Low Coercivity Values Research Articles

Grain Boundary Segregation and Interdiffusion Effects in Nickel–Copper Alloys: An Effective Means to Improve the Thermal Stability of Nanocrystalline Nickel

Nanocrystalline (nc) Ni films show pronounced grain growth and suffer from concomitant deterioration of their mechanical and magnetic properties after annealing at relatively low temperatures (T(ANN) ≥ 475 K). This constitutes a drawback for their applicability as coatings or in components of miniaturized devices. This work reveals that the thermal stability of nc Ni is significantly improved by controllably alloying Ni with Cu, by means of electrodeposition, to form a Ni(1-x)Cu(x) solid solution. To tune the composition of such nc alloys, Ni(1-x)Cu(x) films are deposited galvanostatically using an electrolytic bath containing Ni and Cu sulfate salts as electroactive species, saccharine as grain-refining agent, and applying current densities ranging from -10 to -40 mA cm(-2). The enhanced thermal stability is ascribed to segregation of a Cu-rich phase at the Ni(1-x)Cu(x) grain boundaries, which acts as a shielding layer against grain growth. As a result, high values of hardness (in excess of 6 GPa) remain in nc Ni(1-x)Cu(x) for x ≥ 0.3, even after annealing at T(ANN) ≥ 575 K. From a magnetic point of view, Ni(1-x)Cu(x) films possess lower coercivity values than pure nc Ni films, both in the as-prepared and annealed states, thus offering potential advantages for certain soft magnetic applications.

Magnetic properties of (Sm,Y)5Fe17 melt-spun ribbons

An investigation of the synthesis of the (Sm1−xYx)5Fe17 (x = 0 to 0.5) phase and its magnetic properties is presented. (Sm1−xYx)5Fe17 (x = 0 to 0.5) melt-spun ribbons that fully or mainly consisted of the amorphous phase and which showed low coercivity were prepared. Heat treatment of the melt-spun ribbons resulted in the formation of the (Sm,Y)5Fe17 phase. The highest remanence, 50 emu/g, was achieved in the annealed (Sm0.7Y0.3)5Fe17 melt-spun ribbon. However,the annealed (Sm1−xYx)5Fe17 (x = 0.4 to 0.5) melt-spun ribbons did not contain the (Sm,Y)5Fe17 phase and showed low coercivity values.

Simple synthesis of Pd–Fe3O4 heterodimer nanocrystals and their application as a magnetically recyclable catalyst for Suzuki cross-coupling reactions

A simple gram-scale synthesis of Pd-Fe(3)O(4) heterodimer nanocrystals was achieved by controlled one-pot thermolysis of a mixture solution composed of iron acetylacetonate, palladium acetylacetonate, oleylamine, and oleic acid. The heterodimer nanocrystals are composed of a 6 nm-sized Pd nanosphere and a 30 nm-sized faceted Fe(3)O(4) nanocrystal and they are soft ferrimagnetic with high saturation magnetization value and low coercivity value. The heterodimer nanocrystals exhibited good activities for various Suzuki coupling reactions. Furthermore, the nanocrystal catalyst could be easily separated from the product mixture by using a magnet and could be recycled 10 times without losing catalytic activity.

Structural and Magnetic Properties of Cobal Ferrite - Barium Titanate Nanotube Arrays

ABSTRACTCobalt Ferrite/Barium Titanate nanotube arrays were obtained in anodic aluminum oxide templates (AAO) of 100 nm pore diameter by a two step sol-gel process. Each phase was grown in several wetting – drying cycles starting from the cobalt ferrite layers with the barium titanate on top. As-dried composite structures were sintered at 700 C. The composite nanotubes showed a fine polycrystalline microstructure with an average grain size of 5 nm. The formation of both spinel and perovskite structures was verified by High Resolution Transmission Electron Microscopy (HR-TEM) on isolated nanotubes. The growing rate by layer was found to be lower for the BaTiO3 on top of CoFe2O4 than the later on top of the AAO. Wall sizes were estimated by Z-contrast as 9.8 nm for one layer of CoFe2O4 and 6.6 nm for six layers of BaTiO3. Magnetic properties were studied by VSM. Samples showed ferromagnetic behavior with low coercive values. By means of a finite element model the deformation and stress on the piezoelectric phase was estimated and used to simulate the magnetization reversal under stress in the composite nanotubes, using an updated micromagnetic framework to include the magnetostriction effect. Simulation results showed that a curling mode is expected with opposite vortex states at the end of the nanotubes. The change in the vortex domain structure under voltage driven applied stress is presented and discussed.

Synthesis, phase study and magnetic characterisation of Co50Fe40Cu10 ternary alloy nanopowders prepared by mechanochemical alloying process

Mechanical milling and hydrogen reduction of pure oxide mixture and magnetic characterisation of Co–Fe–Cu ternary alloy nanopowders were investigated. A powder mixture of Co3O4, CuO and Fe2O3 with Co50Fe40Cu10 stoichiometry was first milled by a high energy planetary ball mill and then reduced in a hydrogen reduction system.The optimum condition of the reduction under the hydrogen atmosphere was 650°C and 1 h. The X-ray diffraction patterns exhibit that the powder has ordered bcc structure with b2–bcc space group and 2·87 Å lattice parameter. Mean crystallite sizes calculated from X-ray diffraction results and mean particle size observed from electron microscopes were over 75 nm. Magnetic evaluation of ternary alloy nanopowders showed a saturation magnetisation value about 143 Am2 kg–1 and a low coercivity value of 0·93 Am–1.

Effect of minor additions on the glass forming ability and magnetic properties of Fe–Nb–B based metallic glasses

The glass forming ability, thermal stability, nanocrystallization, and magnetic properties of Fe 65+ x Nb 10B 25− x ( x = 0,5,10), Fe 75+ y Nb 9− y B 15Cu 1 ( y = 0,2,4) and Fe 77− z Co z Nb 7B 15Cu 1 ( z = 2,4,8) metallic glasses have been studied. The results show differences in the nanocrystallization changing from the metastable Fe 23B 6 phase to bcc-Fe when increasing the relative Fe/B content. The ternary FeNbB alloys are good soft magnets in the glassy state and glass forming ability is enhanced with the B content. However, their nanocrystallization leads to large increase of coercivity. The (Fe,Co)–Nb–B–Cu alloys show bcc-Fe or bcc-(Fe,Co) grains with a refined microstructure (4–6 nm) after nanocrystallization with relatively low coercivity values (in the range 4–15 A/m). The replacement of Nb by Fe and the addition of Co enhance the values of both the Curie temperature and saturation polarization from 346 K to 493 K (for Fe 69Co 8Nb 7B 15Cu 1) and from 1.0 T to 1.4 T (for Fe 79Nb 5B 15Cu 1), respectively.

Composition-Dependent Crystal Structure and Magnetism in Nanocrystalline Co-Rich Alloy

We report the magnetism of a ferromagnetic two phase mixture system by investigating the microstructural evolution and relevant property changes as a function of alloy composition in CoFeSiB film. The crystal structure evolved from an amorphous to <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">hcp</i> Co nanocrystal embedded two-phase mixture when Co concentration exceeded the critical amount (~75 at.%) confirmed through TEM and XRD. Very low coercivity value was observed in the amorphous samples. The soft magnetic properties are due to the fact that the magnetic moments of Co-rich phase are exchange-coupled via the amorphous matrix, as a consequence the magnetic anisotropies are averaged out in the amorphous system. Meanwhile, beyond 75 at.% of Co, an abrupt increase in coercivity was observed in the nanocrystalline samples where highly anisotropic Co nanocrystals were precipitated. The hard magnetic properties resulted from the magnetic decoupling (breaking of the nanocrystal-amorphous matrix-nanocrystal coupling chain) because the exchange forces are not able to overcome the magnetocrystaline anisotropy. We also found that antiferromagnetic exchange coupling existed at the nanocrystal-amorphous matrix interfaces.

Phase Transformation in Silica-Coated FePt Nanoparticles

Chemically synthesized FePt nanoparticles (NPs), with an average size of 5.8 nm, were coated with silica (SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) shells. The thickness of the silica shell could be controlled between 7.5-25 nm. A modified heating profile has been adopted with which silica shell cracking at high temperatures (1000degC) could be prevented. Even with prolonged heating for 24 h at 800degC, no sintering of the particles is observed. Low temperature (7 K) and room temperature magnetization scans show low coercivity values which are possibly due to a compositional distribution among the individual NPs.

Study on morphology and magnetic behavior of SmCo 5 and SmCo 5/Fe nanoparticles synthesized by surfactant-assisted ball milling

A surfactant-assisted ball milling was employed for the processing of SmCo 5 and SmCo 5/Fe nanopowders in presence of oleic acid as surfactant. Two types of samples: one is the sediment powders slightly larger particle size in the range of 0.2–1 μm and other is the suspension in the milling medium with colloidal particles in the range of 4–20 nm were obtained. Structural and magnetic properties of SmCo 5 and SmCo 5/Fe sediment powders, as well as their constituent colloidal nanoparticles were investigated by a transmission electron microscopy (TEM), small angle X-ray scattering (SAXS) and vibrating sample magnetometer. TEM analysis on the colloidal particles revealed a plate-like faceted morphology with particle diameter in the range of 4–20 nm, while SAXS analysis suggested that the shape of particles are lamellar with average thicknesses in the range of 0.5–1.0 nm. A possible mechanism for the formation of lamellar particles with faceted morphologies due to the milling process is discussed. Magnetic measurements on these nanoparticles showed superparamagnetic nature, with very low coercivity and remanence values.

Metal oxide/polyaniline nanocomposites: Cluster size and composition dependent structural and magnetic properties

Nanocomposites of iron oxide with conducting polymer in the form of powders of varying compositions have been studied to understand the effects of particle size, cluster size and magnetic inter-particle interactions. The sizes of the nanoparticles were estimated to be ∼ 10–20 nm from the X-ray diffraction (XRD) and the transmission electron micrographs (TEM). XRD shows a single crystalline phase for the γ-Fe2O3. The presence of conducting polymer was confirmed through Fourier transform infrared (FTIR) spectroscopy. The amount of polymer present in the composite, the transition temperature of iron oxide and the thermal stability of polymer was determined through thermogravimetric and differential thermal analysis (TGA-DTA). The room temperature magnetic hysteresis measurements show reduction in saturation magnetization with increasing polymer concentrations. A low value of coercivity was observed for low polymer composites. On increasing the polymer concentration, the coercivity and remanence become negligible indicating a superparamagnetic phase at room temperature. Beyond a certain composition, the system shows paramagnetic behaviour which is also confirmed through zero field cooled-field cooled (ZFC-FC) measurements. We also report preliminary results on the magnetic properties of self standing sheets prepared using γ-Fe2O3 and NiFe2O4 nanoparticles and conducting polymers.

Effect of liquid phase composition on the microstructure and properties of (W,Ti)C cemented carbide cutting tools

The microstructure and properties of (W,Ti)C cemented carbides with a different metal binder composition of Ni and Co fabricated by powder technology were investigated. The densifications of the prepared materials were accomplished by using vacuum sintering at 1450 °C. Nearly fully dense (W,Ti)C cemented carbides were obtained with a relative density of up to 99.7% with 30 wt% Co and 99.9% with 30 wt% Ni. The average grain size of carbides for the (W,Ti)C–Co and the (W,Ti)C–Ni was decreased with increasing the metal binder content. The hardness of the dense (W,Ti)C–15 wt%Co and (W,Ti)C–15 wt%Ni, was greater than the hardness of other different compositions of (W,Ti)C–Co and (W,Ti)C–Ni cemented carbides respectively; in addition, the (W,Ti)C–Co cemented carbides had greater hardness values than the (W,Ti)C–Ni cemented carbides. The magnetic properties, which were measured at an applied field of 0.8 T, indicate that the (W,Ti)C–Co cemented carbides have ferromagnetic properties with a higher magnetic saturation with higher cobalt content as well as higher coercivity with lower cobalt content. On the other hand, the (W,Ti)C–Ni cemented carbides are non-magnetic on account of the very low magnetic saturation and they have lower coercivity values than the (W,Ti)C–Co cemented carbides.

Magnetic properties and magneto-impedance effect of CoNiFe/Cu composite wires by electroplating

In this study, CoNiFe/Cu composite wires were deposited using electroplating. The deposited material composition was controlled by adjusting the concentration of Co ions. The results showed that the surface morphology and particle size did not change much with different compositions. However, the coercivity of the composite wires varied significantly for different compositions. A magnetic layer, with the composition of Co30Ni50Fe20, was found to possess a low coercivity value of 0.65 Oe. This is due to the low magneto-strictive factor for the alloy with this composition. In addition, the results showed that the concentration of Fe ions played an important role in achieving low coercivity and high magneto-impedance (MI) effect in the CoNiFe/Cu wires.

FePt films fabricated by electrodeposition

In this work, we have fabricated FePt films with a thickness in the range of 0.1–1μm using the combination of electrodeposition and postannealing. FePt films with a composition around Fe50Pt50 were formed by electrodeposition onto the Si (100) substrates with an underlayer of Au, Ag, or Cu, and subsequently annealed at a temperature in the range of 200–900°C for 20min. From our x-ray diffraction analysis, the L10 FePt phase started to form after annealing at 400°C for the film deposited on the Au underlayer. The highest coercivity (10kOe) was found after annealing at 600°C. When the FePt was deposited on the Ag underlayer, a high coercivity over 15kOe with an out-of-plane anisotropy has been achieved after annealing at 700–800°C. The magnetic anisotropy was associated with the crystallographic texture. The magnetic properties of FePt films deposited on the Cu underlayer were relatively poor with lower values of coercivity (4–5kOe as the maximum coercivity), probably due to the large grain size.

Structure, magnetization, and magnetostriction of Sm-R-Fe (R = Pr, Nd) thin films

The structure, magnetization, and magnetostriction of Sm 0.9Pr 0.1Fe x and Sm 1− x Nd x Fe 1.9 thin films have been investigated using X-ray diffraction, vibrating sample magnetometer, and optical cantilever method. It is found that the structure of Sm 0.9Pr 0.1Fe x thin films consists of an Sm-Pr-Fe amorphous phase when x ≤ 2.69 and that of Sm 1− x Nd x Fe 1.9 thin films consists of an Sm-Nd-Fe amorphous phase. The in-plane magnetization of Sm 0.9Pr 0.1Fe x thin films increases with increase in the Fe content, and low values of the in-plane coercivity occur in the range of 1.62 ≤ x ≤ 2.28. The magnetostriction value of Sm 0.9Pr 0.1Fe x thin films increases with increasing the Fe content when x ≤ 1.94 and decreases when x > 1.94. The in-plane magnetostriction of Sm 1− x Nd x Fe 1.9 thin films under low magnetic fields has been improved by the substitution of Nd for Sm when x = 0.2.

Effects of ion irradiation on cobalt nanocomposite

The effects of ion irradiation of 3-dimensional arrays of Co nanoparticles were investigated. Arrays were obtained by electron beam deposition of 15 Co/SiO2 bilayers 0.5 and 20nm thick, respectively. The Co layers consist of Co nanoparticles 3.2nm in diameter with a standard deviation of 16%. Irradiation was carried out using combinations of 150kV Ar2+ and 90kV Ar+ ion bombardments with a Ar+:Ar2+ fluence ratio of 1:4. The effects of ion irradiation were followed by Rutherford backscattering, measurements of hysteresis loops at 5K and of temperature-dependent field-cooled (FC) and zero-field-cooled (ZFC) measurements of the magnetic susceptibility. A decrease of the peak temperature in the ZFC curve for displacements per atom (DPA) up to 1.1 was observed. Irradiation also induces progressively lower coercivity values. The Co particles showed a remarkable high resistance to ion irradiation, surviving damage up to 33DPA.

Soft magnetic properties of NiFe compacted powder alloys

It is known that Ni–Fe based alloys (permalloys) are important soft magnetic materials, which have been widely applied in the field of electronic devices and industry. The most suitable permalloys for application exhibit low value of coercivity and magnetostriction (for about 80 at% Ni), high saturation magnetic induction (for about 50 at% Ni), higher electrical resistivity (for about 35 at% Ni). The aim of this work was to investigate the structure and magnetic properties of Ni 81 Fe 19 (wt%) compacted powder material in the form of small cylinders.

Interaction effects in magnetic oxide nanoparticle systems

The interaction effects in magnetic nanoparticle system were studied through a Monte Carlo simulation. The results of simulations were compared with two different magnetic systems, namely, iron oxide polymer nanocomposites prepared by polymerization over core and nanocrystalline cobalt ferrite thin films prepared by sol-gel process. The size of the particles in the nanocomposites were estimated to be ∼15 nm with very little agglomeration. The low values of the coercivity obtained from the hysteresis measurements performed confirm that the system is superparamagnetic. SEM studies showed the cobalt ferrite films to have a nanocrystalline character, with particle sizes in the nanometer range. Hysteresis measurements performed on the thin films coated on silicon do not give evidence of the superparamagnetic transition up to room temperature and the coercivity is found to increase with decreasing film thickness. Comparison with simulations indicate that the nanocomposites behave like a strongly interacting array where exchange interactions lead to high blocking temperatures, whereas the films are representative of a semi-infinite array of magnetic clusters with weak interactions and thickness-dependent magnetic properties.

Dependence of the structural and magnetic properties of FePt foil magnets on annealing ramp rate

Abstract Bulk Fe/Pt multilayers were annealed to form hard magnetic L 1 0 FePt-based permanent magnet foils. The phase composition of the foils, and thus their magnetic properties, were found to depend not only on the temperature and time of annealing, but also on the heating ramp rate used. For short annealing times ( ⩽ 1 h ) and low annealing temperatures ( ⩽ 450 ∘ C ), a slow ramp rate (900 K/h) resulted in the formation of an FePt diffusion barrier at the multilayer interfaces, while a fast ramp rate ( 142 × 10 3 K/h) promoted a more complete interdiffusion of the Fe and Pt atoms. The relatively low value of coercivity of samples annealed for short times ( ⩽ 1 h ) following a fast ramp, is attributed to the presence of the soft-magnetic disordered FCC FePt phase, exchange-coupled to the dominant hard magnetic L 1 0 FePt phase.

Effects of annealing temperatures on the electrical properties of pulsed laser deposited Bi 3.25La 0.75Ti 3O 12 thin films for field effect transistor-type memory device

Electrical characterizations of a metal-ferroelectric-insulator-semiconductor field effect transistor (MFISFET) using Au/Bi 3.25La 0.75Ti 3O 12/Si 3N 4/Si(1 1 1) were investigated by studying the effects of the annealing conditions. Ferroelectric Bi 3.25La 0.75Ti 3O 12 (BLT) was prepared on 15 nm Si 3N 4 coated p-type Si(1 1 1) substrates by the pulsed laser deposition method at a deposition temperature of 400 °C and an oxygen pressure of 200 m Torr. The crystalline structures showed polycrystalline films with preferable c-axis orientations at higher annealing temperatures. The capacitance–voltage ( C– V) measurements for the as-deposited films, 600 and 700 °C annealed films showed a decreasing memory window value of 1.31, 0.78, and 0.36 V, respectively. The generation of low coercive values for higher annealed films is known to be the main cause for such results.

Microwave sintering of Ni–Zn ferrites: comparison with conventional sintering

Ni 1− x Zn x Fe 2O 4 with x=0.0–1.0 in steps of 0.2 have been densified to >95% of theoretical both by conventional and microwave sintering procedures. Comparison studies were performed on these samples sintered at 1275 °C for 30 min. Microstructural study revealed large, but fewer pores in the microwave sintered specimens and small, but substantially more pores in the case of conventionally sintered specimens. Magnetic properties measurements showed lower coercivity and higher magnetization values for microwave sintered specimens. Lower coercivity values observed in microwave sintered specimens are attributed to larger grain size and higher magnetization values may be explained by the contribution of the ‘microwave field’ to the ‘uncoupling spin effect’. Most significantly samples sintered in a microwave field, generally, showed lower dielectric constant values compared with the samples sintered conventionally, making microwave sintering particularly suitable for high frequency applications.