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1 Aug 2011

Volume 99, Issue 5, Articles (05xxxx)

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Appl. Phys. Lett. 99, 051112 (2011); http://dx.doi.org/10.1063/1.3617421 (3 pages)

Wei Li, Jun Chen, Gerard Nouet, Liang-yao Chen, and Xunya Jiang
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Linewidth reduction in a spin-torque nano-oscillator caused by non-conservative current-induced coupling between magnetic layers

D. Gusakova, M. Quinsat, J. F. Sierra, U. Ebels, B. Dieny, L. D. Buda-Prejbeanu, M.-C. Cyrille, V. Tiberkevich, and A. N. Slavin

Appl. Phys. Lett. 99, 052501 (2011); http://dx.doi.org/10.1063/1.3615283 (3 pages) | Cited 7 times

Online Publication Date: 1 August 2011

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We demonstrate by macrospin simulations that in a spin-torque nano-oscillator having synthetic antiferromagnet fixed layer, the non-conservative dynamic coupling between the free and fixed layers caused by spin-torque effect leads to a substantial reduction of the linewidth of the current-induced spin wave mode, involving oscillations in all three magnetic layers. By analysing the phase and amplitude noise extracted from the simulated signal, we prove that the obtained linewidth reduction is related to the reduction of the dimensionless non-linear amplitude-phase coupling parameter ν.
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85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
84.30.Ng Oscillators, pulse generators, and function generators
85.35.-p Nanoelectronic devices

d0 ferromagnetism in undoped sphalerite ZnS nanoparticles

Daqiang Gao, Guijin Yang, Jing Zhang, Zhonghua Zhu, Mingsu Si, and Desheng Xue

Appl. Phys. Lett. 99, 052502 (2011); http://dx.doi.org/10.1063/1.3622303 (3 pages) | Cited 9 times

Online Publication Date: 1 August 2011

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We report the sulfur vacancies-related d0 ferromagnetism in undoped sphalerite ZnS nanoparticles. Systematically tune of sulfur deficiency in ZnS nanoparticles was done by selecting different synthesized temperatures and varying the ratio of hydrogen and argon in post-annealing processes. Our study suggests that such sulfur vacancies can induce the room temperature ferromagnetism. Importantly, the ferromagnetism can be modulated by changing the concentration of sulfur vacancies in the samples. This finding should be the focus of future electronic and spintronic devices.
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75.75.Cd Fabrication of magnetic nanostructures
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Pp Magnetic semiconductors
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance

Magnetic phase separation in SrCoOx (2.5 ≤ x ≤ 3)

C. K. Xie, Y. F. Nie, B. O. Wells, J. I. Budnick, W. A. Hines, and B. Dabrowski

Appl. Phys. Lett. 99, 052503 (2011); http://dx.doi.org/10.1063/1.3622644 (3 pages) | Cited 2 times

Online Publication Date: 2 August 2011

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This work presents a study of the electronic phase separation resulting from oxygen non-stoichiometry in SrCoOx. We report here that for oxygen content 2.88 ≤ x ≤ 3, SrCoOx exhibits a magnetic phase separation while maintaining a single crystallographic phase. Two magnetic components are formed which match those found in SrCoO2.88 and SrCoO3 with TC = 220 K and 280 K, respectively. In addition, a value of TC = 160 K is assigned to the previously identified SrCoO2.75 phase. A magnetic phase diagram with four line phases is proposed for SrCoOx (2.5 ≤ x ≤ 3).
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75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.30.Wx Spin crossover
75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)
75.50.Dd Nonmetallic ferromagnetic materials
61.66.Fn Inorganic compounds

Effects of nanocrystal formation on the soft magnetic properties of Fe-based bulk metallic glasses

J. E. Gao, H. X. Li, Z. B. Jiao, Y. Wu, Y. H. Chen, T. Yu, and Z. P. Lu

Appl. Phys. Lett. 99, 052504 (2011); http://dx.doi.org/10.1063/1.3621832 (3 pages) | Cited 3 times

Online Publication Date: 2 August 2011

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We developed several Fe-based bulk metallic glasses with a unique combination of large glass-forming ability and excellent soft magnetic properties by minor doping of Cu in the Fe76C7.0Si3.3B5.0P8.7 alloy. Proper additions of the non-magnetic copper element which has a positive heat of mixing with Fe, coupled with adequate annealing, can stimulate formation of ∼5 nm αFe ferromagnetic nanocrystals, which results in the increment in the saturation magnetization. Over-annealing which induced coarsening of the α-Fe nanocrystals reduces the ferromagnetic exchange interaction between the nanosized α-Fe crystals and increases the effective magneto-crystalline anisotropy, thereby deteriorating the soft magnetic properties.
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75.75.-c Magnetic properties of nanostructures
75.30.Et Exchange and superexchange interactions
75.30.Gw Magnetic anisotropy
75.50.Bb Fe and its alloys
75.50.Kj Amorphous and quasicrystalline magnetic materials
75.50.Tt Fine-particle systems; nanocrystalline materials
64.75.Ef Mixing

Ferromagnetism in C-doped SnO2 thin films

Nguyen Hoa Hong, J.-H. Song, A. T. Raghavender, T. Asaeda, and M. Kurisu

Appl. Phys. Lett. 99, 052505 (2011); http://dx.doi.org/10.1063/1.3617439 (3 pages) | Cited 14 times

Online Publication Date: 3 August 2011

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Room temperature ferromagnetism (FM) was observed in laser ablated C-doped SnO2 thin films grown on LaAlO3 substrates. The greatest value of the saturated magnetization (at 1 T) is found in SnO2 films doped with 1% of C (about 16 emu/cm3) and reduces lightly as the C concentration increases. By subtracting the base of pure SnO2 that is also magnetic, one can find that C-doping actually enhances ferromagnetism in the host SnO2. It suggests that carbon really causes defect-induced magnetism of about 3.91 μB/C into SnO2. Measurements on C-doped SnO2 bulks show that the observed properties are unique for films (lower dimensions, having surface/interface effect). The investigation on thickness dependence shows a change in magnetization when going from thin to thick films, implying that somehow if magnetism is due to defects then those must be located more on or near the surface than in deeper layers. Anisotropy is also observed, enforcing the assumption for the origin of magnetism due to defects in C-doped SnO2.
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75.70.Ak Magnetic properties of monolayers and thin films
61.72.up Other materials
75.30.Gw Magnetic anisotropy
75.50.Dd Nonmetallic ferromagnetic materials

Spin exchange interactions in hexagonal manganites RMnO3 (R = Tb, Dy, Ho, Er) epitaxial thin films

Xiang-Bai Chen, Nguyen Thi Minh Hien, D. Lee, S.-Y. Jang, T. W. Noh, and In-Sang Yang

Appl. Phys. Lett. 99, 052506 (2011); http://dx.doi.org/10.1063/1.3622768 (3 pages) | Cited 1 time

Online Publication Date: 3 August 2011

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We present the results of an optical method of quantitatively estimating the spin exchange interactions in hexagonal manganites RMnO3 (R = Tb, Dy, Ho, Er) epitaxial thin films. The two in-plane (a-b plane) spin exchange integrals J1 (intratrimer Mn-Mn interaction) and J2 (intertrimer Mn-Mn interaction) are deduced from the magnon scattering peak wavenumbers. We found that J2 decreases systematically when the R ionic radius increases, while J1 is nearly independent of R ionic radius, contrary to the expectation in single crystals. We show that the R dependence of J1 could be understood in terms of the stress in the thin films. Our result indicates that the stress has stronger effect on the atomic displacement of the intratrimer Mn-Mn distance than the intertrimer Mn-Mn distance.
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75.30.Et Exchange and superexchange interactions
75.50.Ee Antiferromagnetics
75.70.Ak Magnetic properties of monolayers and thin films
78.30.Hv Other nonmetallic inorganics
75.30.Ds Spin waves

Magnetization reversal in composition-controlled Gd1–xCox ferrimagnetic films close to compensation composition

A. Hrabec, N. T. Nam, S. Pizzini, and L. Ranno

Appl. Phys. Lett. 99, 052507 (2011); http://dx.doi.org/10.1063/1.3609860 (3 pages) | Cited 2 times

Online Publication Date: 3 August 2011

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We report on a model system for micromagnetic studies, i.e., ferrimagnetic Gd1−xCox thin films with controlled composition gradient and, therefore, a controlled magnetization gradient along the film. By employing extraordinary Hall effect measurements and Kerr microscopy, we have studied magnetization reversal and shown that, around compensation, varying magnetization with temperature or composition is equivalent. In particular, the coercive field diverges close to the compensation temperature or close to the compensation interface. The position of the compensation interface is very sensitive to temperature and can be used as a probe of sample heating.
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75.60.Jk Magnetization reversal mechanisms
75.70.Ak Magnetic properties of monolayers and thin films
72.15.Gd Galvanomagnetic and other magnetotransport effects
73.61.At Metal and metallic alloys
75.50.Gg Ferrimagnetics
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

A possible pressure-induced superconducting-semiconducting transition in nearly optimally doped single crystalline YBa2Cu3O7-δ

T. Muramatsu, D. Pham, and C. W. Chu

Appl. Phys. Lett. 99, 052508 (2011); http://dx.doi.org/10.1063/1.3623475 (3 pages) | Cited 1 time

Online Publication Date: 4 August 2011

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The pressure effect on the single-crystalline, nearly optimally doped high temperature cuprate superconductor YBa2Cu3O7-δ with a sharp ambient transition temperature of 90.7 K has been investigated resistively up to 29 GPa. An abrupt disappearance of superconductivity at 60 K under ∼24 GPa was detected, accompanied by a thermally activated resistance, suggesting a possible pressure-induced superconducting-semiconducting transition. By comparing the present results with those of recent high pressure work on Bi2Sr2Ca2Cu3O10-δ, it is proposed that the control of such electronic instabilities may further help raise the superconducting transition temperature record of 164 K in tri-layer cuprate superconductors by pressures.
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74.62.Fj Effects of pressure
74.72.-h Cuprate superconductors
74.25.F- Transport properties

High TC half-metallic fully-compensated ferrimagnetic Heusler compounds

I. Galanakis and E. Şaşıoğlu

Appl. Phys. Lett. 99, 052509 (2011); http://dx.doi.org/10.1063/1.3619844 (3 pages) | Cited 11 times

Online Publication Date: 4 August 2011

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Extensive ab-initio electronic structure calculations on Heusler alloys suggest that Cr2CoGa is the alloy of choice to achieve the half-metallic fully-compensated ferrimagnetism since (1) it has been already grown experimentally [T. Graf et al., Z. Anorg. Allg. Chem. 635, 976 (2009)], (2) half-metallic XA structure is favored energetically over all the studied lattice constant range with respect to the L21 which is not half-metallic, (3) the half-metallic gap is wide and the Fermi level falls at the middle of the gap and thus, it presents high degree of spin-polarization for a wide range of lattice constants, and (4) the Curie temperature is extremely high reaching the 1520 K.
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71.20.Gj Other metals and alloys
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Gg Ferrimagnetics
61.66.Dk Alloys
71.15.Pd Molecular dynamics calculations (Car-Parrinello) and other numerical simulations

High-power rf oscillation induced in half-metallic Co2MnSi layer by spin-transfer torque

R. Okura, Y. Sakuraba, T. Seki, K. Izumi, M. Mizuguchi, and K. Takanashi

Appl. Phys. Lett. 99, 052510 (2011); http://dx.doi.org/10.1063/1.3624470 (3 pages) | Cited 4 times

Online Publication Date: 4 August 2011

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The rf oscillation induced in a current-perpendicular-to-plane device with Co2MnSi (CMS) layers by spin-transfer torque was investigated to enhance the rf output power due to the large magnetoresistance (MR) ratio. A large MR ratio of 12.5% was obtained due to the large spin-polarization of CMS, and fundamental and second harmonic rf oscillations were clearly observed in the CMS layer. A high rf output power of 1.1 nW was achieved in spite of a small precession angle of 8.6°.
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85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
84.30.Ng Oscillators, pulse generators, and function generators

Oscillating magnetocaloric effect

M. S. Reis

Appl. Phys. Lett. 99, 052511 (2011); http://dx.doi.org/10.1063/1.3615296 (3 pages) | Cited 6 times

Online Publication Date: 5 August 2011

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This Letter presents the oscillatory behavior found in the magnetic entropy change of diamagnetic materials. We show that this quantity depends on the oscillating term and, as a consequence, the magnetocaloric potential can be tuned as either inverse or normal, depending on the value of the magnetic field change. A quite small change (≈10−3 T) of the magnetic field change is able to invert the magnetic entropy change. These results open doors for applications at quite low temperatures and can be further developed to be incorporated into adiabatic demagnetization refrigerators, as well as sensible magnetic field sensors.
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75.30.Sg Magnetocaloric effect, magnetic cooling
07.20.Mc Cryogenics; refrigerators, low-temperature detectors, and other low-temperature equipment

Slow magnetization dynamics and energy barriers near vortex state nucleation in circular permalloy dots

G. N. Kakazei, M. Ilyn, O. Chubykalo-Fesenko, J. Gonzalez, A. A. Serga, A. V. Chumak, P. A. Beck, B. Laegel, B. Hillebrands, and K. Y. Guslienko

Appl. Phys. Lett. 99, 052512 (2011); http://dx.doi.org/10.1063/1.3619846 (3 pages) | Cited 1 time

Online Publication Date: 5 August 2011

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Time decay of the magnetization of the arrays of Permalloy circular dots of submicron sizes was measured on a long time scale (hours) near the vortex nucleation field. A considerable influence of external magnetic field and temperature on the slow magnetization dynamics was detected. The observed effects are explained by overcoming the field dependent energy barriers in the process of vortex nucleation. The magnetic viscosity and energy barriers were found from the magnetization time decay dependencies.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.60.Lr Magnetic aftereffects

Oxygen enhanced ferromagnetism in Cr-doped ZnO films

Ze Xiong, Xue-Chao Liu, Shi-Yi Zhuo, Jian-Hua Yang, Er-Wei Shi, and Wen-Sheng Yan

Appl. Phys. Lett. 99, 052513 (2011); http://dx.doi.org/10.1063/1.3624589 (3 pages) | Cited 3 times

Online Publication Date: 5 August 2011

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Cr-doped ZnO films have been prepared by inductively coupled plasma enhanced physical vapor deposition, and an in-depth study is performed on the chromium doping and oxygen partial pressure dependence of ferromagnetism. The x-ray diffraction and photoluminescence results indicate that the moderate oxygen can relax the lattice strain thus enhancing ferromagnetism which, as confirmed by soft x-ray absorption spectroscopy, is mainly attributed to the Cr valence state transition from Cr3+ to Cr6+. However, excessive oxygen suppresses the oxygen vacancies and the ferromagnetic exchange. Furthermore, the parabola-like dependence of ferromagnetism on oxygen partial pressure is consistent with the bound magnetic polaron scenario.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
78.66.Hf II-VI semiconductors
78.55.Et II-VI semiconductors
68.55.A- Nucleation and growth
61.72.U- Doping and impurity implantation
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
78.70.Dm X-ray absorption spectra
68.55.ag Semiconductors
61.72.uj III-V and II-VI semiconductors
71.38.-k Polarons and electron-phonon interactions

Restoration of bulk magnetic properties by strain engineering in epitaxial CoFe2O4 (001) ultrathin films

S. Matzen, J.-B. Moussy, R. Mattana, F. Petroff, C. Gatel, B. Warot-Fonrose, J. C. Cezar, A. Barbier, M.-A. Arrio, and Ph. Sainctavit

Appl. Phys. Lett. 99, 052514 (2011); http://dx.doi.org/10.1063/1.3622307 (3 pages) | Cited 2 times

Online Publication Date: 5 August 2011

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We report on the significantly enhanced in-plane magnetic properties of CoFe2O4 (001) ultrathin layers (5 nm) grown on MgAl2O4 (001) in comparison to films deposited on MgO (001). The predicted inverse spinel structure is confirmed by x-ray magnetic circular dichroism measurements and transmission electronic microscopy studies reveal a significant in-plane compressive strain, responsible for the strong film anisotropy. These results show that strain engineering can be used to tailor the magnetic properties of oxide ultrathin films. A large compressive strain restores bulk magnetic properties for CoFe2O4 films at tunnel barrier thicknesses.
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75.70.Ak Magnetic properties of monolayers and thin films
78.20.Ls Magneto-optical effects
68.55.aj Insulators
75.30.Gw Magnetic anisotropy
75.50.Dd Nonmetallic ferromagnetic materials
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