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14 Feb 2011

Volume 98, Issue 7, Articles (07xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 98, 073101 (2011); http://dx.doi.org/10.1063/1.3554360 (3 pages)

Wei Xu, Rajesh Leeladhar, Yao-Tsan Tsai, Eui-Hyeok Yang, and Chang-Hwan Choi
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Asymmetric double-shifted characteristics in epitaxial (002) exchange-biased IrMn/CoFe bilayers

Hsin-Hung Huang, Chih-Yen Yang, Hsiu-Hau Lin, and Chih-Huang Lai

Appl. Phys. Lett. 98, 072501 (2011); http://dx.doi.org/10.1063/1.3554384 (3 pages) | Cited 1 time

Online Publication Date: 15 February 2011

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Symmetric and asymmetric double-shifted loops were observed at room temperature in epitaxial (002) Ir20Mn80/Co50Fe50 samples placed in a setting field applied along collinear and noncollinear directions with the deposition-field direction, respectively. A coherent rotation model with extra unidirectional exchange and uniaxial anisotropies induced by the noncollinear setting field is proposed to explain the asymmetric double-shifted loops. Due to the existence of the biaxial magnetocrystalline anisotropy, the noncollinear setting field results in two interfacial Ir20Mn80 spin orientations, where the major spins determine the direction to observe double-shifted loops and the minor spins determine the field-shift and the asymmetry of double-shifted loops.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Et Exchange and superexchange interactions
75.30.Gw Magnetic anisotropy
75.30.Ds Spin waves

Exchange-spring behavior in nanopowders of CoFe2O4–CoFe2

J. M. Soares, F. A. O. Cabral, J. H. de Araújo, and F. L. A. Machado

Appl. Phys. Lett. 98, 072502 (2011); http://dx.doi.org/10.1063/1.3552677 (3 pages) | Cited 6 times

Online Publication Date: 15 February 2011

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The nature of the magnetic interactions among core-shell nanoparticles of CoFe2O4–CoFe2 having an average particle size of 32 nm were investigated by δm plots. It was found that the interactions are mainly demagnetizing in nature for nanoparticles with core diameters larger than 26 nm. For smaller values of core diameters, the exchange-spring phenomenon takes place due to the exchange-coupling at the core-shell interface. The critical thickness of the shell was estimated to be 7.8 nm. The overall results are in excellent agreement with current theoretical models.
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75.75.-c Magnetic properties of nanostructures
75.50.Tt Fine-particle systems; nanocrystalline materials
75.30.Et Exchange and superexchange interactions
75.60.-d Domain effects, magnetization curves, and hysteresis

Josephson tunnel junctions in a magnetic field gradient

R. Monaco, J. Mygind, and V. P. Koshelets

Appl. Phys. Lett. 98, 072503 (2011); http://dx.doi.org/10.1063/1.3555464 (3 pages) | Cited 1 time

Online Publication Date: 15 February 2011

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We measured the magnetic field dependence of the critical current of high-quality Nb-based planar Josephson tunnel junctions in the presence of a controllable nonuniform field distribution. We found skewed and slowly changing magnetic diffraction patterns quite dissimilar from the Fraunhofer-like ones typical of a homogeneous field. Our findings can be well interpreted in terms of recent theoretical predictions [R. Monaco, J. Appl. Phys. 108, 033906 (2010)] for a uniform magnetic field gradient, leading to Fresnel-like magnetic diffraction patterns. We also show that Fiske resonances can be suppressed by an asymmetric magnetic field profile.
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74.50.+r Tunneling phenomena; Josephson effects

Flux requirements for the growth of RFeAsO (R = rare earth) superconductors

J.-Q. Yan, B. Jensen, K. W. Dennis, R. W. McCallum, and T. A. Lograsso

Appl. Phys. Lett. 98, 072504 (2011); http://dx.doi.org/10.1063/1.3555632 (3 pages)

Online Publication Date: 16 February 2011

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Millimeter-sized LaFeAsO single crystals have been successfully grown out of NaAs flux starting with (LaAs+1/2Fe2O3):20NaAs. The factors which allow the growth of these crystals out of NaAs but not out of many other fluxes, such as FeAs, have been investigated. X-ray powder diffraction found that NaAs synthesized for the growth of RFeAsO superconductors has monoclinic LiAs structure. Thermal analysis confirmed that NaAs melts congruently at about 600 °C. The ability to extract RFeAsO crystals from this NaAs flux suggests that NaAs has a significant oxygen solubility, possibly due to the formation of the ternary compound NaAsO2, and enough oxygen diffusivity to transport solute to the growth front. Oxygen solubility and diffusivity are two important factors in searching for an alternative environmentally benign flux for the growth of RFeAsO superconductors.
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81.10.Fq Growth from melts; zone melting and refining
61.66.Fn Inorganic compounds
74.70.Xa Pnictides and chalcogenides

Cation-ordering effects in the single layered manganite La2/3Sr4/3MnO4

B. B. Nelson-Cheeseman, A. B. Shah, T. S. Santos, S. D. Bader, J.-M. Zuo, and A. Bhattacharya

Appl. Phys. Lett. 98, 072505 (2011); http://dx.doi.org/10.1063/1.3554387 (3 pages) | Cited 4 times

Online Publication Date: 16 February 2011

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Epitaxial La1−xSrxMnO3⋅SrO (x = 1/3) films were synthesized as random alloys and cation-ordered analogs to probe how cation ordering affects the properties of a two dimensional manganite. The films show evidence for a spin glass like phase below 20 K and weak ferromagnetic ordering up to 130 K. There is a dramatic difference in magnetic anisotropy and the low temperature transport mechanism depending on the cation order, which is consistent with differences in Mn 3d orbital occupancies. This work indicates that cation ordering can significantly alter the Mn 3d orbital ground state in these correlated electron systems.
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75.70.Ak Magnetic properties of monolayers and thin films
75.30.Gw Magnetic anisotropy
75.50.Dd Nonmetallic ferromagnetic materials
73.61.Ng Insulators
75.50.Lk Spin glasses and other random magnets

Phonon softening induced by striped antiferromagnetic order in LiFeAs

B. Li, Z. W. Xing, and M. Liu

Appl. Phys. Lett. 98, 072506 (2011); http://dx.doi.org/10.1063/1.3555457 (3 pages) | Cited 3 times

Online Publication Date: 16 February 2011

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Using first-principles calculations in a supercell model with structural optimization, we investigate the magnetic ground state of LiFeAs and its phonon spectrum. The obtained magnetic ground state is the marginal striped antiferromagnetic state with partial structural relaxations. It is found that the anisotropic spin-lattice coupling in LiFeAs results in phonon softening of Fe–As atomic motion and so enhances the electron-phonon coupling. As a result, the electron-phonon interactions through the spin channel play an important role in the superconductivity of LiFeAs.
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74.25.Kc Phonons
63.20.kd Phonon-electron interactions
75.30.Ds Spin waves
71.15.-m Methods of electronic structure calculations

Stochastic resonance of a domain wall in a stripe with two pinning sites

Eduardo Martinez, Giovanni Finocchio, and Mario Carpentieri

Appl. Phys. Lett. 98, 072507 (2011); http://dx.doi.org/10.1063/1.3556314 (3 pages) | Cited 4 times

Online Publication Date: 17 February 2011

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We report on the observation of stochastic resonance phenomenon of a single domain wall in a ferromagnetic stripe with two pinning sites. Under a weak oscillating field, the wall performs irregular transitions between both constrictions in the presence of thermal fluctuations. Ours results indicate that synchronized wall transitions with the driving field can be achieved at the optimal level of noise. The stochastic resonance is quantified by computing the output signal power as a function of temperature. The analysis points out that this system could be used to design well-controlled amplification devices, which could find application as nanodetectors.
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76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
02.50.Fz Stochastic analysis
75.60.Ch Domain walls and domain structure
75.50.Bb Fe and its alloys
05.40.Ca Noise

Investigation of critical behavior in Pr0.55Sr0.45MnO3 by using the field dependence of magnetic entropy change

Jiyu Fan, Li Pi, Lei Zhang, Wei Tong, Langsheng Ling, Bo Hong, Yangguang Shi, Weichun Zhang, Di Lu, and Yuheng Zhang

Appl. Phys. Lett. 98, 072508 (2011); http://dx.doi.org/10.1063/1.3554390 (3 pages) | Cited 8 times

Online Publication Date: 17 February 2011

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One method of calculating critical exponents based on the field dependence on magnetic entropy change was applied to study critical behavior in Pr0.55Sr0.45MnO3. By using the obtained critical exponents, the modified Arrott plot [ A. Arrott and J. E. Noakes, Phys. Rev. Lett. 19, 786 (1967) ] is consistent with that by using conventional method. The calculated critical exponents not only obey the scaling theory, but also inosculate the deduced results from the Kouvel–Fisher method [ J. S. Kouvel and M. E. Fisher, Phys. Rev. 136, A1626 (1964) ]. It eliminates the drawback due to utilization of multistep nonlinear fitting in a conventional manner. Therefore, this means can be applied to investigate critical behavior.
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75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
64.60.fh Studies of specific substances in the critical region
75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)

Dipolar ferromagnetic phase transition in Fe3O4 nanoparticle arrays observed by Lorentz microscopy and electron holography

Kazuo Yamamoto, Charles R. Hogg, Saeki Yamamuro, Tsukasa Hirayama, and Sara A. Majetich

Appl. Phys. Lett. 98, 072509 (2011); http://dx.doi.org/10.1063/1.3556562 (3 pages) | Cited 9 times

Online Publication Date: 17 February 2011

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Dipolar ferromagnetism formed in Fe3O4 nanoparticle arrays is revealed by Fresnel Lorentz microscopy and electron holography. Dipolar domain walls do not lie preferentially along macrograin boundaries but depend on the overall shape of the assembly, meaning magnetostatic energy dominates. The domain structures are imaged at different temperatures for both monolayer and bilayer arrays. The domain wall contrast in the monolayer region is visible until 575 °C, and the magnetic order parameter steeply drops toward the temperature. In the bilayer region, finer and more complicated domains are formed.
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75.75.-c Magnetic properties of nanostructures
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
61.72.Mm Grain and twin boundaries
75.70.Ak Magnetic properties of monolayers and thin films
75.75.Fk Domain structures in nanoparticles
75.60.Ch Domain walls and domain structure

Enhanced magnetization drift velocity and current polarization in (CoFe)1−xGex alloys

M. Zhu, B. D. Soe, R. D. McMichael, M. J. Carey, S. Maat, and J. R. Childress

Appl. Phys. Lett. 98, 072510 (2011); http://dx.doi.org/10.1063/1.3554755 (3 pages) | Cited 2 times

Online Publication Date: 17 February 2011

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A spin-wave Doppler technique is used to measure the drift velocity of the magnetization in current-carrying (CoFe)1−xGex alloys. For a current density of 1011 A/m2, we obtain a large enhancement of drift velocity with increased Ge concentration from 3.1±0.2 m/s for CoFe to 8.2±0.6 m/s for (CoFe)0.7Ge0.3. Interpretation of these values yields current polarization increasing from 0.84±0.04 to 0.95±0.05 as the Ge doping increases. While both spin-dependent conductivities decrease with increasing Ge concentration, the maximum in polarization is associated with a stronger relative change in the minority spin conductivity.
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75.47.-m Magnetotransport phenomena; materials for magnetotransport
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Ds Spin waves

Quantum-mechanical method for estimating ion distributions in spinel ferrites

G. D. Tang, D. H. Ji, Y. X. Yao, S. P. Liu, Z. Z. Li, W. H. Qi, Q. J. Han, X. Hou, and D. L. Hou

Appl. Phys. Lett. 98, 072511 (2011); http://dx.doi.org/10.1063/1.3553774 (3 pages) | Cited 5 times

Online Publication Date: 17 February 2011

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A quantum-mechanical method for estimating the cation distribution in spinel ferrites is proposed, by which the ionization energy of the cations and the Pauli repulsion energy is considered, together with the magnetic ordered energy and the tendency toward charge density balance. Using this method, not only can the difference between the observed and the traditional theoretical magnetic moments of the spinel structure ferrites MFe2O4 (M = Mn,Fe,Co,Ni,Cu) be explained, but also the dependence of the magnetic moments of the ferrites M1−xZnxFe2O4 (M = Mn,Fe,Co,Ni,Cu) on the doping level x can be fitted.
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61.72.up Other materials
75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Gg Ferrimagnetics

Effect of resistance-area product on spin-transfer switching in MgO-based magnetic tunnel junction memory cells

Z. M. Zeng, P. Khalili Amiri, G. Rowlands, H. Zhao, I. N. Krivorotov, J.-P. Wang, J. A. Katine, J. Langer, K. Galatsis, K. L. Wang, and H. W. Jiang

Appl. Phys. Lett. 98, 072512 (2011); http://dx.doi.org/10.1063/1.3556615 (3 pages) | Cited 4 times

Online Publication Date: 18 February 2011

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We use ultrafast current-induced switching measurements to study spin-transfer switching performance metrics, such as write energy per bit (EW) and switching current density (Jc), as a function of resistance-area product (RA) (hence MgO thickness) in magnetic tunnel junction cells used for magnetoresistive random access memory (MRAM). EW increases with RA, while Jc decreases with increasing RA for both switching directions. The results are discussed in terms of RA optimization for low write energy and current drive capability (hence density) of the MRAM cells. Switching times <2 ns and write energies <0.3 pJ are demonstrated for 135 nm×65 nm CoFeB/MgO/CoFeB devices.
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85.70.Ec Magnetostrictive, magnetoacoustic, and magnetostatic devices
85.75.Dd Magnetic memory using magnetic tunnel junctions
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