Single domain SmCo5@Co exchange-coupled magnets prepared from core/shell Sm[Co(CN)6]·4H2O@GO particles: a novel chemical approach.
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ABSTRACT:
SUBMITTER: Yang C
PROVIDER: S-EPMC3868969 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:Iron-rich 2:17-type Sm-Co magnets are important for their potential to achieve high coercivity and maximum magnetic energy product. But the evolution of phase structure, which determines magnetic properties, remains an unsolved issue. In this study, the phase constitution and microstructure of solution-treated 2:17-type Sm-Co alloys are studied. The increase of Fe content promotes the ordering transformation from the 1:7H phase to partially ordered 2:17R and lamellar Zr-rich 1:3R phase. This ordering transformation is mainly due to the competitive atoms occupation of Zr, Fe and Sm in the 1:7H phase. To ease this competition, we modify Sm content in iron-rich 2:17-type Sm-Co magnets. Different solution precursors and corresponding cellular structures are observed. Solution precursor with 1:7H, partially ordered 2:17R, 2:17H and 1:3R phase evolves into uneven and incomplete cellular structures, while solution precursor with partially ordered 2:17R phase forms larger cell size with less lamellar phase, thus poor coercivity and magnetic energy product. However, solution precursors with single 1:7H phase evolve into uniform cellular structures and perform high coercivity and magnetic energy product. Our results indicate if a single 1:7H phase could be obtained in solution-treated 2:17-type Sm-Co magnets with higher iron content, much higher magnetic properties could be achieved.
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Project description:During the past decade, CoFe2O4 (hard)/Co-Fe alloy (soft) magnetic nanocomposites have been routinely prepared by partial reduction of CoFe2O4 nanoparticles. Monoxide (i.e., FeO or CoO) has often been detected as a byproduct of the reduction, although it remains unclear whether the formation of this phase occurs during the reduction itself or at a later stage. Here, a novel reaction cell was designed to monitor the reduction in situ using synchrotron powder X-ray diffraction (PXRD). Sequential Rietveld refinements of the in situ data yielded time-resolved information on the sample composition and confirmed that the monoxide is generated as an intermediate phase. The macroscopic magnetic properties of samples at different reduction stages were measured by means of vibrating sample magnetometry (VSM), revealing a magnetic softening with increasing soft phase content, which was too pronounced to be exclusively explained by the introduction of soft material in the system. The elemental compositions of the constituent phases were obtained from joint Rietveld refinements of ex situ high-resolution PXRD and neutron powder diffraction (NPD) data. It was found that the alloy has a tendency to emerge in a Co-rich form, inducing a Co deficiency on the remaining spinel phase, which can explain the early softening of the magnetic material.
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Project description:BackgroundThe Prussian blue analogues represent well-known and extensively studied group of coordination species which has many remarkable applications due to their ion-exchange, electron transfer or magnetic properties. Among them, Co-Fe Prussian blue analogues have been extensively studied due to the photoinduced magnetization. Surprisingly, their suitability as precursors for solid-state synthesis of magnetic nanoparticles is almost unexplored. In this paper, the mechanism of thermal decomposition of [Co(en)3][Fe(CN)6] ?? 2H2O (1a) is elucidated, including the topotactic dehydration, valence and spins exchange mechanisms suggestion and the formation of a mixture of CoFe2O4-Co3O4 (3:1) as final products of thermal degradation.ResultsThe course of thermal decomposition of 1a in air atmosphere up to 600°C was monitored by TG/DSC techniques, (57)Fe Mössbauer and IR spectroscopy. As first, the topotactic dehydration of 1a to the hemihydrate [Co(en)3][Fe(CN)6] ?? 1/2H2O (1b) occurred with preserving the single-crystal character as was confirmed by the X-ray diffraction analysis. The consequent thermal decomposition proceeded in further four stages including intermediates varying in valence and spin states of both transition metal ions in their structures, i.e. [Fe(II)(en)2(?-NC)Co(III)(CN)4], Fe(III)(NH2CH2CH3)2(?-NC)2Co(II)(CN)3] and Fe(III)[Co(II)(CN)5], which were suggested mainly from (57)Fe Mössbauer, IR spectral and elemental analyses data. Thermal decomposition was completed at 400°C when superparamagnetic phases of CoFe2O4 and Co3O4 in the molar ratio of 3:1 were formed. During further temperature increase (450 and 600°C), the ongoing crystallization process gave a new ferromagnetic phase attributed to the CoFe2O4-Co3O4 nanocomposite particles. Their formation was confirmed by XRD and TEM analyses. In-field (5 K / 5 T) Mössbauer spectrum revealed canting of Fe(III) spin in almost fully inverse spinel structure of CoFe2O4.ConclusionsIt has been found that the thermal decomposition of [Co(en)3][Fe(CN)6] ?? 2H2O in air atmosphere is a gradual multiple process accompanied by the formation of intermediates with different composition, stereochemistry, oxidation as well as spin states of both the central transition metals. The decomposition is finished above 400°C and the ongoing heating to 600°C results in the formation of CoFe2O4-Co3O4 nanocomposite particles as the final decomposition product.
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