Showing papers on "Saturation (magnetic) published in 2011"

Finite size and surface effects on the magnetic properties of cobalt ferrite nanoparticles

Abstract: Cobalt ferrite, CoFe2O4, nanoparticles in the size range 2–15 nm have been prepared using a non-aqueous solvothermal method. The magnetic studies indicate a superparamagnetic behavior, showing an increase in the blocking temperatures (ranging from 215 to more than 340 K) with the particle size, D TEM. Fitting M versus H isotherms to the saturation approach law, the anisotropy constant, K, and the saturation magnetization, M S, are obtained. For all the samples, it is observed that decreasing the temperature gives rise to an increase in both magnetic properties. These increases are enhanced at low temperatures (below ~160 K) and they are related to surface effects (disordered magnetic moments at the surface). The fit of the saturation magnetization to the T 2 law gives larger values of the Bloch constant than expected for the bulk, increasing with decreasing the particle size (larger specific surface area). The saturation magnetization shows a linear dependence with the reciprocal particle size, 1/D TEM, and a thickness of 3.7 to 5.1 A was obtained for the non-magnetic or disordered layer at the surface using the dead layer theory. The hysteresis loops show a complex behavior at low temperatures (T ≤ 160 K), observing a large hysteresis at magnetic fields H > ~1000 Oe compared to smaller ones (H ≤ ~1000 Oe). From the temperature dependence of the ac magnetic susceptibility, it can be concluded that the nanoparticles are in magnetic interaction with large values of the interaction parameter T 0, as deduced by assuming a Vogel–Fulcher dependence of the superparamagnetic relaxation time. Another evidence of the presence of magnetic interactions is the almost nearly constant value below certain temperatures, lower than the blocking temperature T b, observed in the FC magnetization curves.

Channel Length Scaling in Graphene Field-Effect Transistors Studied with Pulsed Current—Voltage Measurements

Abstract: We investigate current saturation at short channel lengths in graphene field-effect transistors (GFETs). Saturation is necessary to achieve low-output conductance required for device power gain. Dual-channel pulsed current−voltage measurements are performed to eliminate the significant effects of trapped charge in the gate dielectric, a problem common to all oxide-based dielectric films on graphene. With pulsed measurements, graphene transistors with channel lengths as small as 130 nm achieve output conductance as low as 0.3 mS/μm in saturation. The transconductance of the devices is independent of channel length, consistent with a velocity saturation model of high-field transport. Saturation velocities have a density dependence consistent with diffusive transport limited by optical phonon emission.

Synthesis of cubic SrCoO3 single crystal and its anisotropic magnetic and transport properties.

Abstract: A large-size single crystal of nearly stoichiometric SrCoO(3) was prepared with a two-step method combining the floating-zone technique and subsequent high oxygen pressure treatment. SrCoO(3) crystallizes in a cubic perovskite structure with space group Pm3m, and displays an itinerant ferromagnetic behavior with the Curie temperature of 305 K. The easy magnetization axis is found to be along the [111] direction, and the saturation moment is 2.5 µ(B)/f.u., in accord with the picture of the intermediate spin state. The resistivity at low temperatures (T) is proportional to T(2), indicative of the possible effect of orbital fluctuation in the intermediate spin ferromagnetic metallic state. Unusual anisotropic magnetoresistance is also observed and its possible origin is discussed.

Magnetic nanoparticle heating efficiency reveals magneto-structural differences when characterized with wide ranging and high amplitude alternating magnetic fields

Abstract: Magnetic nanoparticles can create heat that can be exploited to treat cancer when they are exposed to alternating magnetic fields (AMF). At a fixed frequency, the particle heating efficiency or specific power loss (SPL) depends upon the magnitude of the AMF. We characterized the amplitude-dependent SPL of three commercial dextran-iron oxide nanoparticle suspensions through saturation to 94 kA/m with a calorimeter comprising a solenoid coil that generates a uniform field to 100 kA/m at ∼150 kHz. We also describe a novel method to empirically determine the appropriate range of the heating curve from which the SPL is then calculated. These results agree with SPL values calculated from the phenomenological Box-Lucas equation. We note that the amplitude-dependent SPL among the samples was markedly different, indicating significant magneto-structural variation not anticipated by current models.

Method and system for providing high magnetic flux saturation CoFe films

Abstract: A method and system plates CoFeX, where X is an insertion metal. The method and system include providing a plating solution including hydroxymethyl-p-tolylsulfone (HPT). The plating solution being configured to provide a CoFeX film having a high saturation magnetic flux density of greater than 2.3 Tesla and not more than 3 weight percent of X. The method and system also include plating the CoFeX film on a substrate in the plating solution. In some aspects, the plated CoFeX film may be used in structures such as main poles of a magnetic recording head.

Simple synthesis of Pd–Fe3O4 heterodimer nanocrystals and their application as a magnetically recyclable catalyst for Suzuki cross-coupling reactions

Abstract: A simple gram-scale synthesis of Pd–Fe3O4 heterodimer nanocrystals was achieved by controlled one-pot thermolysis of a mixture solution composed of iron acetylacetonate, palladium acetylacetonate, oleylamine, and oleic acid. The heterodimer nanocrystals are composed of a 6 nm-sized Pd nanosphere and a 30 nm-sized faceted Fe3O4 nanocrystal and they are soft ferrimagnetic with high saturation magnetization value and low coercivity value. The heterodimer nanocrystals exhibited good activities for various Suzuki coupling reactions. Furthermore, the nanocrystal catalyst could be easily separated from the product mixture by using a magnet and could be recycled 10 times without losing catalytic activity.

Effect of oxygen defects on ferromagnetic of undoped ZnO

Abstract: Undoped ZnO nano-particles were synthesized by a solution route and annealed in N2, O2, and Ar, respectively. X-ray diffraction and X-ray photoelectron spectroscopy measurements show that the samples possess typical wurtzite structure and have no other impurity phases. Magnetization loops for the ZnO samples were measured and clearly show typical ferromagnetic saturation behavior. With the combination of defect analysis based on photoluminescence spectroscopy and first-principles calculations of the possible magnetic defect centers in ZnO, the effect of defects on the nature and origin of ferromagnetism was investigated. The results suggest oxygen defects, especially singly ionized oxygen vacancies, play a crucial role in mediating ferromagnetism in the undoped ZnO system.

Large change in perpendicular magnetic anisotropy induced by an electric field in FePd ultrathin films

Abstract: The magnetic properties of FePd ultrathin films and their variation under the influence of an electric field are investigated by magneto-optical Kerr effect (MOKE) measurements. L10-ordered FePd shows a spin reorientation transition when varying the thickness. The easy axis of magnetization is found to be normal to the plane at thicknesses above 9 monolayers (MLs) and in-plane below 9 ML. The coercive field, the perpendicular magnetic anisotropy and the MOKE signal at saturation vary with the applied electric field. The sensitivity of the interface magnetic anisotropy is estimated to be 602 fJ/V m.

Evidence of Vacancy-Induced Room Temperature Ferromagnetism in Amorphous and Crystalline Al2O3 Nanoparticles

Abstract: Amorphous and crystalline Al2O3 nanoparticles were synthesized by a sol–gel method with postannealing at different temperatures. Magnetism measurements have indicated that all Al2O3 nanoparticles exhibit intrinsic room temperature ferromagnetism, and the saturation magnetism of the samples increases after vacuum annealing, whereas bulk Al2O3 presents paramagnetism. Electron spin resonance and fitting results of O 1s X-ray photoelectron spectroscopy reveal that the origin of the ferromagnetism in Al2O3 nanoparticles could be attributed to the singly charged oxygen vacancies (F+ centers). The variation of the relative area of oxygen vacancies and the number of free electrons is consistent with the change of saturation magnetization for the samples. Combined with these results, a direct correlation of ferromagnetism with F+ centers exchange mechanism is established.

Amide proton transfer imaging with improved robustness to magnetic field inhomogeneity and magnetization transfer asymmetry using saturation with frequency alternating RF irradiation

Abstract: Amide proton transfer (APT) imaging has shown promise as an indicator of tissue pH and as a marker for brain tumors Sources of error in APT measurements include direct water saturation, and magnetization transfer (MT) from membranes and macromolecules These are typically suppressed by postprocessing asymmetry analysis However, this approach is strongly dependent on B(0) homogeneity and can introduce additional errors due to intrinsic MT asymmetry, aliphatic proton features opposite the amide peak and radiation damping-induced asymmetry Although several methods exist to correct for B(0) inhomogeneity, they tremendously increase scan times and do not address errors induced by asymmetry of the z-spectrum In this article, a novel saturation scheme-saturation with frequency alternating RF irradiation (SAFARI)-is proposed in combination with a new magnetization transfer ratio (MTR) parameter designed to generate APT images insensitive to direct water saturation and MT, even in the presence of B(0) inhomogeneity The feasibility of the SAFARI technique is demonstrated in phantoms and in the human brain Experimental results show that SAFARI successfully removes direct water saturation and MT contamination from APT images It is insensitive to B(0) offsets up to 180 Hz without using additional B(0) correction, thereby dramatically reducing scanning time

Fast magnetization precession observed in L10-FePt epitaxial thin film

Abstract: Fast magnetization precession is observed in L10-FePt alloy epitaxial thin films excited and detected by all-optical means. The precession frequency varies from 45 to 65 GHz depending on the applied magnetic field strength and direction, which can be explained by a uniform precession model taking account of first- and second-order uniaxial magnetic anisotropy. The lowest effective Gilbert damping constant has a minimum value of 0.055, which is about half that in Co/Pt multilayers and is comparable to Ni/Co multilayers with perpendicular magnetic anisotropy.

Annealing effects on the magnetic dead layer and saturation magnetization in unit structures relevant to a synthetic ferrimagnetic free structure

Abstract: The changes in the magnetic dead layer (MDL) and saturation magnetization of the CoFeB layers are investigated as a function of the annealing temperature for four different unit structures, that are relevant to the synthetic ferrimagnetic free structure in MgO-based magnetic tunnel junctions. The MDL results for these unit structures are then converted into those for the constituent interfaces of the free structure. Most of the changes in the MDL thickness occur during annealing at a low temperature of 150 °C while those in the saturation magnetization occur at a high annealing temperature of 350 °C. These results for the MDL and saturation magnetization are critically tested by using the synthetic ferrimagnetic free structures with various thickness asymmetries. The observed switching properties of these tested structures are in good agreement with those expected from the results for the MDL and saturation magnetization, confirming the accuracy of the present results. The accuracy of the saturation magne...

In situ synthesis of graphene/cobalt nanocomposites and their magnetic properties

Abstract: Graphene, which possesses unique nanostructure and excellent properties, is considered as a low cost alternative to carbon nanotubes in nanocomposites. In this study, we present a simple in situ approach for the deposition of cobalt (Co) nanoparticles onto surfaces of graphene sheets by hydrazine hydrate reduction. The as-synthesized composites were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, Raman spectroscopy, transmission electron microscopy (TEM) and thermogravimetry and differential scanning calorimetry. It was shown that the as-formed Co nanoparticles were densely and homogeneously deposited on the surfaces of the graphene sheets and as a result, the restacking of the as-reduced graphene sheets was effectively inhibited. Magnetic studies reveal that the graphene/Co nanocomposite displays ferromagnetic behavior with saturation magnetizations of 53.4 emu g−1, remanent magnetization of 6.0 emu g−1 and coercivity of 226 Oe at room temperature, which make it promising for practical applications in future nanotechnology.

Unique Magnetic Properties of Single Crystal γ-Fe2O3 Nanowires Synthesized by Flame Vapor Deposition

Abstract: Single crystal γ-Fe2O3 nanowires with 40–60 nm diameters were grown for the first time by single-step atmospheric flame vapor deposition (FVD) with axial growth rates up to 5 μm/minute. Because of their superior crystallinity, these FVD γ-Fe2O3 nanowires are single magnetic domains with room temperature coercivities of 200 Oe and saturation magnetizations of 68 emu/g.

Controlling the electronic structure of Co 1 − x Fe 2 + x O 4 thin films through iron doping

Abstract: The electronic, magnetic and transport properties of iron-doped cobalt ferrite (Co{sub 1-x}Fe{sub 2+x}O{sub 4}) thin films grown epitaxially on MgO (001) substrates are investigated by soft x-ray absorption and photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, superconducting quantum interference device magnetometry, and resistivity measurements. The crystal structure for Co{sub 1-x}Fe{sub 2+x}O{sub 4} is determined to be nearly inverse spinel, with the degree of inversion increasing for increased doping until it becomes fully inverse spinel for Fe{sub 3}O{sub 4}. The doped iron cations have a valency of 2+ and reside solely on octahedral sites, which allows for conduction owing to hopping between Fe{sup 2+} and Fe{sup 3+} octahedral cations. The addition of Fe{sup 2+} cations increases the electron density of states near the Fermi energy, shifting the Fermi level from 0.75 to 0 eV with respect to the top of the valence band, as the doping increases from x = 0.01 to 1. This change in electronic structure results in a change in resistivity by over two orders of magnitude. In contrast, the magnetic properties of CoFe{sub 2}O{sub 4} thin films, characterized by a significantly reduced saturation magnetization compared to the bulk and large magnetic anisotropies, are affected less significantly by doping inmore » the range from 0 to 0.63. These results show that Co{sub 1-x}Fe{sub 2+x}O{sub 4} has tunable electronic properties while maintaining magnetic properties similar to CoFe{sub 2}O{sub 4}.« less

New high magnetic field phase of the frustrated S = 1/2 chain compound LiCuVO4

Abstract: Magnetization of the frustrated S = 1/2 chain compound LiCuVO4, focusing on high magnetic field phases, is reported. Besides a spin-flop transition and the transition from a planar spiral to a spin modulated structure observed recently, an additional transition was observed just below the saturation field. This newly observed magnetic phase is considered as a spin nematic phase, which was predicted theoretically but was not observed experimentally. The critical fields of this phase and its dM/dH curve are in good agreement with calculations performed in a microscopic model (M.E. Zhitomirsky and H. Tsunetsugu, Europhys. Lett. 92, 37001 (2010)).

Magnetic moment and chemical order in off-stoichiometric Ni–Mn–Ga ferromagnetic shape memory alloys

Abstract: Recent studies have shown that the total magnetic moment in off-stoichiometric Ni–Mn–Ga alloys depends not only on electronic concentration but also on the degree of chemical order in the alloy. We have performed neutron diffraction experiments and magnetization measurements for determining the preferential atomic order and saturation moment in off-stoichiometric compounds (44–52 at.% Ni), having excess Mn and deficient in Ga. These alloys include isoelectronic alloys with different magnetic moments and were chosen in an effort to study the impact of chemical order on the magnetic moment distribution. In this work, we present an improved model of magnetic interaction between Mn atoms, which carry most of the localized magnetic moment of the alloys. The Mn atoms at Ga sites, which are nearest neighbors to properly sited Mn, couple antiferromagnetically to the dominant moment. In contrast, Mn atoms at Ga sites, which are nearest neighbors to Mn at Ni sites, couple ferromagnetically. Mn at Ni sites is always antiferromagnetic (AF). The new model is supported by the exchange variation with the Mn–Mn distance and demonstrates excellent agreement between experimental and calculated magnetic moments. The proposed model is shown to better explain the observed experimental results as compared to the rigid band model and previous localized moment models that assumed AF coupling for all off-site Mn atoms.

Probing the Chemical Stability of Mixed Ferrites: Implications for Magnetic Resonance Contrast Agent Design

Abstract: Nanomaterials with mixed composition, in particular magnetic spinel ferrites, are emerging as efficient contrast agents for magnetic resonance imaging (MRI). Many factors, including size, composition, atomic structure, and surface properties are crucial in the design of such nanoparticle-based probes due to their influence on the magnetic properties. Silica-coated iron oxide (IO-SiO(2)) and cobalt ferrite (CoIO-SiO(2)) nanoparticles were synthesized using standard high temperature thermal decomposition and base-catalyzed water-in-oil microemulsion techniques. Under neutral aqueous conditions, it was found that 50-75% of the cobalt content in the CoIO-SiO(2) nanoparticles leached out of the core structure. Leaching caused a 7.2-fold increase in longitudinal relaxivity and an increase in the saturation magnetization from ~48 emu/g core to ~65 emu/g core. X-ray absorption fine structure studies confirmed that the atomic structure of the ferrite core was altered following leaching, while TEM and DLS confirmed that the morphology and size of the nanoparticle remained unchanged. The CoIO-SiO(2) nanoparticles converted from a partially inverted spinel cation arrangement (unleached state) to an inverse spinel arrangement (leached state). The control IO-SiO(2) nanoparticles remained stable with no change in structure and negligible changes in magnetic behavior. This detailed analysis highlights how important understanding the properties of nanomaterials is in the development of reliable agents for diagnostic and therapeutic applications.

Redistribution of cations and enhancement in magnetic properties of sol–gel synthesized Cu0.7−xCoxZn0.3Fe2O4 (0 ≤ x ≤ 0.5)

Abstract: Cu0.7− x Co x Zn0.3Fe2O4 (0 ≤ x ≤ 0.5) nanoparticles are prepared by sol–gel auto combustion method, using copper nitrate, zinc nitrate, ferric nitrate, cobalt nitrate, and citric acid as the starting materials. The process takes only a few minutes to obtain as-received Co-substituted Cu–Zn ferrite powders. X-ray diffraction (XRD), vibrational sample magnetometer and thermo gravimetric analysis are utilized in order to study the effect of variation in the Co substitution and its impact on particle size, lattice constant, density, cation distribution and magnetic properties like magnetization, coercivity, remanent magnetization, ferritization temperature and associated water content. Lattice parameter found to increase with increasing Co content, whereas X-ray density, bulk density, particle size showed decreasing trend with the Co content. Cation distribution indicates that the Co and Cu ion show preference towards octahedral [B] site, Zn occupy tetrahedral (A) site whereas Fe occupy both tetrahedral (A) and octahedral [B] site. Redistribution of cations takes place for x > 0.3. Saturation magnetization (Ms) increases from 52.99 to 79.62 emu/g (x ≤ 0.3), for x > 0.3 Ms decreases with increase in Co content x. However, coercivity, magnetocrystalline anisotropy and remanent magnetization increases with the Co2+ substitution.

Magnetic bacterial protein Mms6 controls morphology, crystallinity and magnetism of cobalt-doped magnetite nanoparticles in vitro

Abstract: Magnetic nanoparticles (MNPs) are in high demand within biomedical and nanotechnological industries. Size, shape, material and crystal quality directly affect the particle's properties, namely their magnetic characteristics, and must be tuned and controlled to meet the specification of the application. A key challenge is to refine synthetic methods to tailor the MNP properties with precision, but using cheap, high-yield, industrially robust and environmentally friendly methods. In this study we compare simple high-yield precipitation methods of producing cobalt-doped magnetite MNPs. We explore the variation of magnetic coercivity and saturation with increasing Co-doping from 0–15% in magnetite MNPs, which increases coercivity from 5–62 mT, but decreases saturation from 91–28 emu g−1. An optimum of 6% was further investigated as this produced the greatest increase in coercivity to 34 mT with a relatively small reduction in saturation magnetisation to 79 emu g−1. The methods compared are refined with the addition of the recombinant biomineralisation protein Mms6 from a magnetic bacterium, as this has been shown to help control magnetite MNP morphology and grainsize distribution in vitro. Similar control is seen here over our Co-doped magnetite synthesis. Mms6 increases the size and decreases the size distribution of room temperature co-precipitated particles from 11.7 nm to 31.7 nm. The affinity tagged protein his6Mms6 also controls the size (23.2 nm) but less effectively than Mms6. Therefore the Mms6 mediated Co-doped MNP particles are found to be single domain and thus give very clear, square magnetic hysteresis with a coercivity of 48 mT at 10 K. Hysteresis of the smaller particles (Co-doped MNP with no protein and with his-tagged protein) clearly shows both superparamagnetic and single-domain magnetic behaviours. Powder X-ray diffraction shows that both the addition of Mms6 and cobalt increases the crystal quality of the MNP. Thus Mms6 protein mediated room temperature co-precipitation offers an environmentally friendly, industrially robust route towards tailored, uniform, single-domain, high-quality Co-doped magnetite MNPs.

Improved magnetoelectric coupling in Mn and Zn doped CoFe2O4–PbZr0.52Ti0.48O3 particulate composite

Abstract: In this letter, Mn and Zn simultaneously substituted CoFe2O4, Co0.6Zn0.4Fe1.7Mn0.3O4 has been proposed as magnetostrictive component having the maximum in plane piezomagnetic coupling coefficient (q11+q12∼0.070 ppm/Oe) at a field of H∼300 Oe with appreciable saturation values of longitudinal and transverse magnetostriction coefficients (λ11∼−20 ppm and λ12∼10 ppm). On incorporating this in composite (i.e., (x) PbZr0.52Ti0.48O3−(1−x) Co0.6Zn0.4Fe1.7Mn0.3O4) preparation, maximum magnetoelectric voltage coefficients (αE∼122 mV/cm Oe) was obtained in sample x=0.90 at ac magnetic field of amplitude ∼1 Oe and frequency ∼1 kHz.

N site ordering effect on partially ordered Fe16N2

Abstract: Partially ordered Fe16N2 thin films have been fabricated on Fe (001)-buffered GaAs (001) single-crystal substrates by a facing target sputtering process. The saturation magnetization has been systematically investigated as a function of N site ordering in partially ordered Fe16N2 thin films, which is found to be increased monotonically with the increase in the N site ordering parameter, reaching up to 2.68 T at high ordering case. A model discussion is provided based on the partial localization of 3d electron states in this material system, which successfully rationalizes the formation of the giant saturation magnetization in chemically ordered Fe16N2. We further demonstrate that the average magnetic moment of partially ordered Fe16N2 sensitively depends on the special arrangement of Fe6N clusters, which is the key to realize high magnetic moment in this material system.

Coexistence of ferromagnetism and superconductivity: magnetization and Mössbauer studies of EuFe2(As1 − xPx)2

Abstract: Magnetization and 57Fe and 151Eu Mossbauer studies of EuFe2(As1 − xPx)2 (x = 0–10) at temperatures (5–300 K) have been performed The magnetization studies show a decrease of the divalent Eu sublattice antiferromagnetic transition temperature from TAFM = 20 K for x = 0 to 16 K at x≈02 For x > 02, the Eu sublattice is ferromagnetically ordered at TFM, which increases up to 27 K for x = 10 For 02 02, yet at 5 K exhibit transferred magnetic hyperfine fields (~1 T) from the ferromagnetically ordered Eu sublattice, even in the superconducting region Superconductivity in the presence of ferromagnetism is generally not observable However, transferred magnetic hyperfine fields in the superconducting state are observed here for the first time