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15 Feb 2008

Volume 103, Issue 4, Articles (04xxxx)

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Ni implanted ZnO single crystals: Correlation between nanoparticle formation and defect structure

Shengqiang Zhou, K. Potzger, K. Kuepper, J. Grenzer, M. Helm, J. Fassbender, E. Arenholz, and J. D. Denlinger

J. Appl. Phys. 103, 043901 (2008); http://dx.doi.org/10.1063/1.2837058 (5 pages) | Cited 14 times

Online Publication Date: 20 February 2008

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We show that metallic secondary phase formation inside ZnO(0001) single crystals implant-doped with Ni at an atomic concentration of 5% can be suppressed. All the Ni ions are in the 2+ valence state after mild postannealing. The suppression is achieved by means of annealing of the crystals in high vacuum prior to implantation and is correlated with the introduction of structural disorder. The observed ferromagnetic properties of the preannealed crystals are evidently induced by defects and not primarily by the Ni doping. They degrade at ambient temperature within several days.
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61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
81.07.Bc Nanocrystalline materials
61.72.Mm Grain and twin boundaries
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
61.72.Cc Kinetics of defect formation and annealing

Structural and magnetic properties of magnetite-containing epitaxial iron oxide films grown on MgO(001) substrates

T. Kado

J. Appl. Phys. 103, 043902 (2008); http://dx.doi.org/10.1063/1.2840118 (4 pages) | Cited 5 times

Online Publication Date: 21 February 2008

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The structural and magnetic properties of three kinds of Fe oxide films—Fe3O4, a berthollide type of Fe oxide, and a composite of Fe3O4 and FeO—grown on MgO(001) substrates epitaxially at 423 K by pulsed laser deposition were investigated by magnetoresistance measurement and grazing incidence x-ray diffraction. The Fe3O4 film had the largest negative magnetoresistance and the roughest film-substrate interface. The very close lattice matching of single-phase Fe3O4 and MgO facilitates the formation of antiphase boundaries due to natural growth defects and of a rough interface probably due to cation interdiffusion.
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72.20.My Galvanomagnetic and other magnetotransport effects
68.55.-a Thin film structure and morphology
75.70.Ak Magnetic properties of monolayers and thin films
81.15.Fg Pulsed laser ablation deposition

First-order ferromagnetic to helimagnetic transition in MgMn6Ge6

T. Mazet, H. Ihou-Mouko, and B. Malaman

J. Appl. Phys. 103, 043903 (2008); http://dx.doi.org/10.1063/1.2840126 (5 pages) | Cited 3 times

Online Publication Date: 21 February 2008

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The magnetic and magnetocaloric properties of the new HfFe6Ge6-type (P6/mmm) MgMn6Ge6 compound have been studied by magnetic measurements and powder neutron diffraction experiments. MgMn6Ge6 magnetically orders at TC ∼ 345 K in a ferromagnetic easy-plane arrangement. At TAF ∼ 235 K, it undergoes a first-order transition to a helical structure, characterized by a temperature dependent propagation vector k = 〈0,0,qz〉 (qz ∼ 0.116 reciprocal lattice units at 1.4 K), without structural modification. The Mn atoms carry a magnetic moment of about 2 μB. A metamagnetic transition with a low critical field (Hcr<1 T) is observed below TAF. At low temperature, both magnetic and magnetocaloric data strongly suggest that some kinds of intermediate noncollinear ferromagnetic arrangements are stabilized above the critical field while the pure ferromagnetic state is obtained for significantly higher magnetic fields (H>5 T). The magnitude of the magnetic entropy change at TC (−ΔSM ∼ 20.5 mJ cm−3 K−1 for ΔH = 5 T) is about 25% that of Gd metal. The magnetocaloric effect at the order-order transition is of opposite sign and of lower magnitude.
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75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.30.Sg Magnetocaloric effect, magnetic cooling
64.70.kd Metals and alloys
75.30.Cr Saturation moments and magnetic susceptibilities
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
75.50.Cc Other ferromagnetic metals and alloys

Analysis of squareness in demagnetization curve of Nd–Fe–B magnet produced by the hydrogenation-disproportionation-desorption-recombination process

Tomohito Maki and Satoshi Hirosawa

J. Appl. Phys. 103, 043904 (2008); http://dx.doi.org/10.1063/1.2885151 (4 pages)

Online Publication Date: 21 February 2008

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An analysis of squareness in demagnetization curve of Nd–Fe–B fine-grained alloys prepared by the hydrogenation-disproportionation-desorption-recombination (HDDR) process was carried out in reference to the melt-spun and sintered magnets by measurements of their recoil curves. The recoil curves of the HDDR magnet and the melt-spun magnet suggested that the magnetization process associated with the recoil behavior of the HDDR magnet is dominated by rotation of magnetization, while that of the sintered magnet suggested that domain wall movement is involved. The recoil susceptibilities for recoil curves that returns to nearly demagnetized state were 0.095 and 0.048 T−1 in the isotropic and anisotropic HDDR magnets, respectively, when normalized by saturation magnetization. The normalized susceptibilities could be reproduced quantitatively, assuming that the magnetization recoil process within a grain in the HDDR magnet is described by the coherent rotation of magnetization and a simple model of orientation distribution of the easy axes. It appears that the anisotropic HDDR magnet has a wide distribution of orientation of the easy axes, which should be one of the reasons for poor squareness of the demagnetization curve of the HDDR magnet.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Bb Fe and its alloys

Room-temperature magnetocaloric effect in La0.7Sr0.3Mn1−xMx′O3 (M′ = Al,Ti)

D. N. H. Nam, N. V. Dai, L. V. Hong, N. X. Phuc, S. C. Yu, M. Tachibana, and E. Takayama-Muromachi

J. Appl. Phys. 103, 043905 (2008); http://dx.doi.org/10.1063/1.2840121 (5 pages) | Cited 13 times

Online Publication Date: 25 February 2008

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Magnetic entropy and adiabatic temperature changes in and above the room-temperature region have been measured for La0.7Sr0.3Mn1−xMx′O3 (M′ = Al,Ti) by means of magnetization and heat capacity measurements in magnetic fields up to 6 T. The magnetocaloric effect becomes largest at the ferromagnetic ordering temperature Tc that is tuned (from 364.5 K for x = 0) to ∼ 300 K by the substitution of Al or Ti for Mn. While the substitution of Al for Mn drastically reduces the maximum magnetic entropy (−ΔSm) and adiabatic temperature (ΔTa) changes, it extends considerably the working temperature span and therefore improves the relative cooling power (RCP). Under a magnetic field change ΔH = 2 T, −ΔSm (or ΔTa) of La0.7Sr0.3Mn1−xAlxO3 decreases from 2.66 J/kg K (or 1.65 K) for x = 0 to 1.18 J/kg K (or 0.69 K) for x = 0.1, while RCP increases from 80.3 to 108.8 J/kg, respectively. While Tc is largely suppressed, the magnetocaloric effect is only lightly affected by the Ti substitution. With ΔH = 2 T, the La0.7Sr0.3Mn0.95Ti0.05O3 sample exhibits −ΔSm (or ΔTa) = 2.44 J/kg K (or 1.38 K) with RCP = 89.9 J/kg. The decrease of magnetic moment is found to be one possible reason behind the suppression of the MCE. The magnetocaloric effect in manganite materials seems to be inhibited by the existence of short-range ferromagnetic correlations above Tc.
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75.30.Sg Magnetocaloric effect, magnetic cooling
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Cr Saturation moments and magnetic susceptibilities
75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Effective area reduction in directly coupled superconducting quantum interference device based micromagnetometers

A. Prigiobbo and E. Sarnelli

J. Appl. Phys. 103, 043906 (2008); http://dx.doi.org/10.1063/1.2840138 (4 pages) | Cited 2 times

Online Publication Date: 25 February 2008

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We have designed and tested a set of YBa2Cu3O7−δ (YBCO) directly coupled superconducting quantum interference devices (SQUIDs). The proposed layout introduces a reduction of the device effective area that leads to high design flexibility for micromagnetometer sensors. SQUID magnetometers have been realized using YBCO grain boundary junctions fabricated on symmetric 30° [001] bicrystal substrates. Sensors have been characterized in a highly shielded environment confirming theoretical predictions.
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85.25.Dq Superconducting quantum interference devices (SQUIDs)
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
07.10.Cm Micromechanical devices and systems

Theoretical analysis of magnetic force microscopy contrast in multidomain states of magnetic superlattices with perpendicular anisotropy

N. S. Kiselev, I. E. Dragunov, V. Neu, U. K. Rößler, and A. N. Bogdanov

J. Appl. Phys. 103, 043907 (2008); http://dx.doi.org/10.1063/1.2844312 (7 pages) | Cited 5 times

Online Publication Date: 26 February 2008

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Recently synthesized magnetic multilayers with strong perpendicular anisotropy exhibit unique magnetic properties including the formation of specific multidomain states. In particular, antiferromagnetically coupled multilayers own rich phase diagrams that include various multidomain ground states. Analytical equations have been derived for the stray field components of these multidomain states in perpendicular multilayer systems. In particular, closed expressions for stray fields in the case of ferromagnetic and antiferromagnetic stripes are presented. The theoretical approach provides a basis for the analysis of magnetic force microscopy (MFM) images from this class of nanomagnetic systems. Peculiarities of the MFM contrast have been calculated for realistic tip models. These characteristic features in the MFM signals can be employed for the investigations of the different multidomain modes. The obtained results are applied for the analysis of multidomain modes that have been reported earlier in the literature from experiments on [Co/Cr]Ru superlattices.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Ee Antiferromagnetics
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
85.70.Li Other magnetic recording and storage devices (including tapes, disks, and drums)

Systematic investigation of structural and magnetic properties in molecular beam epitaxial growth of metastable zinc-blende CrTe toward half-metallicity

M. G. Sreenivasan, J. F. Bi, K. L. Teo, and T. Liew

J. Appl. Phys. 103, 043908 (2008); http://dx.doi.org/10.1063/1.2885108 (5 pages) | Cited 19 times

Online Publication Date: 27 February 2008

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We report a systematic investigation on the structural and magnetic properties of molecular-beam epitaxial growth of CrTe thin films with different layer thicknesses and Cr/Te flux ratios. A phase diagram of the growth parameters is established based on the detailed analyses of the reflection high-energy electron diffraction patterns, atomic force microscopy, and magnetization. Our high-resolution transmission electron microscopy results show that under appropriate growth conditions, a metastable zinc-blende (ZB) phase of CrTe film can be achieved with a nominal thickness of 5 nm. The magnetic properties of ZB CrTe exhibit a strong in-plane anisotropy with an easy axis along the [001] direction and hard axes along the [011] and [0math1] directions. Correspondingly, the uniaxial (KU) and cubic (KC) anisotropy constants are obtained through the fitting of the [011] hard-axis direction. The temperature dependence of the remanent magnetization indicates the TC ∼ 100 K of ZB CrTe is attained.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.70.Ak Magnetic properties of monolayers and thin films
75.30.Gw Magnetic anisotropy
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.37.Lp Transmission electron microscopy (TEM)
68.37.Ps Atomic force microscopy (AFM)

Improvement of photovoltaic response based on enhancement of spin-orbital coupling and triplet states in organic solar cells

Zhihua Xu, Bin Hu, and Jane Howe

J. Appl. Phys. 103, 043909 (2008); http://dx.doi.org/10.1063/1.2885349 (8 pages) | Cited 15 times

Online Publication Date: 27 February 2008

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This article reports an improvement of photovoltaic response by dispersing phosphorescent Ir(ppy)3 molecules in an organic solar cell of poly[2-methoxy-5-(2′-ethylhexyloxy)-1 4-phenylenevinylene] (MEH-PPV) blended with surface-functionalized fullerene 1-(3-methyloxycarbonyl)propy(1-phenyl [6,6]) C61 (PCBM). The magnetic field–dependent photocurrent indicates that the dispersed Ir(ppy)3 molecules increase the spin-orbital coupling strength with the consequence of changing the singlet and triplet ratios through intersystem crossing due to the penetration of the delocalized π electrons of MEH-PPV into the large orbital magnetic field of Ir(ppy)3 dopants. The tuning of singlet and triplet exciton ratios can lead to an enhancement of photovoltaic response due to their different contributions to the two different photocurrent generation channels: exciton dissociation and exciton-charge reaction in organic materials. In addition, the photoluminescence temperature dependence reveals that the dispersed Ir(ppy)3 reduces the recombination of dissociated charge carriers in the PCBM doped MEH-PPV. As a result, adjusting singlet and triplet ratios by introducing heavy-metal complex Ir(ppy)3 provides a mechanism to improve the photovoltaic response through controlling exciton dissociation, exciton-charge reaction, and recombination of dissociated charge carriers in organic bulk-heterojunction solar cells.
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84.60.Jt Photoelectric conversion
72.40.+w Photoconduction and photovoltaic effects

Modeling of the Barkhausen jump in low carbon steel

J. A. Pérez-Benitez, J. Capó-Sánchez, and L. R. Padovese

J. Appl. Phys. 103, 043910 (2008); http://dx.doi.org/10.1063/1.2840058 (6 pages) | Cited 5 times

Online Publication Date: 28 February 2008

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This work presents a model for the magnetic Barkhausen jump in low carbon content steels. The outcomes of the model evidence that the Barkhausen jump height depends on the coercive field of the pinning site and on the mean free path of the domain wall between pinning sites. These results are used to deduce the influence of the microstructural features and of the magnetizing parameters on the amplitude and duration of the Barkhausen jumps. In particular, a theoretical expression, establishing the dependence of the Barkhausen jump height on the carbon content and grain size, is obtained. The model also reveals the dependence of the Barkhausen jump on the applied frequency and amplitude. Theoretical and experimental results are presented and compared, being in good agreement.
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75.50.Bb Fe and its alloys
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.60.Ch Domain walls and domain structure

Structural, magnetic, and mechanical properties of 5 μm thick SmCo films suitable for use in microelectromechanical systems

A. Walther, D. Givord, N. M. Dempsey, K. Khlopkov, and O. Gutfleisch

J. Appl. Phys. 103, 043911 (2008); http://dx.doi.org/10.1063/1.2840131 (5 pages) | Cited 11 times

Online Publication Date: 28 February 2008

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5 μm thick SmCo films were deposited onto Si substrates using triode sputtering. A study of the influence of deposition temperature (Tdep ⩽ 600 °C) on the structural, magnetic, and mechanical properties has shown that optimum properties [highest degree of in-plane texture, maximum in-plane coercivity and remanence (1.3 and 0.8 T, respectively), and no film peel-off] are achieved for films deposited at the relatively low temperature of 350 °C. This temperature is compatible with film integration into microelectromechanical systems. The deposition rate was increased from 3.6 to 18 μm/h by increasing the surface area of the target from 7 to 81 cm2 while keeping the target potential fixed. Mechanically intact films could be prepared by deposition onto prepatterned films or deposition through a mask.
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68.55.J- Morphology of films
07.10.Cm Micromechanical devices and systems
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.70.Ak Magnetic properties of monolayers and thin films
81.15.Cd Deposition by sputtering
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

Morphology evolution and magnetic properties improvement in FePt epitaxial films by in situ annealing after growth

F. Casoli, L. Nasi, F. Albertini, S. Fabbrici, C. Bocchi, F. Germini, P. Luches, A. Rota, and S. Valeri

J. Appl. Phys. 103, 043912 (2008); http://dx.doi.org/10.1063/1.2885339 (8 pages) | Cited 13 times

Online Publication Date: 29 February 2008

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This paper presents a detailed study of structure, morphology, and magnetic properties in FePt thin films epitaxially grown on MgO(100) at intermediate temperatures ( ≅ 380 °C). It focuses on the effects obtained by in situ annealing the FePt films after deposition. We have demonstrated that the annealing allows one to complete the ordering, thus obtaining an epitaxial L10-FePt film with large perpendicular anisotropy (up to 3×107 erg/cm3) and high perpendicular squareness and remanence ratio (both higher than 0.85). At the same time, we have found a peculiar morphology evolution by increasing the annealing time: the interconnected mazelike grains evolve towards interconnected dotlike grains of reduced size (down to 10 nm). Due to the interconnection at the grains basis, the increase in the film perpendicular coercivity with the annealing process is moderate (up to 4.1 kOe), leading to an increasing anisotropy/coercivity ratio with the annealing time. As a further step towards the understanding of the properties evolution with annealing, we have investigated the magnetic domain pattern and analyzed the domain-period dependence on the anisotropy constant in comparison with the behavior expected by the Kooy–Enz theory [ C. Kooy and U. Enz, Philips Res. Rep. 15, 7 (1960) ]. The main magnetic properties and reduced grain size obtained after the in situ annealing are promising to realize perpendicular recording media with reduced bit size, high thermal stability, and suitable switching field.
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75.30.Gw Magnetic anisotropy
75.60.Ch Domain walls and domain structure
81.40.Gh Other heat and thermomechanical treatments
68.55.-a Thin film structure and morphology
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
68.60.Dv Thermal stability; thermal effects
75.70.Ak Magnetic properties of monolayers and thin films

Influence of growth temperature on the vortex pinning properties of pulsed laser deposited YBa2Cu3O7−x thin films

Z. Chen, F. Kametani, S. I. Kim, D. C. Larbalestier, H. W. Jang, K. J. Choi, and C. B. Eom

J. Appl. Phys. 103, 043913 (2008); http://dx.doi.org/10.1063/1.2885716 (7 pages) | Cited 7 times

Online Publication Date: 29 February 2008

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Epitaxial high-temperature superconducting YBa2Cu3O7−x thin films grown on 2° miscut (001) (LaAlO3)0.3-(SrAl0.5Ta0.5O3)0.7 substrates by pulsed laser deposition show significant and systematic changes in flux pinning properties on changing the substrate temperature from 730 to 820 °C. The bulk pinning force is highest for the 760 °C growth, rising to a maximum of 4.4 GN/m3 at 77 K, though there are indications that vortex pinning strength is even higher for the 730 °C growth once allowance for the current-blocking effects of a-axis oriented grains is made. Cross-sectional transmission electron microscope images show that the density of antiphase boundaries, stacking faults, and edge dislocations increases strongly with decreasing growth temperature, and is highest at 730 °C. In spite of the enhanced density of the pinning defects mentioned above, their vortex pinning effect is still much smaller than for insulating nanoparticles of high density and optimum size, where pinning forces can be four to five times higher.
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74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
74.72.-h Cuprate superconductors
74.78.-w Superconducting films and low-dimensional structures
74.62.Dh Effects of crystal defects, doping and substitution
81.15.Fg Pulsed laser ablation deposition
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
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