Zero Field Cooling/field Cooling Research Articles

Formation of skyrmion phase in the Fe–Co–Si system by mechanochemical activation

Fe, Co and Si powders were exposed to mechanochemical activation by high-energy ball milling for 0, 2, 4, 8 and 12 h. The samples were subsequently characterized by Mössbauer spectroscopy, X-ray powder diffraction (XRPD), magnetic measurements and optical diffuse reflectance spectroscopy. The room temperature Mössbauer measurements were consistent with the occurrence of FeCo2Si and Fe0.5Co0.5Si crystalline phases. The low temperature Mössbauer spectra confirmed the absence of superparamagnetism up to 44 K in the milled system. XRPD patterns supported the phase sequence derived from Mössbauer spectroscopy. The coercive field was found to increase with the ball milling time (BMT). Zero-field-cooling-field-cooling (ZFC-FC) measurements performed at 200 Oe in the temperature range 5–300 K evidenced the transition to the skyrmion phase of the Fe0.5Co0.5Si material below the critical temperature of 44 K. The optical absorption in the UV–Vis–NIR region of the spectrum was found to increase with BMT.

Synthesis and Characterization of Gadolinium Oxide-Hematite Magnetic Ceramic Nanostructures

Mixed-oxide nanostructures of the type xGd2O3-(1-x)α-Fe2O3 (x=0.1, 0.3, 0.5 and 0.7) were synthesized by mechanochemical activation for ball milling times of 0, 2, 4, 8 and 12 hours. The systems were subsequently analyzed by Mӧssbauer spectroscopy, X-ray powder diffraction (XRPD), magnetic measurements and optical diffuse reflectance spectroscopy. The magnetic hyperfine field was studied as function of ball milling time for all sextets involved and found to be consistent with the formation of a limited solid solution in the systems investigated. The end-product was the gadolinium perovskite, represented by a doublet whose abundance was derived as function of the milling time. The XRPD patterns recorded for the equimolar composition were dominated by the diffraction peaks of GdFeO3 after 12 hours of milling. The hysteresis loops were recorded at 300 and 5 K in an applied magnetic field of 5 T and were interpreted as a superposition of paramagnetic behavior of gadolinium oxide and weak ferromagnetic behavior of hematite and gadolinium perovskite. The Morin transition of hematite was inferred from zero-field-cooling-field-cooling (ZFC-FC) curves measured with a magnetic field of 200 Oe in the 5-300 K temperature range and was found to depend on the ball milling time. Optical diffuse reflectance spectra showed that the compounds were semiconductors with an optical band gap of 2.1 eV.

Temperature dependence of magnetic anisotropy in a cylindrical Fe65Pd35 nanowire array

Fe65Pd35 cylindrical nanowires (NWs) were synthesized in commercial anodic alumina porous templates via potentiostatic electrodeposition. The morphology, composition, and crystalline structure were analyzed by scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. The NWs obtained are (10 ± 2) μm long and (200 ± 20) nm in diameter, with a polycrystalline microstructure. Magnetic properties were investigated by measuring the Zero Field Cooling-Field Cooling magnetization curves and hysteresis loops in the temperature range 10 K to 350 K. The magnetization easy axis is determined to be along the NWs axis, indicating strong anisotropy in this direction, which cannot be accounted for by only considering shape anisotropy. The hysteresis loops were fitted using the law of approach to saturation, obtaining the effective anisotropy field HA and the effective anisotropy constant Keff value for different temperatures. The blocking temperature TB dependence on the magnetic field H was also analyzed, indicating that a blocked magnetic contribution is present, arising from the smaller grains in the NWs. The observed enhancement in the effective magnetic anisotropy of the array throughout the studied temperature range is explained by considering magnetic interactions among these nanograins.

Magnetic properties of iron oxide nanoparticles with a DMSA-modified surface

The magnetic properties of magnetite nanoparticles (Fe3O4 NPs) strongly depend on their chemical and physical parameters, which can be regulated by a controlled synthesis process. To improve the quality of the obtained nanoparticles, their surface is often modified with organic compounds (from the group of surfactants, sugars, proteins, or organic acid). In this study, we synthesized magnetite nanoparticles with a surface modified with the organic compound DMSA. Then, the nanocrystallites were characterized in terms of structure and morphology. To investigate the role of DMSA and to understand the adsorption mechanism, FTIR measurements were carried out. Using Mössbauer spectroscopy, we investigated temperature-induced changes in the magnetic properties of prepared samples. The spectra were recorded in a wide temperature range (from 4 K to 390 K) for two types of samples: powders and ferrofluids with various concentrations. In the case of powder samples, the superparamagnetic doublet appeared at room temperature. For magnetic suspensions, the spectra were more complicated. They consisted of superposition of asymmetrically broadened sextets and doublets, which was caused by the occurrence of long-range dipole-dipole interactions. These interactions affected the magnetic properties of the material and increased the blocking temperature. Additionally, the magnetic hysteresis and zero field cooling-field cooling (ZFC/FC) curves were measured with the use of a vibrating sample magnetometer.

Critical Current and Pinning Features of a CaKFe4As4 Polycrystalline Sample.

We analyze the magnetic behavior of a CaKFe4As4 polycrystalline sample fabricated by a mechanochemically assisted synthesis route. By means of DC magnetization (M) measurements as a function of the temperature (T) and DC magnetic field (H) we study its critical parameters and pinning features. The critical temperature Tc has been evaluated by M(T) curves performed in Zero Field Cooling-Field Cooling conditions. These curves show the presence of a little magnetic background for temperatures above Tc, as also confirmed by the hysteresis loops M(H). Starting from the M(H) curves, the critical current density Jc of the sample has been calculated as a function of the field at different temperatures in the framework of the Bean critical state model. The Jc(H) values are in line with the ones reported in the literature for this typology of samples. By analyzing the temperature dependence of the critical current density Jc(T) at different magnetic fields, it has been found that the sample is characterized by a strong type pinning regime. This sample peculiarity can open perspectives for future improvement in the fabrication of this material.

Effect of graphene on the mechanochemical activation of cobalt ferrite nanoparticles

Cobalt ferrite nanoparticles were exposed to mechanochemical activation, with and without equimolar amounts of graphene nanoparticles, for time periods ranging from 0 to 12 h. Their structural and magnetic properties were detailed from Mӧssbauer spectroscopy and magnetic measurements. The Mӧssbauer spectrum corresponding to the unmilled cobalt ferrite powder was analyzed using 2 sextets, corresponding to the tetrahedral and octahedral sites of ferrites. The rest of the spectra was deconvoluted using an additional quadrupole-split doublet, with an abundance close to 30% and was assigned to superparamagnetic particles. Moreover, the spectra corresponding to milling with graphene at the longest times needed a third sextet, which could be assigned to iron carbide. The degree of inversion was determined from the Mӧssbauer spectra and found to decrease with milling time, both for the set with and that without graphene. The canting angle was derived and studied as function of the ball milling time for both sets of samples. Hysteresis loops were recorded at 5 K in an applied magnetic field of 5 T and was found to exhibit a wasp-waist shape. Magnetization was plotted as function of temperature in the range 5–300 K with an applied magnetic field of 200 Oe using zero-field-cooling-field-cooling (ZFC-FC) measurements. These made it possible to determine the blocking temperature of the samples. Our data exhibit new characteristics of the cobalt ferrite nanopowders milled with and without graphene nanoparticles.

Zero-Dimensional Graphene and Its Behavior under Mechanochemical Activation with Zinc Ferrite Nanoparticles

ABSTRACTEquimolar mixtures of zero-dimensional graphene (SkySpring Nanomaterials, 1-5 nm particle size) and zinc ferrite nanoparticles (Alfa Aesar, 50 nm particle size) were exposed to mechanochemical activation by high-energy ball milling for time intervals of 0-12 hours. Their structural and magnetic properties were analyzed by Mӧssbauer spectroscopy and magnetic measurements. The spectra of zinc ferrite milled without graphene were fitted with one quadrupole-split doublet (quadrupole splitting 0.5 mm/s, isomer shift 0.23 mm/s) and indicated that zinc ferrite was superparamagnetic. The line width of the doublet increased from 0.41 to 0.64 mm/s, which correlates with a reduction in particle size as effect of the ball milling processing performed. When graphene was added to the milling powders, the Mӧssbauer spectra showed the appearance of another quadrupole doublet, with a quadrupole splitting of 0.84 mm/s and an isomer shift of -0.38 mm/s. Its abundance to the spectrum remained constant to 4.48% while the milling time was increased. This second doublet could be related to carbon atoms occupying neighborhoods in the proximity of iron atoms. Hysteresis loops were recorded in an applied magnetic field of 5 T at a temperature of 5 K. A change in the approach to saturation of the loop was observed, with saturation being achieved for the sample milled for 12 hours with graphene. Zero-field-cooling-field-cooling (ZFC-FC) was performed on all samples between 5-300 K with an applied magnetic field of 200 Oe. Graphene was found to stabilize the magnetic properties of the milled system of powders to a blocking temperature of about 90 K.

Fundamental studies of hafnia-hematite nanoparticles

xHfO2-(1-x)α-Fe2O3 (x = 0.1, 0.3, 0.5 and 0.7) nanoparticles system was obtained using mechanochemical activation by high energy ball milling for time periods ranging from 0 to 12 h. X-ray diffraction patterns revealed the presence of two phases, a hafnium-doped hematite and an iron-doped hafnia, which competed with each other to form a solid solution. The trend observed in the lattice parameters using Rietveld refinement was consistent with differences in ionic radii between Hf and Fe. Crystallite sizes for both hematite and hafnia were analyzed as function of milling time using the Scherrer method and found to decrease down to ~15 nm for all molar concentrations used. Mössbauer spectroscopy revealed the presence of 1–5 sextets corresponding to different numbers of Hf nearest neighbors of Fe in the hematite structure. The different hyperfine magnetic fields could be resolved in the model of local atomic environment. Substitutions of Fe in hafnia gave rise to a nonmagnetic phase represented by a quadrupole split doublet, whose abundance was found to increase with ball milling time. Hysteresis loops recorded at 5 K showed that the Hf-doped system does not saturate in an applied magnetic field of 5 T. The Morin transition was observed during zero-field-cooling-field cooling (ZFC-FC) in an external magnetic field of 200 Oe. Differential scanning calorimetry with thermal gravimetric analysis (DSC-TGA) evidenced an exothermic peak for the starting oxides and an endothermic peak for the solid solution.

DC magnetic characterization and pinning analysis on Nd1.85Ce0.15CuO4 cuprate superconductor

Abstract We report the analysis of the magnetic response detected on the cuprate superconductor Nd1.85Ce0.15CuO4. In particular the magnetic behavior of the sample has been studied by means of DC magnetization measurements as a function of the temperature (T) and DC magnetic field (H). The superconducting critical temperature Tc has been obtained by analyzing the m(T) curve performed in Zero Field Cooling-Field Cooling conditions. Moreover, the m(T) curve shows the presence of a magnetic background for temperatures above Tc. By considering the superconducting m(H) hysteresis loop at different temperatures, it can be noted that the width of the curves appears narrow corresponding to a weak superconductivity. This is confirmed by the field dependence of the critical current densities Jc extracted from the superconducting hysteresis loops m(H) at different temperatures within the Bean critical state model. In fact, at the lowest measurement temperature, Jc is close to zero already at low magnetic fields. Nevertheless, by means of the temperature dependence of Jc, the sample shows a strong pinning behavior that can open perspectives for future improvement in the fabrication of this material.

Magnetic nanocomposites based on shape memory polyurethanes

Shape memory composites based on a commercial segmented polyurethane and magnetite (Fe3O4) nanoparticles (MNPs) were prepared by a simple suspension casting method. The average sizes of individual magnetic particles/clusters were determined by TEM microscopy and corroborated from SAXS patterns. The magnetization properties of selected samples were evaluated using zero field cooling/field cooling (ZFC/FC) measurements and magnetization loops obtained at different temperatures. The results showed that magnetization at high field (20 k Oe) and coercitivity measured at 5 K increase with magnetite content and that all the composite films exhibit superparamagnetic behavior at 300 K. The specific absorption rate (SAR) of the nanocomposites was calculated by experimentally determining both the specific heat capacity and the heating rate of the films exposed to an alternant magnetic field. All nanocomposites were able to increase their temperature when exposed to an alternant magnetic field, although the final temperature reached resulted dependent of the MNPs concentration. What is more, a fast and almost complete recovery of the original shape of the nanocomposites containing more than 3 nominal wt.% MNP was obtained by this remote activation applied to the previously deformed samples.

Mixed state properties of iron based Fe(Se,Te) superconductor fabricated by Bridgman and by self-flux methods

The superconducting and transport properties of iron based Fe(Se,Te) superconductors fabricated by means of Bridgman (B) and Self-flux (S) methods have been compared using dc Magnetization (M) measurements as a function of temperature (T) and magnetic field (H). The M(T) measurements performed in Zero Field Cooling-Field Cooling conditions show higher critical temperature Tc and a lower spurious magnetic background signal for the sample (B) rather than the (S) one. By considering the superconducting M(H) hysteresis loops, the sample (B) shows a stronger superconducting signal together with the presence of a peak effect. The field and temperature dependence of the critical current densities Jc are extracted from the superconducting hysteresis loops M(H) within the Bean critical state model, and the high ratio between the JcB and the JcS, relative to the two typologies of samples, together with the comparison between their upper critical field Hc2, points out that the Bridgman method is most attractive for exploiting superconducting and transport properties in view of applications.

Structural, Magnetic and Hyperfine Properties of Molybdenum Dioxide-Hematite Mixed Oxide Nanostructures

ABSTRACTMoO2-Fe2O3 nanoparticle system was successfully synthesized by mechanochemical activation of MoO2 and α-Fe2O3 equimolar mixtures for 0-12 hours of ball milling time. The study aims at exploring the formation of magnetic oxide semiconductors at the nanoscale. X-ray powder diffraction (XRD), Mössbauer spectroscopy and magnetic measurements were used to study the phase evolution of MoO2-Fe2O3 nanoparticle system under the mechanochemical activation process. The Mössbauer studies showed that the spectrum of the mechanochemically activated composites evolved from a sextet for hematite to sextet and a doublet upon duration of the milling process with molybdenum dioxide. Recoilless fraction was determined using our dual absorber method and was found to decrease with increasing ball milling time. Magnetic measurements recorded at 5 and 300 K in an applied magnetic field of 50,000 Oe showed the magnetic properties in the antiferromagnetic and canted ferromagnetic states. The Morin transformation was evidenced by zero-field cooling-field cooling (ZFC-FC) measurements in 200 Oe and the transformation characteristic temperatures were shifted to lower values.

Mechanochemical Synthesis and Characterization of Molybdenum Dioxide-Hematite Nanostructures with Different Molarities

xMoO2-(1-x)α-Fe2O3 nanoparticle system with molarities x = 0.1, 0.3, 0.5 and 0.7 was successfully synthesized by mechanochemical activation of MoO2 and α-Fe2O3 mixtures for 0 - 12 hours of ball milling time. X-ray powder diffraction (XRD), Mössbauer spectroscopy and magnetic measurements were used to study the phase evolution of the molybdenum dioxide-hematite nanoparticle system under the mechanochemical activation process. Rietveld refinement of the XRD patterns yielded the values of the crystallite size and lattice parameters as function of milling times and indicated the presence of Mo-substituted hematite and Fe-doped molybdenum dioxide at long milling times. The Mössbauer studies yielded the magnetic hyperfine fields and the relative abundance of a quadrupole-split doublet as function of the milling time for all molar concentrations involved. Recoilless fraction was determined using our dual absorber method and was found to decrease with increasing ball milling time. Magnetic measurements recorded at 5 and 300 K in an applied magnetic field of 50,000 Oe showed the magnetic properties in the antiferromagnetic and canted ferromagnetic states. The Morin transformation was evidenced by zero-field cooling-field cooling (ZFC-FC) measurements in a magnetic field of 200 Oe.

Nanoscale Ferromagnetic Cobalt‐Doped ZnO Structures Formed by Deep‐UV Direct‐Patterning

AbstractCobalt (II) acetate is mixed with zinc methacrylate (ZnMAA) to form a photopatternable Co‐doped zinc oxide precursor. By using deep‐UV (DUV) interference lithography, Co‐doped ZnMAA precursor can be patterned as negative tone resist and transformed into ferromagnetic Co:ZnO films after thermal treatment. Moreover, Co:ZnO patterns as small as 300 nm line‐width can be easily obtained. To have an in‐depth understanding to the effect of DUV‐patterning process as well as thermal annealing on Co:ZnO films derived from Co‐doped ZnMAA precursor, optical, magnetic, and electrical characterizations are performed on Co:ZnO films prepared in different conditions. For the Co:ZnO film prepared without DUV‐patterning, large zero‐field‐cooling (ZFC)–field‐cooling (FC) irreversibility appears in superconducting quantum interference device measurements after vacuum annealing, indicating that Co clusters have formed inside the film. On the other hand, no ZFC–FC bifurcation can be observed for the DUV‐patterned Co:ZnO film after the vacuum annealing, suggesting that the uniformity of Co ion distribution inside ZnO lattice is improved by DUV‐patterning.

Doping-Concentration-Induced Ferromagnetism and Antiferromagnetism in In2S3:Dy3+ Quantum Dots

Diluted magnetic semiconductor (DMS) quantum dots have been researched extensively due to their potential applications in next-generation spin-based devices. Herein, the cubic In2S3:Dy3+ DMS quantum dots (3–5 nm) with different doping concentrations were synthesized via a gas–liquid phase chemical deposition method. The effect of Dy3+ content on the photoluminescence (PL) and ferromagnetism was investigated. The PL emission spectra exhibit a blue-shift compared with those reported previously due to the increased quantum size confinement and enhanced intensity attributed to the Dy3+ doping. The distinct and stronger room-temperature ferromagnetism is observed from vibrating sample magnetometer (VSM) measurement. The coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) phases and antiferromagnetic interation plays a dominant position after a certain doping concentration value can be further confirmed according to the zero field cooling/field cooling (ZFC/FC) curves. As revealed in the magnetic origin study from first-principles calculations, the ferromagnetism obtained arises not only from the Dy atoms but also from the In vacancies. In addition, we also proposed a spontaneous mechanism based on the bound magnetic polaron theory to explain the change of saturation magnetizations along with Dy3+ doping concentration. This work provides experimental and theoretical guidance for designing and synthesizing unique spintronic materials, which can promote development of spintronic applications.

Fracture problem for an external circumferential crack in a functionally graded superconducting cylinder subjected to a parallel magnetic field

In this study, the multiple isoparametric finite element method (MIFEM) is used to investigate external circumferential crack problem of a functionally graded superconducting cylinder subjected to electromagnetic forces. The superconducting cylinder is composed by Bi2223/Ag composite with material parameters varying. A crack reference region is defined to reflect the effects of crack on flux and current densities, and the magnetically impermeable crack surface condition and the generalized Irie–Yamafuji critical state model outside the crack region are adopted. The distributions of magnetic flux density in the superconducting cylinder are obtained analytically for both the zero-field cooling (ZFC) and the field cooling (FC) activation processes. Based on the MIFEM, the stress intensity factors (SIFs) at crack fronts in the process of field ascent and/or descent are then numerically calculated. It is interesting to note from numerical results that for the present crack model in the ZFC activation process, the crack is easily propagate and grow with the applied field increases, and that in the field descent process of either the ZFC case or FC case, the crack generally does not propagate. In addition, in the field ascent process of the ZFC case, the SIFs depend on not only the crack depths and model parameters but also the applied field. The present study should be helpful to the design and application of high-temperature superconductors with external edge cracks.

Magnetic characterization of chitosan–magnetite nanocomposite films

Magnetic nanocomposites using chitosan as a matrix and magnetite nanoparticles (MNP) generated “in situ” were prepared and magnetically characterized. The content of nanoparticles on the composites was varied from 2 to 10wt.% and their effects, as well as the addition of 30wt.% of glycerol as plasticizer in the formulation, were analyzed. The magnetization properties were evaluated using the zero field cooling/field cooling (ZFC/FC) measurements and magnetization loops obtained at different temperatures. The results showed that magnetization at high field (20 KOe) and coercivity increase with magnetite content. Super-paramagnetic behavior was observed for all non-plasticized samples with exception of the film with 2wt.% of magnetite. Glycerol affected significantly the composite magnetization values and the magnetic interactions between particles, which are reflected in the blocking and irreversibility temperatures of the different systems. Moreover, the size of the precipitated magnetic nanoparticles depends on their concentration as well as on the addition of plasticizer to the formulation, as was corroborated by TEM and SAXS measurements.

Magnetic properties of colloidal cobalt nanoclusters

Co nanoclusters were synthesized by an inverse-micelle chemical route. The magnetic and microstructural properties of the nanoparticles have been analyzed as a function of the surfactant (AOT and DEHP) and the drying method. Microstructural analysis has been performed by TEM and XANES; magnetic properties have been studied by hysteresis loops and zero-field cooling – field cooling (ZFC-FC) curves. TEM images show 2 to 4 nm sized particles spherical in shape. XANES measurements point out a significant presence of Co3O4with metallic Co and some Co2+ bound to the surfactant. The presence of antiferromagnetic Co3O4 explains the magnetic transition observed at low T in both ZFC-FC measurements and hysteresis loops. Finally, the presence of magnetic interactions explains the bigger effective cluster size obtained from hysteresis loops fits (6-10 nm) compared to the sizes observed by TEM (2-4 nm).

Spin-glass-like behavior in Zn xCr yAl zSe 4

The magnetic and electrical properties of the Al-doped polycrystalline spinels Zn x Cr y Al z Se 4 (0.13≤ z≤0.55) with the antiferromagnetic (AFM) order and semiconducting behavior were investigated. A complex antiferromagnetic structure below a Néel temperature T N≈23 K for the samples with z up to 0.4 contrasting with the strong ferromagnetic (FM) interactions evidenced by a large positive Curie–Weiss temperature θ CW decreasing from 62.2 K for z=0.13 to 37.5 K for z=0.55 was observed. Detailed investigations revealed a divergence between the zero-field-cooling (ZFC) and field-cooling (FC) susceptibilities at temperature less than T N suggesting bond frustration due to competing ferromagnetic and antiferromagnetic exchange interactions in the compositional range 0.13≤ z≤0.4. Meanwhile, for z=0.55 a spin-glass-like behavior of cluster type with randomly oriented magnetic moments is observed as the ZFC–FC splitting goes up to the freezing temperature T f=11.5 K and the critical fields connected both with a transformation of the antiferromagnetic spin spiral via conical magnetic structure into ferromagnetic phase disappear.

Structural, Magnetic, and Magnetoresistive Properties of Electrodeposited Ni5Zn21 Alloy Nanowires

Ni5Zn21 alloy nanowires were fabricated through template-assisted electrochemical deposition method. The morphology and microstructures of as-deposited nanowires were determined by field-emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), electron diffraction (ED), and electron probe microanalysis (EPMA). The accurate composition was measured via induced coupling plasma atomic emission spectroscopy. SEM results show that Ni5Zn21 nanowires are deposited in most of the nanopores of the template, and they are continuous and dense throughout the whole length. The XRD result demonstrates that the nanowires are mainly composed of a cubic gamma phase Ni5Zn21 alloy, but there also exists a trace of Zn-rich eta phase. HRTEM and ED reveal that the alloy nanowires are polycrystalline with the crystallite size of several tens of nanometers. EPMA of a single nanowire illustrates that there exist Ni-rich microzones in as-deposited nanowires. Subsequent magnetic measurements of the array also confirmed the existence of them. In addition, it can be further inferred that the shape of Ni-rich microzones is probably barlike or disklike, from the anisotropy of zero field cooling/field cooling (ZFC/FC) curves as well as the vortex magnetization behavior of the Ni5Zn21 nanowire array. The low-temperature magnetoresistance of the Ni5Zn21 nanowire array was also measured. Giant magnetoresistance instead of anisotropic magnetoresistance is suggested to be responsible for contributing to the magnetoresistance.