CoO3 (LSCO) are demonstrated in this work The films are fabricated by the sol–gel method and effort was focused on thermal processing to obtain the desired epitaxial heterostructure The dielectric and ferroelectric properties of PZT thin films were measured and it was found that they are as good as in the films deposited by other thin-film methods The dielectric constant and the dissipation factor of the PZT films are, respectively, about 500 and 006 below 20 kHz The remanent polarization Pr is about 27 μC/cm2 and the coercive field Ec is about 50 kV/cm It was found that the ferroelectric properties were significantly influenced by the microstructure of the PZT layers The present study also showed that the fatigue characteristics of the epitaxial heterostructure with LSCO electrodes under a reversed electrical field are far superior to those obtained with a polycrys

Properties of SrBi2Ta2O9 ferroelectric thin films prepared by a modified metalorganic solution deposition technique

Abstract: Polycrystalline SrBi2Ta2O9 thin films having a layered-perovskite structure were fabricated by a modified metalorganic solution deposition technique using room temperature processed alkoxidecarboxylate precursor solution. It was possible to obtain a complete perovskite phase at an annealing temperature of 650 °C and no pyrochlore phase was observed even up to 600 °C. In addition, the SrBi2Ta2O9 thin films annealed at 750 °C exhibited better structural, dielectric, and ferroelectric properties than those reported by previous techniques. The effects of postdeposition annealing on the structural, dielectric, and ferroelectric properties were analyzed. The electrical measurements were conducted on Pt/SrBi2Ta2O9/Pt capacitors. The typical measured small signal dielectric constant and dissipation factor at 100 kHz were 330 and 0.023 and the remanent polarization and the coercive field were 8.6 μC/cm2 and 23 kV/cm, respectively, for 0.25-μm-thick films annealed at 750 °C. The leakage current density was lower th...

Effect of crystal defects and internal stress on the domain structure and magnetic properties of magnetite

Abstract: Domain structures in magnetite are very sensitive to crystal imperfections, which play a major role in hysteresis and remanence by hindering the motion of domain walls. Using the Bitter colloid technique, we have observed spike and closure domains of the style predicted by Neel [1944] around nonmagnetic inclusions, chemically altered regions, and grain boundaries in a natural single crystal of magnetite. Isolated inclusions within body domains have pairs of attached Neel spikes which reduce magnetostatic energy by diluting magnetic poles. In one example we calculated that spikes reduced the energy by a factor of 6–7. In some cases 71 °, 109° and 180° domain walls are pinned to defects either through spikes or via chains of subsidiary closure domains. One example of pinning by a spike gave a calculated microcoercivity of 0.54 mT, similar to the bulk coercive force of 0.5 mT for the crystal. “Colloid gaps” in 180° and other walls form lines parallel to a easy axis and are evidence of underlying line defects, for example, dislocations, whose stress fields deflect the spins locally, weakening the magnetic field gradient above the walls. We have also observed bending of 180° walls anchored at pinning sites on the grain boundary, the first direct experimental evidence of the effect of internal stresses on the domain structure of magnetite. We determined internal stress magnitudes in the range 7–34 MPa from the observed linear dimension and transverse displacement of each bowed wall. Finally, we measured hysteresis curves on a companion magnetite crystal at temperatures T from ambient to the Curie point of 585°C. Observed coercivity Hc varies with T as λ111W0.5/Ms, in agreement with theoretical predictions of impedance of a wall of width w by dislocation stress fields. We therefore propose that the stability of remanence in multidomain magnetite is mainly due to pinning of domain walls by crystal defects.

Magnetic hardening in FePt nanostructured films

Abstract: FePt films have been prepared by sputtering Fe/Pt multilayers onto glass or silicon substrates. The thickness of the Fe and Pt layers was adjusted with the Fe:Pt atomic ratio from about 1:1 to 2:1. Magnetic hardening is observed after heat treatment at elevated temperatures, which led to coercivity values exceeding 20 kOe in samples with an Fe:Pt ratio around 1.2:1. The hardening originates from the formation of the tetragonal FePt phase with high magnetocrystalline anisotropy and a favorable microstructure. Two-phase composite films containing hard and soft phases were obtained when the Fe:Pt ratio increased. Under optimized processing conditions, composite films with energy products larger than 30 MG Oe at room temperature have been successfully produced.

Ferroelectric properties of PZT thin films prepared by sputtering with stoichiometric single oxide target: Comparison between conventional and rapid thermal annealing

Abstract: The influence of growth conditions and post-annealing treatments on radio frequency (r.f.) magnetron sputtered lead zirconate titanate (PZT) thin films have been investigated. By adjusting the plasma discharge parameters, it is possible to control, with stoichiometric single oxide target, the film composition precisely. No excess lead was used either during sputtering (in the target) or during post-deposition annealing. The structural, microstructural and electrical properties have been systematically examined as a function of the annealing treatments. Perovskite structure was obtained by conventional annealing as well as by rapid thermal annealing. The films were dense and crack-free; the film orientation, the microstructure and the surface morphology are directly related to the thermal processes. The ferro-electric properties in terms of coercive field and remanent polarization are also very sensitive to the annealing treatments.