Showing papers on "Saturation (magnetic) published in 2012"

Electric field-induced magnetization reversal in a perpendicular-anisotropy CoFeB-MgO magnetic tunnel junction

Abstract: The electric field-induced ∼180° magnetization reversal is realized for a sputtered CoFeB/MgO-based magnetic tunnel junction with perpendicular magnetic easy axis in a static external magnetic field. Application of bias voltage with nanoseconds duration results in a temporal change of magnetic easy axis in the free layer CoFeB to in-plane, which induces precessional motion of magnetization in the free layer. The magnetization reversal takes place when the bias voltage pulse duration is adjusted to a half period of the precession. We show that the back and forth magnetization reversal can be observed by using successive application of half-period voltage pulses.

Experiments and modeling of iron-particle-filled magnetorheological elastomers

Abstract: Magnetorheological elastomers (MREs) are ferromagnetic particle impregnated rubbers whose mechanical properties are altered by the application of external magnetic fields. Due to their coupled magnetoelastic response, MREs are finding an increasing number of engineering applications. In this work, we present a combined experimental and theoretical study of the macroscopic response of a particular MRE consisting of a rubber matrix phase with spherical carbonyl iron particles. The MRE specimens used in this work are cured in the presence of strong magnetic fields leading to the formation of particle chain structures and thus to an overall transversely isotropic composite. The MRE samples are tested experimentally under uniaxial stresses as well as under simple shear in the absence or in the presence of magnetic fields and for different initial orientations of their particle chains with respect to the mechanical and magnetic loading direction. Using the theoretical framework for finitely strained MREs introduced by Kankanala and Triantafyllidis (2004) , we propose a transversely isotropic energy density function that is able to reproduce the experimentally measured magnetization, magnetostriction and simple shear curves under different prestresses, initial particle chain orientations and magnetic fields. Microscopic mechanisms are also proposed to explain (i) the counterintuitive effect of dilation under zero or compressive applied mechanical loads for the magnetostriction experiments and (ii) the importance of a finite strain constitutive formulation even at small magnetostrictive strains. The model gives an excellent agreement with experiments for relatively moderate magnetic fields but has also been satisfactorily extended to include magnetic fields near saturation.

Enhanced magnetic and dielectric properties of Eu and Co co-doped BiFeO3 nanoparticles

Abstract: Bi1−xEuxFe1−yCoyO3 (x = 0, 0.01; y = 0, 0.01) nanoparticles, having an average size of 13 nm, were prepared by a simple sol gel route. Strong electronegativity of Eu3+ and smaller oxidation-reduction potential of Co3+/Co2+ (0.55 eV) than Fe3+/Fe2+ (1.3 eV) increase the concentration of Fe3+ ions with doping. Distinct magnetic hysteresis and complete saturation of magnetisation indicate the presence of ferromagnetic phase. The successful co-doping of Eu and Co into BiFeO3 (BFO) lattice dramatically enhances the saturation magnetization (Ms) and coercivity (Hc) by about 20 times than that of pure BiFeO3. A large value of dielectric constant of about 650, low loss (<0.001), and small leakage current density (1.79 × 10−8 A/cm2) are observed for the co-doped sample.

Light-induced magnetization reversal of high-anisotropy TbCo alloy films

Abstract: Magnetization reversal using circularly polarized light provides a new way to control magnetization without any external magnetic field and has the potential to revolutionize magnetic data storage. However, in order to reach ultra-high density data storage, high anisotropy media providing thermal stability are needed. Here, we evidence all-optical magnetization switching for different TbxCo1-x ferrimagnetic alloy composition and demonstrate all-optical switching for films with anisotropy fields reaching 6 T corresponding to anisotropy constants of 3x106 ergs/cm3. Optical magnetization switching is observed only for alloys which compensation temperature can be reached through sample heating.

Synthesis of Nonspherical Superparamagnetic Particles: In Situ Coprecipitation of Magnetic Nanoparticles in Microgels Prepared by Stop-Flow Lithography

Abstract: We present the synthesis of nonspherical magnetic microparticles with multiple functionalities, shapes, and chemistries. Particle synthesis was performed in two steps: polymeric microparticles functionalized homogenously with carboxyl groups were generated using stop-flow lithography, and then in situ coprecipitation was used to grow magnetic nanoparticles at these carboxyl sites. With successive growth of magnetic nanoparticles, we obtained polymeric particles with saturation magnetizations of up to 42 emu/g microparticle. The growth in the magnetic nanoparticle mean size and polydispersity was determined from the magnetization curves obtained following each growth cycle; nanoparticle sizes were limited by the physical constraint of the effective mesh within the hosting gel microparticle. Particles with spatially segregated domains of varying magnetic properties (e.g., Janus particles, particles with step changes in magnetite concentration, etc.) can be synthesized readily using this approach.

Enhancement of perpendicular magnetic anisotropy in FeB free layers using a thin MgO cap layer

Abstract: We prepared magnetic tunnel junction films with PtMn/CoFe/Ru/CoFeB/MgO tunnel barrier/FeB free layer/MgO cap layer/Ta multilayers using sputtering and measured magnetic and magnetoresistive properties of the films at room temperature. The magnetization curves of the FeB plane film measured under perpendicular-to-plane magnetic fields showed much smaller saturation fields (Hs) than those expected from the demagnetizing field. Hs decreased from 4 to 0.4 kOe with increasing MgO cap layer thickness. The small Hs is due to the perpendicular magnetic anisotropy (PMA) induced at both MgO barrier–FeB and FeB–MgO cap interfaces. After microfabrication, the small free layer cells having a 1.6 nm thick MgO cap layer showed a magnetization easy axis in the perpendicular-to-plane direction. By inducing PMA from both upper and lower interfaces, we can stabilize the magnetization of the relatively thick (2 nm) FeB free layer in the perpendicular-to-plane direction.

Insulating behavior at the neutrality point in dual-gated, single-layer graphene

Abstract: The fate of the low-temperature conductance at the charge-neutrality (Dirac) point in a single sheet of graphene is investigated down to 20 mK. As the temperature is lowered, the peak resistivity diverges with a power-law behavior and becomes as high as several Megohms per square at the lowest temperature, in contrast with the commonly observed saturation of the conductivity. As a perpendicular magnetic field is applied, our device remains insulating and directly transitions to the broken-valley-symmetry, nu=0 quantum Hall state, indicating that the insulating behavior we observe at zero magnetic field is a result of broken valley symmetry. Finally we discuss the possible origins of this effect.

Structural, Magnetic And Optical Studies Of nickel Ferrite Thin Films

Abstract: In the present work, structural, morphological, magnetic and optical properties of nickel ferrite thin films having different thickness are reported. All the films were deposited on Si (100) substrate by pulsed laser deposition technique. Thicknesses of the films determined by x-ray reflectivity vary from 62 to 176nm as the deposition time varies from 16 min to 40 min. The films were characterised by x-ray diffractogram, Fourier transform infrared (FTIR) and Raman spectroscopy for structural and phase confirmation. FTIR and Raman spectra confirm mixed spinel nature of nickel ferrite. Surface morphology is studied by Atomic force microscopy. All the films have granular nature. Magnetic properties were studied by vibrating sample magnetometer and magnetic hysteresis curves were recorded for all the films at room temperature and at10K. At 10K, saturation magnetisation was found to increase while coercivity deceases with thickness. The results are explained on the basis of anisotropy induced by cation inversion and strain. Optical properties were studied by UV-vis reflectance spectra. The value of optical band gap (5.7eV) was found to be independent of thickness of the film. Copyright © 2011 VBRI press.

Magnetic anisotropy, magnetostatic interactions and identification of magnetofossils

Abstract: [1] Single-domain magnetite particles produced by magnetotactic bacteria (MTB) and aligned in chains, called magnetosomes, are potentially important recorders of paleomagnetic, paleoenvironmental and paleolife signals. Rock magnetic properties related to the anisotropy of magnetosome chains have been widely used to identify fossilized magnetosomes (magnetofossils) preserved in geological materials. However, ambiguities exist when linking magnetic properties to the chain structure because of the complexity of chain integrity and magnetostatic interactions among magnetofossils that results from chain collapse during post-depositional diagenesis. In this paper, magnetic properties of three sets of samples containing extracted magnetosomes of the culturedMagnetospirillum magneticumstrain AMB-1 were analyzed to determine how chain integrity and particle concentration influence magnetic properties. Intact MTB and well-dispersed magnetosome chains are characterized by strong magnetic anisotropy and weak magnetostatic interactions, but progressive chain breakup and particle clumping significantly increase the degree of magnetostatic interaction. This results in a change of the magnetic signature toward properties typical of interacting, single-domain particles, i.e., a decrease of the ratio of anhysteretic remanent magnetization to the saturation isothermal remanent magnetization, decreasing in the crossing point of the Wohlfarth-Cisowski test and in the delta ratio between losses of field and zero-field cooled remanent magnetization across the Verwey transition, as well as vertical broadening of the first-order reversal curve distribution. We propose a new diagram that summarizes the Verwey transition properties, with diagnostic limits for intact and collapsed chains of magnetosomes. This diagram can be used, in conjunction with other parameters, to identify unoxidized magnetofossils in sediments and rocks.

Surface spin disorder and spin-glass-like behaviour in manganese-substituted cobalt ferrite nanoparticles

Abstract: Manganese-substituted cobalt ferrite nanoparticles coated with triethylene glycol (TREG) have been prepared by the glycothermal reaction. The effect of Mn substitution and coating on temperature-dependent magnetic properties of the TREG-coated Mn x Co1−x Fe2O4 nanoparticles (0.0 ≤ x ≤ 0.8) with size of ~5–7 nm has been investigated in the temperature range of 10–300 K in a magnetic field up to 9 T. After the irreversible processes of the magnetic hysteresis curves were completed, the high-field regions of these curves were fitted by using a ‘law of approach to saturation’ to extract the magnetic properties, such as the effective anisotropy constant (K eff) and the anisotropy field (H A) etc. High coercive field of 12.6 kOe is observed in pure cobalt ferrite coated with TREG at 10 K. The low temperature unsaturated magnetization behaviour indicates the core–shell structure of the Mn x Co1−x Fe2O4 NPs. Zero-field-cooled (ZFC) and field-cooled (FC) measurements revealed superparamagnetic phase of TREG-coated Mn x Co1−x Fe2O4 nanoparticles at room temperature. The blocking and irreversibility temperatures obtained from ZFC–FC curves decrease at highest Mn concentration (x = 0.8). The existence of spin-glass-like surface layer with freezing temperature of 215 K was established with the applied field dependence of the blocking temperatures following the de Almeida–Thouless line for the Mn0.6Co0.4Fe2O4 NPs. The shifted hysteresis loops with exchange bias field of 60 Oe and high-field irreversibility up to 60 kOe in FC M–H curve at 10 K show that spin-glass-like surface spins surrounds around ordered core material of the Mn0.6Co0.4Fe2O4 NPs. FMR measurement show that all the TREG-coated Mn x Co1−x Fe2O4 nanoparticles absorb microwave in broad field range of about ten thousands Oe. The spectra for all the samples have broad linewidth because of angular distributions of easy axis and internal fields of nanoparticles.

Electric-field-control of magnetic anisotropy of Co0.6Fe0.2B0.2/oxide stacks using reduced voltage

Abstract: We have demonstrated purely electrical manipulation of the magnetic anisotropy of a Co0.6Fe0.2B0.2 film by applying only 8 V across the CoFeB/oxide stack. A clear transition from in-plane to perpendicular anisotropy was observed. The quantitative relationship between interface anisotropy energy and the applied electric-field was determined from the linear voltage dependence of the saturation field. By comparing the dielectric stacks of MgO/Al2O3 and MgO/HfO2/Al2O3, enhanced voltage control was also demonstrated, due to the higher dielectric constant of the HfO2. These results suggest the feasibility of purely electrical control of magnetization with small voltage bias for spintronics applications.

Suppression of leakage magnetic field from a wireless power transfer system using ferrimagnetic material and metallic shielding

Abstract: This paper describes a method to suppress the leakage magnetic field from a wireless power transfer (WPT) system through the use of a ferrimagnetic material and metallic shielding To demonstrate the advantages of the coil structure with the ferrimagnetic material and metallic shielding, magnetic field distributions and the electrical performance of three different coil structures are investigated via 3D electromagnetic (EM) field solver and SPICE simulation Results show that the suggested method considerably reduces the leakage magnetic field in the vicinity of the WPT system without significant loss of electrical performance The simulation results of the suggested coil structure are experimentally verified with a 100 W-class WPT system for an LED TV

Fabrication of $\hbox{Fe}_{16}\hbox{N}_{2}$ Films by Sputtering Process and Experimental Investigation of Origin of Giant Saturation Magnetization in $\hbox{Fe}_{16}\hbox{N}_{2}$

Abstract: We present a systematic study to address a longstanding mystery in magnetic materials and magnetism, whether there is giant saturation magnetization in Fe16N2 and why. Experimental results based on sputtered thin film samples are presented. The magnetism of Fe16N2 is discussed systematically from the aspects of material processing, magnetic characterization and theoretical investigation. It is observed that thin films with Fe16N2+Fe8N mixture phases and high degree of N ordering, exhibit a saturation magnetization up to 2.68T at room temperature, which substantially exceeds the ferromagnetism limit based on the traditional band magnetism understanding. From X-ray magnetic circular Dichorism (XMCD) experiment, transport measurement and first-principle calculation based on LDA+U method, it is both experimentally and theoretically justified that the origin of giant saturation magnetization is correlated with the formation of highly localized 3d electron states in this Fe-N system. A large magnetocrystalline anisotropy for such a material is also discussed. Our proposed “cluster+atom” theory provides promising directions on designing novel magnetic materials with unique performances.

Magnetic and optical properties of multifunctional core-shell radioluminescence nanoparticles

Abstract: When X-rays irradiate radioluminescence nanoparticles, they generate visible and near infrared light that can penetrate through centimeters of tissue. X-ray luminescence tomography (XLT) maps the location of these radioluminescent contrast agents at high resolution by scanning a narrow X-ray beam through the tissue sample and collecting the luminescence at every position. Adding magnetic functionality to these radioluminescent particles would enable them to be guided, oriented, and heated using external magnetic fields, while their location and spectrum could be imaged with XLT and complementary magnetic resonance imaging. In this work, multifunctional monodispersed magnetic radioluminescent nanoparticles were developed as potential drug delivery carriers and radioluminescence imaging agents. The particles consisted of a spindle-shaped magnetic γ-Fe2O3 core and a radioluminescent europium-doped gadolinium oxide shell. Particles with solid iron oxide cores displayed saturation magnetizations consistent with their ∼13% core volume, however, the iron oxide quenched their luminescence. In order to increase the luminescence, we partially etched the iron oxide core in oxalic acid while preserving the radioluminescent shell. The core size was controlled by the etching time which in turn affected the particles' luminescence and magnetic properties. Particles with intermediate core sizes displayed both strong magnetophoresis and luminescence properties. They also served as MRI contrast agents with relaxivities of up to 58 mM−1 s−1 (r2) and 120 mM−1 s−1 (r2*). These particles offer promising multimodal MRI/fluorescence/X-ray luminescence contrast agents. Our core–shell synthesis technique offers a flexible method to control particle size, shape, and composition for a wide range of biological applications of magnetic/luminescent nanoparticles.

Nanoparticle magnetization measurements by a high sensitive nano-superconducting quantum interference device

Abstract: A high sensitive nano superconducting quantum interference device (nanoSQUID) operating as a magnetic flux to critical current transducer with a suitable feedback circuit is employed to measure the magnetization of ferrimagnetic iron oxide nanoparticles An improved SQUID responsivity has been obtained by using a loop inductance asymmetry Iron oxide nanoparticles having a mean diameter of 8 nm have been excited by applying a polarizing field in the plane of the nanoSQUID loop The field dependence of the nanoparticle magnetization at T = 42 K shows magnetic hysteresis Magnetic relaxation measurements are reported and compared with those obtained by using a commercial measurement system

Temperature dependence of magnetic anisotropy constant in iron chalcogenide Fe(3)Se(4): Excellent agreement with theories.

Abstract: Magnetic hysteresis loops were measured for ferrimagnetic iron chalcogenide Fe3Se4 nanoparticles in the whole temperature range below the Curie temperature TC (315 K). The coercivity of the material is huge, reaching about 40 kOe at 10 K. The magnetic anisotropy constant K was determined from the magnetic hysteresis loop using the law of approach to saturation. The deduced anisotropy constant at 10 K is 5.22×106 erg/cm3, which is over one order of magnitude larger than that of Fe3O4. We also demonstrated that the experimental magnetic hysteresis loop is in good agreement with the theoretical curve calculated by Stoner and Wohlfarth for a noninteracting randomly oriented uniaxial single-domain particle system. Moreover, we show that K is proportional to the cube of the saturation magnetization Ms, which confirms earlier theoretical models for uniaxial magnets.

Absorption saturation in optically excited graphene

Abstract: We investigate the saturation of the optical absorption in graphene induced by ultrafast optical pulses. Within a microscopic theory, we study the momentum-, angle-, and time-resolved interplay of anisotropic excitation, carrier-carrier, and carrier-phonon scattering, and its influence on the saturation of absorption and transmission. In agreement with performed experiments, we observe a linear regime for the intensity-dependence of the transmission at low pump fluences and a nonlinear saturation in the high excitation regime. Applying 10 fs-pulses, we obtain a saturation fluence of approximately 0.65 mJ/cm2. We demonstrate how the interplay of Pauli-blocking and intensity-dependent relaxation determines the saturation behavior.

Physical, electrical and magnetic properties of nano-sized Co-Cr substituted magnesium ferrites

Abstract: Co-Cr substituted magnesium ferrite nanomaterials (Mg1−xCoxCrxFe2−xO4 with x = 0.0−0.5) have been prepared by the polyethylene glycol assisted micro emulsion method. X-ray diffraction analysis confirms the single-phase cubic close-packed lattice formation of synthesized materials. Hysteresis loops are measured up to field of 4 MA/m and high field region of these loops are modeled using the Law of Approach to saturation to calculate the magnetocrystalline anisotropy constant. The saturation magnetization of the samples increases initially from 148 kA/m for x = 0.0 to 299 kA/m (x = 0.3) and then decreases to 187 kA/m (x = 0.5). Curie temperature for this series is found to be in the range of 618-766 K. Room temperature resistivity increases gradually from 7.5 × 108 Ω cm (x = 0.0) to 3.47 × 109 Ω cm (x = 0.5). Additionally, dielectric measurements are carried out at room temperature in a frequency range of 100 Hz to 3 MHz. With improvement in the values of the above-mentioned properties, the synthesized materials could be suitable for potential application in some magnetic and microwave devices.

Doping effect of Y3+ ions on the microstructural and electromagnetic properties of Mn–Zn ferrites

Abstract: Mn–Zn ferrites doped with different contents of Y3+ ions were prepared by conventional two-step synthesis method. The microstructure and electromagnetic properties of the as-prepared Mn–Zn ferrites were investigated. It was found that all the samples consisted of ferrite phases of typical spinel cubic structure, and when Y3+ ion content was upto 1.5 mol%, yttriumirongarnet (Y3Fe5O12) phase with garnet structure was detected. With increasing doping content of Y3+ ions, the lattice constant and grain size increased, and after an increase to its maximum value, the sample apparent and relative densities dropped down. Through the analysis of magnetic properties, it was revealed that the saturation magnetization, and both the real and imaginary parts of permeability of the as-prepared samples raised with increasing doping content of Y3+ ions but decreased with more Y3+ ions, while their coercivity showed an opposite change trend; and the Curie temperature increased monotonously. The measurement of dielectric properties indicated that the dielectric constant of the doped Mn–Zn ferrites presented a rise with increasing Y3+ ion content, and dropped down gradually when more Y3+ ions were doped, while the dielectric loss tangent would decrease with Y3+ content upto 1.5 mol%, but after that, it increased.

Cascade of magnetic field induced spin transitions in LaCoO3.

Abstract: We present magnetization and magnetostriction studies of LaCoO3 in magnetic fields approaching 100 T. In contrast with expectations from single-ion models, the data reveal two distinct first-order transitions and well-defined magnetization plateaus. The magnetization at the higher plateau is only about half the saturation value expected for spin-1 Co3+ ions. These findings strongly suggest collective behavior induced by interactions between different electronic configurations of Co3+ ions. We propose a model that predicts crystalline spin textures and a cascade of four magnetic phase transitions at high fields, of which the first two account for the experimental data.

Structural and magnetic properties of Mg substituted NiCuZn Nano Ferrites

Abstract: The present paper examines the effect of magnesium substitution on structural and magnetic properties of NiCuZn nano ferrites synthesised by sol-gel method. The prepared samples were characterised by using X-ray Diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Field Emission Scanning Electron Microscopy (FE-SEM) and Vibration sample magnetometer (VSM) techniques. The phase identification, unit cell parameter and crystallite size was determined using XRD analysis. The lattice constant reduced with increasing Mg content. Room temperature saturation magnetisation and coercivity showed reverse trend with increasing Mg content. Curie temperature ( T c ) obtained from the thermo magnetic curves increases with Mg concentration. The initial permeability ( μ i ) decreased with increasing Mg content. This is due to reduced magnetisation, grain size and increased magneto-crystalline anisotropy constant. Simultaneously, there is an upward shift of domain wall relaxation frequency with increasing Mg content. Also the permeability is observed to be constant up to 30 MHz frequency range showing compositional stability and quality of the material. The prepared samples were suitable for applications in Multilayer Chip Inductors due to their invariable permeability up to 30 MHz frequency and high thermal stability along with low sintering temperature.

Ga1−xMnxN epitaxial films with high magnetization

Abstract: We report on the fabrication of pseudomorphic wurtzite Ga1−xMnxN grown on GaN with Mn concentrations up to 10% using molecular beam epitaxy. According to Rutherford backscattering, the Mn ions are mainly at the Ga-substitutional positions, and they are homogeneously distributed according to depth-resolved Auger-electron spectroscopy and secondary-ion mass-spectroscopy measurements. A random Mn distribution is indicated by transmission electron microscopy, and no Mn-rich clusters are present for optimized growth conditions. A linear increase of the c-lattice parameter with increasing Mn concentration is found using x-ray diffraction. The ferromagnetic behavior is confirmed by superconducting quantum-interference measurements showing saturation magnetizations of up to 150 emu/cm3.

Mn2PtIn: A tetragonal Heusler compound with exchange bias behavior

Abstract: The structural and magnetic properties of a tetragonal Heusler compound Mn2PtIn are reported Low temperature (5 K) magnetic measurements of the material show a hard magnetic behavior with relatively low saturation magnetization of 16 μB/fu, suggesting ferrimagnetic ordering in the system Moreover, observance of shifted field-cooled hysteresis loops indicates the presence of unidirectional exchange anisotropy resulting from nano-scale interaction between ferromagnetic/antiferromagnetic clusters An exchange bias (EB) field of 16 mT is measured at 5 K, with the value steadily decreasing with increasing temperature to zero at temperatures around 200 K The EB behavior likely originates from the glassy nature of the low temperature magnetic state, as indicated by ac susceptibility and zero-field-cooled relaxation measurements