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9 Feb 2009

Volume 94, Issue 6, Articles (06xxxx)

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Appl. Phys. Lett. 94, 062105 (2009); http://dx.doi.org/10.1063/1.3079078 (3 pages)

Ikai Lo, Chia-Ho Hsieh, Yu-Chi Hsu, Wen-Yuan Pang, and Ming-Chi Chou
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Direct observation of oxygen induced room temperature ferromagnetism in MoO2 thin films by x-ray magnetic circular dichroism characterizations

P. Thakur, J. C. Cezar, N. B. Brookes, R. J. Choudhary, Ram Prakash, D. M. Phase, K. H. Chae, and Ravi Kumar

Appl. Phys. Lett. 94, 062501 (2009); http://dx.doi.org/10.1063/1.3080679 (3 pages) | Cited 11 times

Online Publication Date: 9 February 2009

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We report the element specific polarized near edge x-ray absorption fine structure (NEXAFS) and x-ray magnetic circular dichroism (XMCD) experiments on well characterized undoped MoO2 thin films that show ferromagnetism at room temperature. The polarization dependent of O K edge NEXAFS spectra indicate a strong hybridization of O 2p-4d Mo orbitals followed by a strong anisotropy in the electronic properties. An unquenched orbital magnetic moment within the O 2p shell is clearly evident from the XMCD O K edge, which is ferromagnetically coupled to the neighboring Mo moments as confirmed by Mo M3,2 edge XMCD experiment.
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75.70.Ak Magnetic properties of monolayers and thin films
78.20.Ls Magneto-optical effects
75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Dd Nonmetallic ferromagnetic materials
71.20.-b Electron density of states and band structure of crystalline solids

Exchange bias in laterally oxidized Au/Co/Au nanopillars

Ll. Balcells, B. Martinez, O. Iglesias, J. M. García-Martín, A. Cebollada, A. García-Martín, G. Armelles, B. Sepúlveda, and Y. Alaverdyan

Appl. Phys. Lett. 94, 062502 (2009); http://dx.doi.org/10.1063/1.3078411 (3 pages) | Cited 10 times

Online Publication Date: 9 February 2009

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Au/Co/Au nanopillars fabricated by colloidal lithography of continuous trilayers exhibit an enhanced coercive field and the appearance of an exchange bias field with respect to the continuous layers. This is attributed to the lateral oxidation of the Co interlayer that appears upon disk fabrication. The dependence of the exchange bias field on the Co nanodots size and on the oxidation degree is analyzed and its microscopic origin clarified by means of Monte Carlo simulations based on a model of a cylindrical dot with lateral core/shell structure.
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75.30.Et Exchange and superexchange interactions
75.50.Tt Fine-particle systems; nanocrystalline materials
81.16.Nd Micro- and nanolithography
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Ee Antiferromagnetics

Supercurrent decay in nano-superconducting quantum interference devices for intrinsic magnetic flux resolution

C. Granata, A. Vettoliere, R. Russo, E. Esposito, M. Russo, and B. Ruggiero

Appl. Phys. Lett. 94, 062503 (2009); http://dx.doi.org/10.1063/1.3078519 (3 pages) | Cited 4 times

Online Publication Date: 9 February 2009

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Supercurrent decay measurements of nano-superconducting quantum interference devices (SQUIDs) based on niobium constrictions (Dayem bridges) are reported. Such measurements provide useful information for applications that employ the SQUID as a trigger where the sensor works on the zero voltage state. The nanodevice consists of a niobium thin film (30 nm thick) ring with a hole side length of 200 nm including two Dayem bridges of 120×200 nm2. The measurements of the switching current distribution from the zero voltage state and the related escape rate, as function of the bias current, have been performed by a low noise fly time technique. The experimental data have shown an intrinsic current fluctuation less than 0.2% of the critical current at liquid helium temperature, corresponding to an intrinsic sensor magnetic flux resolution of a few mΦ0. The theoretical predictions based on the thermal escape process theory in the moderately damping limit are in a reasonable agreement with the experimental data.
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85.25.Dq Superconducting quantum interference devices (SQUIDs)
74.25.Sv Critical currents
74.25.Ha Magnetic properties including vortex structures and related phenomena
74.78.Na Mesoscopic and nanoscale systems

Current- and field-driven magnetic antivortices for nonvolatile data storage

André Drews, Benjamin Krüger, Guido Meier, Stellan Bohlens, Lars Bocklage, Toru Matsuyama, and Markus Bolte

Appl. Phys. Lett. 94, 062504 (2009); http://dx.doi.org/10.1063/1.3072342 (3 pages) | Cited 12 times

Online Publication Date: 9 February 2009

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We demonstrate by micromagnetic simulations that magnetic antivortices are potential candidates for fast nonvolatile data-storage elements. These storage elements are excited simultaneously by alternating spin-polarized currents and their accompanying Oersted fields. Depending on the antivortex-core polarization p and the orientation of the in-plane magnetization c around the core, the superposition of current and field leads to either a suppression of gyration (logical “zero”) or an increased gyration amplitude (logical “one”). Above an excitation threshold the gyration culminates in the switching of the antivortex core. The switching can be seen as a cp-dependent writing of binary data, allowing to bring the antivortex into a distinct state. Furthermore a read-out scheme using an inductive loop situated on top of the element is investigated.
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85.70.Li Other magnetic recording and storage devices (including tapes, disks, and drums)
85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
84.30.Sk Pulse and digital circuits

Nonlinear magnetoimpedance and parametric excitation of standing spin waves in a glass-covered microwire

L. Kraus, M. Vázquez, G. Infante, G. Badini-Confalonieri, and J. Torrejón

Appl. Phys. Lett. 94, 062505 (2009); http://dx.doi.org/10.1063/1.3079659 (3 pages) | Cited 8 times

Online Publication Date: 9 February 2009

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The giant magnetoimpedance of an 8.5 μm glass-covered amorphous microwire was investigated in the frequency range of 10 MHz–3.5 GHz. It was found that when the exciting microwave current exceeds some threshold value, a periodic fine structure appears in the frequency dependence of the complex impedance. The appearance of this nonlinear phenomenon is interpreted to be a consequence of the parametric excitation of standing spin waves.
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75.47.De Giant magnetoresistance
75.30.Ds Spin waves

Local stress engineering of magnetic anisotropy in soft magnetic thin films

Norbert Martin, Jeffrey McCord, Andreas Gerber, Thomas Strache, Thomas Gemming, Ingolf Mönch, Nayel Farag, Rudolf Schäfer, Jürgen Fassbender, Eckhard Quandt, and Ludwig Schultz

Appl. Phys. Lett. 94, 062506 (2009); http://dx.doi.org/10.1063/1.3079664 (3 pages) | Cited 4 times

Online Publication Date: 9 February 2009

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The magnetic anisotropy of amorphous thin films was modified laterally by masked ion irradiation without alteration of the intrinsic magnetic properties. The changes were introduced by local ion implantation in a protection layer, causing additional stress-induced magnetic anisotropy in the magnetostrictive layer. The underlying local variation in magnetic anisotropy was modeled and confirmed experimentally. The described method, relying purely on magnetoelastics, introduces a new path to the alteration of magnetic properties subsequent to magnetic film preparation. With the use of the resulting artificial magnetization patterns, it is possible to tailor the ferromagnetic thin film structure used in magnetoelectronic applications.
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75.30.Gw Magnetic anisotropy
75.70.Ak Magnetic properties of monolayers and thin films
75.50.Kj Amorphous and quasicrystalline magnetic materials
75.80.+q Magnetomechanical effects, magnetostriction
61.80.Jh Ion radiation effects

Tunneling spectroscopy in core/shell structured Fe/MgO nanospheres

C. Martínez-Boubeta, Ll. Balcells, C. Monty, P. Ordejon, and B. Martínez

Appl. Phys. Lett. 94, 062507 (2009); http://dx.doi.org/10.1063/1.3080657 (3 pages) | Cited 7 times

Online Publication Date: 9 February 2009

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We report on tunneling conductance properties of a single Fe island enclosed by an epitaxial MgO shell. The results obtained show clear oscillations in the current-voltage curves that, along with the theoretical modeling, allow determining the electronic density of states of the iron core. Moreover, the correlation of these results with the temperature dependence of the electrical conductivity of assemblies of Fe/MgO nanocrystals provides evidence of the Δ1 symmetry-sensitive tunneling across a crystalline magnesia barrier.
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73.40.Gk Tunneling
73.22.Dj Single particle states

Effects of twin boundary mobility on domain microstructure evolution in magnetic shape memory alloys: Phase field simulation

Yongmei M. Jin

Appl. Phys. Lett. 94, 062508 (2009); http://dx.doi.org/10.1063/1.3081011 (3 pages) | Cited 8 times

Online Publication Date: 10 February 2009

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Effects of twin boundary mobility on domain microstructure evolution during magnetic field-induced deformation in magnetic shape memory alloys are studied by phase field micromagnetic microelastic modeling. The simulations show that different twin boundary mobilities lead to drastically different domain microstructures and evolution pathways, yielding very different magnetization and strain responses, even with opposite signs. The study also reveals complex domain phenomena in magnetic shape memory alloys.
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61.72.Mm Grain and twin boundaries
81.05.Bx Metals, semimetals, and alloys
81.40.Lm Deformation, plasticity, and creep
62.20.fg Shape-memory effect; yield stress; superelasticity
75.80.+q Magnetomechanical effects, magnetostriction
75.60.Ch Domain walls and domain structure

Lower-temperature crystallization of CoFeB in MgO magnetic tunnel junctions by using Ti capping layer

Takahiro Ibusuki, Toyoo Miyajima, Shinjiro Umehara, Shin Eguchi, and Masashige Sato

Appl. Phys. Lett. 94, 062509 (2009); http://dx.doi.org/10.1063/1.3080208 (3 pages) | Cited 16 times

Online Publication Date: 11 February 2009

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Effects of capping materials on magnetoresistance (MR) properties of MgO magnetic tunnel junctions (MTJs) with a CoFeB free layer were investigated. MR ratios of samples with various capping materials showed a difference in annealing temperature dependence. MTJ with a Ti capping layer annealed at 270 °C showed a MR ratio 1.4 times greater than that with a conventional Ta or Ru capping layer. Secondary ion mass spectroscopy and high-resolution transmission electron microscopy images revealed that crystallization of CoFeB was remarkably affected by adjacent materials and the Ti capping layer adjoining CoFeB acted as a boron-absorption layer. These results suggest that the crystallization process can be controlled by choosing proper capping materials. Ti is one of the effective materials that accelerate the crystallization of CoFeB layers at lower annealing temperature.
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75.47.-m Magnetotransport phenomena; materials for magnetotransport
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)

Field orientation dependent vortex formation in individual multilayer triangular rings

S. Jain and A. O. Adeyeye

Appl. Phys. Lett. 94, 062510 (2009); http://dx.doi.org/10.1063/1.3081419 (3 pages) | Cited 4 times

Online Publication Date: 12 February 2009

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We present the giant magnetoresistance responses of individual pseudo-spin-valve triangular ring using synchronous transport measurement technique. We observed that unlike single layer triangular rings, the formation of intermediate vortex state is strongly dependent on the applied field orientation (θ). For nπ/6<θ<(n+1)π/6, n = 0,1,2…, the Co layer in the ring undergoes a transition from forward onion state to reverse onion state via an intermediate vortex state, otherwise there is a direct transition from forward onion state to reverse onion states. This phenomenon in triangular rings may be useful for applications in future spin logic devices.
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75.47.De Giant magnetoresistance
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Tt Fine-particle systems; nanocrystalline materials
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)
75.75.-c Magnetic properties of nanostructures

Small anisotropy, weak thermal fluctuations, and high field superconductivity in Co-doped iron pnictide Ba(Fe1−xCox)2As2

A. Yamamoto, J. Jaroszynski, C. Tarantini, L. Balicas, J. Jiang, A. Gurevich, D. C. Larbalestier, R. Jin, A. S. Sefat, M. A. McGuire, B. C. Sales, D. K. Christen, and D. Mandrus

Appl. Phys. Lett. 94, 062511 (2009); http://dx.doi.org/10.1063/1.3081455 (3 pages) | Cited 80 times

Online Publication Date: 13 February 2009

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We performed high-field magnetotransport and magnetization measurements on a single crystal of the 122-phase iron pnictide Ba(Fe1−xCox)2As2. Unlike the high-temperature superconductor cuprates and 1111-phase oxypnictides, Ba(Fe1−xCox)2As2 showed practically no broadening of the resistive transitions under magnetic fields up to 45 T. We report the temperature dependencies of the upper critical field Hc2 both parallel and perpendicular to the c-axis, the irreversibility field Hirrc(T), and a rather unusual symmetric volume pinning force curve Fp(H) suggestive of a strong pinning nanostructure. The anisotropy parameter γ = Hc2ab/Hc2c deduced from the slopes of dHc2ab/dT = 4.9 T/K and dHc2c/dT = 2.5 T/K decreases from ∼ 2 near Tc, to ∼ 1.5 at lower temperatures, much smaller than γ for 1111pnictides and high-Tc cuprates.
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74.25.Ha Magnetic properties including vortex structures and related phenomena
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
74.40.-n Fluctuation phenomena
61.72.sd Impurity concentration
74.25.F- Transport properties
75.30.Gw Magnetic anisotropy
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
74.25.Op Mixed states, critical fields, and surface sheaths

High-quality antiferromagnetic EuTiO3 epitaxial thin films on SrTiO3 prepared by pulsed laser deposition and postannealing

Koji Fujita, Naoki Wakasugi, Shunsuke Murai, Yanhua Zong, and Katsuhisa Tanaka

Appl. Phys. Lett. 94, 062512 (2009); http://dx.doi.org/10.1063/1.3072598 (3 pages) | Cited 17 times

Online Publication Date: 13 February 2009

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We report on epitaxial growth and magnetic properties of EuTiO3 thin films with a perovskite structure. Single crystalline EuTiO3 films with atomically flat surface are grown on (001) surface of SrTiO3 by pulsed laser deposition (PLD) and subsequent annealing in reducing atmosphere. The as-deposited films possess the crystal structure with an elongated c-axis and tend to stabilize ferromagnetically ordered Eu2+ spins at low temperatures. Postannealing at 1000 °C relaxes the out-of-plane lattice strain, and brings about a drastic change in magnetic structure; the annealed film becomes an antiferromagnet below Néel temperature of 5.1 K. The change in magnetic properties accompanied by the modification in lattice constant is discussed.
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75.70.Ak Magnetic properties of monolayers and thin films
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
75.50.Ee Antiferromagnetics
81.15.Fg Pulsed laser ablation deposition
81.40.Gh Other heat and thermomechanical treatments
68.55.-a Thin film structure and morphology

Enhanced flux pinning by BaZrO3 and (Gd,Y)2O3 nanostructures in metal organic chemical vapor deposited GdYBCO high temperature superconductor tapes

Yimin Chen, Venkat Selvamanickam, Yifei Zhang, Yuri Zuev, Claudia Cantoni, Eliot Specht, M. Parans Paranthaman, Tolga Aytug, Amit Goyal, and Dominic Lee

Appl. Phys. Lett. 94, 062513 (2009); http://dx.doi.org/10.1063/1.3082037 (3 pages) | Cited 21 times

Online Publication Date: 13 February 2009

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We have formed BaZrO3 nanocolumns and (Gd,Y)2O3 nanoprecipitates in reel-to-reel metal organic chemical vapor deposition (MOCVD) processed (Gd,Y)Ba2Cu3O7−x coated conductors and increased the critical currents (Ic) of the conductors in applied magnetic fields to remarkable levels. A (Gd,Y)Ba2Cu3O7−x tape of 1 m in length with 6.5% Zr-additions and 30% composition rich in both Gd and Y showed Ic values of 813 A/cm width at (self-field, 77 K) and above 186 A/cm width at (1 T, 77 K). The strongly enhanced flux pinning over a wide range of magnetic field orientations can be attributed to the bidirectionally aligned defect structures of BaZrO3 and (Gd,Y)2O3 created by optimized MOCVD conditions.
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74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
74.78.Na Mesoscopic and nanoscale systems
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
74.25.Sv Critical currents
74.72.-h Cuprate superconductors
84.71.Mn Superconducting wires, fibers, and tapes

Control of ultranarrow Co magnetic domain wall widths in artificially patterned H-bar structures

Z. J. Yang, L. Sun, X. P. Zhang, M. Cao, X. Y. Deng, An Hu, and H. F. Ding

Appl. Phys. Lett. 94, 062514 (2009); http://dx.doi.org/10.1063/1.3082046 (3 pages) | Cited 2 times

Online Publication Date: 13 February 2009

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Micromagnetic simulations of Co domain walls on nanometer crossbars that join two oppositely magnetized parallel legs of “H” shaped patterns are studied. The crossbar domain walls can twist in the plane of the H-pattern, out of the plane, or swirl, forming Néel, Bloch, or vortex structures, respectively, depending on the initial configurations. An energy phase diagram as a function of the crossbar constriction yields the Néel wall as the energetically most favorable, followed by the Bloch wall, which becomes unstable and changes into a vortex-like wall with increasing crossbar size. Most interestingly, the Néel wall width can either shrink or expand depending on the crossbar dimensions. In the case that both the crossbar length and width are small, desirable, ultranarrow domain walls can be obtained. These findings are useful for spintronic device design based on domain wall pinning via nanonotch and domain-wall magnetoresistance approaches.
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75.60.Ch Domain walls and domain structure
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
72.15.Gd Galvanomagnetic and other magnetotransport effects

Induced half-metallicity in Cr-based ferromagnetic chalcospinels with anion substitutions: CuCr2S(Se)4−xEx (E = F, Cl, Br), Cu(Cd)Cr2S(Se)4−x, and CdCr2S(Se)4−xDx (D = N, P, As)

Y.-H. A. Wang, A. Gupta, M. Chshiev, and W. H. Butler

Appl. Phys. Lett. 94, 062515 (2009); http://dx.doi.org/10.1063/1.3080210 (3 pages) | Cited 6 times

Online Publication Date: 13 February 2009

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First-principles calculations of the electronic structures of chalcogenide spinels with anion vacancy and substitutions are presented. Our calculations predict that Cu(Cd)Cr2S(Se)4−x, CuCr2S(Se)4−xEx (E = F, Cl, Br), and CdCr2S(Se)4−xDx (D = N, P, As) can be half-metallic over a range of concentrations. The magnetic moment is found to scale approximately linearly on adding or withdrawing electrons and varies between 5.0 and 6.0 μB/f.u.
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71.23.Cq Amorphous semiconductors, metallic glasses, glasses
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
75.50.Pp Magnetic semiconductors
75.50.Dd Nonmetallic ferromagnetic materials
75.30.Cr Saturation moments and magnetic susceptibilities
61.72.jd Vacancies

Reduction in critical current for spin transfer switching in perpendicular anisotropy spin valves using an in-plane spin polarizer

Randall Law, Ei-Leen Tan, Rachid Sbiaa, Thomas Liew, and Tow Chong Chong

Appl. Phys. Lett. 94, 062516 (2009); http://dx.doi.org/10.1063/1.3083546 (3 pages) | Cited 17 times

Online Publication Date: 13 February 2009

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We describe a strategy to reduce spin transfer switching (STS) currents in CoFe/Pd-based perpendicular anisotropy single spin valves (SSVs) by the insertion of an in-plane spin polarizer, thus creating a modified-dual spin valve (m-DSV). For SSV devices, concurrent STS of both magnetic layers was observed for positive currents, making the parallel-to-antiparallel (P→AP) transition impossible. In m-DSV devices, we observed a 60% reduction in the energy barrier for AP→P transitions and a 40% reduction in JcAP→P with 10 ns STS current pulses compared to SSV devices. Furthermore, the m-DSV structure enabled the soft layer to switch independently from the hard layer via STS.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.60.-d Domain effects, magnetization curves, and hysteresis
85.70.-w Magnetic devices
75.50.Bb Fe and its alloys
75.30.Gw Magnetic anisotropy
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