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23 Sep 2002

Volume 81, Issue 13, pp. 2319-2480

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Operation of high-temperature superconductor magnetometer with submicrometer bicrystal junctions

M. I. Faley, U. Poppe, K. Urban, V. Yu. Slobodchikov, Yu. V. Maslennikov, A. Gapelyuk, B. Sawitzki, and A. Schirdewan

Appl. Phys. Lett. 81, 2406 (2002); http://dx.doi.org/10.1063/1.1508419 (3 pages) | Cited 9 times

Online Publication Date: 16 September 2002

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We investigated the noise properties of dc superconducting quantum interference device flip-chip magnetometers with submicrometer-wide bicrystal junctions operating at 77.4 K. The noise of the magnetometers with electronics was about 6 fT/√Hz at frequencies above 100 Hz increasing up to about 20 fT/√Hz at 1 Hz. The operation of the magnetometers was characterized in an electronic axial first order gradiometer system, which was employed for biomagnetic measurements. The system demonstrated a gradient resolution of about 1 fT/cm√Hz at 77.4 K and stable operation in a standard magnetically shielded room under clinical conditions. © 2002 American Institute of Physics.
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85.25.Dq Superconducting quantum interference devices (SQUIDs)
07.55.Ge Magnetometers for magnetic field measurements

Thermal energy barrier distribution measurements in perpendicular media

X. W. Wu, H. Zhou, R. J. M. van de Veerdonk, G. Ju, B. Lu, and D. Weller

Appl. Phys. Lett. 81, 2409 (2002); http://dx.doi.org/10.1063/1.1510177 (3 pages) | Cited 6 times

Online Publication Date: 16 September 2002

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dc demagnetized remanent curves at different applied field time durations have been applied to measure the thermal energy barrier distribution of two sets of perpendicular alloy media. Sharrock’s formula is applied at all levels of percentages of magnetization switching. The measured energy barrier distribution width decreases with increasing intergranular exchange coupling and with reducing intergranular magnetostatic interactions. The results agree well with micromagnetic simulations. © 2002 American Institute of Physics.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Ss Magnetic recording materials
75.30.Et Exchange and superexchange interactions

Minor and major-loop studies of magnetic and reversal properties for synthetic antiferromagnetically coupled media

Z. S. Shan, S. S. Malhotra, D. C. Stafford, G. Bertero, and D. Wachenschwanz

Appl. Phys. Lett. 81, 2412 (2002); http://dx.doi.org/10.1063/1.1509864 (3 pages) | Cited 4 times

Online Publication Date: 16 September 2002

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An approach has been developed to determine the magnetic properties for both the top and bottom layers of synthetic antiferromagnetically coupled (SAF) media by measuring the minor and major loops at different H field sweep rates with an alternating gradient force magnetometer. Using this approach, the effects of the bottom layer thickness on the magnetic and thermally activated properties of both the top and bottom layers were investigated systematically. These results are valuable for optimal design of SAF media. © 2002 American Institute of Physics.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.60.Jk Magnetization reversal mechanisms
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Soft anisotropic high magnetization Cu/FeCo films

H. S. Jung, W. D. Doyle, J. E. Wittig, J. F. Al-Sharab, and J. Bentley

Appl. Phys. Lett. 81, 2415 (2002); http://dx.doi.org/10.1063/1.1510163 (3 pages) | Cited 28 times

Online Publication Date: 16 September 2002

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A remarkable reduction in the coercivity Hc of sputtered Fe65Co35 films from 9.6 to 0.7 kA/m was observed using a Cu underlayer as thin as 2.5 nm. The FeCo without Cu exhibited a wide distribution of anisotropy fields up to >80 kA/m while the FeCo with Cu showed a well-defined in-plane uniaxial anisotropy field of 2.3 kA/m up to FeCo thicknesses of at least 1 μm. The saturation magnetostriction was (4.7±0.4)×10−5, independent of Cu thickness while the in-plane tensile stress gradually decreased from 2 to 0.2 GPa as the Cu thickness increased to 10 nm. The Cu changed the preferred orientation of the FeCo from (100) to (110) but more significantly reduced the average grain size from ∼ 50 to ∼9 nm. This alone is sufficient to explain quantitatively the reduction in Hc using Hoffmann’s ripple theory. © 2002 American Institute of Physics.
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75.70.Ak Magnetic properties of monolayers and thin films
75.50.Vv High coercivity materials
75.50.Bb Fe and its alloys
75.30.Gw Magnetic anisotropy
75.80.+q Magnetomechanical effects, magnetostriction

Room-temperature magnetism in Cr-doped AlN semiconductor films

S. G. Yang, A. B. Pakhomov, S. T. Hung, and C. Y. Wong

Appl. Phys. Lett. 81, 2418 (2002); http://dx.doi.org/10.1063/1.1509475 (3 pages) | Cited 81 times

Online Publication Date: 16 September 2002

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Synthesis and characterization of magnetic semiconductors Al1−xCrxN, in which the atomic fraction of chromium x is up to 0.357, are reported. The films, grown by reactive co-sputtering on silicon, glass, and kapton substrates, have a crystal structure of aluminum nitride. Magnetic and transport properties were studied in the temperature range of 50 to 340 K. The materials are in the dielectric regime and have variable-range-hopping type of conductance. The films are ferromagnetic at temperatures over 340 K. © 2002 American Institute of Physics.
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75.70.Ak Magnetic properties of monolayers and thin films
73.61.Ey III-V semiconductors
75.50.Pp Magnetic semiconductors
81.15.Cd Deposition by sputtering
75.50.Dd Nonmetallic ferromagnetic materials
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
73.50.Dn Low-field transport and mobility; piezoresistance

Formation of Co nanoclusters in epitaxial Ti0.96Co0.04O2 thin films and their ferromagnetism

D. H. Kim, J. S. Yang, K. W. Lee, S. D. Bu, T. W. Noh, S.-J. Oh, Y.-W. Kim, J.-S. Chung, H. Tanaka, H. Y. Lee, and T. Kawai

Appl. Phys. Lett. 81, 2421 (2002); http://dx.doi.org/10.1063/1.1509477 (3 pages) | Cited 113 times

Online Publication Date: 16 September 2002

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Anatase Ti0.96Co0.04O2 films were grown epitaxially on SrTiO3 (001) substrates by using pulsed laser deposition with in-situ reflection high-energy electron diffraction. The oxygen partial pressure, PO2, during the growth was systematically varied. As PO2 decreased, the growth behavior was changed from a two-dimensional layer-by-layer-like growth to a three-dimensional island-like one, which resulted in an increase in the saturation magnetization. These structural and magnetic changes were explained in terms of the formation of cobalt clusters whose existence was proved by transmission-electron-microscope studies. Our work clearly indicates that the cobalt clustering will cause room-temperature ferromagnetism in the Co-doped TiO2 films. © 2002 American Institute of Physics.
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Cc Other ferromagnetic metals and alloys
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.70.Ak Magnetic properties of monolayers and thin films
61.46.-w Structure of nanoscale materials
81.15.Fg Pulsed laser ablation deposition

Critical current density and third-harmonic voltage in superconducting films

Yasunori Mawatari, Hirofumi Yamasaki, and Yoshihiko Nakagawa

Appl. Phys. Lett. 81, 2424 (2002); http://dx.doi.org/10.1063/1.1510159 (3 pages) | Cited 41 times

Online Publication Date: 16 September 2002

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When a sinusoidal drive current I0 cos ωt flows in a small coil close to the surface of a superconducting film, third-harmonic voltage V3 cos(3ωt+θ3) is induced in the coil if the film causes a nonlinear response. We have developed an approximate theoretical method yielding the relationships among I0, V3, and θ3, thus providing the scientific basis for a widely used inductive method for measuring the critical current density Jc in large-area superconducting films. Our results show that V3 is near zero when I0 is smaller than a threshold value Ic0∝Jcd, where d is the film thickness. When I0>Ic0, on the other hand, the third-harmonic voltage is expressed as V3 exp(−iθ3) = ωIc0G(I0/Ic0), where G(x) is a scaling function determined by the configuration of the coil. We demonstrate the scaling law of V3/Ic0 vs I0/Ic0 in a YBa2Cu3O7−δ film. © 2002 American Institute of Physics.
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74.78.-w Superconducting films and low-dimensional structures
74.25.Sv Critical currents
74.72.-h Cuprate superconductors

Magnetocaloric effect in random magnetic anisotropy materials

Xavier Bohigas, Javier Tejada, Francesc Torres, José Ignacio Arnaudas, Enrique Joven, and Agustín del Moral

Appl. Phys. Lett. 81, 2427 (2002); http://dx.doi.org/10.1063/1.1506777 (3 pages) | Cited 10 times

Online Publication Date: 16 September 2002

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In this letter we report the results of entropy variations in random anisotropy magnets composed of TbxY1−xAl2, with x = 0.15, 0.20, 0.25, 0.35, 0.40, and 0.50. We discovered large entropy variation associated with the spin glass to paramagnetic transition. Both temperature transition and entropy changes were studied at different temperatures and with different compositions. Our conclusion is that these materials are suitable candidates for use as magnetic refrigerants in a temperature range below 40 K. © 2002 American Institute of Physics.
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75.50.Lk Spin glasses and other random magnets
75.30.Sg Magnetocaloric effect, magnetic cooling
75.30.Gw Magnetic anisotropy
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.20.En Metals and alloys
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