Showing papers by "Laura H. Lewis published in 2003"
TL;DR: In this paper, a large positive magnetic entropy change ΔSM is observed in the metallic antiperovskite Mn3GaC near its first-order metamagnetic transition temperature 159 K where the stoichiometric compound transforms from an antiferromagnetic to a canted ferromagnetic state accompanied by a discontinuous volume change of −0.46% without change of symmetry.
Abstract: A large positive magnetic entropy change ΔSM is observed in the metallic antiperovskite Mn3GaC near its first-order metamagnetic transition temperature 159 K where the stoichiometric compound transforms from an antiferromagnetic to a canted ferromagnetic state accompanied by a discontinuous volume change of −0.46% without change of symmetry. The unusual field dependence of the ΔSM of Mn3GaC shows a very rapid linear increase from zero to a saturation value within a field interval smaller than 1 T. The broadening of the peak of ΔSM to low temperature with increasing field change creates an ΔSM plateau in the temperature dependence of the MCE, which is of significance for practical application of these materials in the Ericsson-cycle magnetic refrigerator.
128 citations
TL;DR: In this article, a simple method of enhancing the magnetocaloric effect (MCE) in ferromagnetic materials is described, where thin layers of pure Fe of 0.1- and 0.2-μm thickness were evaporated onto both sides of polished 0.6-mm, slices of the giant magnetoric material Gd5Si1.5Ge2.
Abstract: A simple method of enhancing the magnetocaloric effect (MCE)) in ferromagnetic materials is described. Thin layers of pure Fe of 0.1- and 0.2-μm thickness were evaporated onto both sides of polished 0.6-mm, slices of the giant magnetocaloric material Gd5Si1.5Ge2.5. The slices were stacked for magnetic measurement in the temperature range from 165 to 235 K to assess the MCE. Sample geometries with the iron layers oriented both parallel and perpendicular to the applied field were measured. In the metamagnetic transition temperature range, the Fe layer in the parallel geometry lowers the onset field of the metamagnetic transition by 4240 Oe for the 0.1-μm layer thickness and by 4940 Oe for the 0.2-μm layer thickness from the base value of 8700 Oe at 187 K in the absence of Fe layers. Furthermore, the 0.1-μm layer of Fe oriented perpendicular to the applied field is found to enhance the entropy change, and thus the magnetocaloric effect, by approximately 11% above its base value at 191.5 K; intriguingly, the ...
20 citations
TL;DR: In this article, the magnetic entropy change associated with the ferromagnetic Curie transition has been studied in La 0.85 Sr 0.15 MnO 3 (LSM) perovskites obtained by both the citrate-nitrate gel autoignition and combustion thermal spray techniques.
Abstract: The magnetic entropy change associated with the ferromagnetic Curie transition has been studied in La 0.85 Sr 0.15 MnO 3 (LSM) perovskites obtained by both the citrate–nitrate gel autoignition and combustion thermal spray techniques. A modest magnetic entropy change can be observed in La 0.85 Sr 0.15 MnO 3 by sintering the precursor-derived powder at 1200 °C for 16 h or by combustion spraying the solution precursor onto an Al substrate and annealing at 800 °C for 6 h. The combustion thermal spray process with rapid deposition rates and relatively low deposition temperature directly produces a microcrystalline perovskite phase. This microcrystalline powder can be processed more rapidly and at lower temperatures than that derived from the sol–gel syntheses process to produce material with fine crystallinity and an obvious magnetocaloric effect.
16 citations
TL;DR: The relationship between the formation of crystalline nuclei or nanophases and the thermal history of the melt was studied in this paper, where it was shown that a lower melt ejection temperature produced a nanocrystalline microstructure, while higher melt ejections (T > 1723 K) largely eliminated the presence of nuclei and associated nano-phases and produced an amorphous product.
16 citations
TL;DR: In this article, large magnetization loops shifts along the field axis, of the same order as those observed in the archetypical exchange biased system Co/CoO, have been measured in a Co/MnO thin film made using reactive ion beam assisted deposition.
15 citations
TL;DR: In this article, the magnetic properties and microstructure in nanocrystalline ferrimagnetic (FIM) Y 3 Fe 3 Al 2 O 12 (Al-YIG) powder precipitated during an auto-ignition of a citrate-nitrate gel precursor onto commercial ferromagnetic (FM) CrO 2 powders were studied.
Abstract: The magnetic properties and microstructure in nanocrystalline ferrimagnetic (FIM) Y 3 Fe 3 Al 2 O 12 (Al-YIG) powder precipitated during an auto-ignition of a citrate–nitrate gel precursor onto commercial ferromagnetic (FM) CrO 2 powders were studied. The synthesis objective was to create an ideal core–shell structure of CrO 2 surrounded by Al-YIG with possible application for magnetocaloric applications; however, agglomeration of the acicular CrO 2 particles prevented individual particles from complete encapsulation by Al-YIG. This result was confirmed by electron microscopy. Due to the metastability of CrO 2 , magnetic interaction between the ferromagnetic CrO 2 and the ferrimagnetic Al-YIG is found to be mediated by antiferromagnetic (AFM) Cr 2 O 3 . Low-temperature magnetic measurement provides distinct evidence of an exchange bias shift in the major hysteresis loop with features that are consistent with interfacial exchange from two kinds of interfaces: CrO 2 (FM)/Cr 2 O 3 (AFM) and Y 3 Fe 3 Al 2 O 12 (FIM)/Cr 2 O 3 (AFM). Subsequent ex situ heat treatment of the reacted particles accelerates the CrO 2 /Cr 2 O 3 transformation and produces additional alterations to the low-temperature magnetic hysteresis loops. The results of this work provide evidence that the magnetic couplings at the FM/AFM and FIM/AFM interfaces are flexible under the action of applied fields, a conclusion that may be traced to the unique spin arrangement expected at the phase interfaces.
15 citations
TL;DR: In this article, the exchange field acting on the soft phase was determined as the field of maximum susceptibility in the soft-phase reversal portion of the second quadrant of the hysteresis loop.
Abstract: Exchange coupling across the interface in polycrystalline exchange-spring thin films of L10 CoPt and hcp Co has been investigated with magnetic measurements and micromagnetic modeling. For thick enough Co films, the reversal in the second quadrant of the hysteresis loop is multiphase, with full reversal of the soft Co phase accomplished before reversal of the hard CoPt phase. Therefore the exchange field acting on the soft phase is determined as the field of maximum susceptibility in the soft-phase reversal portion of the second quadrant. Experimentally, it is found that the exchange field is a linear function of the inverse thickness of the soft phase. This result is in agreement with the exchange field calculated from the associated micromagnetic modeling only under the condition that the magnitude of the exchange coupling constant between spins across the CoPt-Co interface is similar to that characterizing the bulk CoPt. Experimental and modeling-derived results do not agree for decreased interfacial e...
13 citations
TL;DR: In this paper, annealing at low temperatures (300-450 °C) or for short times resulted in a strengthening of the hcp Co fiber texture and disappearance of the minor fcc phase.
12 citations
TL;DR: In this article, a model for the study of interphase interactions in exchange-spring systems in thin-film form is proposed, and coupled magnetic and transmission electron microscopy studies reveal simple relationships between the volume percent of ordered L10 phase, coercivity and the magnetic exchange within the samples.
Abstract: Ferromagnetic “exchange-spring” nanocomposite systems derive their technical magnetic properties, such as the remanence and coercivity, from the details of the interphase coupling between the magnetically soft component and the magnetically hard component. An ideal model material for the study of the interphase interactions in exchange-spring systems is the CoPt system in thin-film form. Depending on the details of post-deposition annealing treatment, CoPt consists of two phases in varying proportions: the chemically disordered A1 phase with low coercivity and the chemically ordered L10 phase with high coercivity. Coupled magnetic and transmission electron microscopy studies reveal simple relationships between the volume percent of ordered L10 phase, coercivity and the magnetic exchange within the samples.
9 citations
TL;DR: In this paper, a complement of experimental characterization techniques, including positron annihilation spectroscopy (PAS), transmission electron microscopy, synchrotron X-ray diffraction and elevated temperature ac and dc magnetic measurements were applied to the study of melt-quenched stoichiometric Nd 2 Fe 14 B ribbons modified by various amounts of the alloying additions Ti and C. The experimental results indicate that an excess of free volume in the glass is associated with increased glass stability, a counterintuitive conclusion.
Abstract: A complement of experimental characterization techniques – positron annihilation spectroscopy (PAS), transmission electron microscopy, synchrotron X-ray diffraction and elevated temperature ac and dc magnetic measurements – were applied to the study of melt-quenched stoichiometric Nd 2 Fe 14 B ribbons modified by various amounts of the alloying additions Ti and C. These alloying additions are known to enhance the glass-forming ability in Nd 2 Fe 14 B melts, allowing for a wider processing window to produce homogeneous nanoscale materials with tailored magnetic properties. The experimental techniques used in this study reveal the complex multi-phase and multi-scale nature of the ribbons, a result that had escaped detection by lower-resolution techniques. The as-quenched ribbons were found to consist of at least three phases: α-Fe nanocrystals, poorly crystalline Nd 2 Fe 14 B and glass. The measured weight fraction of glass does not show a direct correspondence with quenching wheel speed, a result attributed to the complexity of the melt-spinning heat-transfer process. The Curie temperature of the glassy component of the ribbons varies in a non-systematic way with both Ti and C alloying addition content and wheel speed. PAS provides quantitative measurements of the S (or ‘shape’)-parameter which represents the size of a void or open volume in the material volume probed. The experimental results indicate that an excess of free volume in the glass is associated with increased glass stability, a counterintuitive conclusion. However, the results are consistent with the model of Sietsma and Thijsse [Phys. Rev. B 52 (5) (1995) 3248] who propose that thermal relaxation in glass causes the larger free volume regions in the amorphous structure to break up into smaller voids, which necessarily increases the total number of voids, but decreases the volume per void. This void breakup fosters the processes of cooperative diffusion and subsequent devitrification. It is concluded that the free volume concentration in the amorphous component of melt-spun Nd 2 Fe 14 B alloys produced by the highest quenching wheel speeds and Ti/C alloying addition content is thus not sufficient to allow cooperative diffusion to take place, resulting in an increased stability against devitrification.
5 citations