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Li, Jing

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Interphase exchange coupling in Fe/Sm–Co bilayers with gradient Fe thickness

2005-09-30 , Yu, Ming-hui , Hattrick-Simpers, Jason , Takeuchi, Ichiro , Li, Jing , Wang, Z. L. (Zhong Lin) , Liu, J. Ping , Lofland, S. E. , Tyagi, Somdev , Freeland, J. W. , Giubertoni, D. , Bersani, M. , Anderle, M.

We have fabricated Fe/Sm–Co bilayers with gradient Fe thicknesses in order to systematically study the dependence of exchange coupling on the thickness of the Fe layer. The Fe layer was deposited at two different temperatures (150 and 300 °C) to study the effect of deposition temperature on the exchange coupling. Magneto-optical Kerr effect and x-ray magnetic circular dichroism (XMCD) have been employed as nondestructive rapid characterization tools to map the magnetic properties of the gradient samples. Systematic enhancement in exchange coupling between the soft layer and the hard layer is observed as the soft layer thickness is decreased. Separate exchange couplings of the Fe layer with Co and Sm in the hard layer are revealed through measuring the element-specific hysteresis curves using XMCD. The single-phase-like magnetization reversal critical thickness increases from 12 nm for Fe deposited at 150 °C to 24 nm for Fe deposited at 300 °C, indicating an important role of the state of the interface in the exchange coupling.

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Bulk nanocomposite magnets produced by dynamic shock compaction

2004-07-15 , Chen, K. H. , Jin, Z. Q. , Li, Jing , Kennedy, G. , Wang, Z. L. (Zhong Lin) , Thadhani, Naresh N. , Zeng, Hao , Cheng, S.-F. , Liu, J. Ping

Exchange-coupled R₂Fe₁₄B/α -Fe (R = Nd or Pr) nanocomposite bulk magnets with nearly full density have been successfully produced by shock compaction of melt-spun powders. X-ray diffraction and transmission electronic microscopy analyses of the shock-consolidated compacts showed no grain growth upon compaction, in fact, a decrease in the crystallite size of both the hard and soft phases was observed. As a consequence, magnetic properties were retained and even improved after compaction. Hysteresis loops of the shock-consolidated powder compacts showed a smooth single-phaselike behavior, indicating effective exchange coupling between hard and soft magnetic phases.

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Interparticle interactions in annealed FePt nanoparticle assemblies

2002-09 , Zeng, Hao , Li, Jing , Wang, Z. L. (Zhong Lin) , Liu, J. Ping , Sun, Shouheng

Thermal treatment of self-assembled FePt nanoparticles reduces the interparticle distances, resulting in dramatic changes in the type and strength of interparticle interactions. Consequently, magnetic properties such as the hysteresis and magnetization reversal mechanisms are strongly affected. It is suggested that controlled annealing of self-assembled nanoparticles may offer a novel approach for producing hard magnetic nanocomposites.

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Grain size dependence of magnetic properties in shock synthesized bulk Pr₂Fe₁₄B/α-Fe nanocomposites

2004-09-15 , Jin, Z. Q. , Thadhani, Naresh N. , McGill, M. , Li, Jing , Ding, Yong , Wang, Z. L. (Zhong Lin) , Zeng, Hao , Chen, M. , Cheng, S.-F. , Liu, J. Ping

The structural and magnetic properties of the melt-spun Pr₂Fe₁₄B/α-Fe nanocomposite powders consolidated via shock-wave compression and subjected to postshock thermal treatment were investigated. Shock compression results in grain refinement, which leads to a reduction of an effective anisotropy and therefore an increase in the ferromagnetic exchange length, resulting in an enhanced exchange coupling in fully consolidated bulk magnets. A small amount of amorphous phase formed during the shock compression were observed to crystallize into Pr₂Fe₁₄B upon annealing above 600 °C. The heat treatment also results in the recovery of coercivity partially lost during the consolidation, which can be related directly to the dependence of the effective anisotropy on the grain size, as illustrated by the transmission electron microscopy observation of grain refinement in the shock-consolidated bulk samples. A uniform grain morphology is suggested as a means for further increasing the magnetic properties of bulk nanocomposites.

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Interface structures in FePt/Fe₃Pt hard-soft exchange-coupled magnetic nanocomposites

2003-05-26 , Li, Jing , Wang, Z. L. (Zhong Lin) , Zeng, Hao , Sun, Shouheng , Liu, J. Ping

Self-assembly of FePt and Fe₃O₄ nanoparticles of different sizes led to various FePt–Fe₃O₄ nanocomposites. Annealing the composite under reducing atmosphere at 650 and 700 °C induced magnetically hard FePt phase and magnetically soft Fe₃Pt phase. The FePt and Fe₃Pt phases were either linked by a common interface or coexisted within one grain as domains with sizes <10 nm. This ensures the effective exchange coupling of magnetically hard and soft phases. High-resolution transmission electron microscopy studies provide detailed structural characterization for the FePt based nanocomposites.

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Characterization of nanometer-scale defects in metallic glasses by quantitative high-resolution transmission electron microscopy

2002-03-25 , Li, Jing , Wang, Z. L. (Zhong Lin) , Hufnagel, T. C.

Although defects can have a significant effect on the properties of amorphous materials, in many cases these defects are poorly characterized and understood. This is at least partly due to the difficulty of imaging defects in amorphous materials in the electron microscope. In this work, we demonstrate the utility of quantitative analysis of high-resolution transmission electron microscopy for the identification and characterization of nanometer-scale defects in metallic glasses. For a proper identification of such defects, it is important to carefully consider the effects of the imaging conditions and thickness variations in the sample, both of which we describe in detail. As an example, we show that regions of localized plastic deformation (shear bands) in bulk metallic glasses contain a high concentration of nanometer-scale voids. These voids apparently result from the coalescence of excess free volume once the applied stress is removed.

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Tailoring magnetic properties of core/shell nanoparticles

2004-08-02 , Zeng, Hao , Sun, Shouheng , Li, Jing , Liu, J. Ping , Wang, Z. L. (Zhong Lin)

Bimagnetic FePt/MFe₂O₄ sM=Fe,Cod core/shell nanoparticles are synthesized via high-temperature solution phase coating of 3.5 nm FePt core with MFe₂O₄ shell. The thickness of the shell is controlled from 0.5 to 3 nm. An assembly of the core/shell nanoparticles shows a smooth magnetization transition under an external field, indicating effective exchange coupling between the FePt core and the oxide shell. The coercivity of the FePt/Fe₃O₄ particles depends on the volume ratio of the hard and soft phases, consistent with previous theoretical predictions. These bimagnetic core/shell nanoparticles represent a class of nanostructured magnetic materials with their properties tunable by varying the chemical composition and thickness of the coating materials.

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Synthesis, chemical ordering, and magnetic properties of FePtCu nanoparticle films

2003-05-15 , Sun, Xiangcheng , Kang, Shishou , Harrell, J. W. , Nikles, David E. , Dai, Z. R. , Li, Jing , Wang, Z. L. (Zhong Lin)

FePtCu nanoparticles with varying composition were prepared by the simultaneous polyol reduction of platinum acetylacetonate and copper bis(2,2,6,6-tetramethyl-3,5-heptanedionate) and the thermal decomposition of iron pentacarbonyl. As prepared the particles had a fcc structure with an average diameter of 3.5 nm and were superparamagnetic. Heat treatment of the self-assembled films at temperatures above 550 °C transformed the particles from the fcc to the L1₀ phase, give in-plane coercivities as high as 9000 Oe. X-ray diffraction revealed that the Cu remained in the films and the presence of an extra peak, indicating a second phase was present. Consistent with two or more phases, the magnetic hysteresis curves could be decomposed into a hard component (H[subscript c]c>5000 Oe) and a soft component (H[subscript c]c<2000 Oe). Unlike our earlier results for Ag in FePt, adding Cu to FePt did not lower the temperature required for phase transformation from the fcc to the L1₀ phase.

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