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7 May 2013

Volume 113, Issue 17, Articles (17xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 113, 174302 (2013); http://dx.doi.org/10.1063/1.4798262 (4 pages)

Yuichiro Kurokawa, Takehiko Hihara, and Ikuo Ichinose
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back to top Magnetic Nanoparticles

Enhanced controllability of domain-wall pinning by selective domain-wall injection

Sung-Min Ahn

J. Appl. Phys. 113, 17B501 (2013); http://dx.doi.org/10.1063/1.4794135 (3 pages)

Online Publication Date: 4 March 2013

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It is experimentally reported that depinning fields of domain walls (DWs) under an interaction between magnetic charges distributed at a nanobar and at a notch can be enhanced by controlling injection fields for injecting DWs into the ferromagnetic nanowire with an asymmetrical nucleation pad. The DWs injected from the asymmetrical pad show an asymmetrical dependence of the injection field on the saturation angle and are pinned by the notch with the nanobar vertical to it. We have found that the shape of the pinning potential energy experienced by the DW is affected by the uniformly magnetized direction of the nanobar and the pinning potential energy is locally modified by selectively controlling the DW-injection. This is consistent with our estimation based on micromagnetic simulation.
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75.60.Ch Domain walls and domain structure
75.78.Cd Micromagnetic simulations
75.50.Bb Fe and its alloys
75.50.Dd Nonmetallic ferromagnetic materials

Nanoimprint-lithography patterned epitaxial Fe nanowire arrays with misaligned magnetocrystalline and shape anisotropies

Wei Zhang, Mark E. Bowden, and Kannan M. Krishnan

J. Appl. Phys. 113, 17B502 (2013); http://dx.doi.org/10.1063/1.4794358 (3 pages)

Online Publication Date: 5 March 2013

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We fabricated large area (>1 × 1 cm2), epitaxial Fe nanowire arrays on MgO(001) substrates by nanoimprint lithography with a direct metallization of epitaxial materials through a metallic mask, which avoided the disadvantageous metal-etching process in conventional methods. The magnetization reversals, as revealed by magneto-optic Kerr effect, showed competing effects between Fe cubic magnetocrystalline anisotropy and lithographically induced uniaxial shape anisotropy. Unlike the weakly induced uniaxial anisotropy observed in continuous films, both the magnitude and direction of the uniaxial shape anisotropy can be easily modulated in the nanowires. Complex magnetization reversal processes including two-step and three-step loops were observed when magnetizing the samples along different Fe cubic easy axes, respectively. These modified magnetization reversal processes were explained by the nucleation and propagation of the domain walls along the non-superimposed easy axes of the competing magnetocrystalline and shape anisotropies.
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81.16.Nd Micro- and nanolithography
81.16.Rf Micro- and nanoscale pattern formation
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)
81.07.Gf Nanowires
75.30.Gw Magnetic anisotropy
75.60.Jk Magnetization reversal mechanisms

Geometric and magnetic properties of Co adatom decorated nitrogen-doped graphene

Sangho Lee, Minho Lee, and Yong-Chae Chung

J. Appl. Phys. 113, 17B503 (2013); http://dx.doi.org/10.1063/1.4794359 (3 pages)

Online Publication Date: 11 March 2013

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The potential for nitrogen doped graphene with dispersed Co metal atoms as a candidate material for future spintronic applications was studied using first-principles calculations. Among the three types of defective structures, the pyridinic and pyrrolic systems were demonstrated to be structurally stable since the binding energy between the Co adatom and these substrate layers exceeded the cohesive energy of Co metal alone. Also, Co adatom on these two structures showed high magnetic moment values, which surpassed that of the Co metal bulk. From these results, it is shown that the pyridinic and pyrrolic N defects on graphene have a positive effect on the geometric stability and magnetic property of decorated Co atoms.
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75.30.Cr Saturation moments and magnetic susceptibilities
68.43.Bc Ab initio calculations of adsorbate structure and reactions
68.43.Mn Adsorption kinetics

Role of oxygen defects on the magnetic properties of ultra-small Sn1−xFexO2 nanoparticles

Kelsey Dodge, Jordan Chess, Josh Eixenberger, Gordon Alanko, Charles B. Hanna, and Alex Punnoose

J. Appl. Phys. 113, 17B504 (2013); http://dx.doi.org/10.1063/1.4794140 (3 pages)

Online Publication Date: 12 March 2013

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Although the role of oxygen defects in the magnetism of metal oxide semiconductors has been widely discussed, it is been difficult to directly measure the oxygen defect concentration of samples to verify this. This work demonstrates a direct correlation between the photocatalytic activity of Sn1−xFexO2 nanoparticles and their magnetic properties. For this, a series of ∼2.6 nm sized, well characterized, single-phase Sn1−xFexO2 crystallites with x = 0−0.20 were synthesized using tin acetate, urea, and appropriate amounts of iron acetate. X-ray photoelectron spectroscopy confirmed the concentration and 3+ oxidation state of the doped Fe ions. The maximum magnetic moment/Fe ion, μ, of 1.6 × 10−4 μB observed for the 0.1% Fe doped sample is smaller than the expected spin-only contribution from either high or low spin Fe3+ ions, and μ decreases with increasing Fe concentration. This behavior cannot be explained by the existing models of magnetic exchange. Photocatalytic studies of pure and Fe-doped SnO2 were used to understand the roles of doped Fe3+ ions and of the oxygen vacancies and defects. The photocatalytic rate constant k also showed an increase when SnO2 nanoparticles were doped with low concentrations of Fe3+, reaching a maximum at 0.1% Fe, followed by a rapid decrease of k for further increase in Fe%. Fe doping presumably increases the concentration of oxygen vacancies, and both Fe3+ ions and oxygen vacancies act as electron acceptors to reduce e−-h+ recombination and promote transfer of electrons (and/or holes) to the nanoparticle surface, where they participate in redox reactions. This electron transfer from the Fe3+ ions to local defect density of states at the nanoparticle surface could develop a magnetic moment at the surface states and leads to spontaneous ferromagnetic ordering of the surface shell under favorable conditions. However, at higher doping levels, the same Fe3+ ions might act as recombination centers causing a decrease of both k and magnetic moment μ.
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.75.Cd Fabrication of magnetic nanostructures
71.55.Ht Other nonmetals
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Pp Magnetic semiconductors

Nonylphenol polyethoxylate coated body-center-cubic iron nanocrystals for ferrofluids with technical applications

D. Ortega, N. Pérez, J. L. Vilas, J. S. Garitaonandia, K. Suzuki, J. R. Marín, and M. Rodríguez

J. Appl. Phys. 113, 17B505 (2013); http://dx.doi.org/10.1063/1.4794880 (3 pages)

Online Publication Date: 12 March 2013

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An approach to the design of a suitable system for technological applications, such as magneto-rheological fluids with controllable performance, showing high saturation magnetization and low coercivity and remanence, is presented. This approach is based on the synthesis of stable iron nanoparticles with a relatively thick polymeric coating—the non-ionic surfactant nonylphenol polyethoxylate—by a microemulsion method with NaBH4 as a reducing agent. X-ray diffractometry, Mössbauer spectroscopy, and high resolution electron microscopy reveal a body-center-cubic structure in the iron cores. The resulting nanoparticles are predominantly spherical, having an average core size of 7 nm and a constant shell thickness of 3 nm. Magnetic measurements reveal a higher saturation magnetization (127.4 Am2 kg−1 at 300 K and 153.2 Am2 kg−1 at 5 K) than in other approaches and a small coercive field of 12 mT. X-ray diffractometry results account for the presence of iron borate traces as a secondary phase, formed at the initial stages of the synthesis during the reduction process of precursors. Preliminary studies under oxidant conditions show a reduction in saturation magnetization lower than 9% over a six months period.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
76.80.+y Mössbauer effect; other γ-ray spectroscopy
81.16.-c Methods of micro- and nanofabrication and processing
81.65.-b Surface treatments
75.75.-c Magnetic properties of nanostructures
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Single-domain shape anisotropy in near-macroscopic Ni80Fe20 thin-film rectangles

Yi Li, Yiran Lu, and W. E. Bailey

J. Appl. Phys. 113, 17B506 (2013); http://dx.doi.org/10.1063/1.4794876 (3 pages)

Online Publication Date: 13 March 2013

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Shape anisotropy provides a simple mechanism to adjust the local bias field in patterned structures. It is well known that for ellipsoidal particles <1 μm in size, a quasi-single domain state can be realized with uniform anisotropy field. For larger patterned ferromagnetic thin-film elements, domain formation is thought to limit the effectiveness of shape anisotropy. In our work, we show that very soft lithographically patterned Ni80Fe20 films with control of induced magnetic anisotropy can exhibit shape anisotropy fields in agreement with single-domain models, for both hysteresis loop measurements at low field and ferromagnetic resonance measurements at high field. We show the superiority of the fluxmetric form over the magnetometric form of anisotropy estimate for thin films with control dimensions from 10 μm to 150 μm and in-plane aspect ratios above 10.
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75.30.Gw Magnetic anisotropy
75.50.Bb Fe and its alloys
75.50.Dd Nonmetallic ferromagnetic materials
75.70.Ak Magnetic properties of monolayers and thin films

Grain size effect on magnetic and electric properties of LuMnO3 nanocrystalline materials

Tai-Chun Han, Jia-Wern Chen, Yuan-Hsun Liu, and Yu-Min Hu

J. Appl. Phys. 113, 17B507 (2013); http://dx.doi.org/10.1063/1.4794977 (3 pages)

Online Publication Date: 13 March 2013

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Magnetic and electric properties are investigated for the nanosized LuMnO3 samples with different grain sizes (30 nm to 500 nm) synthesized by a modified Pechini method. It shows that magnetic and electric properties are strongly dependent on the grain size. The magnetic characterization indicates that with increasing grain size, the antiferromagnetic (AFM) transition temperature increases from 72 to 89 K. The temperature-dependent dielectric measurements show an anomaly in the dielectric constant at temperatures close to the AFM ordering temperature for all samples. A corresponding shift of the peak-positions of dielectric anomaly and magnetic ordering indicates a strong correlation between the magnetic ordering and the electric polarization. Further analysis suggests that the rising of AFM transition temperature with increasing grain size should be from the structural origin, in which the strength of AFM interaction as well as the electrical polarization is dependent on the in-plane lattice parameters.
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75.75.Cd Fabrication of magnetic nanostructures
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
77.22.Ej Polarization and depolarization
77.22.Ch Permittivity (dielectric function)
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
81.16.-c Methods of micro- and nanofabrication and processing

Synthesis and magnetic properties of core/shell FeO/Fe3O4 nano-octopods

Hafsa Khurshid, Sayan Chandra, Wanfeng Li, M. H. Phan, G. C. Hadjipanayis, P. Mukherjee, and H. Srikanth

J. Appl. Phys. 113, 17B508 (2013); http://dx.doi.org/10.1063/1.4794978 (3 pages)

Online Publication Date: 15 March 2013

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We report the synthesis and magnetic properties of core/shell FeO/Fe3O4 nanoparticles with an average size of 30 nm in a complex quasi-octopod shape. FeO nanoparticles were synthesized by a wet chemical synthesis route followed by partial oxidation to form core/shell structured FeO/Fe3O4 octopods. X-ray diffraction and transmission electron microscopy confirmed the presence of iron oxide phases and the formed core/shell FeO/Fe3O4 morphology. Magnetic measurements revealed two distinct temperatures corresponding to the thermally activated Verwey transition (TV ∼ 120 K) of the ferrimagnetic Fe3O4 shell and the Neel temperature (TN ∼ 230 K) of the antiferromagnetic FeO core. The nanoparticles exhibited a strong horizontal shift in the field-cooled hysteresis loop (the so-called exchange bias (EB) effect) accompanied by enhanced coercivity. The Meiklejohn-Bean model has been implemented to quantify the amount of frozen spins that locate at the interface between FeO and Fe3O4 and are responsible for the observed EB effect.
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75.75.Cd Fabrication of magnetic nanostructures
81.16.Pr Micro- and nano-oxidation
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Gg Ferrimagnetics

Structure and magnetism of dilute Co(Zr) nanoclusters

B. Das, B. Balamurugan, R. Skomski, X. Z. Li, P. Mukherjee, G. C. Hadjipanayis, and D. J. Sellmyer

J. Appl. Phys. 113, 17B509 (2013); http://dx.doi.org/10.1063/1.4795318 (3 pages)

Online Publication Date: 18 March 2013

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Co(Zr) nanoclusters having a small fraction of Zr (≤7.8 at. %) were produced using a cluster-deposition method and aligned using a magnetic field of about 5 kOe prior to deposition. This study shows that Zr addition to Co nanoclusters improves the fraction of hexagonal close-packed structure, magnetic anisotropy, and easy-axis alignment process. Co(Zr) nanoclusters having 7.8 at. % of Zr exhibit a considerably enhanced magnetic anisotropy constant K1 ≈ 6.7 Mergs/cm3 and coercivity Hc ≈ 700 Oe at 300 K as compared to those of Co nanoclusters (K1 ≈ 2.9 Mergs/cm3 and Hc ≈180 Oe).
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75.75.-c Magnetic properties of nanostructures
75.30.Gw Magnetic anisotropy
75.50.Tt Fine-particle systems; nanocrystalline materials

Self-assembled growth and magnetic properties of a BiFeO3-MgFe2O4 nanocomposite prepared by pulsed laser deposition

Dong Hun Kim, Nicolas M. Aimon, and Caroline A. Ross

J. Appl. Phys. 113, 17B510 (2013); http://dx.doi.org/10.1063/1.4795327 (3 pages)

Online Publication Date: 20 March 2013

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The fabrication and magnetic properties are described for a nano-composite epitaxial thin film system consisting of perovskite phase BiFeO3 and spinel phase MgFe2O4 grown on a (001) SrTiO3 substrate by combinatorial pulsed laser deposition. The MgFe2O4 phase formed well-defined vertical pillars in a BiFeO3 matrix, and both phases were epitaxial with the substrate. The BiFeO3-MgFe2O4 composite films showed soft magnetic behavior and magnetically isotropic properties which is explained by the weak magnetoelastic effects in MgFe2O4.
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81.16.Dn Self-assembly
81.16.Mk Laser-assisted deposition
68.55.aj Insulators
75.75.Cd Fabrication of magnetic nanostructures
75.50.Tt Fine-particle systems; nanocrystalline materials
75.80.+q Magnetomechanical effects, magnetostriction

Room temperature ferromagnetism in non-magnetic doped TiO2 nanoparticles

C. Gómez-Polo, S. Larumbe, and J. M. Pastor

J. Appl. Phys. 113, 17B511 (2013); http://dx.doi.org/10.1063/1.4795615 (3 pages)

Online Publication Date: 20 March 2013

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Room-temperature ferromagnetism in non-magnetic doped TiO2 semiconductor nanoparticles is analyzed in the present work. Undoped and N-doped TiO2 nanoparticles were obtained employing sol-gel procedure using urea as the nitrogen source. The obtained gels were first dried at 70 °C and afterwards calcined in air at 300 °C. A residual carbon concentration was retained in the samples as a consequence of the organic decomposition process. Post-annealing treatments at 300 °C under air and vacuum conditions were also performed. The crystallographic structure of nanoparticles was analyzed by X-ray diffraction, obtaining a single anatase crystalline phase after the calcinations (mean nanoparticle diameters around 5–8 nm). SQUID magnetometry was employed to analyze the magnetic response of the samples. Whereas for the undoped samples synthesized with hydrolysis rate h = 6, paramagnetic like behavior is observed at room temperature, the N-doped nanoparticles (h = 3) show a weak ferromagnetic response (saturation magnetization ≈10−3 emu/g). Moreover, a clear reinforcement of the room-temperature ferromagnetism response is found with the post-annealing treatments, in particular that performed in vacuum. Thus, the results indicate the dominant role of the oxygen stoichiometry and the oxygen vacancies in the room temperature ferromagnetic response of these TiO2 nanoparticles.
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81.16.Rf Micro- and nanoscale pattern formation
61.72.up Other materials
61.72.Cc Kinetics of defect formation and annealing
75.50.Pp Magnetic semiconductors
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
81.07.Wx Nanopowders

Magnetic properties and phase transitions of gadolinium-infused carbon nanotubes

Abdiel Quetz, Igor Dubenko, Tapas Samanta, Herbert Vinson, Saikat Talapatra, Naushad Ali, and Shane Stadler

J. Appl. Phys. 113, 17B512 (2013); http://dx.doi.org/10.1063/1.4796151 (3 pages)

Online Publication Date: 25 March 2013

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Carbon nanotube (CNT)/metal-cluster-based composites are envisioned as new materials that possess unique electronic properties which may be utilized in a variety of future applications. Superparamagnetic behavior was reported for CNTs with Gd ions introduced into the CNT openings by internal loading with an aqueous GdCl3 chemical process. In the current work, the magnetic properties of the CNT/Gd composites were obtained by the joining and annealing of Gd metal and CNTs at 850 °C for 48 h. Energy dispersive X-ray analysis shows the presence of Gd intermingled with the CNT walls with maximum and average Gd concentrations of about 20% and 4% (by weight), respectively. The Gd clusters have a non-uniform distribution and are mostly concentrated at the ends of the CNTs. A ferromagnetic-type transition at TC ∼ 320 K, accompanied by jump like change in magnetization and temperature hysteresis typical for the temperature induced first order phase transitions has been observed by magnetization measurements. It was found that Gd infused into the CNTs by the annealing results in a first order paramagnetic-ferromagnetic transition at TC = 320 K.
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81.05.uj Diamond/nanocarbon composites
81.07.De Nanotubes
75.50.Tt Fine-particle systems; nanocrystalline materials
75.75.-c Magnetic properties of nanostructures
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Effect of the Zn content in the magnetic properties of Co1−xZnxFe2O4 mixed ferrites

A. Franco, Jr. and F. C. e Silva

J. Appl. Phys. 113, 17B513 (2013); http://dx.doi.org/10.1063/1.4796173 (3 pages)

Online Publication Date: 25 March 2013

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In this work, the effect of Zn content in the magnetic properties of Co1−xZnxFe2O4 (0.0 ≤ x ≤ 1.0) nanoparticles synthesized by combustion reaction method was investigated by applying magnetic fields up to 20 kOe at room temperature. All the samples were found to have a cubic spinel structure and the lattice parameter increases linearly with increasing Zn-content. The hysteresis loops yield a saturation magnetization (Ms), coercive field (Hc), and remanent magnetization (Mr) that varies significantly with Zn-content. For instance, Ms, Hc, and Mr are 70 emu/g, 600 Oe, and 20 emu/g and 8 emu/g, 0.0 Oe, and 0.0 emu/g for x = 0.0 and x = 1.0, respectively. The Curie temperature determined by means of the inverse susceptibility versus temperature decreases with increasing x, being ∼787 K and ∼634 K for x = 0.0 and 1.0, respectively. The magnetic cubic anisotropy constant for different Zn-contents, determined by a “law of approach” to saturation, was found to be smaller than those values for pure cobalt ferrites nanoparticles and strongly dependent on x, being 3.87 × 106 erg/cm3 and 0.22 × 106 erg/cm3 for x = 0.0 and 1.0, respectively, at room temperature. A discussion on the implications of the Zn-content in the determination of the anisotropy constant in these materials will also be presented.
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75.75.Cd Fabrication of magnetic nanostructures
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
82.33.Vx Reactions in flames, combustion, and explosions
75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Gg Ferrimagnetics

Large magnetic heat transport in a Haldane chain material Ni(C3H10N2)2NO2ClO4

X. F. Sun, X. G. Liu, L. M. Chen, Z. Y. Zhao, and X. Zhao

J. Appl. Phys. 113, 17B514 (2013); http://dx.doi.org/10.1063/1.4796180 (3 pages)

Online Publication Date: 25 March 2013

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We report a study on the heat transport of an S = 1 Haldane chain compound Ni(C3H10N2)2NO2ClO4 at low temperatures and in magnetic fields. The zero-field thermal conductivities show a remarkable anisotropy for the heat current along the spin-chain direction (κb) and the vertical direction (κc), implying a magnetic contribution to the heat transport along the spin-chain direction. The magnetic-field-induced change of the spin spectrum has obviously opposite impacts on κb and κc. In particular, κb(H) and κc(H) curves show peak-like increases and dip-like decreases, respectively, at ∼9 T, which is the critical field that minimizes the spin gap. These results indicate a large magnetic thermal transport in this material.
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81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.
75.30.Sg Magnetocaloric effect, magnetic cooling

Indirect exchange of magnetic impurities in zigzag graphene ribbon

J. H. Sun, F. M. Hu, H. K. Tang, W. Guo, and H. Q. Lin

J. Appl. Phys. 113, 17B515 (2013); http://dx.doi.org/10.1063/1.4797625 (3 pages)

Online Publication Date: 27 March 2013

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We use quantum Monte Carlo method to study the indirect coupling between two magnetic impurities on the zigzag edge of graphene ribbon. If the two adatoms are located on the sites belonging to opposite sublattices, their antiferromagnetic spin-spin correlation is drastically suppressed at the zero-energy, and is first enhanced and then decreased as we lower the chemical potential. If the two adatoms are adsorbed on the same sublattice, we find similar behavior of spin-spin correlation except for a crossover from ferromagnetic to antiferromagnetic correlation as we lower the chemical potential. We also calculate the weight of different components of d-electron wave function and local magnet moment for various values of parameters, and all the results are consistent with those of spin-spin correlation between two magnetic impurities.
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81.05.ue Graphene
75.50.Ee Antiferromagnetics
75.50.Dd Nonmetallic ferromagnetic materials
65.80.Ck Thermal properties of graphene
75.30.Et Exchange and superexchange interactions
75.30.Hx Magnetic impurity interactions

Arrays of ordered nanostructures in Fe-Pt thin films by self-assembling of polystyrene nanospheres

Paola Tiberto, Gabriele Barrera, Luca Boarino, Federica Celegato, Marco Coïsson, Natascia De Leo, Franca Albertini, Francesca Casoli, and Paolo Ranzieri

J. Appl. Phys. 113, 17B516 (2013); http://dx.doi.org/10.1063/1.4797626 (3 pages)

Online Publication Date: 27 March 2013

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Large-area arrays of dots having diameter ranging in the interval 80–400 nm were obtained by polystyrene nanosphere (PN) lithography in L10-FePt thin films. Fe53Pt47 thin films (thickness 10 nm) were epitaxially deposited by rf sputtering on a MgO(100) substrate heated at 400 °C to promote the formation of the L10 tetragonal phase. Patterned films were obtained by assembling PN nanospheres monolayer with starting mean diameter of 100 and 500 nm on a continuous thin film; subsequently, the PNs size has been reduced by reactive ion etching, obtaining a final dots diameter of 80 and 400 nm, respectively. The patterning process resulted to disorder the tetragonal phase therefore reducing the coercive field. A post-annealing at 550 °C for 1 h resulted to be effective in re-inducing the precipitation of the ordered phase. The effect of patterning (i.e., dot diameter and mutual distance) on the magnetic properties is discussed in the light of the presence of the high-anisotropy L10 tetragonal phase, by means of both structural and magnetic characterization techniques.
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68.55.at Other materials
75.75.-c Magnetic properties of nanostructures
75.70.Ak Magnetic properties of monolayers and thin films
75.30.Gw Magnetic anisotropy
81.15.Cd Deposition by sputtering
81.16.Nd Micro- and nanolithography

Effect of growth temperature on ordering of two-dimensional square spin ice

Simon John Greaves and Hiroaki Muraoka

J. Appl. Phys. 113, 17B517 (2013); http://dx.doi.org/10.1063/1.4798371 (3 pages)

Online Publication Date: 29 March 2013

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The growth of two-dimensional square spin ices at various temperatures was simulated. Slower growth rates and higher growth temperatures both increased the degree of ordering. Slightly better ordering was achieved by gradually increasing the temperature during growth, expanding the range of thicknesses over which thermal activation was able to reduce the energy of the island arrays.
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75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)

Spin alignment of surface oxidized CoxNi1–x/Cu(001)

Ying-Ta Shih, Chung-Wei Tsai, Chien-Yu Su, Wei Pan, Der-Hsin Wei, Yuet-Loy Chan, and Hui-Ching Chang

J. Appl. Phys. 113, 17B518 (2013); http://dx.doi.org/10.1063/1.4798504 (3 pages)

Online Publication Date: 1 April 2013

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We investigated the ferromagnetic (FM)/antiferromagnetic (AF) spin alignment of a 13 monolayer oxidized CoxNi1–x/Cu(001) (x = 0 or 0.05) surface by X-ray magnetic circular dichroism and X-ray magnetic linear dichroism photoemission electron microscopy (XMCD-PEEM and XMLD-PEEM). Surface NiO and the underlying Ni (CoxNi1–x) were found to be AF and FM by analyzing the gray scale of XMLD-PEEM and XMCD-PEEM images; this indicates the spin orientation with respect to the polarization of the incident X-ray. We found both collinear and perpendicular alignment of the FM and AF spins. This suggests that the AF NiO is magnetically random, resulting in coercivity enhancement without exchange bias in the surface-oxidized CoxNi1–x/Cu(001) films.
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75.70.Ak Magnetic properties of monolayers and thin films
78.20.Ls Magneto-optical effects
68.37.Xy Scanning Auger microscopy, photoelectron microscopy
75.50.Cc Other ferromagnetic metals and alloys
75.50.Ee Antiferromagnetics
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Magnetic properties of ferrite-titanate nanostructured composites synthesized by the polyol method and consolidated by spark plasma sintering

Ulises Acevedo, Thomas Gaudisson, Raul Ortega-Zempoalteca, Sophie Nowak, Souad Ammar, and Raul Valenzuela

J. Appl. Phys. 113, 17B519 (2013); http://dx.doi.org/10.1063/1.4798604 (3 pages)

Online Publication Date: 1 April 2013

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Multiferroic systems formed by a mixing of a ferroelectric phase and a ferrimagnetic phase are receiving significant attention because of their wide possibilities for tailoring properties. In this work, the magnetic properties of the cobalt ferrite-barium titanate system were investigated on samples prepared by an original combination of synthesis methods. Cobalt ferrite and barium titanate nanoparticles were synthesized separately by hydrolysis of the metal acetates in a polyol method. Both materials were mixed in a 1:1 ratio and consolidated by spark plasma sintering at 500 °C for 5 min. A high density nanostructured ceramic was obtained with grains smaller than 100 nm and a density about 80% of the theoretical value. Magnetic hysteresis loops showed a hard magnet behavior, with a coercive field larger than cobalt ferrite alone prepared under the same conditions. δM reversible magnetization plots exhibited dipolar interactions with a maximum at the coercive field. These results are interpreted in terms of an efficient mixing of the components, which strongly decreases the magnetic percolation in the composite by separating ferrite grains by titanate grains.
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75.75.-c Magnetic properties of nanostructures
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
81.05.Mh Cermets, ceramic and refractory composites
81.07.Bc Nanocrystalline materials
81.16.-c Methods of micro- and nanofabrication and processing

Magnetic properties of GaAs/Fe core/shell nanowires

R. E. Pimpinella, D. Zhang, M. R. McCartney, D. J. Smith, K. L. Krycka, B. J. Kirby, B. J. O'Dowd, L. Sonderhouse, J. Leiner, X. Liu, M. Dobrowolska, and J. K. Furdyna

J. Appl. Phys. 113, 17B520 (2013); http://dx.doi.org/10.1063/1.4799252 (3 pages)

Online Publication Date: 8 April 2013

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We describe the magnetic properties of nanoscale Fe shells grown on GaAs nanowires (NWs) by molecular beam epitaxy. The ferromagnetic character of these tubular Fe shells has been confirmed by dc magnetization measurements, and is further studied by ferromagnetic resonance (FMR), small-angle neutron scattering (SANS), and off-axis electron holography (EH).
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
75.30.Gw Magnetic anisotropy
75.60.Ch Domain walls and domain structure
75.70.Kw Domain structure (including magnetic bubbles and vortices)
81.16.-c Methods of micro- and nanofabrication and processing

Standing spin waves in perpendicularly magnetized circular dots at millimeter waves

S. V. Nedukh, S. I. Tarapov, D. P. Belozorov, A. A. Kharchenko, V. O. Golub, I. V. Kilimchuk, O. Y. Salyuk, E. V. Tartakovskaya, S. A. Bunyaev, and G. N. Kakazei

J. Appl. Phys. 113, 17B521 (2013); http://dx.doi.org/10.1063/1.4799528 (3 pages)

Online Publication Date: 8 April 2013

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Spin wave spectra of 40-nm thick perpendicularly magnetized circular Permalloy dots of 250 nm radius were measured using ferromagnetic resonance technique in 70-80 GHz range at 4.2 K and in 10 GHz at room temperature. The five sharp resonance peaks were observed for both frequency ranges. The resonance fields can be well described by a magneto-exchange dispersion relation, implying that the observed resonances correspond to circular “drumhead” modes with Bessel-function profiles. The relative distances between neighbor peaks for different frequency ranges were almost the same, while the absolute interpeak distances in millimeter range were ∼30% bigger than at 10 GHz, as predicted by the theory.
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75.30.Ds Spin waves
75.30.Et Exchange and superexchange interactions
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
78.70.Gq Microwave and radio-frequency interactions
75.50.Ss Magnetic recording materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Band structure, magnetic, and transport properties of two dimensional compounds Sr2−xGdxCoO4

Q. W. Yao, X. L. Wang, H. Kimura, S. X. Dou, K. Konstantinov, Z. X. Cheng, F. Hong, H. Y. Zhao, H. Qiu, and K. Ozawa

J. Appl. Phys. 113, 17B522 (2013); http://dx.doi.org/10.1063/1.4799780 (3 pages)

Online Publication Date: 9 April 2013

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The layered perovskite compound Sr2−xGdxCoO4 has not yet been subjected to detailed study so far. In this report, structures, transport, magnetic properties, and first principle calculations will be reported for the two dimensional compounds Sr2−xGdxCoO4 (x = 0.5, 0.75, 1, 1.25). Rietveld refinement revealed that these compounds are crystallized in K2NiF4-type structures with space group I4/mmm. It was found that the lattice parameter c decreases as x increases. Through the Curies Weiss fitting of the temperature dependent magnetization, it was found that the Sr1.25Gd0.75CoO4 sample exhibits a weak ferromagnetic to paramagnetic transition at about 62 K, with a Curie constant of 0.113 emu K/mol. Band structure calculations indicated that electrons of these compounds are spin polarized at the Fermi level. The 2-D Variable Range Hopping model fitting indicated that the two dimensional variable range hopping mechanism could be used to account for the conducting mechanism for these samples.
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71.20.Ps Other inorganic compounds
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
61.66.Fn Inorganic compounds
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
72.20.Ee Mobility edges; hopping transport
75.30.Cr Saturation moments and magnetic susceptibilities

Nanophase stability in a granular FeRh-Cu system

Radhika Barua, Felix Jimenez-Villacorta, J. E. Shield, D. Heiman, and L. H. Lewis

J. Appl. Phys. 113, 17B523 (2013); http://dx.doi.org/10.1063/1.4800742 (3 pages)

Online Publication Date: 9 April 2013

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A granular system of FeRh-based nanoprecipitates (∼10–15 nm diameter) embedded in a rapidly solidified copper ribbon matrix was found to transit from a metastable tetragonal L10 (AuCu-1-type) structure to a stable B2 (CsCl-type) structure upon annealing-induced coarsening to ∼94 nm. The hysteretic magnetic transition observed at ∼100 K develops a gradual broadening that accompanies the L10 → B2 crystal structure transition. It is proposed that the Cu matrix influences the structural and magnetic properties of the FeRh-based nanoparticles through interfacial strain and chemical effects. These results emphasize the sensitivity of the magnetostructural response of FeRh to changes in the nanostructural scale, and provide pathways for tailoring the transition.
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75.75.Cd Fabrication of magnetic nanostructures
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization

Correlation between site preference and magnetic characteristics of self assembled strontium ferrite dot array on functionalized multi-walled carbon nanotubes

Ali Ghasemi, Vladimir Sepelak, Xiaoxi Liu, and Akimitsu Morisako

J. Appl. Phys. 113, 17B524 (2013); http://dx.doi.org/10.1063/1.4798802 (3 pages)

Online Publication Date: 10 April 2013

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In this research work, ferrite nanoparticles with composition of SrFe12−x(Ni0.5Co0.5Ti)x/2O19 (x = 0-2.5 in a step of 0.5) were synthesized by a reverse micelle. Multiwalled carbon nanotubes (MWCNT) were also functionalized by employing poly(acrylic acid). Then the ferrite nanoparticles were deposited on the functionalized surface of carbon nanotubes by hetero-coagulation process. The volume percentage of carbon nanotubes was kept constant at 8 vol. % for synthesizing nanocomposites. The site preference of substituted cations in ferrite crystal structure was determined by 57Fe Mössbauer spectroscopy. It was proved that the substituted cations were distributed in 12 k crystallographic sites. The morphology of ferrite dot array on carbon nanotubes was studied by field emission scanning electron microscopy (FESEM). Quantum Device MPMS-5S SQUID magnetometer was used to probe the variation of magnetization with applied magnetic field. It was found that with an increase in substitution content, the saturation of magnetization and coercivity decrease.
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81.05.uj Diamond/nanocarbon composites
81.07.De Nanotubes
75.75.-c Magnetic properties of nanostructures
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Gg Ferrimagnetics
75.50.Tt Fine-particle systems; nanocrystalline materials

Magnetic behavior of reduced graphene oxide/metal nanocomposites

P. K. Sahoo, Bharati Panigrahy, Dan Li, and D. Bahadur

J. Appl. Phys. 113, 17B525 (2013); http://dx.doi.org/10.1063/1.4799150 (3 pages)

Online Publication Date: 10 April 2013

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The dispersion of metal nanoparticles on reduced graphene oxide (RGO) sheets potentially provides a new way to develop novel catalytic, magnetic, adsorbing, and electrode materials. In this work, we report the structural and magnetic properties of RGO/metal (Bi, Pt, Ni, and Pt-Ni) nanocomposites. Such nanocomposites are successfully synthesized by a facile in situ co-reduction route. The structure, composition, and morphology of the synthesized materials are systematically investigated by X-ray diffraction, inductively coupled plasma-atomic emission spectrometer , and high resolution transmission electronic microscopy. The M–H curve and zero-field-cooled and field-cooled data for RGO, RGO/Ni, and RGO/Pt-Ni nanocomposites exhibit ferromagnetic behaviour. RGO/Pt nanocomposite shows diamagnetic, while RGO/Bi nanocomposite shows lower magnetization compared to that of RGO. Detailed magnetic studies on these nanocomposites and its correlation with microstructural features are presented here.
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81.16.Pr Micro- and nano-oxidation
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
75.75.Cd Fabrication of magnetic nanostructures
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Tt Fine-particle systems; nanocrystalline materials
81.07.Wx Nanopowders
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