Magnetic anisotropy in Ni–Si nanoparticle films produced by ultrashort pulsed laser deposition

Pulsed laser deposition (uPLD) in vacuum by means of subpicosecond laser pulses is a powerful, versatile technique for the production of films constituted by nanoparticles. On impact with the deposition substrate, the nanodrops ejected from the target assume an oblate ellipsoidal shape, solidifying with the major cross-section parallel to the substrate plane. These features and the difficult coalescence among the deposited nanoparticles are peculiar characteristics specific to the films obtained by uPLD. In the case of magnetic nanoparticle films obtained by means of this technique, a magnetization isotropy in the film plane and a hard magnetization axis orthogonal to the film plane are expected. This simple assumption, generated by the specific shape and orientation of the deposited nanoparticles, was not experimentally verified up to now. The present investigation represents the first experimental validation of magnetic anisotropy, determined by the peculiar morphology and topology of the constituent particles, in the uPLD Ni_xSi_100−x nanoparticle films. The in-plane isotropic magnetization behaviour, as well as the presence of a hard magnetization axis perpendicular to the sample surface were demonstrated for all investigated films. The difficult coalescence among the magnetic nanoparticles, even at high Ni volume fractions, is confirmed by the behaviour of the initial magnetization curve, typical for single-domain nanoparticles systems.     

On the correlation between microstructure and magnetic losses in electrical steel

The magnetic properties of electrical steel such as magnetization curves, magnetization behavior and specific magnetic losses are related to the microstructure and the texture of the steel. A quantitative model, which describes the effect of microstructure and texture and their interplay on the magnetic losses P, is still missing. Based on experimental data for nonoriented electrical steels and FeSi-samples with high (Si, Al)-content, a more general formula is proposed for the dependence of P, at a given value of magnetic induction B, as a function of the mean value of the grain size d of the material and of the intensities of the relevant magnetic texture components.     

Magnetic studies in evaporated Ni/Pd multilayers

The magnetic properties of Ni/Pd multilayers, prepared by sequential evaporation in ultrahigh vacuum, have been studied. The Ni thickness dependence of the magnetization and magnetic anisotropy is discussed. The temperature dependence of the spontaneous magnetization is well described by a T^3/2 law in all multilayers. A spin-wave theory has been used to explain the temperature dependence of the spontaneous magnetization, and the approximate values for the exchange interactions for various Ni layer thicknesses have been obtained.     

Highly Acid‐Resistant,Magnetically Steerable Acoustic Micromotors Prepared by Coating Gold Microrods with Fe3O4 Nanoparticles via pH Adjustment

There is mounting interest in designing magnetically steerable nano‐ and micromotors for next generation medical nanorobotics, which requires biocompatibility for each individual component. Although various magnetic materials (e.g., Ni, Co, and Fe₃O₄) have been incorporated into micromotors, their acid resistance remains largely unexplored. In this article, a simple one‐step method to prepare magnetic microrods via electrostatic attraction between paramagnetic magnetite nanoparticles (Fe₃O₄ NPs) and gold microrods at appropriate pH values is reported. The as‐prepared Fe₃O₄‐coated micromotors can be powered by MHz ultrasound and easily steered by external magnetic fields, and perform well in harsh working conditions such as high acidity, high viscosity, and high ionic strength. In particular, extended exposure to solution of pH as low as 0.9 has a minimal effect on the speed, steerability, or cargo‐transporting capability of micromotors coated with Fe₃O₄ NPs, in stark contrast with those containing Ni segments. Considering the many challenges of biomedical applications, acid‐resistant, magnetically steerable Fe₃O₄‐coated micromotors powered by MHz ultrasound can be a promising prototype for the future development of medical nano‐ and microrobotics.     

Magnetic properties of SiO2-coated iron oxide nanoparticles studied by polarized small angle neutron scattering

The structural and magnetic properties of non-coated and SiO₂-coated iron oxide (Fe₃O₄) nanoparticles (NPs) were investigated by a polarized small-angle neutron scattering (P-SANS) method. Measurement of the P-SANS allowed us to obtain nuclear and magnetic scattering cross sections of the NPs under applied magnetic field. The analysis of the scattering intensity provided the structural parameters and the spatial magnetization distribution of the non-coated and the SiO₂ coated core–shell NPs. The measured radius of both NPs and the shell thickness of the core–shell NPs were in consistent with those measured by the transmission electron microscopy. In comparison, the magnetic core radii of both NPs were 0.12–0.6 nm smaller than the nuclear radii, indicating the magnetization reduction in the surface region of core Fe₃O₄ in both NPs. However, the reduced magnetization region, which is the surface spin canting region, of the SiO₂-coated NPs was relatively narrower than that of the non-coated NPs. We suggest that the SiO₂ coating on the Fe₃O₄ NPs may stabilize the spin order of atoms and prohibit the oxidation or defect formation at the surface region of the Fe₃O₄ NPs, and enhance the corresponding magnetization of the Fe₃O₄ NPs by the reduction of the spin canting layer thickness.     

Structural and magnetic properties of Co-C composite films and Co/C multilayer films

CoC composite films and Co/C multilayer films have been prepared by a method incorporating ion beam sputtering and plasma chemical vapor deposition. It has been found that the structure and magnetic properties of both the Co-C composite and the Co/C multilayer films depend strongly on the substrate temperature during deposition. The Co-C composite film deposited at room temperature is amorphous, with relatively low saturation magnetization and coercivity. On the other hand, the film deposited at 250 °C is composed of fine Co crystallites separated by amorphous C or Co-C phase. As a result, both the saturation magnetization and coercivity are increased compared with the film deposited at room temperature. When deposited at room temperature, the Co/C multilayer film exhibits good periodicity, with a period of 70 nm (Co: 40 nm, C: 30 nm) and sharp and flat Co-C interfaces. High magnetization (602 emu/cm³) and low coercivity (1.6 Oe) are obtained for such a film. However, increasing the substrate temperature to 250 °C was found to be detrimental to the magnetic properties due to the formation of cobalt carbide at the Co-C interface. Received: 11 July 2000 / Accepted: 13 July 2000 / Published online: 30 November 2000     

Single crystal study of the one dimensional Ca Co O compound: five stable configurations for the Ising triangular lattice

Single crystals of the one-dimensional phase Ca₃Co₂O₆ of several mm length have been grown. The magnetic study of such a crystal confirms the previous observations on polycrystalline samples: it consists of a triangular lattice of ferromagnetic [Co₂O₆] chains ( K) antiferromagnetically coupled ( K). The dynamic of these chains array, probed by AC susceptibility, is very slow as shown from the large shift of the freezing temperature from 12 K to 16.5 K as the excitation frequency increases by three orders of magnitude (10⁰ to 10³ Hz). The origin of this effect is believed to be the result of different arrangements with close energies for the chain ferromagnetic moments on the triangular lattice. Five stable magnetic configurations have been evidenced by the magnetization as a function of applied field curves registered at 2 K. Their relative magnetizations correspond to m =1/4, 1/2, 1, 2, 3 where m =3 represents the ferromagnetic ordering of three chains on the same triangle, each chain having a m =1 magnetization. A magnetic phase diagram is finally proposed. Received 7 December 1999     

Magnetic anisotropy and magnetization dynamics of Fe nanoparticles embedded in Cr and Ag matrices

Static and dynamical magnetic properties of Fe nanoparticles (NPs) embedded in non-magnetic (Ag) and antiferromagnetic (Cr) matrices with a volume filling fraction (VFF) of 10% have been investigated. In both Fe@Ag and Fe@Cr nanocomposites, the Fe NPs have a narrow size distribution, with a mean particle diameter around 2 nm. In both samples, the saturation magnetization reaches that of Fe bulk bcc, suggesting the absence of alloying with the matrices. The coercivity at 5 K is much larger in Fe@Cr than in Fe@Ag as a result of the strong interaction between the Fe NPs and the Cr matrix. Temperature-dependent magnetization and ac-susceptibility measurements point out further evidence of the enhanced interparticle interaction in the Fe@Cr system. While the behaviour of Fe@Ag indicates the presence of weakly interacting magnetic monodomain particles with a wide distribution of blocking temperatures, Fe@Cr behaves like a superspin glass produced by the magnetic interactions between NPs.     

Magnetic and optical properties of Zn1– x Fe x O ( x = 0.05 and 0.10) diluted magnetic semiconducting nanoparticles

We have investigated the magnetic and optical properties of chemically low temperature-synthesized Zn_{1– x} Fe _x O (x = 0.05 and 0.10) diluted magnetic semiconducting nanoparticles (～7 nm). Observed magnetic behaviour of x = 0.05 samples showed that the net magnetic interaction was antiferromagnetic-like, a feature established by Curie–Weiss fit, concave Arrott–Belov–Kouvel (ABK) plots with the absence of spontaneous magnetization even at 5 K and stretched exponential-type time-dependent magnetization behaviour. Optimization of the Fe(x) dopant concentration in Zn_{1– x} Fe _x O gave the most favourable room-temperature ferromagnetism for x = 0.10, as supported by finite coercive field (～94.4 Oe) and remanent magnetization (0.011 µ_B/Fe ion) from strong hysteretic magnetization vs. magnetic-field curves at room temperature. The Curie temperature of the x = 0.10 sample was estimated at ～388 K. The existence of a room-temperature ferromagnetic phase was further established by the convex nature of the ABK plots with finite spontaneous magnetization. The observed magnetic behaviour for different x values is best explained by a magnetic polaron model.     

Synthesis and characterization of LSMO manganite-based biocomposite

In this paper we study the structural, morphological and magnetic properties of La_0.67Sr_0.33MnO₃ (LSMO) manganite nanoparticles (NPs) and its biocomposite, obtained by mixing NPs of hydroxyapatite (HA). From the studies of X-ray diffraction and Fourier transmission of infrared spectroscopy it is evident that in the biocomposite sample both the individual phases are distinguishable from each other. The measurements of direct current (DC) magnetization and hysteresis loops reveal that the basic magnetic behaviour of LSMO–HA is similar to that of LSMO; however, the admixture of HA makes the sample magnetically softer. From the investigation of transmission electron microscopy it is observed that such a biocomposite is composed of the NPs of LSMO surrounded by HA particles, which can be found suitable for biomedical applications.     

Superior values of the initial permeability for electrodeposited Ni–Co–P-BaFe12O19 composite films

In this research, we detected superior values of the initial magnetic permeability for electrodeposited Ni–Co–P-M-type BaFe₁₂O₁₉ (BaM) composite films as magnetic soft–hard composites with a large amount of entrapped BaM particles. Furthermore, the initial magnetic permeability of Ni–Co–P-BaM composite films is significantly large and is comparable with corresponding values of the excellent corresponding values for permalloys, Sendust, Co-based amorphous alloys, nanocrystalline (Fe-based) and commercially sintered MnZn ferrite cores. Therefore, Ni–Co–P-BaM composite films are very attractive for magnetic devices, such as magnetic inductors, transformers, and magnetic recording heads.     

Nonlinear surface magneto-optics of ferromagnetic Ni/Cu(001) from first principles

We present first-principles calculations of the nonlinear optical (NLO) response of a Ni/Cu(001) bilayer. The calculations are based on the full potential linearized augmented plane wave (FLAPW) method with the additional implementation of spin–orbit coupling (SOC). On the basis of this set of eigenstates the magneto-optical transition-matrix elements are evaluated. Using the surface-sheet model the optical reflection properties are determined for the cases of the magnetization vector perpendicular to the surface (polar magneto-optical configuration (MOC)) and for the in-plane magnetization (longitudinal MOC). The nonlinear optical susceptibility tensor elements χ⁽²⁾ _ijk for different magnetization directions as well as the spectral dependence of χ⁽²⁾ _ijk, the resulting intensities, and Kerr angles are presented for the Ni/Cu(001) bilayer. The results show that the magnetic tensor elements of the χ⁽²⁾ _ijk tensor are smaller than the nonmagnetic ones by only one order of magnitude, confirming the important role of magnetic properties in the NLO response. Received: 16 October 2001 / Revised version: 8 March 2002 / Published online: 29 May 2002     

磁性颗粒复合体磁渗流区矫顽力异常的研究

制备了MnZn铁氧体/SiO₂颗粒复合体.研究了磁性颗粒复合体的有效磁导率μ、 比磁化强度σ以及矫顽力H_c随磁性颗粒含量的变化.研究发现，在MnZn铁氧体体积百分含 量为90%—98%的区域，复合体的有效磁导率μ的变化速率发生突变，出现磁渗流现象，从实验得到的体系磁渗流阈值V_c=97.9%.在磁渗流区，矫顽力表现出异常行为.结果表明 ，这种异常行为与复合体微观结构有着密切关系.在磁渗流前，矫顽力H_c的变化主要来 源于磁 关键词： 颗粒复合体 磁渗流 矫顽力     

Microstructure characterization and magnetic properties of nanomaterials

The microstructure and superparamagnetic properties of two systems of magnetic nano-particles are reviewed. A new type of magnetic core-shell Ni—Ce nanocomposite particle (15–50 nm) has been prepared. Typical HREM images and FFT patterns of HREM images showed that many planar defects (nanotwins and stacking faults) exist in the large Ni core zone (10–45 nm). The shell layers (3–5 nm) consist of an innermost Ni—Ce alloy and an outermost NiO oxide. FFT patterns from different regions of typical HREM images show well defined spots characteristic of core-shell nanocomposite materials. Magnetization measurements as a function of magnetic field and temperature showed that superparamagnetic behaviour is exhibited above the average block temperature (T_B = 170 K). This superpara-magnetic relaxation was found to be modified by interparticle interactions that depend on the applied field and size distribution. In addition, antiferromagnetic order occurred with a Neél temperature T_N of about 11 K. A spin-flip transition was observed below T_N at a certain applied field. Novel carbon encapsulated Ni nanoparticles assemblies have been synthesized by modified arc-discharge under a methane atmosphere. The presence of carbon encapsulation is confirmed by HR-TEM lattice imaging, and nanodiffraction. The intimate and contiguous carbon fringe around these Ni nanoparticles is good evidence for complete encapsulation by carbon shell layers. Superparamagnetic property studies show that the blocking temperature T_B is around 115K at 0.1 T applied field. Above T_B, the magnetization M(H,T) can be described by the classical Langevin function L using the relation M/M_s(T = 0) = coth (μH/kT) — kT/μH. The particle size can be inferred from the Langevin fit (particle moment μ), which is a little larger than the HR-TEM observation. It is suggested that these assemblies of carbon encapsulated Ni nanoparticles have typical single-domain, field-dependent superparamagnetic relaxation properties, and this typical superparamagnetic behaviour is consistent with the Stoner—Wohlfarth theory of single-domain particles.     

Electron spin resonance and quantum critical phenomena in VOx multiwall nanotubes

Basing on the high frequency (60 GHz) electron spin resonance study of the VO_x multiwall nanotubes (VO_x ‐NTs) carried out in the temperature range 4.2–200 K we report: (i) the first direct experimental evidence of the presence of the antiferromagnetic dimers in VO_x ‐NTs and (ii) the observation of an anomalous low temperature growth of the magnetic susceptibility for quasi‐free spins, which obey the power law χ (T)～1/T ^α with the exponent α ≈ 0.6 in a wide temperature range 4.2–50 K. We argue that the observed departures from the Curie–Weiss behaviour manifest the onset of the quantum critical regime and formation of the Griffiths phase as a magnetic ground state of these spin species. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Size-controlled synthesis of superparamagnetic iron oxide nanoparticles and their surface coating by gold for biomedical applications

The size mono-dispersity, saturation magnetization, and surface chemistry of magnetic nanoparticles (NPs) are recognized as critical factors for efficient biomedical applications. Here, we performed modified water-in-oil inverse nano-emulsion procedure for preparation of stable colloidal superparamagnetic iron oxide NPs (SPIONs) with high saturation magnetization. To achieve mono-dispersed SPIONs, optimization process was probed on several important factors including molar ratio of iron salts [Fe³⁺ and Fe²⁺], the concentration of ammonium hydroxide as reducing agent, and molar ratio of water to surfactant. The biocompatibility of the obtained NPs, at various concentrations, was evaluated via MTT (3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide) assay and the results showed that the NPs were non-toxic at concentrations <0.1 mg/mL. Surface functionalization was performed by conformal coating of the NPs with a thin shell of gold (∼4 nm) through chemical reduction of attached gold salts at the surface of the SPIONs. The Fe₃O₄ core/Au shell particles demonstrate strong plasmon resonance absorption and can be separated from solution using an external magnetic field. Experimental data from both physical and chemical determinations of the changes in particle size, surface plasmon resonance optical band, phase components, core–shell surface composition, and magnetic properties have confirmed the formation of the mono-dispersed core–shell nanostructure.     

Studies of magnetic properties of permalloy (Fe-30%Ni) prepared by SLM technology

In the present study, a high permeability induction Fe-30%Ni alloy cubic bulk was prepared by the selective laser melting process. In order to reveal the microstructure effect on soft magnetic properties, the microstructure and magnetic properties of the Fe-30%Ni alloy were carefully investigated by scanning electron microscopy, X-ray diffraction and hysteresis measurements. The bcc-Fe (Ni) phase formation is identified by X-ray diffraction. Meanwhile, it was found that low bcc lattice parameter and high grain size could be obtained when high laser scanning velocity and low laser power were used. Moreover, the lowest value of coercivity is 88 A/m, and the highest value of saturation magnetization is 565 Am²/kg, which can be obtained at a low laser scanning velocity of 0.4 m/s and high laser power input at 110 W.     

Ni80Fe20 permalloy nanoparticles: Wet chemical preparation, size control and their dynamic permeability characteristics when composited with Fe micron particles

Ni₈₀Fe₂₀ permalloy nanoparticles (NPs) have been prepared by the polyol processing at 180 °C for 2 h and their particle sizes can be precisely controlled in the size range of 20-440 nm by proper addition of K₂PtCl₄ agent. X-ray diffraction results show that the Ni-Fe NPs are of FCC structure, and a homogeneous composition and a narrow size distribution of these NPs have been confirmed by scanning electron microscopy assisted with energy dispersion spectroscopy of X-ray (SEM-EDX). The saturation magnetization of ~440nm NPs is 80.8 emu/g that is comparable to that of bulk Ni₈₀Fe₂₀ alloys, but it decreases to 28.7 emu/g for ~20 nm NPs. The coercive force decreases from 90 to 3 Oe with decreasing NP size. The wide range of particle size is exploited to seek for high permeability composite particles. The planar type samples composed of the NiFe NPs exhibit low initial permeability due to the deteriorated magnetic softness and low packing density. However, when they are mixed with Fe micron particles, the initial permeability significantly increases depending on the mixing ratio and the NiFe NP size. A maximum initial permeability is achieved to be ~9.1 at 1 GHz for the Fe-10 vol%NiFe (~20 nmΦ), which is about three times that of pure Fe micron particles. The effects of Ni-Fe particle size, volume percentage and solvent on the static and dynamic permeability are discussed.     

Carbon-coated silicon/crumpled graphene composite as anode material for lithium-ion batteries

Silicon is a promising anode material for high-capacity Li-ion batteries (LIBs). However, its insulating property and large volume change during the lithiation/delithiation process result in poor cycling stability and in pulverization of Si. In this work, glucose-derived carbon-coated Si nanoparticles (C–Si NPs) are in conjunction with crumpled graphene (cGr) particles by a spray-drying method to prepare a novel composite (C–Si/cGr) material. The prepared C–Si NPs are uniformly embedded in the ridges of the cGr particles. The carbon layer of C–Si can make a good contact with the graphene sheet, resulting in enhanced electrical conductivity and fast charge transfer. In addition, the unique crumpled structure of the cGr can buffer the large volume change upon cycling process and facilitate the diffusion of electrolyte into the composite material. When employed as an anode electrode of LIBs, the C–Si/cGr composites deliver enhanced electrochemical performance, including stable cycling with a discharge capacity of 790 mAh·g⁻¹ after 100 cycles and a rate capability of 654 mAh·g⁻¹ at 2C. The synergistic effect of the carbon layer coating of Si NPs and the crumpled structure of the cGr particles results in a composite with improved the electrochemical performance, which is likely related to its high electrical conductivity and good mechanical stability of composite material.     

Cluster glass behaviour in Co0.2Zn0.8Fe2−xRhxO4(x=0–1.0)

We report the low field magnetic behaviour of Zn- and Rh-substituted Co ferrite by AC susceptibility, DC magnetization, hysteresis measurement and relaxation experiment. The present system is a cubic spinel, where 80% of A site is diluted by non-magnetic Zn²⁺ and B site is consequently diluted by non-magnetic Rh³⁺ substitution in place of Fe³⁺. The Rh substitution reduces antiferromagnetic exchange interaction between B site Fe³⁺ ions and destroys the ferrimagnetic nature of the Co_0.2Zn_0.8Fe₂O₄ system. On diluting the B site moments, the system shows a typical cluster freezing phenomena at T⩽T_f due to frustration effect. We propose a zero field cooled relaxation technique that can easily identify the mixture of small frustrated clusters or spins along with the strongly coupled clusters. It has been observed from the field dependence of DC magnetization that for x=1.0, there is a crossover from de Almeida–Thouless (AT)-like line to Gabay–Toulouse (GT)-like line at the freezing temperature.