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15 May 2003

Volume 93, Issue 10, pp. 5855-8792

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Magnetic coupling and magnetoresistance in La0.55Sr0.45MnO3/ La0.67Ca0.33 MnO3 multilayers

M. Sirena, N. Haberkorn, L. B. Steren, and J. Guimpel

J. Appl. Phys. 93, 6177 (2003); http://dx.doi.org/10.1063/1.1565827 (5 pages) | Cited 10 times

Online Publication Date: 9 May 2003

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We have studied the interlayer coupling and the magnetoresistant effect of La0.55Sr0.45MnO3/ La0.67Ca0.33MnO3 structures. Magnetization loops, measured in La0.55Sr0.45MnO3/La0.67Ca0.33MnO3/La0.55Sr0.45MnO3 trilayers, indicate that there is a ferromagnetic coupling of the La0.55Sr0.45MnO3 layers across the La0.67Ca0.33MnO3 spacer up to room temperature, even above the Curie temperature of the La0.67Ca0.33MnO3 layers. However, magnetization versus temperature curves present signatures of the magnetic ordering of both compounds. No extrinsic magnetoresistance associated with the multilayered structure was observed in the whole temperature range due to the presence of the interlayer ferromagnetic coupling. © 2003 American Institute of Physics.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Dd Nonmetallic ferromagnetic materials
75.47.Gk Colossal magnetoresistance
75.47.Lx Magnetic oxides
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Et Exchange and superexchange interactions
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.40.-s Critical-point effects, specific heats, short-range order

Low-temperature magnetic properties and the crystallization behavior of FINEMET alloy

N. Ponpandian, A. Narayanasamy, K. Chattopadhyay, M. Manivel Raja, K. Ganesan, C. N. Chinnasamy, and B. Jeyadevan

J. Appl. Phys. 93, 6182 (2003); http://dx.doi.org/10.1063/1.1565829 (6 pages) | Cited 6 times

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We have synthesized FINEMET alloy by a melt spinning technique and studied in detail its crystallization behavior and low-temperature magnetic properties. The crystallization behavior is characterized by transmission electron microscopy and Mössbauer spectroscopy. At early stages bcc solid solution precipitates from the amorphous matrix. At later stages, they order to yield DO3 ordered Fe3Si coexisting with a small amount of Fe2B. The analysis of Mössbauer spectra supports this observation. The temperature dependence of the magnetization in the temperature range 10–300 K of the FINEMET alloy in its as-quenched state follows the relation M(T)=M0(T) (1−B T3/2−C T5/2−⋅⋅⋅), which is indicative of the presence of spin wave excitations in the alloy. The value of the C/B ratio and the mean-square value of the range of exchange interaction 〈r2〉 are found to be characteristic of the noncrystalline ferromagnets. The small value obtained for the exchange stiffness constant D is an indication of the softening of the exchange interaction. © 2003 American Institute of Physics.
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75.30.Et Exchange and superexchange interactions
64.70.K- Solid-solid transitions
75.50.Kj Amorphous and quasicrystalline magnetic materials
75.50.Tt Fine-particle systems; nanocrystalline materials
61.43.-j Disordered solids
75.50.Bb Fe and its alloys
76.80.+y Mössbauer effect; other γ-ray spectroscopy
81.30.Mh Solid-phase precipitation
64.75.-g Phase equilibria
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.)

Magnetic properties and the tunneling magnetoresistance effect in Co−MgF2 granular films

J. H. Chi, S. H. Ge, C. M. Liu, H. P. Kunkel, X. Z. Zhou, and G. Williams

J. Appl. Phys. 93, 6188 (2003); http://dx.doi.org/10.1063/1.1567054 (4 pages) | Cited 6 times

Online Publication Date: 9 May 2003

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Co−MgF2 granular films were deposited on glass substrates by rf co-sputtering at room temperature (RT). The influence of the Co volume fraction, fv, of these granular films on the tunneling magnetoresistance (TMR) and magnetic properties was studied systematically. In a magnetic field of 1.2 T, the TMR value at RT initially increases gradually with decreasing fv, reaches its maximum value of −8% for fv=0.38, and then decreases. The corresponding magnetization curves indicate a change from ferromagnetism to superparamagnetism. A minimum in the coercivity, Hc, (11 Oe) is obtained in the Co50(MgF2)50 granular film which also has a large zero field resistivity. This magnetically soft granular film may consequently be a good candidate for high frequency applications. These variations of the TMR and magnetic properties can be ascribed to gradual changes in the film microstructure with decreasing fv, from interconnected metallic Co grains to nano-scaled Co particles dispersed in a crystallized insulating MgF2 matrix. The zero field cooled (ZFC) and field cooled (FC) curves for the samples were obtained in the temperature range 5–300 K in various magnetic fields. The peak in the ZFC curve shifts gradually towards lower temperature with increasing applied magnetic field and, with increasing fv, the peak temperature decreases more quickly with increasing field. The latter indicates that the magnetic interactions between grains become stronger, consistent with model predictions. © 2003 American Institute of Physics.
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75.47.Pq Other materials
75.50.Cc Other ferromagnetic metals and alloys
75.50.Tt Fine-particle systems; nanocrystalline materials
75.70.Ak Magnetic properties of monolayers and thin films
81.05.Rm Porous materials; granular materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Topology dependence of domain wall depinning in magnetic hard–soft composites

Z. F. Lin, S. T. Chui, and L. B. Hu

J. Appl. Phys. 93, 6192 (2003); http://dx.doi.org/10.1063/1.1563848 (3 pages) | Cited 3 times

Online Publication Date: 9 May 2003

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We study the depinning field Hd in magnetic hard–soft nanocomposite by finite-temperature Monte Carlo simulation for two different topologies of the composites: (1) a hard phase in a soft matrix and (2) a soft phase in a hard matrix. We find that the depinning field and maximum energy product is higher in the first case. The temperature and composition dependence of the switching field and maximum energy product is reported. © 2003 American Institute of Physics.
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75.60.Ch Domain walls and domain structure
75.40.Mg Numerical simulation studies
02.70.Uu Applications of Monte Carlo methods

Circular susceptibilities of CoFeSiB amorphous wires determined by inductance and second harmonic longitudinal magnetization

D.-X. Chen and L. Pascual

J. Appl. Phys. 93, 6195 (2003); http://dx.doi.org/10.1063/1.1568522 (4 pages) | Cited 2 times

Online Publication Date: 9 May 2003

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After a positive high field premagnetizing, the circular susceptibility χϕ of annealed nearly nonmagnetostrictive amorphous wires determined by measuring ac inductance is anomalously large and asymmetric with respect to the sign of the dc bias field Hz. This phenomenon is explained by comparing with another circular susceptibility χϕ,V2 obtained by measuring second harmonic longitudinal magnetization in terms of a model of connected (for low Hz) or coupled (for elevated Hz) donut structures induced by alinged magnetic inclusions. © 2003 American Institute of Physics.
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75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Kj Amorphous and quasicrystalline magnetic materials
75.50.Bb Fe and its alloys
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Switching speed limitations in perpendicular magnetic recording media

A. Lyberatos

J. Appl. Phys. 93, 6199 (2003); http://dx.doi.org/10.1063/1.1567801 (9 pages) | Cited 3 times

Online Publication Date: 9 May 2003

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A method to study switching speed limitations in high-frequency magnetic recording is presented. The characteristic parameter is the minimum irreversible switching time that is evaluated by numerical simulation of the writing of a dibit of two transitions using Landau–Lifshitz dynamics. The dibit is written on a perpendicular polycrystalline thin film medium using a single-pole head with a soft underlayer. We study the physical factors that determine the ultimate data rate and consider the problem of optimization of switching speed by changing the relative head-medium velocity, the magnetocrystalline anisotropy, the rise time of the head field, the head-medium separation, the exchange intergranular coupling, and the dispersion in the direction and magnitude of the local anisotropy. © 2003 American Institute of Physics.
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85.70.Kh Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.
75.70.Ak Magnetic properties of monolayers and thin films
75.30.Et Exchange and superexchange interactions
75.30.Gw Magnetic anisotropy
75.60.Jk Magnetization reversal mechanisms

Mg(B,O)2 precipitation in MgB2

X. Z. Liao, A. Serquis, Y. T. Zhu, J. Y. Huang, L. Civale, D. E. Peterson, F. M. Mueller, and H. F. Xu

J. Appl. Phys. 93, 6208 (2003); http://dx.doi.org/10.1063/1.1568528 (8 pages) | Cited 44 times

Online Publication Date: 9 May 2003

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MgB2 samples prepared by solid-state reaction were investigated using high-resolution transmission electron microscopy (HREM), x-ray energy-dispersive spectroscopy (EDX), electron energy-loss spectroscopy (EELS), and energy-filtered imaging. Large amounts of coherent precipitates with a size range from about 5 nm up to about 100 nm were found in the MgB2 crystallite matrices. The precipitates are of different shapes including sphere, ellipsoid, and faceted polyhedron depending on the size of the precipitates. EDX and EELS analyses confirm that smaller precipitates contain magnesium, boron and oxygen while larger faceted precipitates contain mainly magnesium and oxygen, implying that the oxygen content increases with precipitate size. HREM and electron diffraction investigations found that the precipitates have the same crystal lattice structure as that of MgB2 but with various composition modulations depending on the composition of the precipitates. The precipitates transform to the MgO phase after long exposure to residual oxygen in flowing Ar gas at high temperatures. The effect of the precipitates in different size ranges on flux pinning is discussed. © 2003 American Institute of Physics.
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74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)
64.75.-g Phase equilibria
61.72.-y Defects and impurities in crystals; microstructure
82.80.Ej X-ray, Mössbauer, and other γ-ray spectroscopic analysis methods
79.20.Uv Electron energy loss spectroscopy
68.49.Jk Electron scattering from surfaces
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
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