Coercitive Field Research Articles

Magnetic, structural and magnetocaloric effect investigations on the substituted spinel Mg1−xZnxFe2O4 (0 ≤ x ≤ 1) prepared by sol-gel method

Polycrystalline Mg1−xZnxFe2O4 (0 ≤ x ≤ 1) ferrites were prepared with sol-gel method. The obtained samples were characterized by powder X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and Mössbauer spectrometry. The X-ray diffraction analysis combined with the Rietveld refinement confirmed that the samples crystallize in a single-phase cubic spinel structure with (SG: Fd3−m) and an average grain size in the range of 29.1–40 nm. Lattice parameter increases with increasing Zn concentration, due to the larger ionic radius of Zn2+ ions in regard of Mg2+ ions. The Curie temperature TC decreases linearly with substitution. Saturation magnetization (MS), magnetic moment of iron (µFe) and magnetic entropy change (-ΔSM) increase for x ≤ 0.5 and decrease for x ≥ 0.5, this behavior is attributed to the cationic distribution of the ions on tetrahedral [A] and octahedral [B] sites, where the structure turns from partially inverse spinel to normal spinel, affecting the remanent magnetization and the coercitive field. Moreover, a new empirical relation is acquired performing linear relation between magnetic moment and the inversion parameter δ. Mössbauer spectra show a transformation from sextuple to an only a doublet which indicates a transformation from ferrimagnetic to paramagnetic phase, this ferrimagnetism degradation is confirmed by the negative values of -ΔSM and the transition of the magnetocaloric effect from normal to inverse through substitution.

Cations distribution by Rietveld refinement and magnetic properties of MgCrxFe2−xO4 spinel ferrites

Magnesium–chromium ferrites MgCrxFe2−xO4 (0 ≤ x ≤ 1) were studied using X-ray diffraction and magnetization measurements. Rietveld refinement of XRD patterns confirmed the cubic spinel structure in the space group Fd-3m and enabled to obtain the cations’ distribution amongst octahedral and tetrahedral sites of this structure. Magnetization measurements were performed using vibrating sample magnetometer (VSM) in the temperature range of 300–700 K and superconducting quantum interference device (SQUID) in the range of 10–300 K. Hysteresis loops for each concentration x at 300 K, show non-linear variation of saturation magnetization (MS) with chromium content. Both coercitive field and remanence decrease with increasing chromium content. MS variations with temperature indicate that the Curie temperature TC decreases with increasing chromium content. On the basis of Neel’s two sub-lattice crystal field model, the cations’ distribution enabled to calculate the theoretical magnetic moments at 0 K which are compared and discussed with those extracted by extrapolation to 0 K from MS experimental data.

Enhanced ultrafast optomagnetic effects in room-temperature ferromagnetic Pt nanoclusters embedded in silica by ion implantation

Ferromagnetic-like behavior at room temperature (300 K) was observed in Pt particles embedded in ion-implanted silica matrices. Results in samples integrated by ultra-small photoluminescent Pt clusters (<2 nm) were compared with samples containing exclusively larger plasmonic Pt nanoparticles (>3 nm). The ferromagnetic behavior coexists simultaneously with a diamagnetic response. Enhanced diamagnetic response of one order of magnitude was observed compared to typical diamagnetism in pure silica, and it is increased with the mean diameter of the Pt particles. Besides, a larger sensitivity to an external field was observed in the ferromagnetic response of the nanostructures with a characteristic saturation at 20 kOe. This ferromagnetic behavior was only observed in the samples with nucleated Pt particles. The magnitude of the saturation magnetization shows up to a fivefold increase in the samples with smaller particle size and larger particle density. Saturation magnetization was observed between 3–15 × 10−4 emu g−1, with remanent magnetization of 0.2–0.6 × 10−4emu g−1, measured at 300 K. Coercitive fields also decrease in samples with smaller size and particles density, with values of 114 and 300 Oe. At lower temperatures (5 K) the saturation magnetization increases, as it would be expected from a ferromagnetic state. Optomagnetic response was studied by inverse Faraday effects and induced photomagnetization with circular polarized picosecond pulsed light at 1064 nm wavelength. Results showed that samples with a stronger ferromagnetic response exhibit larger Faraday rotation up to 5.3 × 103deg cm−1 by light excitations with irradiances between 50 and 180 GW cm−2. These findings have immediate applications in multifunctional solid-state magneto-optical devices such as optical isolators, high-data storage devices and ultrafast all-optical switching of magnetization.

On the physical and semiconducting properties of the crednerite AgMnO2 prepared by sol-gel auto-ignition

AgMnO2 prepared by sol-gel auto-ignition, crystallizes in the crednerite type structure with a monoclinic symmetry (Space Group: C2/m), supported by the Raman spectroscopy. The structure, deriving from the delafossite, is elongated along the c-axis (c/a = 1.12), due to the Jahn-Teller (J-T) Mn3+ ion. The thermal analysis (TGA) shows a stability up to 630 °C before its reduction into Ag and Mn2O3, which is confirmed by the DTA plot. The optical transition (1.82 eV) between the bands edges is directly allowed. The magnetization at saturation (MS), the coercitive field (HC) and remanent field (MR) obtained from the vibration sample magnetometry are 0.16 emu/g, 205 Oe and 6.75 × 10−3 emu/g respectively. The electrical conductivity is frequency dependent and follows an exponential law. Moreover, the metal-metal interaction is weakened by the longer AgAg length (2.89 nm). The thermoelectric power confirms the p-type character of AgMnO2 (hole-doped crednerite). The conduction mechanism is thermally activated and occurs through nearest neighboring hopping among Ag2+/+. The activation energy decreases with increasing the frequency, indicating that the mobility is thermally activated rather than the holes density.

Mean-Field and Monte Carlo Studies of the Magnetic Properties of a Spin-7/2 and Spin-5/2 Ising Bilayer Film

We use standard Monte Carlo simulations based on the Metropolis algorithm and mean-field calculations to investigate the magnetic properties of an Ising bilayer film consisting of two superposed ferromagnetic squared lattices A and B whose magnetic atoms have spin 7/2 and 5/2 respectively. Crystal-field and external magnetic field effects on the spins are considered in the model. The magnetic order parameters and response functions are calculated as functions of the temperature for selected values of the model parameters and this enables one to devise thermal phase diagrams by both methods. Our calculations only reveal second-order phase boundaries. Under appropriate conditions, compensation point phenomena are detected in the calculations below the critical temperatures. When the system is exposed to the external magnetic constraint, attracting hysteresis phenomena are sometimes generated. The temperature dependence of the coercitive field for various values of the crystal-field is singled out.

Preparation and characterization of (CuInTe2)1-x(TaTe)x solid solutions (0

Polycrystalline samples of the (CuInTe2)1-x(TaTe)x alloys system were prepared by the melt and anneal technique in the composition range 0 < x < 1. Products were characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Differential Thermal Analysis (DTA) and SQUID techniques. From XRD and SEM, it was found that the solubility of TaTe in CuInTe2 is around 10%; however, samples up to x = 2/3 are composed by a mean tetragonal CuInTe2-like phase with traces of Ta2Te3. Samples with x > 2/3 show diffraction patterns with several phases with poor crystallization. In addition, the tetragonal CuInTe2-like phase shows spinodal decomposition i.e. there are two phases, one Ta-rich and the other Ta-poor, with the same crystal structure and very close lattice parameters. SEM measurements also show up to six different phases, although they do not produce observed diffraction line in powder XRD patterns.DC magnetic susceptibility measurements as a function of temperature using the ZFC-FC protocol were performed in samples with x ≤ 2/3. With the exception of x = 2/3, all samples show a similar behavior in the ZFC and FC curves which are typical of a system with a distribution of magnetic cluster sizes. Sample x = 2/3 shows ferromagnetic behavior with Tc = 50 K. For sample x = 1/4, measurements of the magnetization as a function of the applied magnetic field and temperature were performed. From the analysis of the curves it was found that the clusters contain 105 Ta-atoms and a coercitive field of 0.22 kOe at T = 5 K.

Magnetic and Electrical Properties of MnxCu1-xFe2O4 Ferrite

In the present work, mixed manganese-copper ferrite of composition MnxCu1-xFe2O4 (within x=0.40, 0.42, 0.44, 0.46, 0.48 and 0.50) have been investigated for their electric and magnetic properties such as dc resistivity, Curie temperature, saturation magnetization. MnxCu1-xFe2O4 ferrite samples were prepared by uniaxial pressing from the oxide mixture, synthesized at 1000°C during 45 hours and finally heated to 1200°C by 5h, at room atmosphere. The X-ray diffractograms show that the samples with different compositions were formed by compact structure spinel with cubic cell. The saturated magnetization of the MnxCu1-xFe2O4 ferrites increased with x up to 0.46, which presented the smallest coercitive field. All samples showed hysteresis characteristic of soft magnetic materials. This electrical behavior is compatible with an insulator. The results were analyzed in the framework of grain/barrier model.

Chemical Solution Deposition of Ferroelectric Hafnium Oxide for Future Lead Free Ferroelectric Devices

The phase formation and the influence of the type of dopant of an improved routine for chemical solution deposition (CSD) of ferroelectric hafnium oxide films are investigated. Ferroelectric properties for yttrium doping and doping with different elements from the lanthanide group are shown. The samples are prepared using platinum electrodes and a hafnium oxide layer with a thickness of 45 nm. The combination of DTA-TG analysis, thickness measurements and temperature dependent XRD study provide insight into the film formation process during the different fabrication steps. Polarization and capacitance measurements are performed to prove the ferroelectric nature of the deposited layers. The yttrium concentration is varied in a range from 0 mol% to 11 mol% proving to have a strong influence on the ferroelectric properties in agreement with the observation made on atomic layer deposition (ALD) prepared samples. A high remanent polarization of 20 μC cm−2 is attained with a coercitive field of 1.4 MV cm−1 for an yttrium doping concentration of 5.2 mol%. The possibility of depositing ferroelectric hafnium oxide films by CSD provides the opportunity of application for lead free piezoelectric and pyroelectric sensors and actuators and can lead to a better insight into the formation of the ferroelectric phase.

Magnetic properties of tin ferrites nanostructures doped with transition metal

Magnetic nanoparticles of tin ferrites doped with transition metal (Sn0.5–TM0.5Fe2O4, TM=Ni; Co; Mn) have been synthesized by co-precipitation route. Particles sizes were found in the range 8–14nm. These compounds exhibit superparamagnetic behavior, the blocking temperature was determined from the Field Cooled (FC) and Zero Field Cooled (ZFC) magnetization curves. The blocking temperature and the coercitive field of Co-doped tin nanoferrite are higher than those of Mn-doped and Ni-doped tin nanoferrites. While, the saturation magnetization of Mn-doped tin nanoferrite is higher than that of Co-doped and Ni-doped tin nanoferrites.

Investigation of the functional properties of Mg x Ni1−x Fe2O4 ceramics

The paper presents a complex study of the effect of Mg substitution on the functional properties of Ni-ferrite ceramics prepared by self-combustion sol–gel method. The sintered ceramics have pure cubic spinel structures with an increase of the lattice parameter and the grain size with Mg content. The electrical properties of Mg x Ni1−x Fe2O4 (x = 0; 0.17; 0.34; 0.5; 0.66; 1) ceramics have been investigated. The complex impedance spectra suggest a grain boundary contribution in the conduction process and reveal that the real part of impedance and the imaginary component (reactance) increase with increasing the Mg amount. The temperature dependence of dielectric properties shows that the hopping of charge carriers is thermally activated. The resistivity as a function of frequency for different degree of humidity was also investigated. All the investigated samples show a typical ferrimagnetic character with a strong non-linearity, small coercitive field (~50 Oe) and a saturation field of ~1kOe, typical to the investigated Mg x Ni1−x Fe2O4 ceramics. The Curie temperature determined from magnetic susceptibility versus temperature dependences presents a decrease with the addition of non-magnetic Mg2+ ion concentration from 603 °C (for x = 0) to 384 °C (for x = 1).

Influence of Ag-doping in La0.5Sr0.5CoO3 on Its Structural and Magnetic Properties

The structural and magnetic properties of the mixed valent cobaltites La0.5Sr0.5−x Ag x CoO3 (0≤x≤0.15) with perovskite structure have been investigated. X-ray diffraction (XRD) analysis using Rietveld refinement shows that all our samples crystallize in the orthorhombic structure with $R\overline{3}c$ space group. Although the unit cell volume remains almost constant, the Co–O bond length and the Co–O–Co bond angle are sensitive to the Ag addition. All our studied samples undergo a paramagnetic-ferromagnetic transition with decreasing temperature. The zero field cooled (ZFC) and field cooled (FC) magnetization curves at 50 mT of La0.5Sr0.5CoO3 (x=0) sample exhibits thermomagnetic irreversibility indicating frustration and competition of both antiferromagnetic and ferromagnetic interactions. It is suggested that Co3+ ions are in the intermediate spin state but Co4+ ions stay in a mixture of intermediate and high spin states. Ag addition leads to dramatic changes in magnetic properties. An interesting result has been obtained for La0.5Sr0.4Ag0.1CoO3, where the ZFC and FC curves coincide. For La0.5Sr0.35Ag0.15CoO3 (x=0.15) sample, the observed thermomagnetic irreversibility is much higher than that observed in x=0 sample. Hysteresis loops were recorded for x=0, 0.05, 0.1, and 0.15. The coercitive field is found to increase with Ag content reaching 0.26 T for x=0.15 whereas the remanent magnetization decreases.

Hall current sensor IC with integrated Co-based alloy thin film magnetic concentrator

This work deals with a cobalt-based alloy thin film magnetic concentrator (MC) which is fully integrated on a Hall sensor integrated circuit (IC) developed in the 0.35 µm Bipolar CMOS DMOS (BCD) technology on 8” silicon wafer. An amorphous magnetic film with a thickness of 1µm, coercitive field Hc MS ) of 0.45T has been obtained with a sputtering process. The Hall sensor IC has shown sensitivity to magnetic field at room temperature of 240V/AT without concentrator and 2550V/AT with concentrator, gaining a factor of 10.5. A current sensor demonstrator has been realized showing linear response in the range -50 to 50A.

On the soft magnetic properties of the compounds of the series NaxMn4.5−x/2(VO4)3 and the magnetic structure of h.t.-Mn3(VO4)2 (x = 1)

The high temperature phase of manganese vanadate h.t.-Mn3(VO4)2 and the solid solution with NaMn4(VO4)3 (Na(x)Mn(4.5-x/2)(VO4)3) were shown to order ferrimagnetically below 55 K for x < 1, whereas NaMn4(VO4)3 is an antiferromagnet. The materials show very soft magnetic properties with low coercitive fields required for demagnetisation (Hc < 0.001 T). The magnetic structure of h.t.-Mn3(VO4)2 was determined by Rietveld analysis of low temperature powder neutron diffraction data, and shows antiferromagnetic alignment of the magnetic moments of the Mn(2+) ions on the 8c and 8d sites. The ferrimagnetic moments were shown to result from the magnetic moments of the Mn(2+) cations located on the 4b site in unusual dodecahedral coordination (Hoard dodecahedron). This coordination can be understood as two penetrating oxygen coordination tetrahedra, one showing shorter and one showing longer Mn-O distances. The magnetic moments of the Mn(2+) ions on the 4b site are aligned parallel to the ones on 8d and antiparallel to the ones on 8c, being in good agreement with the GKA rules. The local exchange interactions between the Mn(2+) ions on the 4b to those on the 8c/8d sites are likely to be similar in strength and competitive and therefore probably contribute to the soft magnetic properties.

On the Synthesis and Physical Properties of Iron Doped SnO2 Nanoparticles

The synthesis of iron doped tin oxide by pulsed laser pyrolysis is reported. The as obtained nanoparticles have a dominant SnO2 phase (as revealed by Wide Angle X-ray Scattering), with particles of the order of 10 nm. The doping with iron or iron oxide triggers magnetic properties as confirmed by SQUID experiments. EDX measurements supported the presence of Fe while Wide Angle X-ray Scattering failed to sense any iron or iron-oxide phase. It is concluded that Fe is well dispersed within the tin-oxide nanoparticles. The coercitive field has a complex dependence on the Fe/Sn content suggesting that the magnetization is not controlled solely by the amount of Fe dispersed within the nanoparticles.

DIELECTRIC AND PIEZOELECTRIC PROPERTIES OF LEAD-FREE (Bi0.5Na0.5)0.94Ba0.06TiO3 + x%Ce2O3 + y%La2O3 + z%Y2O3 COMPOUNDS SYNTHESIZED BY SOL–GEL TECHNIQUE

( Bi 0.5 Na 0.5)0.94 Ba 0.06 TiO 3 + x% Ce 2 O 3 + y% La 2 O 3 + z% Y 2 O 3 compounds (named BNT–BT– x/y/z) were synthesized by a sol–gel technique, for (x,y,z) = (0,0,0), (0.25, 0.25, 0.25), (0.25, 0.5, 0.5), (0.5, 0.25, 0.25) and (0.5, 0.5, 0.25). The structural variation according to the different system compositions is investigated by X-ray diffraction analyses. The results shows that the addition of Ce2O3 , La2O3 or Y2O3 in BNT–BT system, do not modify the crystalline structure, and a rhombohedral–tetragonal morphotropic phase boundary is maintained for different dopant addition. The BNT–BT–x/y/z-based ceramics sintered at relatively low temperature (1100°C) exhibit a good densification ratio. The optimum dielectric and piezoelectric properties are obtained with the BNT–BT–0.5/0.25/0.25 composition. High dielectric properties at room temperature (εr &gt; 1000) and low dielectric losses (tan δ &lt; 4 × 10-2) are obtained for this composition. This compound exhibits a piezoelectric constant d33 of 95 pC/N, a good polarization behavior is observed with high remanent polarization Pr of 9.21 μC/cm2 and low value coercitive field Ec of 2.66 kV/mm.

Magnetic Properties and Hardness of Nanostructured WC-Co Cemented Carbide

Nacrystalline WC-10wt.%Co powders were prepared by high energy milling and liquid phase sintered. The powders with different milling time were characterized by X-ray diffraction and SEM. After sintered the WC-10wt.%Co cemented carbides exhibits ultra fine grain sizes. Coercitive field and Vickers hardness measurements on the consolided samples detected a significant increase and decrease Vickers hardness with the milling time increase in sintered samples.

Coercivity enhancement in FePt nanowires due to the suppression of available paths for domain wall propagation

This paper reports the observation of magnetization reversal in nanometric devices with perpendicular anisotropy using extraordinary Hall effect. Hall crosses were processed by scanning electron microscope lithography in FePt(001)/Pt(001) layers deposited by molecular beam epitaxy. Hysteresis loops are found to be strongly dependent on the wire width, particularly with an increase of the coercitive field at low sizes. We show that this is not due to a lower probability for extrinsic nucleation to occur in smaller structures but rather due to the suppression of available domain wall propagation paths in thin wires. An invasion percolation model, as well as simulations, is proposed to describe this coercivity enhancement, based on a quasisquare lattice of structural defects, the microtwins, being clearly responsible in our samples for the magnetic coercivity. Finally, we report the direct observation of individual Barkhausen jumps in 200-nm-wide crosses, showing that the measured amplitudes of these jumps correspond to the microtwins' areal density.

Atomic structure and reactivity of ferromagnetic Fe deposited on Si(001)

This study presents a correlated study of structural, reactivity, and magnetic properties of ultrathin Fe layers grown on Si(001) by molecular beam epitaxy in ultrahigh vacuum. The interface reactivity is characterized by Auger electron spectroscopy. The surface structure is characterized by low electron energy diffraction with spot profile analysis. The magnetism of the synthesized layers is investigated by magneto-optical Kerr effect. At room temperature, metal Fe layers with poor long-range order are synthesized; these layers are ferromagnetic with an extremely low coercitive field (below 1 Oe). The reactivity with Si is low in this case, with formation of an interface layer of about 8 A Fe equivalent thickness with about 7 at.% Si diffused. Samples synthesized at higher temperatures (500 °C) exhibit better long-range order, though the Fe reactivity with Si is higher and leads to the formation of an interface compound whose approximate stoichiometry is very close to Fe3Si. Once this compound is formed (for an equivalent Fe thickness of about 14 monolayers), disordered metal Fe islands are developing with subsequent Fe deposition, which contain also about 8 at.% Si diffused. These structures exhibit a much lower ferrimagnetism, with saturation magnetization about one order of magnitude lower than in the case of the room temperature synthesis. In this case of high temperature synthesis, two phases are observed, a ferrimagnetic one and a superparamagnetic one.

Magnetic tunnels junctions for all-oxide spin valves devices

We have developed La0.7Sr0.3MnO3 (LSMO) based spin valve tunnel junctions where the hard layer is composed of Ru doped La0.7Sr0.3MnO3 (LSMRO). These perovskite oxide layers have been grown on SrTiO3 by pulsed laser deposition. X ray diffraction, transmission electron microscopy and atomic force microscope measurements show the crystalline quality of the La0.7Sr0.3MnO3 and La0.7Sr0.3MnO3 doped Ruthenium thin films. Magnetization, resistance and noise characterisation of these layers are presented. The doping of Ruthenium increases the coercitive field of the La0.7Sr0.3MnO3 but degrades the metallic properties of the layer and leads to a noise increase of one order of magnitude. We have studied the magnetoresistance and magneto-transport properties of LSMO /STO/ LSMRO-based junctions which show the importance of the interfaces quality.

Nanostructured thin layers of vanadium oxides doped with cobalt, prepared by pulsed laser ablation: chemistry, local atomic structure, morphology and magnetism

Cobalt-doped vanadium oxide thin layers prepared by pulsed laser ablation are investigated from the following points of view: (1) the chemical states by X-ray photoelectron spectroscopy (XPS), (2) the local atomic order by X-ray absorption fine structure at both vanadium and cobalt K-edges, (3) the morphology of the films by atomic force microscopy (AFM) and (4) the magnetic properties by magneto-optical Kerr effect (MOKE). The chemical composition of the host matrix was found to be close to VO2 at the sample surface, with V2O3 in the bulk. Co ions are found near the surface in high ionisation states Co4+ (for the samples synthesised in a high vacuum condition, denoted by VO1), or with Co(4− δ )+ (for the samples synthesised in an oxygen atmosphere, denoted by VO2), whereas in the bulk, Co1.5+ is obtained for VO1 and Co2+ is obtained for VO2. The AFM revealed nanoparticles with sizes 10–25 nm for VO1 samples, whereas a few bigger nanoparticles are observed for VO2 samples. The VO1 samples presented high coercitive fields with a relatively low saturation magnetisation at room temperature, superposed with a superparamagnetic component attributed to the observed nanoparticles, whereas the VO2 samples presented double-loop hysteresis curves, indicating the co-existence of two kinds of magnetic moieties with antiparallel coupling at zero applied field. The proposed two magnetic phases are Co-doped V2O3 and VO2.