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13 Feb 2006

Volume 88, Issue 7, Articles (07xxxx)

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Appl. Phys. Lett. 88, 072905 (2006); http://dx.doi.org/10.1063/1.2172744 (3 pages)

Y. L. Li and L. Q. Chen
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Reducing the operational magnetic field in the prototype magnetocaloric system Gd5Ge4 by approaching the single cluster size limit

J. D. Moore, G. K. Perkins, Y. Bugoslavsky, M. K. Chattopadhyay, S. B. Roy, P. Chaddah, V. K. Pecharsky, K. A. Gschneidner, and L. F. Cohen

Appl. Phys. Lett. 88, 072501 (2006); http://dx.doi.org/10.1063/1.2173631 (3 pages) | Cited 29 times

Online Publication Date: 13 February 2006

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We studied polycrystalline samples of the prototype magnetocaloric system Gd5Ge4 using scanning Hall imaging and global magnetometry. The magnetic field required to complete the phase transition that is critical for magnetocaloric application is reduced by up to 20% when small fragments, each consisting of several randomly oriented crystallites, are removed from the bulk. The removal of competing strain fields from neighboring grains when the fragment is embedded in the bulk underlies the observation. We support the results by showing local Hall imaging of phase separation in the bulk. When the bulk is ground into a fine powder, the effect vanishes.
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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.)
75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)

Significantly enhanced critical current densities in MgB2 tapes made by a scaleable nanocarbon addition route

Yanwei Ma, Xianping Zhang, G. Nishijima, K. Watanabe, S. Awaji, and Xuedong Bai

Appl. Phys. Lett. 88, 072502 (2006); http://dx.doi.org/10.1063/1.2173635 (3 pages) | Cited 89 times

Online Publication Date: 13 February 2006

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Nanocarbon-doped Fe-sheathed MgB2 tapes with different doping levels were prepared by the in situ powder-in-tube method. Compared to the undoped tapes, Jc for all the C-doped samples was enhanced by more than an order of magnitude in magnetic fields above 9 T. At 4.2 K, the transport Jc for the 5 at. % doped tapes reached 1.85×104 A/cm2 at 10 T and 2.8×103 A/cm2 at 14 T, respectively. Moreover, the critical temperature for the doped tapes decreased slightly. Transmission electron microscopy showed a number of intragranular dislocations and the dispersed nanoparticles embedded within MgB2 grains induced by the C doping. The mechanism for the enhancement of flux pinning is also discussed. These results indicate that powder-in-tube-processed MgB2 tape is very promising for high-field applications.
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84.71.Mn Superconducting wires, fibers, and tapes
74.25.Sv Critical currents
74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)
61.72.S- Impurities in crystals
74.62.Dh Effects of crystal defects, doping and substitution
61.72.up Other materials

Magnetization reversal of uncompensated Fe moments in exchange biased Ni/FeF2 bilayers

Elke Arenholz, Kai Liu, Zhipan Li, and Ivan K. Schuller

Appl. Phys. Lett. 88, 072503 (2006); http://dx.doi.org/10.1063/1.2173716 (3 pages) | Cited 14 times

Online Publication Date: 13 February 2006

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The magnetization reversal of uncompensated Fe moments in exchange biased Ni/FeF2 bilayers was determined using soft x-ray magnetic circular and linear dichroism. The hysteresis loops resulting from the Fe moments are almost identical to those of the ferromagnetic Ni layer. However, a vertical loop shift indicates that some Fe moments are pinned in the antiferromagnetically ordered FeF2. The pinned moments are oriented antiparallel to small cooling fields, leading to negative exchange bias, but parallel to large cooling fields, resulting in positive exchange bias. No indication for the formation of a parallel antiferromagnetic domain wall in the FeF2 layer upon magnetization reversal in the Ni layer was found.
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75.50.Ee Antiferromagnetics
75.60.Jk Magnetization reversal mechanisms
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.)

Magnetoresistance of n-type quantum dot solids

Dong Yu, Brian L. Wehrenberg, Inseok Yang, Woowon Kang, and Philippe Guyot-Sionnest

Appl. Phys. Lett. 88, 072504 (2006); http://dx.doi.org/10.1063/1.2174089 (3 pages) | Cited 5 times

Online Publication Date: 15 February 2006

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Solid films of n-type Mn2+/CdSe colloidal quantum dots show a positive magnetoresistance. At 0.3 K the film resistance is increased by up to 150% at 10 T. The variable range hopping model quantitatively accounts for the magnetoresistance at low field.
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75.47.Pq Other materials
75.50.Pp Magnetic semiconductors
73.63.Kv Quantum dots
75.75.-c Magnetic properties of nanostructures

Anomalous diamagnetism of SmNiAl due to hydrogenation and field cooling

Y. T. Wang, H. Zhang, H. Y. Shao, X. G. Li, L. F. Zhang, and S. Takahashi

Appl. Phys. Lett. 88, 072505 (2006); http://dx.doi.org/10.1063/1.2174105 (3 pages) | Cited 2 times

Online Publication Date: 15 February 2006

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The field-cooled (FC) and zero-field-cooled magnetizations of SmNiAl and its hydride were measured as a function of temperature. Hydrogenation and field-cooling cause a remarkable change in magnetic properties. SmNiAl exhibits a positive susceptibility, but an anomalous diamagnetic susceptibility is observed in the FC SmNiAl hydride below 32 K and in H<1000 Oe. The diamagnetism is associated with magnetic layer structures and hydrogen ions.
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75.20.En Metals and alloys
75.30.Cr Saturation moments and magnetic susceptibilities
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Reduction of surface magnetism of Co2Cr0.6Fe0.4Al Heusler alloy films

M. Kallmayer, H. Schneider, G. Jakob, H. J. Elmers, K. Kroth, H. C. Kandpal, U. Stumm, and S. Cramm

Appl. Phys. Lett. 88, 072506 (2006); http://dx.doi.org/10.1063/1.2175486 (3 pages) | Cited 26 times

Online Publication Date: 15 February 2006

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Element specific magnetization has been determined at the surface and in the bulk of Co2Cr0.6Fe0.4Al Heusler alloy films grown on α-Al2O3 and capped by Al, using x-ray magnetic circular dichroism both in transmission and total electron yield. The magnetic moments for Co and Fe are considerably reduced at the upper surface in comparison to their values in the bulk of the film. The large reduction at room temperature of 17% for thick films averaged along the electron escape depth implies an even larger reduction at the topmost layer which is crucial for spin-dependent transport. The surface magnetization decreases additionally with respect to the bulk value with decreasing film thickness below 20 nm.
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75.50.Bb Fe and its alloys
75.70.Rf Surface magnetism
75.70.Ak Magnetic properties of monolayers and thin films
75.30.Cr Saturation moments and magnetic susceptibilities
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
78.20.Ls Magneto-optical effects

Enhancing the charge ordering temperature in thin films of Pr0.5Ca0.5MnO3 by strain

Z. Q. Yang, Y. Q. Zhang, J. Aarts, M.-Y. Wu, and H. W. Zandbergen

Appl. Phys. Lett. 88, 072507 (2006); http://dx.doi.org/10.1063/1.2172715 (3 pages) | Cited 20 times

Online Publication Date: 17 February 2006

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We report the effects of biaxial strain on the charge ordering temperature Tco of the mixed-valent manganite perovskite oxide Pr0.5Ca0.5MnO3. Thin films were grown on SrTiO3, which has a 1.3% larger in-plane lattice parameter. Other substrates were used for comparison. Transport measurements combined with data from electron microscopy show that Tco is considerably enhanced. At thicknesses of the order of 10 nm, where the films are fully strained, Tco is above 320 K, more than 70 K above the bulk value of 250 K, while around 50 nm, where relaxation has set in, the enhancement is around 40 K. The bulk value is only reached at a thickness of about 150 nm.
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68.55.-a Thin film structure and morphology
73.50.−h
75.47.Gk Colossal magnetoresistance
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