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18 May 2009

Volume 94, Issue 20, Articles (20xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 94, 203301 (2009); http://dx.doi.org/10.1063/1.3133902 (3 pages)

Zihong Liu, Joon Hak Oh, Mark E. Roberts, Peng Wei, Bipul C. Paul, Masaki Okajima, Yoshio Nishi, and Zhenan Bao
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Pattern induced phase transition of vortex motion in high-Tc films

R. Wördenweber, E. Hollmann, J. Schubert, R. Kutzner, and Ajay Kumar Ghosh

Appl. Phys. Lett. 94, 202501 (2009); http://dx.doi.org/10.1063/1.3139077 (3 pages) | Cited 2 times

Online Publication Date: 18 May 2009

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A micropattern induced transition in the mechanism of vortex motion and vortex mobility is demonstrated for high-Tc films. The competition between the anomalous Hall effect and the guidance of vortices by rows of microholes (antidots) leads to a sudden change in the direction of vortex motion that is accompanied by a change of the critical current density and microwave losses. The latter demonstrates the difference in vortex mobility in the different phases of vortex motion in between and within the rows of antidots.
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74.78.-w Superconducting films and low-dimensional structures
72.15.Gd Galvanomagnetic and other magnetotransport effects
74.25.Sv Critical currents
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)

Spin polarization of Fe4N thin films determined by point-contact Andreev reflection

A. Narahara, K. Ito, T. Suemasu, Y. K. Takahashi, A. Ranajikanth, and K. Hono

Appl. Phys. Lett. 94, 202502 (2009); http://dx.doi.org/10.1063/1.3140459 (3 pages) | Cited 12 times

Online Publication Date: 18 May 2009

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The spin polarization of (100)-oriented γ′-Fe4N layers grown on MgO(001) substrates by molecular beam epitaxy was deduced from point contact Andreev reflection measurements, and the value was compared with that of α-Fe. The spin polarization (P) for γ′-Fe4N is approximately 0.59 at 7.8 K. This value is distinctly larger than that for α-Fe (P = 0.49 at 7.8 K) measured with an identical setting. The mechanism of enhanced spin polarization in γ′-Fe4N is discussed.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.A- Nucleation and growth

Surface and core magnetic anisotropy in maghemite nanoparticles determined by pressure experiments

Y. Komorida, M. Mito, H. Deguchi, S. Takagi, A. Millán, N. J. O. Silva, and F. Palacio

Appl. Phys. Lett. 94, 202503 (2009); http://dx.doi.org/10.1063/1.3131782 (3 pages) | Cited 9 times

Online Publication Date: 19 May 2009

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In magnetic nanoparticles, anisotropy energy has extra contributions compared to that of the bulk counterparts, being the most relevant surface anisotropy. Here we use pressure to separate core from surface anisotropy in one system of maghemite nanoparticles dispersed in a polymer. The core anisotropy is Kcore = 7.7×105 erg/cm3 while the surface anisotropy is KS = 4.2×10−2 erg/cm2. This in-one-sample separation is possible due to changes in structurally ordered and disordered ratio, which induce changes in the average magnetic anisotropy energy.
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75.30.Gw Magnetic anisotropy
75.70.Rf Surface magnetism
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

Dynamics of single vortices in grain boundaries: I-V characteristics on the femtovolt scale

B. Kalisky, J. R. Kirtley, E. A. Nowadnick, R. B. Dinner, E. Zeldov, Ariando, S. Wenderich, H. Hilgenkamp, D. M. Feldmann, and K. A. Moler

Appl. Phys. Lett. 94, 202504 (2009); http://dx.doi.org/10.1063/1.3137164 (3 pages) | Cited 3 times

Online Publication Date: 19 May 2009

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We employed a scanning Hall probe microscope to detect the hopping of individual vortices between pinning sites along grain boundaries in YBa2Cu3O6+δ thin films in the presence of an applied current. Detecting the motion of individual vortices allowed us to probe the current-voltage (I-V) characteristics of the grain boundary with voltage sensitivity below a femtovolt. We find a very sharp onset of dissipation with V∝In with an unprecedented high exponent of n ≈ 290 that shows essentially no dependence on temperature or grain boundary angle. Our data have no straightforward explanation within the existing grain boundary transport models.
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74.78.-w Superconducting films and low-dimensional structures
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
74.25.F- Transport properties

Nonreciprocal microwave devices based on magnetic nanowires

Bijoy K. Kuanr, V. Veerakumar, Ryan Marson, Sanjay R. Mishra, R. E. Camley, and Z. Celinski

Appl. Phys. Lett. 94, 202505 (2009); http://dx.doi.org/10.1063/1.3124657 (3 pages) | Cited 15 times

Online Publication Date: 20 May 2009

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We use magnetic nanowires in an alumina matrix as the active element in microwave nonreciprocal resonance isolators. The design is related to waveguide E-plane isolators but is planar and much smaller than typical waveguide isolators. There is a nonreciprocal attenuation of the wave in forward and reverse directions. The isolation is about 6 dB/cm at 23 GHz. The bandwidth of the device is relatively large (5–7 GHz) in comparison to ferrite-based devices. The central frequency of the device can be tuned with the application of magnetic field.
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84.40.Az Waveguides, transmission lines, striplines
85.70.-w Magnetic devices

Low frequency magnetoresistive noise in spin-valve structures

A. Ozbay, A. Gokce, T. Flanagan, R. A. Stearrett, E. R. Nowak, and C. Nordman

Appl. Phys. Lett. 94, 202506 (2009); http://dx.doi.org/10.1063/1.3139067 (3 pages) | Cited 19 times

Online Publication Date: 20 May 2009

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We report on resistance noise in spin-valve structures that is due to reconfigurations in domain structure of the magnetic layers. 1/f noise from the free layer and pinned layer is evident and its magnitude is in good agreement with predictions from the fluctuation dissipation relation using the imaginary (dissipative) component of the measured resistance susceptibility. In addition, we find that the imaginary component is dependent on applied magnetic field, being larger for layers that exhibit pronounced magnetic hysteresis. A magnetoresistive 1/f noise parameter is proposed, and benchmark values for a variety of spin-valve devices are reported.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
85.70.Kh Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.
75.60.Ch Domain walls and domain structure
75.47.-m Magnetotransport phenomena; materials for magnetotransport
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Cr Saturation moments and magnetic susceptibilities

Room-temperature simultaneously enhanced magnetization and electric polarization in BiFeO3 ceramic synthesized by magnetic annealing

Wanju Luo, Dongliang Wang, Fangwei Wang, Tao Liu, Jianwang Cai, Liyan Zhang, and Yulong Liu

Appl. Phys. Lett. 94, 202507 (2009); http://dx.doi.org/10.1063/1.3139780 (3 pages) | Cited 13 times

Online Publication Date: 20 May 2009

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Multiferroic BiFeO3 ceramics were synthesized by high temperature magnetic annealing using nanosized precursor powders prepared through microwave combustion. Simultaneously enhanced magnetization and electric polarization were observed at room temperature in the sample annealed under an external magnetic field of 10 T. These enhanced properties might be originated from a spin structure severely modulated or the low temperature magnetic phase driven up to room temperature and above by the large external annealing magnetic fields. These results demonstrate that the strong magnetic annealing method is an alternative way to synthesize high performance BiFeO3 materials.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.60.Nt Magnetic annealing and temperature-hysteresis effects
75.80.+q Magnetomechanical effects, magnetostriction
77.80.-e Ferroelectricity and antiferroelectricity
77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials
77.22.Ej Polarization and depolarization

Transition width limit in magnetic recording

Xiaobin Wang, Bogdan Valcu, and Nan-Hsiung Yeh

Appl. Phys. Lett. 94, 202508 (2009); http://dx.doi.org/10.1063/1.3141455 (3 pages) | Cited 1 time

Online Publication Date: 20 May 2009

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The minimum magnetization transition width between information bits determines achievable recording linear density. The ultimate limit of this width is determined by the continuous media granular structure. The dependence of the transition width limit on grain size mean and standard deviation are given without an a priori assumption of magnetization transition shape. A simplified approach to combine finite grain size and magnetic field gradient for transition width is discussed.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Ss Magnetic recording materials

Microwave assisted magnetization reversal in composite media

Shaojing Li, Boris Livshitz, H. Neal Bertram, Manfred Schabes, Thomas Schrefl, Eric E. Fullerton, and Vitaliy Lomakin

Appl. Phys. Lett. 94, 202509 (2009); http://dx.doi.org/10.1063/1.3133354 (3 pages) | Cited 12 times

Online Publication Date: 22 May 2009

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Magnetic reversal in exchange-coupled composite elements under microwave fields is characterized by several unique properties including reduced reversal fields, microwave fields, microwave resonant frequencies, and reduced sensitivity to anisotropy distributions as compared to homogeneous elements. We find that reversal can occur in uniform and nonuniform regimes. The uniform regime is characterized by coherent spin precession enhancement by the microwave field. In the nonuniform regime domain walls in the soft layer mediate reversal and under linearly polarized microwave fields, can lead to a formation of localized reversal/nonreversal areas in the “applied field-frequency” phase plane.
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75.60.Jk Magnetization reversal mechanisms
75.30.Et Exchange and superexchange interactions
75.50.Ss Magnetic recording materials
75.30.Gw Magnetic anisotropy
75.60.Ch Domain walls and domain structure

Tailoring exchange bias in half-metallic La2/3Sr1/3MnO3 thin films for spin valve applications

P. K. Muduli and R. C. Budhani

Appl. Phys. Lett. 94, 202510 (2009); http://dx.doi.org/10.1063/1.3139770 (3 pages) | Cited 2 times

Online Publication Date: 22 May 2009

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We have utilized the antiferromagnetic nature and structural/chemical compatibility of La0.45Sr0.55MnO3 with highly spin polarized La0.67Sr0.33MnO3 to prepare epitaxial exchange bias (EB) couples. A robust EB shift in magnetization hysteresis with associated interfacial exchange energy J ≈ 0.13 erg/cm2 at 10 K along with enhanced coercivity is reported. The EB effect was engineered to bring coercivity contrast between La0.67Sr0.33MnO3 and cobalt films in La0.45Sr0.55MnO3/La0.67Sr0.33MnO3/SrTiO3/Co magnetic tunnel junctions.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Ee Antiferromagnetics
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
72.25.-b Spin polarized transport
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