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1 Dec 2008

Volume 104, Issue 11, Articles (11xxxx)

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Low write-current magnetic random access memory cell with anisotropy-varied free layers

S. Fukami, H. Honjo, T. Suzuki, and N. Ishiwata

J. Appl. Phys. 104, 113901 (2008); http://dx.doi.org/10.1063/1.3032894 (4 pages) | Cited 4 times

Online Publication Date: 1 December 2008

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We propose a magnetic random access memory (MRAM) cell that utilizes field-induced switching and is applicable to high-speed memories. The MRAM cell, called the shape-varying MRAM cell, has three free layers, each having different shapes and functions, and achieves low write-current switching with high thermal stability and high external field robustness. We show analytically that one of the layers contributes to the low write-current switching and another contributes to the thermal stability. We also show the results of a micromagnetic simulation, in which write current of <0.5 mA, write time of <2 ns, energy barrier (ΔE/kBT)>100, and external field robustness of >32 Oe were obtained.
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85.70.Li Other magnetic recording and storage devices (including tapes, disks, and drums)
75.30.Gw Magnetic anisotropy

Crack problem in a long cylindrical superconductor

Hua-Dong Yong, You-He Zhou, and Jun Zeng

J. Appl. Phys. 104, 113902 (2008); http://dx.doi.org/10.1063/1.2987478 (4 pages) | Cited 5 times

Online Publication Date: 1 December 2008

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In this work, the general problem of a center crack in a long cylindrical superconductor is studied. The dependence of the stress intensity factor on the parameters, including the crack length and the applied field, is investigated. We presented a simple model in which the effect of the crack on the critical current is taken into account. It is assumed that the crack forms a perfect barrier to the flow of current. The Bean model and the Kim model are considered for the critical state. Based on the complex potential and boundary collocation methods, the stress intensity factor under the magnetic field is obtained for a long cylindrical superconductor containing a central crack. The results show that the crack length and the applied field have significant effects on the fracture behavior of the superconductor.
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74.25.Ld Mechanical and acoustical properties, elasticity, and ultrasonic attenuation
74.25.Sv Critical currents
74.20.-z Theories and models of superconducting state

Thermal behavior of the interlayer coupling in a spin-valve Co/Pt multilayer with perpendicular anisotropy

Z. Y. Liu, F. Zhang, N. Li, B. Xu, D. L. Yu, J. L. He, and Y. J. Tian

J. Appl. Phys. 104, 113903 (2008); http://dx.doi.org/10.1063/1.3033519 (4 pages) | Cited 4 times

Online Publication Date: 1 December 2008

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In the spin-valve Co(4 Å)/Pt(36 Å)/[Co(4 Å)/Pt(6 Å)]2 multilayer with perpendicular anisotropy, with the decrease in temperature from 300 to 20 K, our investigations have revealed one transition of the interlayer coupling from antiferromagnetic to ferromagnetic through the Pt spacer at the temperature close to 80 K. Above 80 K, the obtained positive minor-loop shift displays a nonmonotonic variation with temperature, while below 80 K, the obtained negative minor-loop shift demonstrates a fast decrease in magnitude with temperature until a square hysteresis major loop is observed at 20 K. This observed unusual temperature behavior of the interlayer coupling can be attributed to the thermal variations in the magnetization of the bottom Co single layer and the polarization of the Pt spacer with temperature.
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75.70.Ak Magnetic properties of monolayers and thin films
74.78.Fk Multilayers, superlattices, heterostructures

Coplanar waveguide resonators for circuit quantum electrodynamics

M. Göppl, A. Fragner, M. Baur, R. Bianchetti, S. Filipp, J. M. Fink, P. J. Leek, G. Puebla, L. Steffen, and A. Wallraff

J. Appl. Phys. 104, 113904 (2008); http://dx.doi.org/10.1063/1.3010859 (8 pages) | Cited 19 times

Online Publication Date: 1 December 2008

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High quality on-chip microwave resonators have recently found prominent new applications in quantum optics and quantum information processing experiments with superconducting electronic circuits, a field now known as circuit quantum electrodynamics (QED). They are also used as single photon detectors and parametric amplifiers. Here we analyze the physical properties of coplanar waveguide resonators and their relation to the materials properties for use in circuit QED. We have designed and fabricated resonators with fundamental frequencies from 2 to 9 GHz and quality factors ranging from a few hundreds to a several hundred thousands controlled by appropriately designed input and output coupling capacitors. The microwave transmission spectra measured at temperatures of 20 mK are shown to be in good agreement with theoretical lumped element and distributed element transmission matrix models. In particular, the experimentally determined resonance frequencies, quality factors, and insertion losses are fully and consistently explained by the two models for all measured devices. The high level of control and flexibility in design renders these resonators ideal for storing and manipulating quantum electromagnetic fields in integrated superconducting electronic circuits.
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85.25.Qc Superconducting surface acoustic wave devices and other superconducting devices
84.40.Az Waveguides, transmission lines, striplines
85.25.Am Superconducting device characterization, design, and modeling

Magnetic anisotropy, interlayer coupling, and magneto-optical effects in single-crystalline Fe/Cr/Fe/MgO/Fe magnetotunnel structures grown on GaAs(001) substrates

J. Grabowski, M. Przybylski, M. Nyvlt, and J. Kirschner

J. Appl. Phys. 104, 113905 (2008); http://dx.doi.org/10.1063/1.3021094 (5 pages) | Cited 3 times

Online Publication Date: 2 December 2008

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Fe/MgO/Fe single-crystalline magnetotunneling structures were epitaxially grown on GaAs(001). An independent magnetization switching in the Fe electrodes was achieved by pinning the magnetization of the Fe-top electrode by antiferromagnetic coupling across a Cr spacer to another Fe film. Such Fe/Cr/Fe/MgO/Fe/GaAs(001) structures exhibit in-plane anisotropy, which ensures a discrete switching behavior of magnetization simplifying the shape of the hysteresis loop. The field dependence of magnetization was measured along the [110] and [100] crystallographic directions of GaAs(001) substrate and modeled by minimizing the total magnetic energy of the system. A peculiar magneto-optical behavior (similar to a reversed hysteresis loop) is explained by magneto-optical phase angles, which are determined mainly by the optical properties of the semiconducting GaAs substrate.
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75.50.Bb Fe and its alloys
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Ee Antiferromagnetics
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Gw Magnetic anisotropy
78.20.Ls Magneto-optical effects

Effects of engineered Cu spacer on the interlayer coupling and giant magnetoresistance behavior in Pd/[Pd/Co]2/Cu/[Co/Pd]4 pseudo-spin-valves with perpendicular anisotropy

Naganivetha Thiyagarajah and Seongtae Bae

J. Appl. Phys. 104, 113906 (2008); http://dx.doi.org/10.1063/1.3032898 (7 pages) | Cited 7 times

Online Publication Date: 3 December 2008

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Effects of perpendicular interlayer coupling formed between two perpendicularly magnetized ferromagnetic multilayers separated by engineered Cu spacer on the giant magnetoresistance (GMR) behavior were investigated in the Pd (3 nm)/[Pd (1.2 nm)/Co (0.6 nm)]2/Cu (x nm)/[Co (0.3 nm)/Pd (0.6 nm)]4/Pd (3 nm) pseudo-spin-valves. It was clearly observed that an increase in Cu spacer thickness from 1.6 to 4.9 nm, decreased the perpendicular interlayer coupling field through the Cu spacer and correspondingly decreased the magnetoresistance ratio. It was found that this is due to the degradation of the perpendicular anisotropy caused by strain relaxation in the soft [Co/Pd] multilayers. Further experimental and theoretical analyses verified that the physical nature of the perpendicular interlayer coupling field is directly relevant to the topological and Ruderman–Kittel–Kasuya–Yosida (RKKY) type oscillatory coupling created in the [Co/Pd]2/Cu/[Co/Pd]4 multilayers. Unlike the spin valves with in-plane anisotropy, it was clearly verified that the contribution of topological coupling field to the perpendicular interlayer coupling field is negligibly small. In contrast, the oscillatory RKKY coupling field depending on the magnetization angle between the soft and hard [Co/Pd] multilayers dominantly determined the physical characteristics of perpendicular interlayer coupling field. Furthermore, even at a larger surface roughness of greater than 0.25 nm for tCu = 6–16 nm, the slight oscillations in the GMR ratio corresponded well with the oscillations in the interlayer coupling field, although the GMR ratio showed an overall decreasing trend possibly due to shunting through the thicker Cu spacer. This indicates that the perpendicular interlayer coupling rather than the topological coupling is dominant in determining the GMR behavior. Based on the experimental and theoretical results, a physical model enabling the explanation of GMR behavior in [Co/Pd] based pseudo-spin-valves with perpendicular anisotropy is proposed: a perpendicular magnetostatic field, induced in between the soft and hard [Co/Pd] multilayers through Cu spacer by perpendicular anisotropy, is directly relevant to the perpendicular interlayer coupling field and dominantly controls the GMR characteristics. In addition, this model proposes that the GMR ratio is proportional to the sine of the angle between the soft and hard layer magnetizations from the perpendicular direction during magnetic reversal of the soft layer by an applied magnetic field. However this model is only applicable once the magnetization of the soft layer is slightly tilted away from the perpendicular direction by a critical angle (θC1 = 5–10°). Similar perpendicular interlayer coupling characteristics were also observed for the Cu spacers engineered by different input sputtering powers.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Gw Magnetic anisotropy
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
75.50.Cc Other ferromagnetic metals and alloys
75.47.De Giant magnetoresistance
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Influence of magnetic anisotropy and its distribution on the hysteresis loops upon patterning thin films

S. Boukari, J. Venuat, A. Carvalho, J. Arabski, and E. Beaurepaire

J. Appl. Phys. 104, 113907 (2008); http://dx.doi.org/10.1063/1.3032938 (5 pages) | Cited 4 times

Online Publication Date: 3 December 2008

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We compare the hysteresis loops of ferromagnetic films and dots down to 0.2 μm. Samples are made of Co–Pt multilayers with different Co thicknesses so as to vary the perpendicular magnetic anisotropy. We determined the first- and second-order anisotropies of the films, as well as the first-order anisotropy distribution, and show that from these quantities the switching field distribution of dots can be predicted. We also explain through a simple criterion—the relation between the anisotropy distribution and the minimum nucleation field—why in some cases the hysteresis loops recorded along the easy axis on the films undergo a dramatic change after patterning.
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75.30.Gw Magnetic anisotropy
75.75.-c Magnetic properties of nanostructures
75.60.Jk Magnetization reversal mechanisms

Magnetocaloric effect and Griffiths-like phase in La0.67Sr0.33MnO3 nanoparticles

W. J. Lu, X. Luo, C. Y. Hao, W. H. Song, and Y. P. Sun

J. Appl. Phys. 104, 113908 (2008); http://dx.doi.org/10.1063/1.3037236 (6 pages) | Cited 13 times

Online Publication Date: 4 December 2008

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The effect of grain size on electrical and magnetic properties of La0.67Sr0.33MnO3 nanoparticles with average grain size 32–85 nm has been investigated. The metal-insulator transition temperature TP gradually decreases with decreasing grain size, while the ferromagnetic-paramagnetic transition temperature TC remains almost constant. For the 32 nm sample, the larger effective magnetic moments and the deviation of the inverse susceptibility from the Curie–Weiss law are observed, indicating the possible existence of a Griffiths-like cluster phase. The ferromagnetic transition of the samples is further investigated by measuring magnetocaloric effect (MCE). The presence of short-range magnetic order greatly depresses the magnetic entropy of the paramagnetic phase. Moreover, the analysis of the MCE using Landau theory of phase transition confirms the importance of magnetoelastic coupling and electron interaction in magnetocaloric properties of manganites.
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75.30.Sg Magnetocaloric effect, magnetic cooling
75.50.Tt Fine-particle systems; nanocrystalline materials
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.30.Cr Saturation moments and magnetic susceptibilities
72.60.+g Mixed conductivity and conductivity transitions
73.63.Bd Nanocrystalline materials

Composition dependence of field induced anisotropy in ferromagnetic (Co,Fe)89Zr7B4 and (Co,Fe)88Zr7B4Cu1 amorphous and nanocrystalline ribbons

P. R. Ohodnicki, J. Long, D. E. Laughlin, M. E. McHenry, V. Keylin, and J. Huth

J. Appl. Phys. 104, 113909 (2008); http://dx.doi.org/10.1063/1.3021141 (6 pages) | Cited 12 times

Online Publication Date: 4 December 2008

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The composition dependence of field induced anisotropy KU of field annealed soft ferromagnetic (Co1−xFex)89Zr7B4 and (Co1−xFex)88Zr7B4Cu1 amorphous and amorphous/nanocrystalline “nanocomposite” melt spun ribbons is investigated. With the exception of the highest Co-containing alloys (x< ∼ 0.10), the observations are discussed in terms of a superposition of directional pair ordering of Fe,Co atoms and an additional contribution presumably due to the presence of Zr and B in both the field crystallized and field annealed amorphous ribbons. The highest Co-containing alloys (x< ∼ 0.10) contain multiple nanocrystalline phases (bcc, fcc, and hcp) for which a peak in KU is observed (KU ∼ 2000–2500 J/m3). In this framework, asymmetry in the compositional dependence of KU resulting in larger values for Co-rich alloys relative to Fe-rich alloys for both the field crystallized and field annealed amorphous alloys is explained in terms of a strong dependence of the Curie temperature of the amorphous phase on the Co content.
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75.30.Gw Magnetic anisotropy
75.50.Bb Fe and its alloys
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.75.-c Magnetic properties of nanostructures
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Tt Fine-particle systems; nanocrystalline materials

Microwave magnetoelectric effects in bilayers of single crystal ferrite and functionally graded piezoelectric

V. M. Petrov, G. Srinivasan, and T. A. Galkina

J. Appl. Phys. 104, 113910 (2008); http://dx.doi.org/10.1063/1.3033486 (5 pages) | Cited 8 times

Online Publication Date: 4 December 2008

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A model is discussed for magnetoelectric interactions under ferromagnetic resonance (FMR) in bilayers of a ferrite and a functionally graded piezoelectric. A linear variation in the in-plane piezoelectric coefficient is assumed. A dc electric field E applied perpendicular to the sample plane will then result in a varying in-plane strain that is transferred to the ferrite and manifests a uniaxial magnetic anisotropy. Expressions have been obtained for microwave magnetic susceptibility, taking into account the effect of grading and substrate clamping and applied to specific cases of yttrium iron garnet/lead magnesium niobate–lead titanate (PMN-PT) and nickel ferrite/PMN-PT. The model predicts unique E dependence of microwave magnetic susceptibility and FMR absorption. With increasing E, the susceptibility is predicted to decrease, and there will be broadening of FMR absorption. The results are of interest for electric field tunable ferrite-piezoelectric microwave devices.
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75.80.+q Magnetomechanical effects, magnetostriction
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
75.30.Cr Saturation moments and magnetic susceptibilities
77.65.-j Piezoelectricity and electromechanical effects
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.70.Ak Magnetic properties of monolayers and thin films

Optimizing the geometry of an in vitro tunneling magnetoresistance biosensor using an immobilized ferrimagnetic nanoparticle agent

Sunwook Kim and Seongtae Bae

J. Appl. Phys. 104, 113911 (2008); http://dx.doi.org/10.1063/1.3032918 (8 pages)

Online Publication Date: 4 December 2008

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Spatial interactions of the magnetic field produced by an immobilized ferrimagnetic nanoparticle agent on the sensing layer have been numerically calculated considering the longitudinal and transverse components of the magnetic field. Based on the calculations, the sensor geometry to obtain the maximum sensing performance of an in vitro exchange biased tunneling magnetoresistance (TMR) based biosensor can be determined prior to fabrication. Numerical analysis considering the one-dimensional (longitudinal) component of the magnetic field along the magnetization of the ferrimagnetic nanoparticle agent found that the geometrical parameters of the biosensor can be determined by simply considering the effective distance (δ). The effective distance (δ) is determined by the remnant magnetization and radius of the ferrimagnetic nanoparticle agent and the detectable field limit (BDL) relevant to the exchange bias field of the TMR sensor. The optimized sensor geometry in terms of the critical length (lc) and width (wc) is approximated as wc/lc ≈ 3.14. For a more accurate optimization of the sensor geometry, the two-dimensional magnetic field distributions on the sensing surface are numerically analyzed by employing the astroid curve model (or Stoner–Wohlfarth model). According to the numerical results using this model, the effective sensing area is found to have been extended due to the transverse component of the magnetic field. The extended effective sensing area in the optimized sensor geometry resulting from the spatial field interaction caused by the two-dimensional magnetic field is expected to enhance the output signal of the in vitro TMR based biosensor. However, in considering the two-dimensional magnetic field distribution, the undetectable area formed in the vicinity of the center of the optimized sensing area due to the spatial field interaction is a problem in designing a more accurate sensor geometry. Making a TMR sensor with a larger exchange bias field and introducing an especially designed sensor structure with a magnetic shield with a high permeability are suggested as solutions to this technical problem.
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87.85.Ox Biomedical instrumentation and transducers, including micro-electro-mechanical systems (MEMS)
87.85.Rs Nanotechnologies-applications
75.50.Gg Ferrimagnetics
85.70.Kh Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.

Stochastic oscillations of magnetic flux in the toroid sample of lead-doped lanthanum manganite

A. Rinkevich, A. Nosov, V. Vassiliev, and E. Vladimirova

J. Appl. Phys. 104, 113912 (2008); http://dx.doi.org/10.1063/1.2993752 (7 pages) | Cited 3 times

Online Publication Date: 9 December 2008

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The dynamics of magnetic flux variations in a toroidal sample of the La0.6Pb0.4MnO3 manganite was investigated. Experiments were carried out when the ac magnetic field was applied perpendicular to the dc one. It is shown that under some conditions, several regimes of complex oscillations of magnetic flux can be excited. Evolution of these oscillations as a function of frequency of ac and magnitude of dc magnetic fields was investigated. The signs of existence of the stochastic regime of oscillations were found.
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75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)

Strain-controlled anisotropic electronic transport in Bi0.4Ca0.6MnO3 films

Y. Z. Chen, J. R. Sun, S. Liang, W. M. Lu, and B. G. Shen

J. Appl. Phys. 104, 113913 (2008); http://dx.doi.org/10.1063/1.3035914 (4 pages) | Cited 4 times

Online Publication Date: 9 December 2008

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Structural and resistive anisotropy has been studied for the Bi0.4Ca0.6MnO3 films grown on (011)-oriented SrTiO3 substrates. Strong anisotropic transport behaviors are observed when significant lattice strains exist. The ratio of the two resistivities along the a and c axes of the films can be tuned between ∼ 1 and ∼ 13 by adjusting the a/c ratio between ∼ 1.01 and ∼ 1.04, which can be conducted simply by decreasing film thickness from 100 to 10 nm. Considerable anisotropy emerges and develops when film thickness drops below ∼ 60 nm. With the decrease in film thickness, a change in preferred growth direction of the films is also observed. These features of the lattice effects could be useful for the design of artificial materials and devices.
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68.55.jd Thickness
73.90.+f Other topics in electronic structure and electrical properties of surfaces, interfaces, thin films, and low-dimensional structures (Restricted to new topics in section 73)
75.47.Lx Magnetic oxides

Recording performance improvement for perpendicular recording head with shielded sloped pole design

Mark Gubbins, Alexandru Cazacu, Paul Kolbo, Tom Nolan, Robert Lamberton, and Alan Johnston

J. Appl. Phys. 104, 113914 (2008); http://dx.doi.org/10.1063/1.3040018 (4 pages)

Online Publication Date: 10 December 2008

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A sloped write pole design is assessed for perpendicular magnetic recording at high areal densities. Modeling shows that there is a significant increase in magnetic field from the sloped pole design over the conventional pole design at a fixed write width. Spin stand measurements were carried out in order to investigate the recording benefits of the sloped pole design. Writing improvements from this writer were best observed at high media coercivity.
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75.50.Ss Magnetic recording materials
85.70.Kh Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.
75.50.Vv High coercivity materials

Fiske steps and hysteresis in YBa2Cu3O7 grain boundary Josephson junctions: Structural information of the barrier by means of a nondestructive approach

M. A. Navacerrada, M. L. Lucía, F. Sánchez-Quesada, and E. Sarnelli

J. Appl. Phys. 104, 113915 (2008); http://dx.doi.org/10.1063/1.3040083 (6 pages) | Cited 2 times

Online Publication Date: 10 December 2008

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A deep analysis of the current-voltage (I-V) characteristics of YBa2Cu3O7 grain boundary Josephson junctions (GBJJs) allows us to go much farther than the usual calculus of the transport parameters. It is possible to construct a structural image of the barrier by an exhaustive and complementary analysis of both transport and electromagnetic parameters obtained from I-V curves. For such an approach, we have chosen the following three representative bicrystalline geometries: 24° [001] asymmetric, 45° [100] asymmetric, and 24° [001] symmetric +45° [100] asymmetric. The dependence of the product ICRN on the junction normal resistance is of the ICRN−1 type pointing to a SNINS model (S denotes superconductor, I denotes insulator, and N denotes normal metal) for all our GBJJs. A satisfactory explanation of the discrepancy of the capacitance of the barrier estimated from Fiske resonance positions and hysteresis in the I-V curves needs of such a model. Moreover an estimation of the length of the normal regions adjacent to the crystallographic barrier can be made. This comparative analysis is presented in order to extract interesting information about the particular transport mechanisms involved in these GBJJs.
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74.50.+r Tunneling phenomena; Josephson effects
74.25.F- Transport properties
61.72.Mm Grain and twin boundaries
74.45.+c Proximity effects; Andreev reflection; SN and SNS junctions
74.72.-h Cuprate superconductors
81.70.Ex Nondestructive testing: electromagnetic testing, eddy-current testing

Oxygen deficiency as a driving force for metamagnetism and large low field magnetocaloric effect in La0.7Ca0.3−xSrxMnO3−δ manganites

A. N. Ulyanov, J. S. Kim, Y. M. Kang, D. G. Yoo, and S. I. Yoo

J. Appl. Phys. 104, 113916 (2008); http://dx.doi.org/10.1063/1.3040153 (7 pages) | Cited 5 times

Online Publication Date: 10 December 2008

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We report the importance of metamagnetism for the giant magnetocaloric effect in lanthanum manganites and the key role of oxygen deficiency for this phenomenon. It is found that La0.7Ca0.3−xSrxMnO3−δ samples with x = 0.0, 0.025, 0.05, 0.075, 0.1, and 0.125 are orthorhombic (O) phase, while those with x = 0.15, 0.175, 0.2, and 0.3 are rhombohedral (R) phase at room temperature. The samples possess relatively high oxygen deficiency δ. With increasing x, the oxygen content increases from about 2.92 to 2.94 in the O phase and from 2.93 to 2.97 in the R phase, while it decreases in the vicinity of the O-R phase transition. The Curie temperature increases with increasing x in both the structural phases and shows a positive jump around the O-R phase transition. The x = 0 sample shows S-shape magnetization, which is typical for metamagnetic materials. The S shape is continuously suppressed with increase in Sr and disappears for x ≥ 0.075. Interestingly, the magnetic entropy change, adiabatic temperature change, and refrigerant capacity achieved for the present samples are about 20%–50% larger than those previously reported for manganites prepared using different processing routes. The large magnetocaloric effect in the present samples originates from the sharp change in magnetization with respect to temperature at the Curie temperature and the presence of S-shape magnetization near this transition temperature caused by the oxygen deficiency.
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75.30.Sg Magnetocaloric effect, magnetic cooling
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
81.20.-n Methods of materials synthesis and materials processing
65.40.gd Entropy
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

The accelerated formation of MgB2 phase with high critical current density by Cu and SiC multidoping during the low-temperature sintering process

Zongqing Ma, Yongchang Liu, Liming Yu, and Qian Zhao

J. Appl. Phys. 104, 113917 (2008); http://dx.doi.org/10.1063/1.3040703 (3 pages) | Cited 11 times

Online Publication Date: 10 December 2008

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Combined with the thermal analysis and phase identification, it is found that during the low-temperature sintering process of the SiC-doped MgB2 samples, the Cu addition can improve both the reaction between Mg and B and the reaction between Mg and SiC and thus accelerate the formation of the MgB2 phase with effective C substitution for B. Accordingly, we successfully synthesize MgB2 bulks with excellent critical current density (Jc) by proper amount of Cu and SiC multidoping sintered at 575 °C for only 5 h.
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74.25.Sv Critical currents
74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
61.72.up Other materials

Dynamic and temperature effects in spin-transfer switching

Dorin Cimpoesu, Huy Pham, Alexandru Stancu, and Leonard Spinu

J. Appl. Phys. 104, 113918 (2008); http://dx.doi.org/10.1063/1.3032415 (7 pages) | Cited 4 times

Online Publication Date: 10 December 2008

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We have studied the dynamic switching triggered by spin angular momentum transfer in a pulsed current of a spin-valve-type trilayer structure, and its dependence on thermal effects. In order to determine the current pulse parameters, where fast and stable switching can be achieved, we have studied the magnetization’s dynamics properties as a function of applied current pulse amplitude and shape, waiting time, and initial orientation, and also as a function of the Gilbert damping constant. The magnetic layer is assumed to be single domain, ellipsoid shaped. In this paper also we present the thermal fluctuation effects on the switching behavior. The model is based on the Landau–Lifshitz–Gilbert equation and the stochastic Landau–Lifshitz–Gilbert equation with a spin-transfer term included, which are numerically integrated.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Ds Spin waves
75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)
75.40.Mg Numerical simulation studies
85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields

Short range ordering and magnetostriction in Fe–Ga and other Fe alloy single crystals

Sivaraman Guruswamy, Tanjore V. Jayaraman, Robert P. Corson, Gavin Garside, and Swieng Thuanboon

J. Appl. Phys. 104, 113919 (2008); http://dx.doi.org/10.1063/1.3040154 (8 pages) | Cited 4 times

Online Publication Date: 11 December 2008

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The magnetostrictive behaviors of Fe–Ga, Fe–Mo, Fe–W, and other Fe alloys have been observed to be sensitive to their thermal history. In this work, the changes in the structure with thermal history and how they correlate with the observed magnetostriction values in Fe–Ga, Fe–W, and Fe–Mo single crystals are examined. Single crystals were grown using the vertical Bridgman crystal growth technique. The magnetostriction constant (3/2)λ100 measured in (i) as-grown and air-cooled (DG) and (ii) annealed and water quenched single crystal samples of Fe–Ga and Fe–W alloys clearly indicate that annealing in the α-phase region followed by rapid quenching improves the magnetostriction values. High-resolution x-ray diffraction (XRD) studies on [100]-oriented Fe–Ga as well as other Fe alloy single crystals show (i) a diffuse scattering peak indicative of short range ordering, the extent of which depends on composition and thermal history, and (ii) (200) peak splitting and broadening indicative of the presence of long range ordered coherent second phases and associated coherency strains. A detailed examination of the XRD patterns suggests that the changes in magnetostriction with composition and thermal history are related to the local strain modulations associated with the solutes, short range order, long range ordered coherent second phases, incoherent precipitates, and other structural defects.
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75.80.+q Magnetomechanical effects, magnetostriction
81.10.Fq Growth from melts; zone melting and refining
61.72.Cc Kinetics of defect formation and annealing
81.40.Gh Other heat and thermomechanical treatments

The effect of cross-sectional geometry and size on magnetostatic modes in nanorods

M. Krawczyk and H. Puszkarski

J. Appl. Phys. 104, 113920 (2008); http://dx.doi.org/10.1063/1.3041476 (11 pages) | Cited 2 times

Online Publication Date: 11 December 2008

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We study the effect of cross-sectional geometry and size on the spectrum of magnetostatic modes in dipole-coupled magnetic nanorods of circular or square cross section. The scope of this study is confined only to excitations propagating along the central axis of the nanorod; at the same time, the direction of the central axis is assumed to correspond to that of the magnetization of the sample. We find that a modification of the cross-sectional geometry results in nonuniform changes in the local field profile and the effect proves especially strong in the range of nanometer lateral dimensions. Moreover, the effect of cross-sectional geometry on the magnetostatic spectrum of a nanorod is found to be the most significant in peripheral regions of elongated rods. This is due to the occurrence of dipolar local field wells in these very regions. The shape of these peripheral wells affects first of all the bulk-dead and surface modes, which are “stuck” in them; frequencies of these modes prove to increase when the cross-sectional shape changes from square to circle.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.75.-c Magnetic properties of nanostructures
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)
75.30.Cr Saturation moments and magnetic susceptibilities
75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)

Chiral states of electromagnetic fields originated from ferrite-based microwave vortices

M. Sigalov, E. O. Kamenetskii, and R. Shavit

J. Appl. Phys. 104, 113921 (2008); http://dx.doi.org/10.1063/1.3040008 (12 pages)

Online Publication Date: 12 December 2008

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Electromagnetic vortices in a microwave cavity with an inserted piece of a magnetized ferrite appear due to the time-reversal symmetry breaking effect. We reveal numerically that the Poynting-vector vortices are possible in open resonant microwave structures with ferrite inclusions. We demonstrate a pair of resonances which have opposite vortex rotations at the same direction of time given by the direction of the magnetization precession. There are two coalescent resonances with different chirality. We show that the structures of the radiating near and far fields are intimately related to the ferrite-induced topological singularities. The observed far-field polarization structures represent a doublet of chiral vortices in space originated from a doublet of resonant chiral states in a patch resonator with an enclosed ferrite disk.
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41.20.Jb Electromagnetic wave propagation; radiowave propagation
85.70.Ge Ferrite and garnet devices
03.65.Vf Phases: geometric; dynamic or topological
11.30.Er Charge conjugation, parity, time reversal, and other discrete symmetries

A three-dimensional resistor network model for the linear magnetoresistance of Ag2+δSe and Ag2+δTe bulks

Jie Xu, Duanming Zhang, Fengxia Yang, Zhihua Li, and Yuan Pan

J. Appl. Phys. 104, 113922 (2008); http://dx.doi.org/10.1063/1.3035834 (5 pages) | Cited 3 times

Online Publication Date: 15 December 2008

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A network model for the linear magnetoresistance (MR) of the Ag2+δSe and Ag2+δTe bulks is proposed. The bulk sample is considered as a two-phase (the metal phase and the semiconductor phase) composite and dispersed into a three-dimensional resistor network. The network is constructed from six-terminal resistor units and the mobility of carriers within the network has a modified Gaussian distribution, i.e., a Gaussian distribution with two constraint conditions. The modified Gaussian distribution is related with the mobility of carriers in the two phases and the volume fraction of the metal phase. The model predicts that the MR increases linearly with the increasing magnetic fields and does not show the saturation at high field. Moreover, the temperature dependence of the MR predicted by the model is discussed. A good agreement is found between the theoretical MR predictions and the available experimental data.
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75.47.Pq Other materials
72.20.My Galvanomagnetic and other magnetotransport effects
72.80.Ng Disordered solids
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