Small Coercivity Research Articles

Characterization, Photoluminescence and Magnetic Properties of SiC Nanowires Synthesized with Nickel Catalyst via Microwave Heating

Large scale SiC nanowires were synthesized through a rapid and low-cost microwave heating method. Silicon, silica, graphite and nickel powders were used as raw materials and catalyst, respectively, and no inert protective gas was employed during the preparation. The microstructures of the products were comprehensively characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrum (EDS), and transmission electron microscopy (TEM). Results showed that the nanowires have lengths of several dozens of micrometers and diameters of 50nm approximately. The growth of them was governed by vapor-liquid-solid (VLS) growth mechanism. In addition, the photoluminescence (PL) and magnetic properties of the products were subsequently investigated by fluorescent photometer and vibrating sample magnetometer (VSM). The PL spectrum, employing a Xe laser (240 nm) as an excitation source, shows an emission band centered at about 390 nm, indicating that the obtained SiC nanowires possess excellent optical property. The hysteresis loop shows big magnetic saturation (Ms) of 0.96 emu/g and small coercivity (Hc) of 37.92 Oe. So, the obtained SiC nanowires can be characterized as typical soft magnetic material, and the improvement of magnetic properties may be attributed to the existence of Ni2Si phase in the SiC nanowires.

Structural Investigation and Functional Properties of MgxNi1−xFe2O4 Ferrites

The preparation, structural, microstructural, dielectric, and low temperature magnetic properties of MgxNi1−xFe2O4 (x = 0, 0.17, 0.34, 0.50, 0.66, 0.83, 1) ferrites synthesized by using a self‐combustion sol–gel method is presented. Good insulating properties were found for all the compositions at high frequencies (kHz and MHz range), which might drive the present ceramics as interesting for RF/microwaves applications. By increasing the Mg2+ concentration, the total resistivity strongly increases (from ~106 Ωm for the Ni ferrite to 109 Ωm for the Mg ferrite), corresponding to conductivities in the range 10−9–10−6 S/m at f = 1 Hz. Typical ferrimagnetic character with a small coercivity and saturation magnetization in the range (30–50) Am2/kg, which slightly decreases with increasing the Mg content, were found. On the basis of the combined results from the infrared spectroscopy and XRD analysis, it was shown that the magnetic properties depend on the Mg2+ ions distribution on the octahedral and tetrahedral sites and the experimental saturation magnetization allowed to compute the cation distribution for the MgxNi1−xFe2O4 ferrites.

Formation and magnetic properties of metastable phases observed in the Fe50(HfxTa1−x)50 films by ion beam mixing

Metastable phase formation and magnetic properties of the Fe50(HfxTa1−x)50 (x=0, 0.2, …, and 1) thin films induced by ion beam mixing are studied. The results show that unique amorphous phases could be formed in all the six multilayered films. Interestingly, preferred orientations and formation of BCC-Hf phase were observed in the Fe50Hf50 multilayered films at as-deposited state and after irradiated to doses less than 6×1014Xe+/cm2. In addition, good soft magnetic properties were obtained in the formed amorphous phases. The Fe50Hf40Ta10 amorphous phase owns the maximum magnetization of 45emu/g (i.e., 1.21μB per Fe atom) at 5K, with a saturate magnetization of 7.5emu/g and a small coercivity Hc=7Oe at room temperature. The possible mechanisms responsible for the observations are discussed in terms of atomic collision theory and crystallography.

Comparison of surface effects in SiO2 coated and uncoated nickel ferrite nanoparticles

Magnetic properties of uncoated and silica coated nickel ferrite nanoparticles of comparable sizes have been studied in detail. Silica coated and uncoated nanoparticles were prepared by sol–gel and co-precipitation methods, respectively. Average crystallite size determined by X-ray diffraction is 12nm and 14nm for the silica coated and uncoated nanoparticles, respectively. Normalized saturation magnetization value of the coated nanoparticles was found to be lower than of uncoated nanoparticles, while a comparable small coercivity is observed for both the samples. Zero field cooled/field cooled (ZFC/FC) measurements reveal that the average blocking temperature (TB) of coated nanoparticles is lower than of the uncoated nanoparticles and is shifted to lower temperatures at high field. Thermoremanent magnetization (TRM) measurement indicates that the relaxation of coated nanoparticles have not been influenced very much with increasing cooling field as compared to uncoated nanoparticles and is attributed to enhanced surface effects in coated nanoparticles. The main source of enhanced surface effects in the coated nanoparticles is foremost disordered surface spins due to silica matrix. Temperature dependent AC susceptibility exhibits two peaks for the coated nanoparticles’ sample. First peak corresponds to blocking of huge core spin while second peak at lower temperature is may be due to enhanced surface effects (spin-glass behavior). All these findings such as lower saturation magnetization, faster shift of blocking temperature at high field, small effect of high magnetic field on magnetic relaxation, low temperature out-of-phase AC susceptibility peak for the coated nanoparticles signify enhanced surface effects in them as compared to uncoated nanoparticles.

Magnetic performance and corrosion resistance of cobalt based magnetic films

To optimise the composition of magnetic materials applied in the fabrication of ultralarge scale integration (ULSI) devices and microelectromechanical systems (MEMS), the magnetic performance and anticorrosion property of cobalt based magnetic films (CoWP, CoPtP and CoMoP) were investigated using vibration sample magnetometer (VSM) and electrochemical technologies (potentiodynamic polarisation curve and electrochemical impedance spectroscopy). Results indicate that CoWP has a small coercivity (592·73 Oe) and large saturation magnetisation value (144·20 emu.g−1); while CoPtP and CoMoP films are hard magnetic materials with coercivity of 1786·57 and 2745·20 Oe, saturation magnetisation value of 91·72 and 120·74 emu.g−1 respectively. Electrochemical measurements reveal the order of corrosion resistance in 3·5%NaCl solution was CoPtP film>CoWP film>CoMoP film. In the fabrication of ULSI devices and MEMS, CoPtP film was recommend to be one of the most suitable magnetic materials. CoPtP film was thick with finer grains (52–78 nm) and lower roughness (18·212 nm).

Synthesis and magnetic properties of hydrothermal magnesium–zinc spinel ferrite powders

Mg–Zn ferrite powders with the nominal composition Mg0.5Zn0.5Fe2O4 were synthesized via hydrothermal method, and their synthesis, magnetic properties and microstructures were studied. It was found that the pH value affected the amount of impurity Fe2O3 and the purity of ferrites greatly. It was also found excess Zn content (5 at.%) in starting materials was not helpful to synthesize pure spinel ferrite, while the prolonged reaction time was harmful for the formation of pure spinel structure. The specimens presented small coercivity lower than 10 Oe, which showed a typical magnetically soft behavior. With the increase of pH value, the saturation magnetization of specimens with excess Zn ions (5 at.%) kept increasing from 23.90 to 41.82 emu/g due to the decreasing amount of impurity Fe2O3. The study of microstructures showed that the large particles in powders were the aggregates of small nanoscale crystallites. The analysis of actual Zn and Mg content in synthesized ferrites confirmed that the best experimental conditions for synthesis of pure spinel Mg–Zn ferrite are the hydrothermal temperature is 200 °C, the reaction time is 8 h, the pH value is 12 and the excess amount of Zn(NO)3 in starting materials is 5 at.%.

Growth-temperature-dependent ferrimagnetism in Mn3Ga thin films

We report on the growth of and the modified ferrimagnetism in DO22-structured Mn3Ga thin films epitaxially grown on GaSb(001) substrate by using molecular beam epitaxy. We have observed that the magnetic properties strongly depend on the growth temperature. The net magnetic moments of the Mn3Ga films grown at 30 °C and 200 °C are 0.23 µB/Mn and 0.75 µB/Mn atom, respectively. The as-studied Mn3Ga film is found to exhibit a relatively small coercivity around 400 Oe, which differs greatly from the hard magnetic properties of bulk Mn3Ga.

Magnetic properties of Mn–Bi melt-spun ribbons

Mn–Bi melt-spun ribbons with the low temperature phase (LTP) of MnBi were produced by melt-spinning and subsequent annealing. The as-rapidly quenched Mn–Bi melt-spun ribbons contained some LTP MnBi phase and exhibited a high coercivity exceeding 8kOe. Annealing of the melt-spun ribbons resulted in an increase in the amount of the LTP MnBi phase. A maximum remanence value of 42emu/g was achieved in Mn50Bi50 melt-spun ribbon annealed at 673K for 1h. High-temperature measurements revealed that the coercivity of the annealed Mn50Bi50 melt-spun ribbon increased with increasing ambient temperature. Although the Mn50Bi50 melt-spun ribbons showed a much smaller coercivity than Nd15Fe77B8 melt-spun ribbon at room temperature, it exhibited a higher coercivity at temperatures of 473K and higher. Therefore, the magnetic properties of Mn50Bi50 melt-spun ribbon are comparable to those of Nd–Fe–B melt-spun ribbon at an ambient temperature of 473K and become superior to those of Nd–Fe–B melt-spun ribbon at 573K.

Tunable interface anisotropy in a Pt/Co1−xFex/Pt multilayer

Interfacial magnetic anisotropy in a Pt/Co1−xFex/Pt multilayer is tuned by doping iron atoms into the cobalt layer. The perpendicular magnetic anisotropy and out-of-plane coercivity are found to decrease with increasing x. For a specific x, the out-of-plane coercivity acquires a maximal value as a function of the thickness of the CoFe layer. At low temperature, the coercivity is enhanced. Small coercivity but reasonably large perpendicular magnetic anisotropy can be obtained by controlling the x and CoFe layer thickness.

Exchange bias of perpendicularly magnetized [Co/Pt]3/IrMn multilayer on porous anodized alumina

The exchange bias of [Co/Pt]3/IrMn multilayers deposited on porous anodized alumina oxide template has been investigated. The perpendicular exchange bias has been successfully established on the anodized alumina template and Si substrate. For samples on the anodized alumina templates, a larger exchange field was obtained but smaller coercivity was observed, in comparison with those on flat Si substrate. Unlike conventional observations, the asymmetry of angular dependence of coercive fields of two branches for nanostructured samples is less prominent than that of the continuous samples.

Structural and magnetic properties of patterned perpendicular media with linearly graded anisotropy

L10-FePt thin films with linearly distributed anisotropy were fabricated by continuously varying substrate temperatures from 650 °C to 290 °C. An average ordering parameter S = 0.6 was obtained. The hysteresis loop has a clear bow-tie shape with low remanence and coercivity before patterning. The patterned nanopillar arrays exhibit an enhancement in both remanence and coercivity. Dynamic coercivity measurement shows that the nanopillar arrays with linearly graded anisotropy have a relatively small intrinsic coercivity and a relatively high value of the figure of merit. The switching field exhibits a narrow distribution, which indicates a strong coupling between soft and hard phases in nanopillar arrays with graded anisotropy.

Granular L10 FePt:X (X = Ag, B, C, SiOx, TaOx) thin films for heat assisted magnetic recording

Ordered L10 FePt thin films are of interest as potential Heat Assisted Magnetic Recording media. In order to achieve the microstructure and magnetic properties to support recording at densities beyond 1 Tbit/in2, it is necessary to add segregants into the FePt films. In this work, the effects of a number of segregants, X, on the microstructure and magnetic properties of FePt:X (X = Ag, B, C, SiO x , TaO x ) thin films, deposited by RF sputtering with various volume content (0–50%), various in-situ heating temperatures (450–600 °C), various Ar pressures (10–40 mtorr) and various sputtering powers (25–200 W) onto 1′′ Si substrates with a MgO texture (002) underlayer (20 nm), were investigated. It was observed that introducing segregants (B, C, SiO x , and TaO x ) into the FePt reduced ordering and grain size of the FePt:X thin films. Ag was found to offset the reduction of ordering in the FePt:X films. The B, SiO x and TaO x promoted columnar growth whereas C promoted a secondary nucleation layer but produced the least reduction of ordering. By varying the volume content of the segregants, the grain size of the FePt:X can be controllably reduced throughout the 2.5–10 nm range. It was found that TaO x produced the best exchange decoupling, thermal durability, grain isolation and hindered grain coalescence as compared with the films deposited with B, C or SiO x . With the FePt:C:Ag films sputtered at 450 °C, a perpendicular coercivity measured at room temperature as high as 25 kOe was achieved; whereas with B, SiO x , and TaO x , perpendicular coercivities as high as 11 kOe were obtained. These FePt:X thin films with small grain size, columnar microstructure and high coercivity are believed to be favorable for application in Heat Assisted Magnetic Recording. The role of surface energies of FePt and the segregant in columnar growth of FePt grains is discussed.

5 Tdots/in2 bit patterned media fabricated by a directed self-assembly mask

FePt bit patterned media (BPM) was fabricated with a self-assembled polymer mask with a feature size of 12 nm pitch (equivalent to 5 Tdots/in2 ). A 3.5 nm FePt film with high c-axis crystal orientation was prepared for the magnetic recording layer. A solvent vapor annealing process was applied for obtaining uniform directed self-assembling of polystyrene (PS)-polydimethylsiloxane (PDMS) diblock copolymer pattern. Pattern transfer from a polymer mask to FePt layer was achieved by employing a carbon hard mask. In spite of excellent magnetic characteristics of FePt layer, the fabricated FePt BPM showed small coercivity (Hc) of 6 kOe and large switching field distribution (SFD) of 21%. These results are due to the etching damage of FePt dots. Disordering of FePt L10 phase by the etching damage reduced magnetic anisotropy energy (Ku). The damaged portion became a nucleus of the magnetization reversal and reduced Hc. Distribution of the damaged volume and the extent of the Ku reduction contributed to large SFD. This model is supported by the experimental data of magnetic field angle dependence of switching field. The result suggests the domain wall motion type of magnetization reversal mode, where the domain wall is created at the interface between the damaged portion and the internal high-Ku region.

Perpendicular magnetic anisotropy in piezoelectric- and dielectric–ferromagnetic heterostructures based on Co/Pt multilayers

We have optimized the perpendicular magnetic anisotropy of piezoelectric/ferromagnetic and dielectric/ferromagnetic heterostructures based on Co/Pt multilayers. These hybrid systems are promising for electric field control of magnetism for applications to ultra-low power spintronics devices. Epitaxial (PbMg1/3Nb2/3O3)0.7-(PbTiO3)0.3 has been used for the piezoelectric layer and several dielectric layers (MgO, Al2O3, HfO2, Ta2O5 and TiO2) have been tested. We show that for both heterostructures perpendicular magnetic anisotropy of Co/Pt multilayers can be obtained with small coercivity.

Structural, magnetic, electrical and dielectric properties of MnxNi1−xFe2O4 spinel nanoferrites prepared by PEG assisted hydrothermal method

Abstract MnxNi1−xFe2O4 (x=0.2, 0.4, 0.6) nanoparticles were synthesized by a polyethylene glycol (PEG)-assisted hydrothermal route. We present a systematic investigation on the structural, magnetic, electrical and dielectric properties of the products by using XRD, FT-IR, SEM, TGA, VSM and dielectric spectroscopy, respectively. Single phased cubic spinel structure was confirmed for all samples and the average crystallite size of the products was estimated using Line profile fitting and ranges between 6.5 and 11 nm. The nanoparticles have ferromagnetic nature with small coercivity. The samples showed semiconducting behavior which is revealed from temperature dependent conductivity measurements. Temperature and frequency dependent dielectric property; dielectric permittivity (e) and ac conductivity (σAC) studies for the samples indicated that the dielectric dispersion curve for all samples showed usual dielectric dispersion confirming the thermally activated relaxation typical for Debye-like relaxation referring to it as the Maxwell–Wagner relaxation for the interfacial polarization of homogeneous double structure. The particle size, saturation magnetization, coercive field, conductivity and dielectric constant of the samples are strictly temperature dependent and increased with Mn concentration.

Synthesis and Magnetic Properties of Co0.5Zn0.5Fe2O4 Nanoparticles by Template-Assisted Combustion Method

In the present article, zinc cobalt ferrite (Co0.5Zn0.5Fe2O4) nanopowders were synthesized by a template-assisted citric combustion method. Polyethylene Glycol (PEG-4000) and carboxymethyl cellulose (CMC) were used as the templating agents for controlling the anisotropy and the microstructure of the Co0.5Zn0.5Fe2O4 nanopowder. The synthesized nanopowders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and a vibrating sample magnetometer (VSM). The results indicated that pure samples were obtained at calcining temperature of 700°C and the templates remarkably changed microstructure and magnetic behavior of the nanopowders. All of the powders synthesized were ferromagnetic. The powder synthesized with template PEG-4000 had a smallest coercivity (75 Oe) but largest magnetization (80.3 emu · g−1) close to that of bulk material. The powders synthesized without template exhibited moderate coercivity (1864 Oe) and smallest magnetization (53.2 emu · g−1). A largest coercivity (3986 Oe) and a moderate magnetization (60.9 emu · g−1) were observed when the template CMC was used. The variation in the magnetic nature of the powders can be related to their anisotropy growth and average particle size.

Growth and properties of GdTiO3 films prepared by hybrid molecular beam epitaxy

The paper reports on the thin film growth of a protoptype Mott insulator, ferrimagnetic GdTiO3, using shuttered molecular beam epitaxy. Substrates were (001) (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT), with and without epitaxial SrTiO3 buffer layers, respectively. It was found that on bare LSAT, the starting monolayer was crucial for stabilizing the GdTiO3 perovskite phase. The quality of the films was evaluated using structural, electric, optical and magnetic characterization. Structural characterization showed that the GdTiO3 layers were free of pyrochlore impurity phases and that the lattice parameter was close to what was expected for coherently strained, stoichiometric GdTiO3. The room temperature film resistivity was 7Ωcm and increased with decreasing temperature, consistent with Mott insulating characteristics. The Curie temperature was 30K and a small coercivity was observed at 2K, in good agreement with bulk GdTiO3 properties reported in the literature.

Robust Room-Temperature Ferromagnetism with Giant Anisotropy in Nd-Doped ZnO Nanowire Arrays

As an important class of spintronic material, ferromagnetic oxide semiconductors are characterized with both charge and spin degrees of freedom, but they often show weak magnetism and small coercivity, which limit their applications. In this work, we synthesized Nd-doped ZnO nanowire arrays which exhibit stable room temperature ferromagnetism with a large saturation magnetic moment of 4.1 μ(B)/Nd as well as a high coercivity of 780 Oe, indicating giant magnetic anisotropy. First-principles calculations reveal that the remarkable magnetic properties in Nd-doped ZnO nanowires can be ascribed to the intricate interplay between the spin moments and the Nd-derived orbital moments. Our complementary experimental and theoretical results suggest that these magnetic oxide nanowires obtained by the bottom-up synthesis are promising as nanoscale building blocks in spintronic devices.

Synthesis and characterization of Piperidine-4-carboxylic acid functionalized Fe3O4 nanoparticles as a magnetic catalyst for Knoevenagel reaction

Piperidine-4-carboxylic acid (PPCA) functionalized Fe3O4 nanoparticles as a novel organic–inorganic hybrid heterogeneous catalyst was fabricated and characterized by XRD, FT-IR, TGA, TEM and VSM techniques. Composition was determined as Fe3O4, while particles were observed to have spherical morphology. Size estimations using X-ray line profile fitting (10nm), TEM (11nm) and magnetization fitting (9nm) agree well, revealing nearly single crystalline character of Fe3O4 nanoparticles. Magnetization measurements reveal that PPCA functionalized Fe3O4 NPs have superparamagnetic features, namely immeasurable coercivity and absence of saturation. Small coercivity is established at low temperatures. The catalytic activity of Fe3O4–PPCA was probed through one-pot synthesis of nitro alkenes through Knoevenagel reaction in CH2Cl2 at room temperature. The heterogeneous catalyst showed very high conversion rates (97%) and could be recovered easily and reused many times without significant loss of its catalytic activity.

Polycrystalline magnetic garnet films comprising weakly coupled crystallites for piezoelectrically-driven magneto-optic spatial light modulators

We have investigated the magnetization process of the polycrystalline magnetic garnet films in order to determine the most suitable composition of garnet films for piezoelectrically-driven magneto-optic spatial light modulators (MOSLMs). For experiment, the bismuth–dysprosium–aluminum-substituted yttrium iron (Bi1.3Dy0.7Y1.0Fe3.1Al1.9O12) garnet films were deposited by an RF magnetron sputter and annealed at 700 °C in air. The annealing time was varied in a range of several minutes to control the grain size. The saturation magnetization, the remanent magnetization and the composition of the fabricated garnet films slightly changed versus the annealing time. Experiments showed that the coercivity and the grain size increased at longer annealing; the coercivity was larger for films with bigger grains. This work shows that garnet films with smaller coercivity are most suitable for controlling the magnetization of garnet and, correspondingly, the magneto-optical rotation of MOSLM pixels driven by piezoelectrics.