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15 Jan 2007

Volume 90, Issue 3, Articles (03xxxx)

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Appl. Phys. Lett. 90, 032108 (2007); http://dx.doi.org/10.1063/1.2431702 (3 pages)

D. Buca, B. Holländer, S. Feste, St. Lenk, H. Trinkaus, S. Mantl, R. Loo, and M. Caymax
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Competing spin-dependent conductance channels in underoxidized tunnel junctions

J. Ventura, J. P. Araujo, J. B. Sousa, R. Ferreira, and P. P. Freitas

Appl. Phys. Lett. 90, 032501 (2007); http://dx.doi.org/10.1063/1.2430482 (3 pages) | Cited 9 times

Online Publication Date: 16 January 2007

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The authors study the transport properties of magnetic tunnel junctions (MTJs) with underoxidized 9 Å AlOx barriers suitable for magnetoresistive sensors in high-density storage devices. Temperature dependent measurements revealed different dominant transport mechanisms in different junctions: tunnel, metallic, or both, depending on the MTJ-magnetic state. This denotes a competition between two conductance channels (tunnel through oxidized AlOx and metallic through unoxidized Al nanoconstrictions), so that the dominance of one over the other is the outcome of small structural and composition variations in the barrier. Furthermore, transport through the Al nanobridges is spin dependent, caused by ballistic and/or diffusive magnetoresistance through nonmagnetic metallic paths.
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85.75.Dd Magnetic memory using magnetic tunnel junctions
85.70.Kh Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
85.75.Ss Magnetic field sensors using spin polarized transport

Observation of a Griffiths-like phase in bilayered manganites

Ren-Fu Yang, Young Sun, Wei He, Qing-An Li, and Zhao-Hua Cheng

Appl. Phys. Lett. 90, 032502 (2007); http://dx.doi.org/10.1063/1.2431061 (3 pages) | Cited 8 times

Online Publication Date: 16 January 2007

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The authors report the observation of a Griffiths-like phase in La2−2xSr1+2xMn2O7 (x = 0.30, 0.33, and 0.40) and (La0.8Eu0.2)2−2xSr1+2xMn2O7 (x = 0.33) single crystals by means of electron spin resonance, magnetic susceptibility, and magnetoresistance measurements. In the temperature region TC<T<350 K, the resonance signal consists of a ferromagnetic resonance line and a paramagnetic resonance line, which suggests that the system is not in a pure paramagnetic phase. The ferromagnetic resonance signal disappears and the magnetic susceptibility starts to deviate from the Curie-Weiss law at the same temperature T* ≈ 350 K, independent of doping level and anisotropy. These results indicate that a Griffiths-like phase exists at TC<T<T* in bilayered manganites, which is due to intrinsic inhomogeneity caused by quenched disorder.
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75.47.Gk Colossal magnetoresistance
76.30.-v Electron paramagnetic resonance and relaxation
75.30.Cr Saturation moments and magnetic susceptibilities
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
75.20.Ck Nonmetals

Improvement switching characteristics of toggle magnetic random access memory with dual polarity write pulse scheme

Yuan-Jen Lee, Chien-Chung Hung, Ding-Yeong Wang, Cheng-Tyng Yen, Wei-Chuan Chen, Shan-Yi Yang, Kuei-Hung Shen, Yung-Hung Wang, Yung-Hsiang Chen, Ming-Jer Kao, and Ming-Jinn Tsai

Appl. Phys. Lett. 90, 032503 (2007); http://dx.doi.org/10.1063/1.2431755 (3 pages)

Online Publication Date: 17 January 2007

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The writing probability of toggle magnetic random access memory (MRAM) at built-in bias field is studied by micromagnetic simulation and a dual polarity write pulse scheme has been proposed to enhance the toggle probability at low writing field. The critical writing field can be reduced to 19 Oe at strong built-in bias field from CoFe(1.0 nm)/CoFeB(2.0 nm)/Ru/CoFe(5.0 nm) pinned layer structure. From the simulation and experimental results, it is proven that the toggle MRAM can be operated at lower writing field by dual polarity write pulse scheme.
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85.70.Li Other magnetic recording and storage devices (including tapes, disks, and drums)
84.30.Sk Pulse and digital circuits

Room-temperature ferromagnetism of Cu-implanted GaN

Jong-Han Lee, In-Hoon Choi, Sangwon Shin, Sunggoo Lee, J. Lee, Chungnam Whang, Seung-Cheol Lee, Kwang-Ryeol Lee, Jong-Hyeob Baek, Keun Hwa Chae, and Jonghan Song

Appl. Phys. Lett. 90, 032504 (2007); http://dx.doi.org/10.1063/1.2431765 (3 pages) | Cited 28 times

Online Publication Date: 17 January 2007

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1 MeV Cu2+ ion was implanted into GaN with a dose of 1×1017 cm−2 at room temperature. After implantation, the samples were subsequently performed by rapid thermal annealing at 700, 800, and 900 °C for 5 min. Both nonmagnetic Cu ion implanted samples annealed at 700 and 800 °C exhibit the ferromagnetism at room temperature, and the saturation magnetization of these samples is estimated to be 0.057μB and 0.27μB per Cu atom from M-H curve, respectively. However, the sample annealed at 900 °C does not show ferromagnetism due to clustering of Cu during the annealing process.
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81.05.Ea III-V semiconductors
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Pp Magnetic semiconductors
61.72.Cc Kinetics of defect formation and annealing
61.72.S- Impurities in crystals

Growth, structural, and magnetic characterizations of nanocrystalline γ′-FeNiN(220) thin films

P. Prieto, K. R. Pirota, J. M. Sanz, E. Jiménez, J. Camarero, F. Maccherozzi, and F. G. Panaccione

Appl. Phys. Lett. 90, 032505 (2007); http://dx.doi.org/10.1063/1.2430920 (3 pages) | Cited 2 times

Online Publication Date: 18 January 2007

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The authors have developed a fabrication process of nanocrystalline γ′-FeNiN[220] thin ferromagnetic films by using a dual ion beam sputtering system. The films show well defined in-plane magnetic uniaxial anisotropy, with high anisotropy field and high macroscopic saturation magnetization, requisites of crucial importance for high-frequency applications. The estimated ferromagnetic resonance frequency goes within the gigahertz regime, and the magnetic response can be tailored by the deposition conditions. Element-selective measurements reveal a reduction of the total Fe and Ni magnetic moments in γ′-FeNiN with respect to pure Fe and Ni due to hybridization between Fe and Ni 3d and N 2p states.
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75.70.Ak Magnetic properties of monolayers and thin films
81.07.Bc Nanocrystalline materials
81.15.Cd Deposition by sputtering
75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Dd Nonmetallic ferromagnetic materials
75.30.Gw Magnetic anisotropy

Assessment of the local supercurrent densities in long superconducting coated conductors

M. Zehetmayer, R. Fuger, M. Eisterer, F. Hengstberger, and H. W. Weber

Appl. Phys. Lett. 90, 032506 (2007); http://dx.doi.org/10.1063/1.2431472 (3 pages) | Cited 9 times

Online Publication Date: 18 January 2007

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The authors report on measurements of the local supercurrent density in long Y1Ba2Cu3O7−δ based coated conductors by the magnetoscan technique. Significant inhomogeneities were found, which are well resolved by the resulting magnetic field map. A single central line scan along the length of the conductor reflects the inhomogeneities over the entire width of the sample, thus offering the possibility of very fast characterization. Modifying the applied field leads to different results highlighting either the overall critical current or details of the defect structure. In addition, numerical simulations of the current dynamics were carried out for a qualitative and quantitative interpretation of the results.
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74.72.-h Cuprate superconductors
74.25.Sv Critical currents
74.25.F- Transport properties

Determination of the entropy changes in the compounds with a first-order magnetic transition

G. J. Liu, J. R. Sun, J. Shen, B. Gao, H. W. Zhang, F. X. Hu, and B. G. Shen

Appl. Phys. Lett. 90, 032507 (2007); http://dx.doi.org/10.1063/1.2425033 (3 pages) | Cited 82 times

Online Publication Date: 18 January 2007

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Entropy changes in the compounds of La1−xPrxFe11.5Si1.5 (x = 0.3 and 0.4) have been experimentally studied. A tower-shaped entropy change of the height of ∼ 27 J/kg K is obtained based on the analyses of heat capacity, while the Maxwell relation predicts an extra entropy peak of the height of ∼ 99 J/kg K, slightly varying with Pr content. A careful study indicates that the Maxwell relation cannot be used in the vicinity of the Curie temperature because of the coexistence of paramagnetic and ferromagnetic phases, and the huge entropy peak is a spurious result. Similar conclusions are applicable to MnAs and Mn1−xFexAs, for which huge entropy changes have been reported. Appropriate methods for the determination of entropy change of the compound with phase separation are discussed based on the magnetic data.
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75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)
65.40.G- Other thermodynamical quantities
65.40.Ba Heat capacity
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.20.Ck Nonmetals
75.50.Dd Nonmetallic ferromagnetic materials

Studies on the cluster sizes in the mixed-phase thin films

Qing-li Zhou, Kui-juan Jin, Hui-bin Lu, Chun-lian Hu, Kun Zhao, Dong-yi Guan, and Guo-zhen Yang

Appl. Phys. Lett. 90, 032508 (2007); http://dx.doi.org/10.1063/1.2432945 (3 pages) | Cited 2 times

Online Publication Date: 19 January 2007

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Based on the phase separation scenario, by simulating the resistivity of La0.33Pr0.34Ca0.33MnO3 film with the known coexisting-phase sizes, the authors have obtained the theoretical ferromagnetic (FM) cluster sizes of La2/3Ca1/3MnO3 film, showing around 0.6 μm near Tc. Subsequently the insulator-metal transition occurs when the cluster grows up to 0.7 μm, and the abrupt drop in resistivity occurs when the size is around 0.9 μm. Furthermore, the FM cluster sizes grow with the magnetic field. The obtained results indicate that the model and method can be used to predict the critical size of the clusters at the phase transition for the mixed-phase materials.
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72.60.+g Mixed conductivity and conductivity transitions
71.30.+h Metal-insulator transitions and other electronic transitions
75.50.Dd Nonmetallic ferromagnetic materials
64.75.-g Phase equilibria

Magnetic anisotropy in the ferromagnetic Cu-doped ZnO nanoneedles

T. S. Herng, S. P. Lau, S. F. Yu, H. Y. Yang, L. Wang, M. Tanemura, and J. S. Chen

Appl. Phys. Lett. 90, 032509 (2007); http://dx.doi.org/10.1063/1.2433028 (3 pages) | Cited 32 times

Online Publication Date: 19 January 2007

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Copper-doped ZnO (ZnO:Cu) nanoneedles exhibiting room-temperature ferromagnetism were fabricated by an ion beam technique using Cu plate and ZnO film. A saturated magnetization moment of 0.698 emu/cm3 was found in the nanoneedles when a field of 10 kOe was applied perpendicular to the substrate, which was 15% larger than the field applied parallel to the substrate. The magnetic ordering of the nanoneedles was enhanced significantly to 0.968 emu/cm3 after annealing of 400 °C for 20 min. However, the magnetic anisotropy at high field is vanished but an “easy plane” ferromagnetism becomes apparent at low field region. The possible mechanisms of the magnetic ordering and anisotropy in the ZnO:Cu nanoneedles are discussed.
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75.30.Gw Magnetic anisotropy
61.72.Cc Kinetics of defect formation and annealing
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Pp Magnetic semiconductors

Irreversibility of magnetization rotation in exchange biased Fe/epitaxial-FeF2 thin films

Justin Olamit, Kai Liu, Zhi-Pan Li, and Ivan K. Schuller

Appl. Phys. Lett. 90, 032510 (2007); http://dx.doi.org/10.1063/1.2431784 (3 pages) | Cited 8 times

Online Publication Date: 19 January 2007

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Magnetization reversal via rotation is typical in ferromagnet/antiferromagnet exchange biased systems. The reversibility of the rotation is a manifestation of the microscopic reversal process. The authors have investigated the magnetization reversal in Fe/epitaxial-FeF2 thin films using vector magnetometry and first-order reversal curves. The reversal is predominantly by rotation as the applied field makes an angle with the antiferromagnet spin axis, mostly irreversible at small angles and reversible at larger angles. A modified Stoner-Wohlfarth model reproduces the overall trend of the irreversibility evolution. The remaining discrepancies between the modeled and measured irreversibilities may be attributed to local incomplete domain walls.
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75.70.Ak Magnetic properties of monolayers and thin films
75.50.Bb Fe and its alloys
75.50.Ee Antiferromagnetics
75.60.Jk Magnetization reversal mechanisms
75.30.Et Exchange and superexchange interactions
75.60.Ch Domain walls and domain structure

Control of hole distribution through isovalent R-cation substitution in Cu2Ba2RCu2O8 superconductors

M. Karppinen, Y. Abe, I. Grigoraviciute, J. M. Chen, R. S. Liu, and H. Yamauchi

Appl. Phys. Lett. 90, 032511 (2007); http://dx.doi.org/10.1063/1.2431458 (3 pages)

Online Publication Date: 19 January 2007

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Superconductive Cu2Ba2RCu2O8 samples with R ranging from Gd to Tm have been synthesized through a sol-gel route. Both iodometric titration and Cu L-edge x-ray absorption near-edge structure (XANES) spectroscopy data indicate that the average Cu valence remains constant, whereas Tc increases with decreasing size of the R constituent. An explanation for this trend is revealed from O K-edge XANES spectra, which show that the smaller-for-larger R-cation substitution results in a shift of holes from the Cu2O2 charge reservoir to the superconductive CuO2 planes. Since Cu2Ba2RCu2O8 samples are underdoped, such a shift of holes raises the value of Tc.
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74.62.Dh Effects of crystal defects, doping and substitution
74.25.F- Transport properties
74.72.-h Cuprate superconductors
74.25.N- Response to electromagnetic fields
78.70.Dm X-ray absorption spectra
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