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20 Aug 2007

Volume 91, Issue 8, Articles (08xxxx)

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Appl. Phys. Lett. 91, 083504 (2007); http://dx.doi.org/10.1063/1.2772752 (3 pages)

Jeong-M. Choi, Jae Hoon Kim, and Seongil Im
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Effects of chemical fluctuations on microstructures and properties of multiferroic BiFeO3 thin films

W. B. Luo, J. Zhu, Y. R. Li, X. P. Wang, D. Zhao, J. Xiong, and Y. Zhang

Appl. Phys. Lett. 91, 082501 (2007); http://dx.doi.org/10.1063/1.2771089 (3 pages) | Cited 16 times

Online Publication Date: 20 August 2007

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BiFeO3 films have been grown on SrTiO3 (001) substrates by pulsed laser deposition. It was found that oxygen partial pressure is crucial to phase purity, surface morphology, and surface chemistry. Single-phase BFO films were obtained at 1 Pa O2, while Bi2O3 appeared in the films deposited at 0.01 Pa as confirmed by x-ray diffractions. It was revealed that Fe2+ and metallic Bi exist in the films fabricated at 0.01 Pa by x-ray photoelectron spectroscopy investigation. Owing to Fe2+ in the samples deposited at 0.01 Pa, the saturation magnetization is much larger than the ones fabricated at 1 Pa. A well-saturated ferroelectric hysteresis loop with a polarization of 23.6 μC/cm2 was observed in the single-phase samples. In contrast, the films deposited at 0.01 Pa exhibited poor ferroelectric properties.
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77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
77.55.-g Dielectric thin films
77.80.Dj Domain structure; hysteresis
75.70.Ak Magnetic properties of monolayers and thin films
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
81.15.Fg Pulsed laser ablation deposition

Substantial reduction in the depinning field of vortex domain walls triggered by spin-transfer induced resonance

T. Nozaki, H. Maekawa, M. Mizuguchi, M. Shiraishi, T. Shinjo, Y. Suzuki, H. Maehara, S. Kasai, and T. Ono

Appl. Phys. Lett. 91, 082502 (2007); http://dx.doi.org/10.1063/1.2772174 (3 pages) | Cited 6 times

Online Publication Date: 20 August 2007

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An influence of spin-transfer-induced resonant excitation of a vortex magnetic domain wall on magnetization reversal process was investigated in Permalloy nanowires. Sizable reduction of the depinning field was observed under an application of radio frequency current when the frequency coincided with resonance of the vortex core. The resonant frequency was found to increase by shrinking the wire width. These results lead the authors to conclude that the spin-transfer-induced resonant excitation assists the depinning of the vortex domain walls. This provides an important insight for the realization of novel magnetic memory and logic devices based on current-driven domain wall manipulation.
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75.60.Ch Domain walls and domain structure
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.60.Jk Magnetization reversal mechanisms
72.25.-b Spin polarized transport

Electrical detection of the ferromagnetic resonance: Spin-rectification versus bolometric effect

Y. S. Gui, N. Mecking, A. Wirthmann, L. H. Bai, and C.-M. Hu

Appl. Phys. Lett. 91, 082503 (2007); http://dx.doi.org/10.1063/1.2772764 (3 pages) | Cited 12 times

Online Publication Date: 21 August 2007

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The authors have investigated the dc resistance change of a Permalloy microstrip under microwave irradiation. The experimental results demonstrate that both the spin-rectification and the bolometric effects significantly affect the dc resistance change, and the contribution of each can be precisely determined due to their different dependences on the modulation frequency of the microwave. Therefore, both the cone angle of magnetization precession and the thermal relaxation time following microwave heating are obtained.
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76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Observation of room temperature ferromagnetic behavior in cluster-free, Co doped HfO2 films

Y. H. Chang, Y. L. Soo, W. C. Lee, M. L. Huang, Y. J. Lee, S. C. Weng, W. H. Sun, M. Hong, J. Kwo, S. F. Lee, J. M. Ablett, and C.-C. Kao

Appl. Phys. Lett. 91, 082504 (2007); http://dx.doi.org/10.1063/1.2773746 (3 pages) | Cited 10 times

Online Publication Date: 21 August 2007

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Extensive structural and magnetic analyses of Hf1−xCoxO2 thin films grown by molecular beam epitaxy are reported. Nearly cobalt cluster-free film with x = 0.04–0.1 was obtained via 100 °C growth, and Co ions are inferred to be located at interstitial site. Ferromagnetic behavior was observed up to 300 K in both magnetization curves and temperature dependence of the moment. Via post-oxygen-annealing studies, a qualitative correlation between saturation magnetization and oxygen vacancy concentration is established, consistent with the impurity-band exchange model, and that the occurrence of ferromagnetic insulator behavior in the Co doped HfO2 is more probable than Co doped ZnO, TiO2, and SnO2 systems for doping concentrations under cation percolation threshold.
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75.70.Ak Magnetic properties of monolayers and thin films
75.50.Dd Nonmetallic ferromagnetic materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
61.72.up Other materials

Magnetic coupling in Ge nanoparticles

Y. Liou, M. S. Lee, and K. L. You

Appl. Phys. Lett. 91, 082505 (2007); http://dx.doi.org/10.1063/1.2773755 (3 pages) | Cited 6 times

Online Publication Date: 21 August 2007

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Ferromagnetism in Ge nanoparticles fabricated by inert gas condensation was investigated. The magnetization was influenced by both the size and the density of Ge nanoparticles. The size was determined by the helium pressure and the density was controlled by depositing Ge nanoparticles on Si substrates at different locations. As the density was increased, the interparticle distance was reduced, which enhanced the magnetic coupling among Ge nanoparticles. The magnetism in Ge nanoparticles was attributed to both the quantum size effect and the magnetic coupling among Ge nanoparticles. By capping a layer over the Ge nanoparticles the magnetization was also enhanced.
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81.07.Bc Nanocrystalline materials
81.05.Cy Elemental semiconductors
75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Pp Magnetic semiconductors
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Electric-current-driven vortex-core reversal in soft magnetic nanodots

Sang-Koog Kim, Youn-Seok Choi, Ki-Suk Lee, Konstantin Y. Guslienko, and Dae-Eun Jeong

Appl. Phys. Lett. 91, 082506 (2007); http://dx.doi.org/10.1063/1.2773748 (3 pages) | Cited 25 times

Online Publication Date: 22 August 2007

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The authors report on electric-current-driven vortex-core (VC) reversal (switching) and the accompanying spin-wave emission, driven by spin-polarized ac currents of different amplitudes and frequencies, investigated by micromagnetic calculations of the dynamic evolution of a magnetic vortex in Permalloy nanodots. The magnetization orientation of the VC is effectively switchable between its upward and downward bistates and controllable by applying current above its threshold density, but with sufficiently small magnitude at frequencies close to the vortex eigenfrequency. This VC reversal phenomenon occurs through the creation of a vortex-antivortex pair and the subsequent annihilation of the initial vortex and the created antivortex, when the velocity of the initial VC reaches its critical value of approximately 340±20 m/s for the given material and geometry. In the course of these serial processes and immediately after VC switching, strong spin waves are emitted. These results provide physical insights into how and when current-driven VC switching takes place, thereby offering a means to manipulate bistate VC orientations.
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Jk Magnetization reversal mechanisms
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Ds Spin waves
75.40.Mg Numerical simulation studies

Touching the properties of NbTi by carbon doped tapes with mechanically alloyed MgB2

Marko Herrmann, Wolfgang Haessler, Christian Rodig, Wolfgang Gruner, Bernhard Holzapfel, and Ludwig Schultz

Appl. Phys. Lett. 91, 082507 (2007); http://dx.doi.org/10.1063/1.2773942 (3 pages) | Cited 32 times

Online Publication Date: 23 August 2007

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Combining mechanical alloying and powder-in-tube processing is a promising way to obtain tapes with excellent properties. Taking advantage of the properties of nanocrystalline precursor powders, it was possible to obtain Jc values of 104 A/cm2 at 12.1 T and 4.2 K. Evidential substitution of carbon into the MgB2 changed the electron scattering and therefore raised the Bc2 up to 12 T at 10 K. Systematic investigation on the influence of the heat treatment showed that, although an interfering Fe2B reaction layer was formed, an excellent Jc of 104 A/cm2 at 14.3 T and 4.2 K was achieved.
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74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)
84.71.Mn Superconducting wires, fibers, and tapes
74.25.Sv Critical currents
74.25.Op Mixed states, critical fields, and surface sheaths

Nanostripes in (Nd0.33Eu0.28Gd0.38)Ba2Cu3Ox superconductors

M. R. Koblischka, M. Winter, A. Koblischka-Veneva, M. Muralidhar, F. Mücklich, and U. Hartmann

Appl. Phys. Lett. 91, 082508 (2007); http://dx.doi.org/10.1063/1.2770774 (3 pages) | Cited 5 times

Online Publication Date: 24 August 2007

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Nanostripes are observed in melt-textured, Gd-enriched light-rare-earth (LRE)-based ternary (Nd0.33Eu0.28Gd0.38)Ba2Cu3Ox (NEG-38) superconductors. Employing atomic force microscopy/scanning tunneling microscopy topographic scans together with electron backscatter diffraction (EBSD) analysis, the authors determine the direction of the nanostripes with respect to the twin structure. Details of the topography measurements and of the EBSD maps reveal that the nanostripes are composed of nanoclusters of the LRE-rich phase. The nanostripes in the NEG-38 samples have a periodicity of about 40–60 nm, and in lateral direction between 300 and 500 nm. The role of these nanostripes for flux pinning in these compounds is discussed.
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74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
74.40.-n Fluctuation phenomena
74.81.Bd Granular, melt-textured, amorphous, and composite superconductors
74.72.-h Cuprate superconductors

Current induced switching of vortex polarity in magnetic nanodisks

Denis D. Sheka, Yuri Gaididei, and Franz G. Mertens

Appl. Phys. Lett. 91, 082509 (2007); http://dx.doi.org/10.1063/1.2775036 (3 pages) | Cited 26 times

Online Publication Date: 24 August 2007

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It is shown that the vortex polarity can be irreversibly switched by injecting a spin-polarized direct electrical current, which flows perpendicular to the disk plane. Intensive numerical spin-lattice simulations demonstrate that the switching process involves a vortex-antivortex pair creation. This differs from magnets with no dipolar interaction, where the spin dc acts similar to a static magnetic field.
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
72.25.-b Spin polarized transport
73.63.-b Electronic transport in nanoscale materials and structures

Magnetic force microscopy study of microwave-assisted magnetization reversal in submicron-scale ferromagnetic particles

Y. Nozaki, M. Ohta, S. Taharazako, K. Tateishi, S. Yoshimura, and K. Matsuyama

Appl. Phys. Lett. 91, 082510 (2007); http://dx.doi.org/10.1063/1.2775047 (3 pages) | Cited 23 times

Online Publication Date: 24 August 2007

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Changes in magnetic-domain configurations caused by the application of radio-frequency (rf) fields ranging from 3 to 18 GHz were observed using magnetic force microscope. rf fields barely altered the magnetization of Co particles with scattered large crystalline anisotropy; however, for NiFe particles, magnetization switching occurred when the rf approached the natural ferromagnetic-resonance frequency of 5.92 GHz. However, the reduction of the switching field fluctuated widely among the particles. Scattering of the effective energy barrier to thermal fluctuations caused by an applied rf field can explain the stochastic switching properties.
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Jk Magnetization reversal mechanisms
75.60.Ch Domain walls and domain structure
75.50.Cc Other ferromagnetic metals and alloys
75.50.Bb Fe and its alloys
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance

Magnetocaloric effect in Ho5Pd2: Evidence of large cooling power

Tapas Samanta, I. Das, and S. Banerjee

Appl. Phys. Lett. 91, 082511 (2007); http://dx.doi.org/10.1063/1.2775050 (3 pages) | Cited 31 times

Online Publication Date: 24 August 2007

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The study of magnetocaloric effect has been performed on polycrystalline Ho5Pd2 compound, which undergoes magnetic field-induced first order metamagnetic transition from antiferromagnetic to ferromagnetic state below antiferromagnetic transition temperature. A large magnetic entropy change associated with the field-induced metamagnetic transition has been observed over a wide temperature range. The evaluated value of relative cooling power is 6.32 J/cm3 for a magnetic field change of 5 T, which is even larger than that for reported best known magnetic refrigerant materials irrespective of the temperature range.
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75.30.Sg Magnetocaloric effect, magnetic cooling
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Half-metallic sp-electron ferromagnets in rocksalt structure: The case of SrC and BaC

G. Y. Gao and K. L. Yao

Appl. Phys. Lett. 91, 082512 (2007); http://dx.doi.org/10.1063/1.2775081 (3 pages) | Cited 26 times

Online Publication Date: 24 August 2007

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First-principles calculations are employed to study the structural and magnetic properties of SrC and BaC in four different phases: rocksalt, CsCl, zinc blende, and NiAs. The obtained results indicate that both compounds show half-metallic behavior in all phases except the CsCl-type structure. Among them the rocksalt structure is found to be the most stable one with a robust half-metallic characteristic with respect to the lattice compression and expansion. The magnetic phase transition temperature Tc of the rocksalt phase is calculated employing both the mean-field approximation and the random-phase approximation. The predicted Tc values of both compounds are considerably above room temperature.
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75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Dd Nonmetallic ferromagnetic materials
61.66.Fn Inorganic compounds

Nanoscale disorder in high critical field, carbon-doped MgB2 hybrid physical-chemical vapor deposition thin films

Y. Zhu, D. C. Larbalestier, P. M. Voyles, A. V. Pogrebnyakov, X. X. Xi, and J. M. Redwing

Appl. Phys. Lett. 91, 082513 (2007); http://dx.doi.org/10.1063/1.2775088 (3 pages) | Cited 10 times

Online Publication Date: 24 August 2007

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Carbon-doped MgB2 thin films deposited by hybrid physical-chemical vapor deposition (HPCVD) can have Hc2(0 K) ∼ 70 T, anomalously high compared to bulk samples which have Hc2 about half this value. Using transmission electron microscopy, the authors show that the carbon content in HPCVD-grown Mg(B1−xCx)2 grains is x ⩽ 5%, and that most carbon forms an amorphous intergranular phase. The authors also find extensive structural disorder, including rotations about the c axis and small tilts of the c axis, with a domain size of 5–20 nm. The authors propose that strong electron scattering from the nanoscale disorder is responsible for the very high Hc2 in such films.
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68.55.-a Thin film structure and morphology
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
61.43.-j Disordered solids
74.78.-w Superconducting films and low-dimensional structures
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