High Remanence Research Articles

Ferromagnetic Fe3O4 nanofibers: Electrospinning synthesis and characterization

In this paper, Fe3O4 nanofibers with average diameters of 100 nm were successfully synthesized by electrospinning of PVP/Fe(NO3)·9H2O composite solution followed by calcinations and reduction. The resulting Fe3O4 nanofibers were characterized with X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscope (TEM), transmission Mössbauer spectra (MS), and vibrating sample magnetometer (VSM). These nanofibers showed a cubic structure and uniform and continuous morphology. The Fe3O4 nanofibers possessed a saturation magnetization Ms of 57.6 emu/g. A large coercivity Hc of 188.4 Oe together with a high remanence ratio Mr/Ms = 0.28 is observed, which result from the shape anisotropy.

Annealing induced compositional changes in SmCo5/Fe/SmCo5 exchange spring trilayers and its impact on magnetic properties

SmCo5/Fe/SmCo5 sandwich configurations have gained renewed interest when compared to the conventional SmCo5/Fe bilayers, as they exhibit enormous scope to impart high energy product values with strong magnetization reversal behavior. The compositional changes that occur in the intermediate soft layer upon annealing play a critical role in determining the phase chemistry, as well as magnetic performance of the trilayers and herein we address such a study. SmCo5/Fe/SmCo5 sandwich films with fixed layer thicknesses of SmCo5 (25nm) and Fe (8nm) were grown by sputtering on Cr buffered Si (100) substrates and subsequently, annealed at different temperatures: 673, 773 and 873K. The effect of post-deposition annealing on the structure, composition and magnetic properties of the trilayer architecture was investigated by X-ray diffraction (XRD), Rutherford back-scattering (RBS) and super-conducting quantum interference device (SQUID), respectively. The XRD studies showed significant decrease in the lattice parameter values with increasing annealing temperature, which suggests an increase in the diffused Co-content in the Fe-intermediate layer. The XRD results were further validated by RBS measurements, which confirmed that both composition and thickness of the Fe-intermediate layer were strongly affected by the annealing temperature. The SQUID measurements demonstrated the existence of in-plane anisotropy and strong exchange coupling between the hard and soft layers for all the annealed sandwich films. The trilayer stack annealed at 773K showed the best magnetic performance such as high coercivity (814kA/m), remanence (944kA/m) and energy product (243kJ/m3).

Effect of Fe layer thickness and Fe/Co intermixing on the magnetic properties of Sm–Co/Fe bilayer exchange-spring magnets

Sm–Co(20 nm)/Fe(tFe) bilayers are fabricated with different Fe layer thicknesses (tFe = 0, 2, 4, 6 and 8 nm) in order to systematically investigate the intermixing effect between Fe and Sm–Co layers and its influence on the magnetic properties. X-ray diffraction analysis indicates that the deposition of soft layer (Fe) promotes crystallization of hard layer (Sm–Co) at a low annealing temperature (400 °C). The annealed films consist of hexagonal Sm2Co7, Sm2Co17 and SmCo5 phases in the hard layer and bcc-structured Fe(Co) in the soft layer. Rutherford backscattering is employed to investigate the atomic composition of the individual layers and thereby the extent of diffusion of Co atoms into the Fe layer. The estimated Co content is higher for lower tFe. The magnetic properties of the bilayers very close to the interface are analysed by the magneto-optical Kerr effect. The calculated Kerr intensities (which represent the magnetization process) are significantly affected by the extent of diffused Co in the Fe layer. Superconducting quantum interference device magnetic measurements demonstrate smooth and single-phase magnetic behaviour for tFe up to 6 nm and a good combination of high coercivity (6.5 kOe) and high magnetic remanence (834 emu cm−3) is obtained for tFe = 4 nm.

Micro-Structure and Magnetic Properties of Hexagonal M-Type Ferrite Thick Films Prepared by Tape Casting

The M-type hexagonal ferrite thick films are prepared by tape casting . The morphology is investigated by scanning electron microscope (SEM), the phase composition is characterized by X-ray diffraction (XRD) and the magnetic properties are characterized by vibrating sample magnetometer (VSM). The result shows that thick film by tape casting has high remanence as 26.607emu/g, high magnetization as 55.236emu/g, high coercivity as 2323.8Oe, and high squareness ratio and a good morphology which is suitable for small and light-weight microwave and millimeter-wave devices .

Research on the Steady Voltage of Permanent-Magnet Generator with Pressure Roller on the Vehicle

The steady voltage theory of permanent-magnet generator with pressure roller is analyzed, permanent-magnet with the high remanence intensity is adopted, generator’s structure is designed with creativity, and output voltage is increased at low rotate speed of generator. Using the parallel electronic voltage stabilizer stabilizes output voltage at high rotate speed, and solves vehicle’s permanent-magnet generator’s problem of generating stable voltage at wide speed and wide load.

Magnetic properties of BaxSr1−xFe12O19 (x = 0.05–0.35) ferrites prepared by different methods

Hexagonal ferrites BaxSr1−xFe12O19 (x=0.05, 0.15, 0.25, 0.35) were prepared by sol–gel and conventional solid state reaction techniques. Their magnetic properties, investigated by magnetometry and Mössbauer spectroscopy, were discussed in correlation to structural aspects. Whereas the hexaferrites obtained by sol–gel are almost single phase, the ones prepared by the conventional solid state reaction technique present a low amount of BaFe2O4 as secondary phase. All samples show a relative high coercive field and remanent magnetization, as specific features for permanent magnet behavior. The five Fe3+ sites of the specific elemental cell have been revealed and their relative occupancy has been derived from the Mössbauer spectra. The analyzed samples show a sensitivity of the magnetic properties (magnetic moment and anisotropy) with respect to the occupancy of the different Fe sites.

Effects of grain microstructure on magnetic properties in FePtAg-C media for heat assisted magnetic recording

L10 FePt has been proposed as a magnetic medium suitable for heat assisted magnetic recording, owing to its high magnetocrystalline anisotropy and moderate Curie temperature. Carbon has previously been used as a segregant creating small magnetically decoupled grains by sputter deposition at high temperature. Here, we deposit granular, segregated FePt films having a high degree of chemical order, coercive fields of 4.8 T, high remanence, and average grain size of 7.2 nm with 17% size distribution. Magnetic characterization reveals a large intrinsic switching field distribution of 15 kOe. We incorporate the results obtained by transmission electron microscopy, magnetometry, and x-ray diffraction into a micromagnetic model to better understand the origin of the intrinsic switching field distribution. We find that the dominant contributions to the observed magnetic properties are (i) a grain-size dependent distribution in anisotropy field and (ii) small angle variations in perpendicular anisotropy axis orientation. The presence of small superparamagnetic grains, contributions originating from the grain size distribution, as well as from grains with longitudinal or randomly oriented anisotropy axis are found to play a relatively small role. The results help understand the observed magnetization loops used to evaluate FePt media.

Anisotropic fully dense MnBi permanent magnet with high energy product and high coercivity at elevated temperatures

The structural and magnetic properties of a fully dense anisotropic MnBi magnet prepared by hot compaction are investigated. Arc-melting and low energy ball milling are employed to synthesize highly anisotropic MnBi powders with very high remanence ratio (Mr/Ms) of 0.97, coercivity (Hc) of 11.7 kOe and maximum energy product [(BH)max] of 9 MG Oe. The bulk magnet fabricated from these powders displays anisotropic characteristics with high Mr/Ms ratio of 0.91 and a (BH)max of 5.8 MG Oe at room temperature. The Hc of bulk magnet increases linearly from 6.5 kOe at 300 K to 28.3 kOe at 530 K with a (BH)max of 3.6 MG Oe at 530 K. Analysis of the temperature dependence of Hc suggests the nucleation of reversed domains as the dominant mechanism for the magnetization reversal.

Paleomagnetism of ca. 1.35 Ga sills in northern North China Craton and implications for paleogeographic reconstruction of the Mesoproterozoic supercontinent

We report paleomagnetic data on precisely dated Mesoproterozoic sills, in zircon and baddeleyite 207Pb/206Pb ages of 1345±12 and 1353±14Ma respectively, intruding the Xiamaling and Wumishan formations in North China with an aim of evaluating how the North China Craton (NCC) was involved in the evolution of the Columbia supercontinent from ca. 1.78 to 1.35Ga. After systematic thermal demagnetization we isolate a high temperature characteristic remanence from 18 sites with a tilt-corrected site-mean direction of D=294.4°, I=−31.7° and α95=4.3°, corresponding to a mean paleomagnetic pole at λ=5.9°N, φ=359.6°E (N=18) with angular standard deviation of 10.0° and A95=4.3°. Rock magnetic experiments, light microscopy and scanning electron microscopy all indicate that the main magnetic minerals in the sampled sill rocks are medium sized titanomagnetites. The positive fold test and fresh titanomagnetite grains suggest that the characteristic remanence is likely to be a primary acquired at ca. 1.35Ga. The preliminary paleogeography reconstruction based upon the well-matched apparent polar wander paths between the Laurentia–Baltica–Siberia united block and the NCC supports the interpretation that the NCC may have drifted together with this united block before ca. 1.35Ga during the late history of the Columbia supercontinent. We argue that the NCC was located in low latitudes and kept tight connections with the Siberian and Indian cratons during the interval between ca. 1.8 and 1.35Ga, and that the ca. 1.35Ga sills widely developed in the Mesoproterozoic Xiamaling and Wumishan formations, as well as the ca. 1.3–1.2Ga magma events well-developed in Siberia and northern margin of the NCC, may represent the complete fragmentation of the NCC with the neighboring cratons.

High-performance α-Fe/Pr2Fe14B-type nanocomposite magnets fabricated by direct melt spinning

High-performance α-Fe/Pr2Fe14B-type nanocomposite magnets based on the compositions of Pr8Fe86B6 microalloyed with Co, Nb and C were fabricated by direct melt spinning. The coercivity was greatly improved from 5.5 kOe for the Pr8Fe86B6 ribbons to 7.4 kOe for the Pr8Fe85NbB5C ribbons. The balanced high coercivity and remanence were obtained in Pr8Fe75Co10NbB5C ribbons due to the Co substitution for Fe, which led to the significant improvement of magnetic properties in these ribbons. A remanence ratio of 0.82, a coercive field of 6.6 kOe and a maximum energy product of 26.2 MGOe in melt-spun Pr8Fe75Co10NbB5C ribbons were obtained at room temperature.

Controlled synthesis, phase formation, growth mechanism, and magnetic properties of 3-D CoNi alloy microstructures composed of nanorods

3-D flower like CoNi alloys nanostructures composed of nanorods have been synthesized by template free hydrothermal method at relatively low temperature (120 °C). The detailed characterizations confirm the formation of good crystalline fcc CoNi alloy, average crystallite size of 18.8 ± 1.0 nm, lattice parameter of 3.531 ± 0.01 Å, and the nearly equiatomic composition (Co50Ni50). Highly uniform flower like nano structures are built up with nanorod of diameter about 100 nm and length in range of 200–400 nm. The nanorods (building blocks of flower) have single crystalline nature with [111] preferred growth direction. The concentration of NaOH plays a vital role in formation of alloys and high concentration promotes the formation of CoNi alloy at low temperature. The concentration of NaOH also affects the morphology remarkably by changing the growth/reaction rate of CoNi nanostructures and results in hollow spheres to nanoplate flower of CoNi alloys. Based on the evolution of the morphology of the products, a step wise growth mechanism is rationally proposed for flower like nanostructures by considering the effects of kinetic parameters on growth. Magnetic measurements show Co50Ni50 flower like nanostructure have high saturation magnetization, coercivity, remanent magnetization, and high effective anisotropy constant of value 101.3 emu g−1, 210.5 Oe, 16.2 emu g−1, and 4.457 × 104 J m−3 respectively. The enhancements of coercivity and effective anisotropy constant are attributed to nanoscale effects such as shape/surface anisotropy.

Controllable synthesis, formation mechanism and magnetic properties of hierarchical α-Fe2O3 with various morphologies

In this paper, a simple and efficient route had been developed for morphology-controlled synthesis of hierarchical α-Fe2O3 superstructures assembled by nanocrystallites. All chemicals used were low-cost compounds and environmentally benign. The morphologies and structures of the α-Fe2O3 crystals were characterized by field emission scanning electron microscope (FESEM), X-ray diffraction (XRD) and Fourier-transform IR spectroscopy (FTIR). The results showed that different shapes of hierarchical α-Fe2O3 nanostructures such as peanuts-like, capsule-like, cantaloupe-like and almond-shaped could be prepared by simply varying the concentration of silicate anions and other inorganic reagents. The as-prepared α-Fe2O3 architectures were composed of nanorods or nanosheets at different synthesis temperature. The possible formation mechanism was described based on the experimental results. Magnetic hysteresis measurements revealed the as-prepared superstructures displayed ferromagnetic behavior with higher remanence and coercivity at room temperature, which was attributed to the superstructure or the shape anisotropy of the samples.

Effects of Sintering Temperature on Microstructure and Magnetic Properties of Nd–Fe–B Magnets

The sintered Nd–Fe–B (neodymium–iron–boron) magnet has been used for many applications in various fields such as acoustics, communications, and automation due to its excellent properties including high remanence, high coercivity, and large energy product. Especially high-coercivity sintered Nd–Fe–B magnets have been extensively applied in the field of permanent magnet motors. In the present work, the effects of sintering temperature on the structural and magnetic properties of a Nd15Fe77B8-type magnet have been investigated. Sintered permanent magnets were produced from a Nd15Fe77B8 commercial alloy. The magnetic properties were evaluated using an Automatic Magnet Tester. The magnets were successfully produced at different temperatures. It was seen that the best magnetic properties were obtained for the magnet produced at 1050 ∘C for 1 h. The structural evolution of the magnets has also been examined by means of X-ray diffraction (XRD) and polarized optical microscope. Nd2Fe14B, Fe3B and some α-iron phases were observed by X-ray diffraction results.

Effect of nitrogen environment on NdFeB thin films grown by radio frequency plasma beam assisted pulsed laser deposition

Abstract NdFeB is a very attractive material for applications in electrical engineering and in electronics, for high-tech devices where high coercive field and high remanence are needed. In this paper we demonstrate that the deposition of nitrogen doped NdFeB thin films by pulsed laser deposition, in the presence of a nitrogen radiofrequency plasma beam, exhibit improved magnetic properties and surface morphology, when compared to vacuum deposited NdFeB layers. A Nd:YAG pulsed laser (3ω and 4ω) was focused on a NdFeB target, in vacuum, or in the presence of a nitrogen plasma beam. Substrate temperature (RT-850 °C), nitrogen gas pressure, and radiofrequency power (75–150 W), were particularly varied. The thin films were investigated by means of X-ray diffraction, atomic force microscopy, scanning electron microscopy, spectroscopic-ellipsometry, and vibrating sample magnetometry.

Magnetic heterostructures with low coercivity for high-performance magneto-optic devices

In this work, we analyse the method of forming magneto-optically active heterostructures based on magnetic layers with different magnetic properties. Layers of one type possess a high effective constant of uniaxial magnetic anisotropy for which the condition is fulfilled, where Ku is the constant of uniaxial magnetic anisotropy and is the demagnetizing energy, and layers of the second type used possess in-plane or quasi-in-plane magnetization, in which the condition holds true. The layers of the first type, which we refer to as layers of positive effective uniaxial magnetic anisotropy, may have the composition Bi2Dy1Fe4Ga1O12 and the layers of second type the composition Bi3Fe5O12, which may have very high magneto-optic (MO) figure of merit and are therefore very attractive for the development of MO transparencies and ultra-fast switches. We discuss the optimization of triple-layer structure parameters aimed at achieving a high MO figure of merit simultaneously with low coercivity and high remanent magnetization and possessing rectangular hysteresis loops. The results of the experimental study of the MO properties achieved in garnet heterostructures fabricated using RF sputtering are also described. We show that the proposed paradigm of using new magnetic material combinations demonstrating significantly improved magnetic and MO properties may be realized when working with heterostructures based on Bi-substituted ferrite garnets grown on (1 1 1)-oriented garnet substrates.

Thermally stimulated processes in samarium-modified lead titanate ferroelectric ceramics

The thermally stimulated processes in a samari- um-modified lead titanate ferroelectric system are analyzed from the thermally stimulated depolarization discharge cur- rent. The discharge due to the space charge injected during the poling process, the pyroelectric response and a conduc- tion process related to oxygen vacancies are evaluated con- sidering a theoretical decomposition by using a numerical method. The pyroelectric response is separated from other components to evaluate the polarization behavior and some pyroelectric parameters. High remanent polarization, pyro- electric coefficient and merit figure values are obtained at room temperature.

The microstructure and ferroelectric properties of Sm and Ta-doped bismuth titanate ferroelectric thin films

Sm–Ta co-doped Bi4Ti3O12 (Bi3.96Sm0.04Ti2.92Ta0.08O12, BSTTO) thin films were fabricated on Pt(111)/Ti/SiO2/Si(100) substrates by sol–gel technology, and the microstructure, dielectric, ferroelectric and leakage current properties of the thin films were investigated. BSTTO thin film exhibits a high remanent polarization (2Pr) of 46.2μC/cm2 and a low coercive field (2Ec) of 102kV/cm, fatigue-free characteristics up to ≧108 switching cycles. The 2Pr of the BSTTO thin film is larger than that of the Bi4Ti3O12 thin film (2Pr=26μC/cm2). In addition, the leakage current density of Sm–Ta co-doped Bi4Ti3O12 thin film is 2.56×10−8A/cm2, which is lower than that of the Bi4Ti3O12 thin film (1.0×10−5A/cm2). These improvements in the ferroelectric and leakage current properties can be attributed to the enhanced stability of the oxygen in the Ti–O octahedron layer by the substitution with Sm and Ta for Bi3+ and Ti4+ in A-site and B-site of the BTO thin film, respectively.

Nd2Fe14B/FeCo Anisotropic Nanocomposite Films with a Large Maximum Energy Product

Anisotropic Nd(2) Fe(14) B/Fe(67) Co(33) nanocomposite thin films are successfully fabricated. The multilayer composite films comprise Nd-rich shell-enveloped Nd(2) Fe(14) B grains and a Fe(67) Co(33) phase. The strong (001) texture of the Nd(2) Fe(14) B grains and the presence of exchange-coupled Fe(67) Co(33) lead to a high remanence and the presence of the Nd-rich shell gives rise to a high coercivity. The unique nanocomposite microstructure provides hints for developing rare-earth-lean high-performance magnets.

Tetragonally strained BiFeO3 thin film on single crystal Rh substrate

Epitaxial (001) BiFeO3 thin films with tetragonally stained structures and a c/a ratio of about 1.04 were fabricated on single crystalline (100) Rh substrates. The BiFeO3 thin film showed high remanent polarization (Pr) of about 91μC/cm2. We suggest that this enhanced remanent polarization is due to a tetragonally strained structure with a c/a ratio up to 1.04 and the large stripe domain structure originating from high domain wall energy. Low leakage current was confirmed and the leakage current mechanism was found to be dominated by space charge limited conduction, which provides an opportunity for non-volatile memory applications.

DIELECTRIC AND PIEZOELECTRIC PROPERTIES OF LEAD-FREE (Bi0.5Na0.5)0.94Ba0.06TiO3 + x%Ce2O3 + y%La2O3 + z%Y2O3 COMPOUNDS SYNTHESIZED BY SOL–GEL TECHNIQUE

( Bi 0.5 Na 0.5)0.94 Ba 0.06 TiO 3 + x% Ce 2 O 3 + y% La 2 O 3 + z% Y 2 O 3 compounds (named BNT–BT– x/y/z) were synthesized by a sol–gel technique, for (x,y,z) = (0,0,0), (0.25, 0.25, 0.25), (0.25, 0.5, 0.5), (0.5, 0.25, 0.25) and (0.5, 0.5, 0.25). The structural variation according to the different system compositions is investigated by X-ray diffraction analyses. The results shows that the addition of Ce2O3 , La2O3 or Y2O3 in BNT–BT system, do not modify the crystalline structure, and a rhombohedral–tetragonal morphotropic phase boundary is maintained for different dopant addition. The BNT–BT–x/y/z-based ceramics sintered at relatively low temperature (1100°C) exhibit a good densification ratio. The optimum dielectric and piezoelectric properties are obtained with the BNT–BT–0.5/0.25/0.25 composition. High dielectric properties at room temperature (εr > 1000) and low dielectric losses (tan δ < 4 × 10-2) are obtained for this composition. This compound exhibits a piezoelectric constant d33 of 95 pC/N, a good polarization behavior is observed with high remanent polarization Pr of 9.21 μC/cm2 and low value coercitive field Ec of 2.66 kV/mm.