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17 Aug 2009

Volume 95, Issue 7, Articles (07xxxx)

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Appl. Phys. Lett. 95, 073703 (2009); http://dx.doi.org/10.1063/1.3152768 (3 pages)

Mohammud R. Noor, Swati Goyal, Shawn M. Christensen, and Samir M. Iqbal
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Room temperature ferromagnetism of ZnO nanocrystals in amorphous ZnO–Al2O3 matrix

Y. W. Ma, J. Ding, D. C. Qi, J. B. Yi, H. M. Fan, H. Gong, A. T. S. Wee, and A. Rusydi

Appl. Phys. Lett. 95, 072501 (2009); http://dx.doi.org/10.1063/1.3206654 (3 pages) | Cited 5 times

Online Publication Date: 17 August 2009

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In this work, we demonstrate room temperature ferromagnetism of ZnO nanocrystals (NCs) embedded in alumina-based amorphous matrix. Our study has shown that solid solution occurred for lower Al concentration [ ≤ x = 0.09 for (Zn1−x,Alx)O]. No ferromagnetism appears in Al-doped ZnO with x ≤ 0.09. With higher Al concentration, alumina-based amorphous structure appeared and fully amorphous was found in the composition of (Zn0.30,Al0.70)O. Ferromagnetism was found in (Zn1−x,Alx)O with a threshold of 16 mol % of Al. The room temperature ferromagnetism was found to be attributed to ZnO NCs in an alumina-based amorphous ZnO–Al2O3 matrix. Ferromagnetism reached a maximum with a volume percentage of ZnO NCs to be ∼ 27 vol %. The detailed investigations (transmission electron microscopy, x-ray photoelectron spectroscopy, and superconducting quantum interference device) show that the optimized size of these NCs is in the range of 4–5 nm. No ferromagnetism appears if the sample contains a fully amorphous structure.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Pp Magnetic semiconductors
75.50.Dd Nonmetallic ferromagnetic materials

Abnormal frequency dependence of ac susceptibility in melt texture Gd–Ba–Cu–O single domain

Ke-Xi Xu, Xing-Da Wu, and Peng-Jun Pan

Appl. Phys. Lett. 95, 072502 (2009); http://dx.doi.org/10.1063/1.3206657 (3 pages)

Online Publication Date: 17 August 2009

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In this paper, the pinning property of the melt texture Gd–Ba–Cu–O single domain has been examined by ac susceptibility measurements. An unexpected frequency dependence behavior of the irreversibility line Birr(T) has been found, in which the power n for Birr(T)∝(1−t)n is affected by frequencies of the ac fields, no longer kept as a constant as in the Yeshurun and Malozemoff theory. We present the abnormal frequency dependent behavior and interpret it in framework of the flux diffusion model.
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74.25.Ha Magnetic properties including vortex structures and related phenomena
74.72.-h Cuprate superconductors
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)

Subtle interplay between native point defects and magnetism in ZnO:Co

M. H. N. Assadi, Y. B. Zhang, and S. Li

Appl. Phys. Lett. 95, 072503 (2009); http://dx.doi.org/10.1063/1.3210793 (3 pages) | Cited 10 times

Online Publication Date: 18 August 2009

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Distribution of Co ions and its effect on magnetic properties of ZnO:Co in the presence of native point defects, oxygen vacancy, and interstitial hydrogen, have been investigated using first-principles density functional calculations. The study provides a fundamental theoretical understanding on the correlation between magnetism and the distribution of magnetic ions and the native point defects in the semiconducting host. Results show that Co ions have a strong tendency toward aggregation via oxygen within ab plane in the presence of point defects. The room temperature ferromagnetism observed experimentally in ZnO:Co is mainly dominated by the interstitial hydrogen rather than oxygen vacancy.
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61.72.jd Vacancies
61.72.jj Interstitials
75.50.Pp Magnetic semiconductors

Modelling the quasistatic and dynamical sensing response of Galfenol-based magnetostrictive devices

Philip Weetman and George Akhras

Appl. Phys. Lett. 95, 072504 (2009); http://dx.doi.org/10.1063/1.3210789 (3 pages) | Cited 4 times

Online Publication Date: 19 August 2009

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A computational model for the quasistatic and dynamical response of Galfenol based magnetostrictive devices in the sensing configuration is presented. The model calculates the fraction of magnetic moments oriented along the preferred orientations within the crystal as a function of time using a self-consistent rate equation technique. These magnetic moment fractions are then used to determine the total magnetization as a function of time. The model is compared to experiments for uniaxial, compressive, and quasistatic loading. Predictive calculations are presented for dynamical loading. Eddy currents and finite transition times lead to increasing hysteresis as the frequency increases.
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85.70.Ec Magnetostrictive, magnetoacoustic, and magnetostatic devices
75.30.Cr Saturation moments and magnetic susceptibilities
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
61.66.Dk Alloys
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Magnetically tuned mechanical resonances in magnetoelectric multilayer capacitors

C. Israel, V. M. Petrov, G. Srinivasan, and N. D. Mathur

Appl. Phys. Lett. 95, 072505 (2009); http://dx.doi.org/10.1063/1.3205477 (3 pages) | Cited 9 times

Online Publication Date: 19 August 2009

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Poled multilayer capacitors based on barium titanate represent model magnetoelectric systems because the interdigitated planar electrodes are nowadays based on a magnetostrictive material, namely nickel. Here we demonstrate electrically excited mechanical resonances that can be magnetically tuned. Nonmonotonic variations in resonant frequency with applied magnetic field are interpreted in terms of the influence of magnetostriction on the Young’s modulus of the electrodes, i.e., the ΔEY effect.
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75.80.+q Magnetomechanical effects, magnetostriction
77.65.Fs Electromechanical resonance; quartz resonators
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
84.32.Tt Capacitors

100 ps precessional spin-transfer switching of a planar magnetic random access memory cell with perpendicular spin polarizer

C. Papusoi, B. Delaët, B. Rodmacq, D. Houssameddine, J.-P. Michel, U. Ebels, R. C. Sousa, L. Buda-Prejbeanu, and B. Dieny

Appl. Phys. Lett. 95, 072506 (2009); http://dx.doi.org/10.1063/1.3206919 (3 pages) | Cited 16 times

Online Publication Date: 20 August 2009

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Ultrafast spin-transfer precessional switching between two stable states of a magnetic random access memory device is demonstrated in structures comprising a perpendicularly magnetized polarizing layer (PL⊥), an in-plane magnetized free layer (FL), and an in-plane magnetized analyzing layer (AL) in a PL⊥/spacer/FL/spacer/AL stack. Back and forth switching can be achieved with sub-ns current pulses of the same polarity. The spin-torque influence from the analyzer leads to an asymmetric dependence of the switching properties as a function of the current sign and initial state.
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85.70.Ec Magnetostrictive, magnetoacoustic, and magnetostatic devices

Effects of thickness on the cation segregation in epitaxial (001) and (110) La2/3Ca1/3MnO3 thin films

S. Estradé, J. M. Rebled, J. Arbiol, F. Peiró, I. C. Infante, G. Herranz, F. Sánchez, J. Fontcuberta, R. Córdoba, B. G. Mendis, and A. L. Bleloch

Appl. Phys. Lett. 95, 072507 (2009); http://dx.doi.org/10.1063/1.3211130 (3 pages) | Cited 8 times

Online Publication Date: 21 August 2009

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Electron-energy-loss spectroscopy is used to map composition and electronic states in epitaxial La2/3Ca1/3MnO3 (LCMO) films of various thicknesses grown on SrTiO3 (001) and (110) substrates. For relatively thick films ( ≥ 20 nm), epitaxial tensile strain in (001) films promotes a compositional La/Ca gradient across the film thickness, being the interface La rich, while the relaxed (110) films are chemically homogeneous. In contrast, much thinner (001) and (110) LCMO films display a different La/Ca distribution, being La rich at the free surface. The observed distinct thickness-dependent composition gradient behavior reflects a balance between strain-induced elastic energy minimization and kinetic effects during growth.
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75.50.Dd Nonmetallic ferromagnetic materials
68.55.jd Thickness
75.30.Mb Valence fluctuation, Kondo lattice, and heavy-fermion phenomena
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
68.60.Bs Mechanical and acoustical properties
79.20.Uv Electron energy loss spectroscopy
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity

Microscopic evidence of a strain-enhanced ferromagnetic state in LaCoO3 thin films

S. Park, P. Ryan, E. Karapetrova, J. W. Kim, J. X. Ma, J. Shi, J. W. Freeland, and Weida Wu

Appl. Phys. Lett. 95, 072508 (2009); http://dx.doi.org/10.1063/1.3206667 (3 pages) | Cited 7 times

Online Publication Date: 21 August 2009

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Strain-induced modification of magnetic properties of lightly hole doped epitaxial LaCoO3 thin films on different substrates were studied with variable temperature magnetic force microscopy (MFM). Real space observation at 10 K reveals the formation of the local magnetic clusters on a relaxed film grown on LaAlO3 (001). In contrast, a ferromagnetic ground state has been confirmed for tensile-strained film on SrTiO3 (001), indicating that strain is an important factor in creating the ferromagnetic state. Simultaneous atomic force microscopy and MFM measurements reveal nanoscale defect lines for the tensile-strained films, where the structural defects have a large impact on the local magnetic properties.
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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
75.30.Cr Saturation moments and magnetic susceptibilities

The “colossal” magnetocaloric effect in Mn1−xFexAs: What are we really measuring?

Mohamed Balli, Daniel Fruchart, Damien Gignoux, and Ryszard Zach

Appl. Phys. Lett. 95, 072509 (2009); http://dx.doi.org/10.1063/1.3194144 (3 pages) | Cited 34 times

Online Publication Date: 21 August 2009

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Recently, a so-called “colossal” magnetocaloric effect (MCE) was reported in Mn1−xFexAs [ A. de Campos et al., Nat. Mater. 5, 802 (2006) ]. However, the value of ΔS that was determined appears markedly overestimated since it results from the inadequate use of the Maxwell relation. Here, we report on recent measurements of ΔS in Mn1−xFexAs from which a correct MCE value is deduced by using the Clausius–Clapeyron equation. This result is asserted by careful use of the Maxwell relation.
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75.30.Sg Magnetocaloric effect, magnetic cooling
75.50.Dd Nonmetallic ferromagnetic materials
65.40.gd Entropy

Effect of hydrostatic pressure in degenerate Ge1−xMnxTe

S. T. Lim, J. F. Bi, K. L. Teo, Feng Y. P, T. Liew, and T. C. Chong

Appl. Phys. Lett. 95, 072510 (2009); http://dx.doi.org/10.1063/1.3211990 (3 pages) | Cited 5 times

Online Publication Date: 21 August 2009

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We utilize the effect of hydrostatic pressure to investigate the magnetotransport properties of degenerate p-Ge1−xMnxTe (x = 0.10) ferromagnetic semiconductor. The Curie temperature was found to increase with pressure as 0.27 K/kbar, which can be understood on the basis of the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction mechanism. For sufficiently high carrier concentration of po ∼ 1021 cm−3, both the light holes from the L valence band and the heavy holes from the Σ valence band contribute to the RKKY interaction. Additionally, a negative magnetoresistance is observed at low temperature and is found to decrease with pressure.
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72.20.My Galvanomagnetic and other magnetotransport effects
72.80.Jc Other crystalline inorganic semiconductors
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.30.Et Exchange and superexchange interactions
71.20.Nr Semiconductor compounds
75.50.Pp Magnetic semiconductors
75.50.Dd Nonmetallic ferromagnetic materials
62.50.-p High-pressure effects in solids and liquids
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