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28 Oct 2002

Volume 81, Issue 18, pp. 3311-3500

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Noninvasive magnetic imaging and magnetization measurement of isolated mesoscopic Co rings

J. Bekaert, D. Buntinx, C. Van Haesendonck, V. V. Moshchalkov, J. De Boeck, G. Borghs, and V. Metlushko

Appl. Phys. Lett. 81, 3413 (2002); http://dx.doi.org/10.1063/1.1518564 (3 pages) | Cited 26 times

Online Publication Date: 22 October 2002

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A high-resolution scanning Hall probe microscope was used as a noninvasive technique to visualize the magnetization reversal in an array of micron-size Co rings. Two stable “onion” states at remanence and “vortex” states at switching fields were found. To rule out a possible influence of dipole–dipole interaction between ring elements on remagnetization processes, an isolated Co ring was deposited on top of a Hall magnetometer and extremely sharp transitions from onion to vortex and from vortex to onion state of opposite polarity were resolved. Our results were supported by MOKE magnetization measurements and micromagnetic simulations. © 2002 American Institute of Physics.
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75.75.-c Magnetic properties of nanostructures
75.60.Jk Magnetization reversal mechanisms
75.50.Cc Other ferromagnetic metals and alloys
07.55.Jg Magnetometers for susceptibility, magnetic moment, and magnetization measurements
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
78.20.Ls Magneto-optical effects

Magnetocaloric effect in layered perovskite manganese oxide La1.4Ca1.6Mn2O7

Hong Zhu, Hao Song, and YuHeng Zhang

Appl. Phys. Lett. 81, 3416 (2002); http://dx.doi.org/10.1063/1.1518160 (3 pages) | Cited 12 times

Online Publication Date: 22 October 2002

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We report the discovery of a large magnetic entropy change in La1.4Ca1.6Mn2O7, a bilayered perovskite manganese oxide that has a Curie temperature (TC) of about 270 K and which allows magnetic refrigeration at room temperature. The magnetic entropy changes reach values of 11.3 J K−1 kg−1 and 16.8 J K−1 kg−1 for field changes of 2 T and 5 T, respectively. The refrigerant capacity of our material is much larger than that of Gd. The large entropy change can be attributed to the fact that the ferromagnetic transition enhances the effect of the applied magnetic field greatly. © 2002 American Institute of Physics.
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75.30.Sg Magnetocaloric effect, magnetic cooling
75.47.Gk Colossal magnetoresistance
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Dd Nonmetallic ferromagnetic materials

Enhancement of the critical current density and flux pinning of MgB2 superconductor by nanoparticle SiC doping

S. X. Dou, S. Soltanian, J. Horvat, X. L. Wang, S. H. Zhou, M. Ionescu, H. K. Liu, P. Munroe, and M. Tomsic

Appl. Phys. Lett. 81, 3419 (2002); http://dx.doi.org/10.1063/1.1517398 (3 pages) | Cited 392 times

Online Publication Date: 22 October 2002

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Doping of MgB2 by nano-SiC and its potential for the improvement of flux pinning were studied for MgB2−x(SiC)x/2 with x = 0, 0.2, and 0.3 and for 10 wt % nano-SiC-doped MgB2 samples. Cosubstitution of B by Si and C counterbalanced the effects of single-element doping, decreasing Tc by only 1.5 K, introducing intragrain pinning centers effective at high fields and temperatures, and significantly enhancing Jc and Hirr. Compared to the undoped sample, Jc for the 10 wt % doped sample increased by a factor of 32 at 5 K and 8 T, 42 at 20 K and 5 T, and 14 at 30 K and 2 T. At 20 K and 2 T, the Jc for the doped sample was 2.4×105 A/cm2, which is comparable to Jc values for the best Ag/Bi-2223 tapes. At 20 K and 4 T, Jc was twice as high as for the best MgB2 thin films and an order of magnitude higher than for the best Fe/MgB2 tapes. The magnetic Jc is consistent with the transport Jc which remains at 20 000 A/cm2 even at 10 T and 5 K for the doped sample, an order of magnitude higher than the undoped one. Because of such high performance, it is anticipated that the future MgB2 conductors will be made using a formula of MgBxSiyCz instead of pure MgB2. © 2002 American Institute of Physics.
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74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)
74.25.Sv Critical currents
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
74.25.Ha Magnetic properties including vortex structures and related phenomena

Influence of near-surface nonstoichiometry on the surface magnetization of mixed-valent manganite: A computer simulation study

U. P. Wad, Abhijit S. Ogale, S. B. Ogale, and T. Venkatesan

Appl. Phys. Lett. 81, 3422 (2002); http://dx.doi.org/10.1063/1.1517399 (3 pages) | Cited 6 times

Online Publication Date: 22 October 2002

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A Monte Carlo simulation study is performed to examine the effects of near-surface nonstoichiometry in a mixed-valent manganite (e.g., La0.7A0.3MnO3, where A = Sr, Ca) on the magnetization of its surface, and that of a few layers underneath. The nonstoichiometry is introduced either in the form of oxygen vacancy gradient or a gradient in the La:A ratio. The corresponding gradient in the exchange constant J is incorporated phenomenologically from the known phase diagram [P. Schiffer et al., Phys. Rev. Lett. 75, 3356 (1995)]. We show that the near-surface nonstoichiometry can account for the temperature dependence of magnetization of the surface layers as revealed by photoemission and x-ray magnetic circular dichroism (X-MCD) experiments [J. Park et al., Nature (London) 392, 794 (1998); J. Park et al., Phys. Rev. Lett. 81, 1953 (1998)]. The demagnetization shows viscous fingeringlike protrusions which may have important consequences for spin-polarized transport across interfaces. © 2002 American Institute of Physics.
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75.70.Rf Surface magnetism
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.47.Gk Colossal magnetoresistance
75.30.Mb Valence fluctuation, Kondo lattice, and heavy-fermion phenomena
75.20.Hr Local moment in compounds and alloys; Kondo effect, valence fluctuations, heavy fermions
75.50.Dd Nonmetallic ferromagnetic materials
78.20.Ls Magneto-optical effects
79.60.-i Photoemission and photoelectron spectra
61.66.Bi Elemental solids
61.66.Dk Alloys
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.30.Et Exchange and superexchange interactions
68.35.B- Structure of clean surfaces (and surface reconstruction)

Antiferromagnetic hysteresis in magnetoresistive multilayers investigated by x-ray resonant scattering

Carlo Spezzani, Piero Torelli, Maurizio Sacchi, Renaud Delaunay, Coryn F. Hague, Vincent Cros, and Frédéric Petroff

Appl. Phys. Lett. 81, 3425 (2002); http://dx.doi.org/10.1063/1.1517403 (3 pages) | Cited 5 times

Online Publication Date: 22 October 2002

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We have used resonant scattering of polarized soft x rays as a direct probe of the magnetic order in a weakly coupled Co/Cu multilayer. Our field dependent results, combined with in situ resistance measurements, show a direct correlation between magnetoresistance and antiparallel magnetic ordering in reversible and irreversible processes. © 2002 American Institute of Physics.
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75.47.De Giant magnetoresistance
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Ee Antiferromagnetics
78.70.Ck X-ray scattering

Magnetization dependence of training effect of exchange coupling in ferromagnet/FeMn bilayers

S. J. Yuan, L. Wang, S. M. Zhou, M. Lu, J. Du, and A. Hu

Appl. Phys. Lett. 81, 3428 (2002); http://dx.doi.org/10.1063/1.1517711 (3 pages) | Cited 11 times

Online Publication Date: 22 October 2002

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The exchange coupling and its training effect are studied as a function of the ferromagnetic layer magnetization by using various ferromagnet/FeMn bilayers with ferromagnetic materials Ni, Ni81Fe19, Ni50Fe50, Co, and Fe. The exchange coupling energy Jex increases with increasing MFM as Jex∝math. The training effect of the exchange field is related to both the ferromagnet magnetization and the magnetization reversal mechanism. For ferromagnet/FeMn bilayers with similar magnetization reversal mechanisms, the relative change of the exchange field decreases with increasing magnetization in an exponential manner. © 2002 American Institute of Physics.
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75.70.Ak Magnetic properties of monolayers and thin films
75.30.Et Exchange and superexchange interactions
75.60.Jk Magnetization reversal mechanisms
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Bb Fe and its alloys
75.50.Cc Other ferromagnetic metals and alloys
75.50.Ee Antiferromagnetics
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