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ItemInterphase exchange coupling in Fe/Sm–Co bilayers with gradient Fe thickness(Georgia Institute of Technology, 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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ItemExplosive shock processing of Pr₂Fe₁₄B/α–Fe exchange-coupled nanocomposite bulk magnets(Georgia Institute of Technology, 2005-03) Jin, Z. Q. ; Thadhani, Naresh N. ; McGill, M. ; Ding, Yong ; Wang, Z. L. (Zhong Lin) ; Chen, M. ; Zeng, Hao ; Chakka, V. M. ; Liu, J. PingExplosive shock compaction was used to consolidate powders obtained from melt-spun Pr₂Fe₁₄B/α–Fe nanocomposite ribbons, to produce fully dense cylindrical compacts of 17–41-mm diameter and 120-mm length. Characterization of the compacts revealed refinement of the nanocomposite structure, with approximately 15 nm uniformly sized grains. The compact produced at a shock pressure of approximately 1 GPa maintained a high coercivity, and its remanent magnetization and maximum energy product were measured to be 0.98 T and 142 kJ/m³, respectively. The compact produced at 4–7 GPa showed a decrease in magnetic properties while that made at 12 GPa showed a magnetic softening behavior. However, in both of these cases, a smooth hysteresis loop implying exchange coupling and a coercivity of 533 kA/m were fully recovered after heat treatment. The results illustrate that the explosive compaction followed by post-shock heat treatment can be used to fabricate exchange-coupled nanocomposite bulk magnets with optimized magnetic properties.
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ItemAmorphization and ultrafine-scale recrystallization in shear bands formed in shock-consolidated Pr₂Fe₁₄B/α -Fe nanocomposite magnets(Georgia Institute of Technology, 2004-09-20) Wang, Z. L. (Zhong Lin) ; Jin, Z. Q. ; Liu, J. Ping ; Thadhani, Naresh N.Amorphization and ultrafine-scale recrystallization within shear bands formed in shock-consolidated Pr₂Fe₁₄B/α -Fe nanocomposite magnetic powder compacts have been observed using transmission electron microscopy. The shear bands span through multiple grain lengths and truncate preexisting ~25 nm hard and soft magnetic phase grains, resulting in further grain size refinement. The shear bands contain nanocrystallites (<10 nm size) interdispersed in an amorphous matrix, which suggests the occurrence of shock-induced phase transition in localized regions of the shear bands, and provides insight into the process of deformation of nanocrystalline materials under coupled high-strain-rate and high-pressure conditions.
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ItemGrain size dependence of magnetic properties in shock synthesized bulk Pr₂Fe₁₄B/α-Fe nanocomposites(Georgia Institute of Technology, 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. PingThe 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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ItemTailoring magnetic properties of core/shell nanoparticles(Georgia Institute of Technology, 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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ItemBulk nanocomposite magnets produced by dynamic shock compaction(Georgia Institute of Technology, 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. PingExchange-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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ItemControlling the crystallization and magnetic properties of melt-spun Pr₂Fe₁₄B/α Fe nanocomposites by Joule heating(Georgia Institute of Technology, 2004-05-12) Jin, Z. Q. ; Cui, B. Z. ; Liu, J. Ping ; Ding, Yong ; Wang, Z. L. (Zhong Lin) ; Thadhani, Naresh N.Pr₂Fe₁₄B/α Fe based nanocomposites have been prepared through crystallization of melt-spun amorphous Pr₇Tb₁Fe₈₅Nb ₀.₅ Zr ₀.₅ B₆ ribbons by means of ac Joule heating while simultaneously monitoring room-temperature electrical resistance R. The R value shows a strong variation with respect to applied current I, and is closely related to the amorphous-to-nanocrystalline phase transformation. The curve of R versus I allows one to control the crystallization behavior during Joule heating and to identify the heat-treatment conditions for optimum magnetic properties. A coercivity of 550 kA/m and a maximum energy product of 128 kJ/m³ have been obtained upon heating the amorphous ribbons at a current of 2.0 A. These properties are around 30% higher than the values of samples prepared by conventionally (furnace) annealed amorphous ribbons.
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ItemInterface structures in FePt/Fe₃Pt hard-soft exchange-coupled magnetic nanocomposites(Georgia Institute of Technology, 2003-05-26) Li, Jing ; Wang, Z. L. (Zhong Lin) ; Zeng, Hao ; Sun, Shouheng ; Liu, J. PingSelf-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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ItemInterparticle interactions in annealed FePt nanoparticle assemblies(Georgia Institute of Technology, 2002-09) Zeng, Hao ; Li, Jing ; Wang, Z. L. (Zhong Lin) ; Liu, J. Ping ; Sun, ShouhengThermal 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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ItemExchange-coupled FePt nanoparticle assembly(Georgia Institute of Technology, 2002-04-08) Zeng, Hao ; Sun, Shouheng ; Vedantam, T. S. ; Dai, Z. R. ; Liu, J. Ping ; Wang, Z. L. (Zhong Lin)We have produced exchange-coupled FePt nanoparticle assemblies by chemical synthesis and subsequent thermal annealing. As the interparticle distances decrease by tuning the annealing conditions, interparticle interactions change from dipolar type to exchange type, and the magnetization reversal mechanism switches from rotation controlled to domain-nucleation controlled. With increasing annealing temperature, the coercivity first increases due to improved chemical ordering, and then drops significantly, resulting from excessive interparticle exchange coupling. For the samples exhibiting exchange coupling, both the remanence ratio and coercive squareness increase.