Change In Coercivity Research Articles

Phase analysis and structure of rapidly quenched Al-Mn systems

Abstract This investigation is focused on the rapidly quenched systems based on binary combination of post-transition and transition metal atom. The metastable Al-Mn alloys are prepared in form of metallic ribbons by planar flow casting and consequently used for elaboration of hard magnets. These systems are interesting as perspective alternative to RE based permanent magnets. The effect of alloying of constituting elements (in ratio about 1/1) on the changes of physical properties caused by structural transformations was studied. Magnetic properties result from the existence of phase τ-MnAl in the system. The structure evolution from as-cast state was analyzed by calorimetric and thermogravimetric measurements. The formation and evolution of crystalline phases is shown by direct structure observation using transmission electron microscopy and by X-ray diffraction methods. The results of in-situ phase analyses by exposure to a defined temperature regime are also presented. The annealing regime has a direct influence on magnetic properties, leading to small changes in coercivity and significantly affecting maximum energy product.

Synthesis, characterization and magnetic properties of NiFe2−xCexO4 nanoribbons by electrospinning

NiFe2−xCexO4 (x = 0–0.03) nanoribbons have been successfully fabricated using electrospinning technique and followed by calcining in air at 500°C. The crystalline, morphologies and compositions of NiFe2−xCexO4 nanoribbons are characterized by X-ray diffraction, selected area electron diffraction, transmission electron microscope, field emission scanning electron microscopy and energy dispersive X-ray spectroscopy (EDX). The results show that the components, mean crystallite sizes and morphologies change along with the content of Ce3+. A formation mechanism of NiFe2-xCexO4 nanoribbons is proposed. The magnetic hysteresis loops of NiFe2−xCexO4 nanoribbons reveals that the coercivity changes from 165Oe to 64Oe and saturation magnetizations change from 40.97emu/g to 25.05emu/g at room temperature. Morevover, the Mössbauer spectra of 57Fe in NiFe2−xCexO4 nanoribbons is discussed in detail. It is believed that this work will play important role in magnetic application with the advantage of excellent magnetic properties, efficient functionalization and relatively low cost.

Optical, magnetic and structural properties of ZnFe2O4 nanoparticles synthesized by conventional and microwave assisted combustion method: A comparative investigation

In the present work, zinc ferrite nanostructured materials were synthesized by a microwave combustion method (MCM) using Okra (Abelmoschus esculentus) as a plant extract and compared with the ones synthesized by a conventional combustion method (CCM). Plant extracts play a dual role of both oxidizing and reducing nature. Microwave assisted combustion method is a potential, attractive and energy saving technique when compared to conventional heating, due to the direct heating of the reaction mixtures. They have several advantages, such as, simple, inexpensive, good stability of nanoparticles, less time consumption, and large-scale synthesis. The synthesized products are investigated by the standard characterization techniques, such as, X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), Rietveld refinement analysis, high resolution scanning electron microscopy (HR-SEM), energy dispersive X-ray analysis (EDX), photoluminescence studies (PL), diffuse reflectance spectroscopy (DRS), and vibrating sample magnetometer (VSM), for the structural phase, morphology, optical and magnetic properties. The development of the single cubic phase ZnFe2O4 is affirmed by XRD and FT-IR. The morphological studies of the obtained ZnFe2O4 nanoparticles were investigated by HR-SEM studies. The optical band gap value was determined by DRS. The change in saturation magnetization (Ms) and coercivity (Hc) was observed by VSM studies. Dye Sensitized Solar Cell made up of ZnFe2O4 nanoparticles prepared by microwave method exhibited maximum conversion efficiency of around 0.22%, which is higher than that obtained from conventional combustion method gave efficiency of approximately 0.15%.

Dependence of grain size and defect density on the magnetic properties of mechanically alloyed Fe90W10 powder

Mechanical alloying was used to synthesize nanocrystalline Fe90W10 powders from high purity Fe and W powders. X-ray diffraction measurements showed that the formation of BCC Fe-W solid solution occurred after 32 h of milling. The Scherrer-Debye and Williamson–Hall equations revealed that grain size reduction and defect creation were achieved during the milling process. There is a decrease of grain size from about 53 nm to about 6 nm after 80 h of milling. Williamson and Smallman's equation was also used to calculate the dislocation density. The result shows an increase in the dislocation density with increasing milling time. The grain size and defect characteristics were correlated with magnetic measurements. Magnetization versus magnetic field curves recorded using a vibrating sample magnetometer were utilized to obtain coercivity and saturation magnetization at different temperatures. As W is continually dissolved in the Fe lattice, the change in coercivity seems to be minimal until the completion of solid solution when there is a sudden increase in coercivity. The increase in coercivity is explained by an increase in anisotropy due to an additional source of anisotropy arising from strain during the milling process. There was also a decrease in saturation magnetization as a result of the grain size reduction.

Strain-mediated multiferroic control of spontaneous exchange bias in Ni-NiO heterostructures

This paper presents the measurement of strain-mediated multiferroic control of spontaneous exchange bias (SEB) in magnetostrictive nickel/nickel oxide (Ni/NiO) bilayers on ferroelectric lead magnesium niobate-lead titanate (PMN-PT). Electric field control of a positive to negative exchange bias shift was measured, with an overall shift of 40.5 Oe, corresponding to a 325% change. Observed changes in coercivity are also reported and provide insight into the role of competing anisotropies in these structures. The findings in this paper provide evidence that magnetoelastic anisotropy can be utilized to control spontaneous exchange bias (SEB). This control of SEB is accomplished by modifying a bulk anisotropy (magnetoelasticity) that adjusts the mobility of interfacial anti-ferromagnetic spins and, therefore, the magnitude of the exchange bias. The demonstrated magnetoelastic control of exchange bias provides a useful tool in the creation of future magnetoelectric devices.

Low-temperature magnetic properties of monoclinic pyrrhotite with particular relevance to the Besnus transition

Monoclinic pyrrhotite (Fe7S8) owes its ferrimagnetism to an ordered array of Fe vacancies. Its magnetic properties change markedly around 30 K, in what is known as the Besnus transition. Plausible explanations for the Besnus transition are either due to changes in crystalline anisotropy from a transformation in crystal symmetry or from the establishment of a two-phase system with magnetic interaction between the two phases. To help resolve this discrepancy, we measured hysteresis loops every 5° and backfield curves every 10° in the basal plane of an oriented single crystal of monoclinic pyrrhotite at 300 K and every 2 K from 50 K through the Besnus transition until 20 K. Between 50 and 30 K, hysteresis loops possess double inflections between crystallographic a-axes and only a single inflection parallel to the a-axes. Magnetization energy calculations and relative differences of the loops show a sixfold symmetry in this temperature range. We propose that the inflections stem from magnetic axis switching, which is both field and temperature dependent, in a manner somewhat analogous to an isotropic point where magnetocrystalline constants change their sign. The Besnus transition is best characterized by changes in magnetic remanence and coercivity over a 6° temperature span (28–34 K) with a maximum rate of change at 30 K. A surprising yet puzzling finding is that the coercivity ratio becomes less than unity below the transition when fourfold symmetry arises. Because the changes in magnetic parameters are linked to the crystal structure, we conclude the Besnus transition owes its origin to a distortion of the crystallographic axes below 30 K rather than an apparition of a two-phase system. An isothermal magnetization of natural pyrrhotite cycled from room temperature to successively lower temperatures through the Besnus transition decreases 2–4 times less than equivalent grain sizes of magnetite, with less than a 10 per cent loss in remanence between 300 and 150 K in pseudo-single-domain (PSD) pyrrhotite. As PSD monoclinic pyrrhotite carries the magnetic remanence in some meteorites, it is likely that low-temperature cycling in space then warming to ambient conditions at the Earth's surface will have only a minor influence on palaeointensity values derived from those meteorites.

Strain mediated magnetoelectric coupling in a NiFe2O4–BaTiO3 multiferroic composite

In this paper we demonstrate significant magnetoelectric coupling in ferrimagnetic, NiFe2O4, and ferroelectric, BaTiO3, multiferroic composite bulk materials by measuring temperature dependent magnetization. X-ray diffraction, scanning electron microscopy and high resolution transmission electron microscopy data show that the two phases coexist with a highly crystalline and sharp interface without any detectable impurities, which enables significant magnetoelectric (ME) coupling. The temperature dependent magnetization data of the composite clearly show the jumps in magnetization curves at the structural phase transitions of BaTiO3, thereby indicating their origin in ME coupling. The change in coercivity of composite sample in different ferroelectric phases of BaTiO3 has been observed compared to the NiFe2O4 sample. The different lattice strains corresponding to different ferroelectric phases of BaTiO3 could be the driving force for modulating the magnetization and coercivity of the composite material. This is clear evidence of strain mediated ME coupling in ferrimagnetic and ferroelectric composite materials.

Optical Writing of Magnetic Properties by Remanent Photostriction.

We present an optically induced remanent photostriction in BiFeO_{3}, resulting from the photovoltaic effect, which is used to modify the ferromagnetism of Ni film in a hybrid BiFeO_{3}/Ni structure. The 75% change in coercivity in the Ni film is achieved via optical and nonvolatile control. This photoferromagnetic effect can be reversed by static or ac electric depolarization of BiFeO_{3}. Hence, the strain dependent changes in magnetic properties are written optically, and erased electrically. Light-mediated straintronics is therefore a possible approach for low-power multistate control of magnetic elements relevant for memory and spintronic applications.

Ferroelectric KNbO3 nanofibers: synthesis, characterization and their application as a humidity nanosensor

By virtue of its non-toxicity, high Tc, and non-linear optical and ferroelectric properties, one-dimensional (1D) potassium niobate (KNbO3) may enable the development of numerous nanoscale devices. Despite the progress in 1D perovskite materials, preparing high aspect ratio KNbO3 nanostructures is still a concern. This report presents the successful synthesis of ultra-long KNbO3 nanofibers using a simple sol–gel assisted far-field electrospinning process. At optimized conditions, centimeters long, orthorhombic KNbO3 nanofibers with an average diameter of 100 nm have been obtained. The nanofibers are composed of uniform grains densely stacked along the direction of the nanofiber axis. Due to large surface–volume ratio, a high sensitive humidity nanosensor based on KNbO3 nanofibers displaying a logarithmic–linear dependence behavior of the conductance with the relative humidity (RH) was demonstrated. The conductance increases dramatically from 10−10 ℧ to 10−6 ℧ while RH varies from 15% to 95% at room temperature. In addition, the nanosensor exhibits excellent sensing performance, including ultrafast response (≤2 s) and recovery times (≤10 s), good linearity and reproducibility. Furthermore, the change in ferroelectric coercivity with respect to the RH and its effect in the sensing behavior were unveiled. This work could enable broad applications in the fields of environmental sensing and nano-electrical-mechanical systems.

Spin-orbit torque induced reversible coercivity change in Co/Pd multilayer thin films

Abstract In this work, we report reversible reduction in coercivity of Co/Pd multilayer thin films under high-density direct current biasing. We carried out in-situ focused magneto optic Kerr effect based hysteresis measurement while the specimen was under DC bias. The experiments show a reversible reduction in coercivity during the application of direct current. We propose this reduction occurs due to the field like spin-orbit torque generated by spin Hall effect. These results are further supported by the anomalous Hall effect measurement, which do not show any change in coercivity. The magneto-optic Kerr measurement probes the surface (penetration depth) whereas anomalous Hall effect is a bulk transport behavior. These complimentary measurements prove that the origin of spin-orbit torques in ferromagnetic metal/ heavy metal multilayer thin films is spin-Hall effect.

Microstructure and mechanical properties of WC-40Co composite obtained by impact sintering in solid state

The influence of the densification temperature on the density, microstructure, mechanical properties, resistivity and coercivity of WC- 40 wt%Co composite produced by impact sintering has been investigated. The studied material possessed a 54% volume content of cobalt and tungsten carbide particles of initial size 1–3 μm. Impact sintering was carried out in a vacuum of 0.013 Pa in a temperature range covering both the solid and the liquid state: 1150, 1200, 1250 and 1290 °C.The samples were held isothermally for 20 min before consolidation and the time of the impact consolidation was 0.003–0.006 s. For comparison conventional liquid-phase sintering of samples was carried out at 1340 °C for a period of 30 min. The results showed that the impact sintering in the solid state temperature range enabled almost pore-free samples with high mechanical properties to be obtained. In particular, the samples that were impact sintered at 1200 °C have the following mechanical properties: transverse rupture strength (TRS) 3116 ± 156 MPa, fracture toughness 30.3 ± 1.2 MPam1/2, compression strength 3095 ± 106 MPa, proof stress at compression 1682 ± 82 MPa, plastic deformation at compression 11.9 ± 1.3%, strain energy density 302 ± 35 J/cm3 and Vickers hardness 7.7 GPa. The maximum TRS = 3143 ± 63 MPa and the maximum plastic deformation at compression 12.3 ± 1.1% have the samples obtained at the impact sintering temperature of 1250 °C. The impact sintering of samples at 1290 °C leads to decrease in TRS, hardness, compressive strength and proof stress at compression. The second halves of the samples were subjected to annealing at 1250 °C within 180 min after sintering. Additional annealing of the impact sintered samples led to significant changes in specific resistivity and coercivity, increased the fracture toughness, but had negligible influence on the TRS. The compressive strength and plastic deformation decreased after annealing.

Realization of two-dimensional ferromagnetism with giant coercivity in ultrathinβ−Ni(OH)2layers grown on aMoS2surface

Due to the charge transfer effect at the contact of transition metal (TM) and $\mathrm{Mo}{\mathrm{S}}_{2}$, the use of ferromagnets in $\mathrm{Mo}{\mathrm{S}}_{2}$ based spin transistor is not suitable. On the other hand, $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ni}{(\mathrm{OH})}_{2}$ is known to be a layered type material with antiparallel Ni spins in alternate layers. Here, an ultrathin layer of antiferromagnetic $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ni}{(\mathrm{OH})}_{2}$ is grown on the $\mathrm{Mo}{\mathrm{S}}_{2}$ surface to achieve complete ferromagnetism with giant coercivity (2925 Oe). The origin of this ferromagnetic ordering is the reduction of Ni spin moments in $\mathrm{Ni}{(\mathrm{OH})}_{2}$ layer adjacent to $\mathrm{Mo}{\mathrm{S}}_{2}$ surface due to charge transfer from S to Ni. The use of antiferromagnetic layered type material to achieve ferromagnetic ordering with giant coercivity is a new concept to realize perfect two-dimensional (2D) ferromagnets which have major advantages due to the huge change in coercivity with thickness.

Magnetism and structure of nanocomposites made from magnetite and vegetable oil based polymeric matrices

The aim of this work is to study the influence of the polymeric matrix composition on particle aggregation, magnetic interparticle interactions and nanoparticle surface effects, which affect the magnetic and structural properties of different ultra-diluted magnetite nanocomposites (MNCPs). Bio-based matrices were selected as a possible response to the increasing demand for renewable materials. To investigate the influence of different bio-based polymeric matrices on the magnetic behavior, three different bio-based polymers were used to prepare MNCPs with 1 wt.% of magnetite nanoparticles (MNPs). One of them was prepared using a tung oil (TO)/styrene (St) weight ratio of 70/30, a second one was prepared by replacing the styrene with methylester (green comonomer obtained from tung oil, ME, 70TO/30ME) and a third one that incorporated a green modifier, acrylated epoxidized soybean oil (AESO), using a tung oil/AESO weight ratio of 90/10. Structural features as nanoparticle aggregation state, nanoparticle and cluster sizes, and fractal dimension were studied and determined from small-angle X-ray Scattering (SAXS). The experimental SAXS data were analyzed by means of fractal aggregate model. Results indicate differences in nanoparticle arrangement depending of the containing matrix. The magnetic characterization of these materials indicates that the matrix strongly affects the physical and chemical properties of the MNCPs. All samples display superparamagnetic behavior at room temperature, but the blocking temperature varies from 75 K (tung oil/styrene with 1 wt.% MNPs) to 126 K (tung oil/AESO 1 wt.% MNPs). Furthermore, the temperature dependence of the coercive field changes for all samples, suggesting a strong influence of the polymer properties on the magnetic properties of the MNCPs.

Voltage induced reversible and irreversible change of magnetic coercivity in Co/ZnO heterostructure

In this study, the application of bias voltage to 4–8 nm Co/275 nm ZnO heterostructures changed the magnetic behavior reversibly or irreversibly, depending on the different voltage-induced mechanisms. The magnetic coercivity (Hc) monotonically decreased 20% when the small voltages of 0–8 V were applied. The Hc reduction was symmetric with the voltage polarity, and the reversibility was demonstrated by cyclically switching the bias voltage between 0 and 7 V. While a large voltage up to 40 V was applied to the Co/ZnO junction, the current heating effect became considerable and the Co-oxide was formed, as confirmed by depth-profiling X-ray photoemission spectroscopy analysis. The presence of Co-oxide in the Co films induced the irreversible reduction of the Kerr signal and Hc at room temperature. The considerable Hc enhancement at 130 K also indicates the exchange bias coupling effect from the antiferromagnetic Co-oxide.

Structural and Magnetic Consequences of Mn0.6Zn0.4Fe2−x Gd x O4 Ferrite

Gadolinium-substituted spinel ferrite nanopowders Mn0.6Zn0.4Fe2−xGdxO4 (x = 0, 0.1, 0.2, 0.3, 0.4) were prepared by sol-gel auto-combustion method and post subsequent heat treatment. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Fourier-transform infrared (FTIR), and vibrating sample magnetometer (VSM) methods were used in order to investigate the effect of Gd 3+ cation substitution on structural and magnetic properties of Mn–Zn ferrite. The results of XRD measurements and FTIR spectra showed the formation of single spinel ferrite for x ≤ 0.2 that is associated with some secondary phases at higher amounts of Gd 3+ doping. The lattice parameter was found to increase with increasing Gd content. The average particle size of substituted Mn–Zn ferrite became smaller by addition of gadolinium. The room temperature saturation magnetization values increased up to 56.4 emu/g for x = 0.1, and then decreased upon further Gd substitution. Similar behavior was observed in the coercivity changes. The observed magnetic properties were explained on the basis of net magnetic moment changes and structural characterization upon Gd substitution in the compound.

Hydrogen Co-Deposition Induced Phase and Microstructure Evolution of Cobalt Nanowires Electrodeposited in Acidic Baths

The growth and deposition mechanism of electrodeposited cobalt nanowires (NWs) at different bath temperatures (25°C–60°C) in strongly acidic bath (pH of 2.0) is studied. CV scans show that hydrogen co-deposition is most pronounced in 50°C bath. X-ray diffraction patterns of Co NWs show that a mixture of hcp and fcc phases grows at 25°C, (200) textured fcc-Co phase grows at 50°C, and and (110) textured hcp-Co phase at 60°C, with these planes normal to the NW-axis. The magnetic properties studied by vibrating sample magnetometer show the change in coercivity, saturation field and magnetization reversal behavior is consistent with the phase of NW. Nucleation mechanism is explained on the basis of intermediates formation induced by the extent of hydrogen co-deposition leading to the change in nucleation overpotential and linked selection of the growth plane as per bath pH and temperature. Chronoamperometry study has confirmed the instantaneous and progressive growth mechanisms operative at 50°C and 60°C respectively and hydrogen co-deposition controlled grain size growth model is also put forth to understand the growth kinetics of fcc and hcp phase Co NWs. The suggested growth mechanism also consistently explains the growth in less acidic bath of pH 4.5.

Manipulation of magnetization switching and tunnel magnetoresistance via temperature and voltage control.

Magnetization switching between parallel and antiparallel alignments of two magnetic layers in magnetic tunnel junctions (MTJs) is conventionally controlled either by an external magnetic field or by an electric current. Here, we report that the manipulation of magnetization switching and tunnel magnetoresistance (TMR) in perpendicularly magnetized CoFeB/MgO/CoFeB MTJs can be achieved by both temperature and voltage. At a certain range of temperature, coercivity crossover between top and bottom magnetic layers is observed in which the TMR ratio of the MTJs is almost unmeasurable. Furthermore, the temperature range can be tuned reversibly by an electric voltage. Magnetization switching driven by the voltage reveals an unconventional phenomenon such that the voltage driven coercivity changes with temperature are quite different for top and bottom CoFeB layers. A model based on thermally-assisted domain nucleation and propagation is developed to explain the frequency and temperature dependence of coercivity. The present results of controlling the magnetization switching by temperature and voltage may provide an alternative route for novel applications of MTJs based spintronic devices.

First-order-reversal-curve (FORC) diagrams of alternative chain of soft/ hard magnetic CoFe/Cu multilayer nanowires

The effect of interactions on the soft and hard phases and interference region that commonly appears in the First Order Reversal Curve (FORC) of interacting two-phase magnetic systems was investigated. To obtain an interacting two-phase system, a new method was introduced for the first time to electrodeposit a two-phase magnetic nanowire (NW) composed of hard and soft phases with high magnetization into nanopores of the anodized aluminum oxide template using the single-bath ac-pulse electrodeposition technique. Two-phase behavior was obtained by multilayer and grainy configurations of the CoFe and Cu layers as two type layers with controllable thickness through the related pulse numbers. It was found that interphase interaction can be observed in FORC diagrams with three factors; (i) the shift in center of the soft phase feature along the interaction field axis without the change in coercivity, (ii) shift in center of the hard phase feature along the coercivity axis and (iii) appearance of an additional interference region. However, order of the shifts directly correlates with the order of demagnetizing intraphase interaction through the hard phase and magnetic moment contribution of the soft phase. The interference region contribution was found strongly correlates with irreversible magnetic moment contribution of the soft and hard phases.

Enhancement of electric field modulation of coercivity in Pt /Co/Al-O structures by tuning Co surface oxidation

The effect of the oxidation of the Co ferromagnetic layer on gate modulation of its coercivity was experimentally investigated in Pt / Co / Al-O heterostructures. The Co surfaces were naturally oxidized by exposing to atmosphere after Co layer growth, and the degree of oxidation was characterized by x-ray photoelectron spectroscopy (XPS). A comparison between XPS and magneto-transport results suggested that a moderate oxidation of the Co interface enhances the electric-field induced coercivity change, indicating the importance of interface engineering for electrical control of magnetization reversal.

Electric current induced coercivity change in Co/Pd multilayer thin films

In this work, we present the experimental results on the effect of electric current-induced diffusion leading to change in ferromagnetic behavior of Co/Pd multilayer thin films. We applied high-density direct current to a Co/Pd multilayer specimen in ambient conditions. We observed an almost 8.8 times change in coercivity and improved squareness of the thin films using magneto-optic Kerr effect measurement. Magnetic force microscope studies further validate an increase in the coercivity for the current-treated thin films. We experimentally observe that this change cannot be attributed only to Joule heating due to electric current. We propose that the underlying mechanism for the observed behavior is electromigration induced diffusion along the grain boundaries and the thin film surface. Surface diffusion of O2 leads to formation of CoO resulting in the permanent coercivity change observed in this work. The composition of the specimens is Ta (1 nm)/ [Co (0.35 nm)/Pd (0.55 nm)]20.