Influence of strain and polycrystalline ordering on magnetic properties of high moment rare earth metals and alloys

Despite being the most suitable candidates for solenoid pole pieces in state-of-the-art superconductor- based electromagnets, the intrinsic magnetic properties of heavy rare earth metals and their alloys have gained comparatively little attention. With the potential of integration in micro- and nanoscale devices, thin films of Gd, Dy, Tb, DyGd and DyTb were plasma-sputtered and investigated for their in-plane magnetic properties, with an emphasis on magnetisation vs. temperature profiles. Based on crystal structure analysis of the polycrystalline rare earth films, which consist of a low magnetic moment FCC layer at the seed interface topped with a higher moment HCP layer, an experimental protocol is introduced which allows the direct magnetic analysis of the individual layers. In line with the general trend of heavy lanthanides, the saturation magnetisation was found to drop with increasing unit cell size. In-situ annealed rare earth films exceeded the saturation magnetisation of a high-moment Fe65Co35 reference film in the cryogenic temperature regime, proving their potential for pole piece applications; however as-deposited rare earth films were found completely unsuitable. In agreement with theoretical predictions, sufficiently strained crystal phases of Tb and Dy did not exhibit an incommensurate magnetic order, unlike their single-crystal counterparts which have a helical phase. DyGd and DyTb alloys followed the trends of the elemental rare earth metals in terms of crystal structure and magnetic properties. Inter-rare-earth alloys hence present a desirable blend of saturation magnetisation and operating temperature.

Supramolecular control of the magnetic anisotropy in two-dimensional high-spin Fe arrays at a metal interface

Magnetic atoms at surfaces are a rich model system for solid-state magnetic bits exhibiting either classical(1,2) or quantum(3,4) behaviour. Individual atoms, however, are difficult to arrange in regular patterns(1-5). Moreover, their magnetic properties are dominated by interaction with the substrate, which, as in the case of Kondo systems, often leads to a decrease or quench of their local magnetic moment(6,7). Here, we show that the supramolecular assembly of Fe and 1,4-benzenedicarboxylic acid molecules on a Cu surface results in ordered arrays of high-spin mononuclear Fe centres on a 1.5nm square grid. Lateral coordination with the molecular ligands yields unsaturated yet stable coordination bonds, which enable chemical modification of the electronic and magnetic properties of the Fe atoms independently from the substrate. The easy magnetization direction of the Fe centres can be switched by oxygen adsorption, thus opening a way to control the magnetic anisotropy in supramolecular layers akin to that used in metallic thin films.

Gluon-field contribution in QCD sum rules for the magnetic moments of the nucleons

The Wilson coefficient corresponding to the gluon-field strength GμνGμν is evaluated for the nucleon current correlation function in the presence of a static external electromagnetic field, using a regulator mass Λ to separate the high-momentum part of the Feynman diagrams. The magnetic-moment sum rules are analyzed by two different methods and the sensitivity of the results to variations in Λ are discussed.

Determination of baryon magnetic moments from QCD sum rules

The magnetic moment μB of a baryon B with quark content (aab) is written as μB=4ea(1+δB)eħ/2cMB, where ea is the charge of the quark of flavor type a. The experimental values of δB have a simple pattern and have a natural explanation within QCD. Using the ratio method, the QCD sum rules are analyzed and the values of δB are computed. We find good agreement with data (≊10%) for the nucleons and the Σ multiplet while for the cascade the agreement is not as good. In our analysis we have incorporated additional terms in the operator-product expansion as compared to previous authors. We also clarify some points of disagreement between the previous authors. External-field-induced correlations describing the magnetic properties of the vacuum are estimated from the baryon magnetic-moment sum rules themselves as well as by independent spectral representations and the results are contrasted.

Quasi-2D XY Magnetic Properties and Slow Relaxation in a Body Centered Metal Organic Network of [Co-4] Clusters

Octahedral Co2+ centers have been connected by mu(3)-OH and mu(2)-OH2 units forming [Co-4] clusters which are linked by pyrazine forming a two-dimensional network. The two-dimensional layers are bridged by oxybisbenzoate (OBA) ligands giving rise to a three-dimensional structure. The [Co-4] clusters bond with the pyrazine and the OBA results in a body-centered arrangement of the clusters, which has been observed for the first time. Magnetic studies reveal a noncollinear frustrated spin structure of the bitriangular cluster, resulting in a net magnetic moment of 1.4 mu B per cluster. For T > 32 K, the correlation length of the cluster moments shows a stretched-exponential temperature dependence typical of a Berezinskii-Kosterlitz-Thouless model, which points to a quasi-2D XY behavior. At lower temperature and down to 14 K, the compound behaves as a soft ferromagnet and a slow relaxation is observed, with an energy barrier of ca. 500 K. Then, on further cooling, a hysteretic behavior takes place with a coercive field that reaches 5 Tat 4 K. The slow relaxation is assigned to the creation/annihilation of vortex-antivortex pairs, which are the elementary excitations of a 2D XY spin system.

Magnetic Properties of Iron Particles Embedded in Multiwall Carbon Nanotubes

Iron nanoparticles are embedded in multiwall carbon nanotubes by the chemical vapor deposition, where benzene and ferrocene are taken as precursor materials. Varying quantity of iron particles are embedded in these tubes by taking different amount of ferrocene. These particles exhibit a magnetic moment up to 98 emu/g and an enhanced coercivity in the range of 500-2000 Oe. Negative magnetoresistance similar to 10% is observed in the presence of magnetic field up to 11 T applied at various temperatures in the range of 1.3 K-300 K. It is argued that the enhanced coercivity is due to the shape anisotropy. The negative magnetoresistance is believed to be due to the weak localization and spin dependent scattering of electrons by the ferromagnetic particles. In addition we also observe a dependence of the magnetoresistance on the direction of applied field and this is correlated with the shape anisotropy of the Fe particles.

Platinum dendritic nanoparticles with magnetic behavior

Magnetic nanoparticles have attracted increasing attention for biomedical applications in magnetic resonance imaging, high frequency magnetic field hyperthermia therapies, and magnetic-field-gradient-targeted drug delivery. In this study, three-dimensional (3D) platinum nanostructures with large surface area that features magnetic behavior have been demonstrated. The well-developed 3D nanodendrites consist of plentiful interconnected nano-arms ∼4 nm in size. The magnetic behavior of the 3D dendritic Pt nanoparticles is contributed by the localization of surface electrons due to strongly bonded oxygen/Pluronic F127 and the local magnetic moment induced by oxygen vacancies on the neighboring Pt and O atoms. The magnetization of the nanoparticles exhibits a mixed paramagnetic and ferromagnetic state, originating from the core and surface, respectively. The 3D nanodendrite structure is suitable for surface modification and high amounts of drug loading if the transition temperature was enhanced to room temperature properly.

Effect of Al substitution on the magnetic properties of amorphous Fe73.5Cu1Mo3Si13.5-xAlxB9 alloy

We have synthesized FINEMET type amorphous Fe73.5Cu1Mo3Si13.5-xAlxB9 alloy by the single wheel melt spinning technique. The effect of Al substitution on the magnetic properties has been studied using a vibrating sample magnetometer, SQUID and Mossbauer spectroscopy. Magnetization and Curie temperature of the amorphous phase of the alloys were found to decrease with A] concentration. The results are attributed to the dilution effect of At on the magnetic moment of Fe and to the increase in Fe-Fe interaction distance resulting in the weakening of exchange interaction. (C) 2007 Elsevier B.V. All rights reserved.

Crystal growth, transport and magnetic properties of rare-earth manganite Pr1-xPbxMnO3

Crystal growth, electrical and magnetic properties are reported for mixed valence manganite Pr1-xPbxMnO3 (x = 0.2, 0.23, and 0.3). The crystals with x = 0.2 and 0.23 are ferromagnetic and insulating, whereas that with x = 0.3 is ferromagnetic below 200 K and shows an insulator-metal transition at 235 K. This composition shows a magnetoresistance of 90% in a field of 5 T. In the paramagnetic region, the temperature dependence of magnetic susceptibility of the crystals follows a Curie-Weiss behavior. The thermal evolution of magnetization in the ferromagnetic phase varies as T-3/2, in accordance with Bloch's law. The spin-stiffness constant D obtained from the Bloch constant is found to increase linearly with x. The magnetization does not reach complete saturation upto a field of 5 T. A possible contribution of the Pr spins to the total magnetic moment is discussed.

Studies on high‐density nickel zinc ferrite and its magnetic properties using novel hydrazine precursors

Nickel zinc ferrites have been very widely used in the high‐frequency applications. In our present study we have prepared Ni1−x Znx Fe2O4 (0≤x≤1) using novel hydrazinium metal hydrazinecarboxylate precursors. High densities (∼99%) have been obtained for all the ferrites sintered at relatively low temperatures, 1100 °C, in comparison with the conventional method (≥1200 °C). The variation of magnetic properties like magnetic moment, Curie temperature, and permeability with zinc concentration have been studied.

Magnetic ordering in La0.85Pb0.15Mn1-xTixO3 (0 <= x <= 0.15)

Powder-neutron diffraction study has been carried out at 300 and 10 K in La0.85Pb0.15Mn1-xTixO3 (0 less than or equal to x less than or equal to 0.15). The samples crystallize in the rhombohedral phase. The magnetic moment reduces nonlinearly with increase in Ti and correlates well with the reported behavior of T-C. The change in the moment and T-C could not be related to change in the one electron bandwidth, W. The reduction is attributed to the effect of dilution and thereby reducing the double exchange ferromagnetic interaction. (C) 2002 Elsevier Science B.V. All rights reserved.

Magnetic and dielectric interactions in nano zinc ferrite powder: Prepared by self-sustainable propellant chemistry technique

The structural, magnetic and dielectric properties of nano zinc ferrite prepared by the propellant chemistry technique are studied. The PXRD measurement at room temperature reveal that the compound is in cubic spinel phase, belong to the space group Fd (3) over barm. The unit cell parameters have been estimated from Rietveld refinement. The calculated force constants from FTIR spectrum corresponding to octahedral and tetrahedral sites at 375 and 542 cm(-1) are 6.61 x 10(2) and 3.77 x 10(2) N m(-1) respectively; these values are slightly higher compared to the other ferrite systems. Magnetic hysteresis and EPR spectra show superparamagnetic property nearly to room temperature due to comparison values between magnetic anisotropy energy and the thermal energy. The calculated values of saturation magnetization, remenant magnetization, coercive field and magnetic moment supports for the existence of multi domain particles in the sample. The temperature dependent magnetic field shows the spin freezing state at 30 K and the blocking temperature at above room temperature. The frequency dependent dielectric interactions show the variation of dielectric constant, dielectric loss and impedance as similar to other ferrite systems. The AC conductivity in the prepared sample is due to the presence of electrons, holes and polarons. The synthesized material is suitable for nano-electronics and biomedical applications. (C) 2014 Elsevier B.V. All rights reserved.

Structural and magnetic properties of ultra-small scale eutectic CoFeZr alloys

Aiming to develop high mechanical strength and toughness by tuning ultrafine lamellar spacing of magnetic eutectic alloys, we report the mechanical and magnetic properties of the binary eutectic alloys Co90.5Zr9.5 and Fe90.2Zr9.8, as well as the pseudo-binary eutectic alloys Co82.4Fe8Zr9.6, Co78Fe12.4Zr9.6 and Co49.2Fe49.2Zr9.6 developed by suction-casting. The lower lamellar spacing around 100 nm of the eutectics Co49.2Fe49.2Zr9.6 yields a high hardness of 713(+/- 20) VHN. Magnetic measurements reveal high magnetic moment of 1.92 mu B (at 5 K) and 1.82 mu B (at 300 K) per formula unit for this composition. The magnetization vs. applied field data at 5 K show a directional preference to some extent and therefore smaller non-collinear magnetization behavior compared to Co11Zr2 reported in the literature due to exchange frustration and transverse spin freezing owing to the presence of smaller Zr content. The decay of magnetization as a function of temperature along the easy axis of magnetization of all the eutectic compositions can be described fairly well by the spin wave excitation equation Delta M/M(0) = BT3/2 + CT5/2. (C) 2014 Elsevier B.V. All rights reserved.

Structure and Magnetic Properties of Tb1-xSrxMnO3

Tb1-xSrxMnO3 (x = 0.1, 0.2, 0.3, 0.4 and 0.5) polycrystalline samples are prepared via conventional solid state synthesis route. All samples crystallize in orthorhombic Pnma space group and possess O'-type distortion. Orthorhombic and octahedral distortion is found to decrease with increase in Sr content. At intermediate distortion, (20% and 30% doping level) Curie-Weiss analysis of inverse dc magnetic susceptibility data yields +ve Curies-Weiss constant, characteristic of FM interaction. Isothermal magnetization measurements give the highest magnitude of magnetic moment at these compositions.

Electronic and Magnetic Properties of Yttrium-doped Silicon Carbide Nanotubes: Density Functional Theory Investigations

The electronic structure of yttrium-doped Silicon Carbide Nanotubes has been theoretically investigated using first principles density functional theory (DFT). Yttrium atom is bonded strongly on the surface of the nanotube with a binding energy of 2.37 eV and prefers to stay on the hollow site at a distance of around 2.25 angstrom from the tube. The semi-conducting nanotube with chirality (4, 4) becomes half mettalic with a magnetic moment of 1.0 mu(B) due to influence of Y atom on the surface. There is strong hybridization between d orbital of Y with p orbital of Si and C causing a charge transfer from d orbital of the Y atom to the tube. The Fermi level is shifted towards higher energy with finite Density of States for only upspin channel making the system half metallic and magnetic which may have application in spintronic devices.