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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 7528-7530 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experimental studies were performed on model bilayers of fully ordered L10 CoPt with thicknesses of 25, 50, or 100 nm, capped by Co of varying thicknesses from 2.6–75 nm. The bilayers were subjected to different annealing conditions; transmission electron microscopy and x-ray diffraction supplied data concerning the crystallographic structure, microstructure, and texture of the constituent phases in the bilayers. The degree of exchange coupling was investigated primarily with magnetic measurement and magnetic force microscopy. Samples subjected to annealing treatments at 300 °C showed evidence of grain growth and increased texture of the hexagonally close-packed (hcp) cobalt layer relative to the as-deposited condition. The degree of exchange coupling in these materials improved only slightly with annealing, as evidenced by both magnetic recoil curve measurement and magnetic force microscopy. Samples subjected to a higher-temperature annealing treatment of 550 °C evidenced dramatic improvement in the magnetic coupling, an improvement attributed to interdiffusion of CoPt and Co and the formation of alloy phases, specifically face-centered-cubic and hcp Co–Pt solid solutions. © 2001 American Institute of Physics.
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 4783-4785 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A model has been developed to examine the effect that different distributions of pinning and nucleation fields have upon the reversible magnetization behavior. The model allows each grain to be in one of three states, saturated in the positive and negative directions or containing a domain wall. The nucleation and pinning field distributions determine the rate of change of the number of grains in the three states with field. The results indicate that reversible magnetization curves display a well-defined minimum where the domain wall pinning fields exceed the reverse domain nucleation fields. Where the domain wall nucleation fields exceed the domain wall pinning fields no minimum is seen. The exact shape of the reversible magnetization curve and the position of the minimum are indicative of the dynamics of the reversal process. This model is able to reproduce the form of reversible magnetization curves measured previously in a sintered isotropic NdFeB based magnet and a Sm2(Co, Fe, Cu, Zr)17 type magnet. © 2000 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 4735-4737 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Elevated-temperature superconducting quantum interference device magnetometry was employed to investigate the detailed phase constitution of nominally glassy stoichiometric Nd2Fe14B melt-spun ribbons modified with 6 wt % Ti/C and subjected to various degrees of quenching by circumferential wheel speed variations of 40, 35, and 20 m/s. While previous microstructural characterization indicated that these materials were completely amorphous, ac susceptibility and magnetization measurements analyzed within the framework of superparamagnetism indicates that the materials actually contain a concentration of crystalline phase clusters of Nd2Fe14B and α-Fe that remained undetected by previous microstructural characterization due to their small size (diameter 〈5 nm) and inhomogeneous spatial distribution. The Curie temperatures of the glassy component increase with decreasing wheel speed, while the amount of glass varies in a systematic manner from 78 to 91 wt %. The remaining phases in the quenched product are Nd2Fe14B and α-Fe. Analysis of the distribution and size of the α-Fe clusters provides insight into the nucleation and growth process that ultimately produces the crystallized microstructure associated with high energy-product melt-spun Nd2Fe14B-based magnets. © 2000 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 4744-4746 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: To understand the effects of nucleation fields and intergranular dipolar interactions on the magnetization reversal mechanism, recoil curves from the major hysteresis loop have been measured on a sample of sintered PrFeB as a function of temperature from 150 to 300 K. At room temperature the reversible magnetization behavior indicates a reversal mechanism of nucleation of domain walls whose motion after nucleation is resisted by dipolar fields. As the temperature is reduced, the coercivity, and hence the nucleation field, is observed to increase while the dipolar fields, dependent on microstructure and saturation magnetization, remain approximately constant. These temperature-dependent changes in the relative magnitudes of the dipolar field and nucleation field cause the reversible magnetization behavior to change from domain wall motion to rotation. This change in behavior is attributed to the supposition that at temperatures where the nucleation field exceeds the dipolar field, once nucleated, domain walls are swept out of the material. © 2000 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 6570-6572 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: DC demagnetization (DCD), isothermal remanent magnetization (IRM), and reversible magnetization measurements were used to examine the magnetic reversal behavior in melt-quenched samples of composition Nd13.9(Fe0.92Co0.08)80.3B5.3Ga0.5 subjected to different levels of thermomechanical deformation (die upsetting). The reversible magnetization measurements indicate that in the thermally demagnetized condition domain walls are present within the material for all levels of deformation. During demagnetization from a saturated state, domain walls are present in a minority of grains at low levels of deformation but these domain walls disappear at higher levels of deformation. This behavior is explained in terms of changes in the relative strengths of nucleation and pinning fields. Comparison of these results to previous results obtained on similar materials indicate that changes in the magnetization behavior with degree of deformation are more marked in samples which are free of Ga, indicating that Ga additions act to suppress the nucleation of new domain walls, presumably in the grain boundaries. © 2000 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 5091-5093 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Determination of the quantitative relationship between crystallographic texture and magnetic properties in advanced permanent magnets may be hampered by complex microstructures, which complicate methods that rely on diffraction, or by interparticulate interactions, which adversely affect methods based on magnetic remanence measurements. To this end, new techniques in the determination of texture of bulk permanent magnets are being explored to overcome these inherent experimental difficulties. The analysis of inverse paramagnetic susceptibility measurements constitutes a new method to investigate crystallographic texture. Such measurements also provide Curie temperature data, which are sensitive to chemical changes that may have occurred in the magnetic phase during processing. The mathematical formalism underlying the analysis of inverse susceptibility measurements is outlined, and is used to evaluate magnetic measurements taken from a series of Nd2Fe14B magnets that have been processed by different means, and thus contain different degrees of texture. While this method does provide qualitative information concerning the relative crystallographic alignment of magnet samples, it needs calibration to obtain an explicit value for a texture order parameter. © 1997 American Institute of Physics.
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  • 7
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Quantitative evidence of a ferromagnetic Fe-rich minor phase present in both melt-quenched/die-upset and sintered magnets based on the Nd2Fe14B composition is presented. Full hysteresis loops were obtained at elevated temperatures (650 K≤T≤800 K) and were subsequently decomposed to obtain the saturation magnetization Ms of the minor phase as well as the paramagnetic slope of the 2-14-1 major phase as a function of temperature. Assuming a composition of pure iron, the calculated volume % of the impurity phase in the die-upset magnets is consistent with that obtained from previous observations of the widths and geometry of the intergranular phase found in the same magnets. The magnetization of the ferromagnetic impurity phase in the melt-quenched magnets decreases more rapidly than that of pure iron; extrapolation indicates a Curie temperature in the range 925 K≤T≤975 K. The paramagnetic susceptibility of the Nd2Fe14B main phases exhibits Curie–Weiss behavior with the same paramagnetic Curie temperature for both sintered and die-upset samples. The Curie constants differ, however, probably due to the different degrees of crystallographic alignment. © 1996 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 351-360 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Melt-quenched and thermomechanically deformed samples with the nominal compositions Pr18Co82, Pr18Co81C, and Pr18Co76C6 were examined with optical and electron microscopy, differential thermal analysis, and superconducting quantum interference device magnetometry that was performed in the temperature range 20 K≤T≤300 K. At room temperature, Pr18Co82 exhibits poor coercivity and remanence, 600 Oe and 2.9 kG, respectively. Pr18Co81C exhibits relatively superior remanence, 7.7 kG, but poor coercivity, 2.7 kOe, while Pr18Co76C6 exhibits the opposite trend, BR=4.5 kG and Hci=12 kOe. The main phase present in all samples, PrCo5, has basically the same character and morphology for all three samples and shows no evidence of intragranular carbon, which is demonstrated to reside in the impurity phases. The superior coercivity found in Pr18Co76C6 is attributed to a previously unknown triclinic phase, Pr3Co4Cx (x≈3–4) that appears to undergo a magnetic–nonmagnetic transition with increasing temperature around T=80 K. The variety of magnetic properties exhibited by each sample is due to the variety of minor phases present in each sample, which may be a product of the effect that carbon has on the solidification rate of the parent alloy. © 1996 American Institute of Physics.
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  • 9
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: It is known that the addition of alloying elements to the Nd2Fe14B-based alloys significantly inhibits the onset of the crystallization in particles produced by the inert gas atomization (IGA) process. Transmission electron microscopy (TEM) was performed on a selection of as-atomized particles synthesized by IGA methods with the addition of 3 wt % TiC. TEM samples were successfully prepared using the wedge technique. Large particles (〉10 μm) tend to consist of well-defined grains of Nd2Fe14B while the submicron size particles are generally amorphous and particles with diameter d in the size range, 1 μm〈d〈10 μm, contain nanocrystallites dispersed evenly throughout the amorphous matrix. The selected area electron diffraction pattern of the nanocrystallites in powder particles with diameters below 10 μm generally show three or more diffused diffraction rings that are indicative of α-Fe, with a particulate size of several tens of angstroms. These observations demonstrate that while the alloying addition of Ti and C in the IGA process significantly suppresses the formation of properitectic α-Fe in the Nd2Fe14B alloy, they do not completely prevent it. © 1997 American Institute of Physics.
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  • 10
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The Jiles–Atherton theory is based on considerations of the dependence of energy dissipation within a magnetic material resulting from changes in its magnetization. The algorithm based on the theory yields five computed model parameters, MS, a, α, k, and c, which represent the saturation magnetization, the effective domain density, the mean exchange coupling between the effective domains, the flexibility of domain walls and energy-dissipative features in the microstructure, respectively. Model parameters were calculated from the algorithm and linked with the physical attributes of a set of three related melt-quenched permanent magnets based on the Nd2Fe14B composition. Measured magnetic parameters were used as inputs into the model to reproduce the experimental hysteresis curves. The results show that two of the calculated parameters, the saturation magnetization MS and the effective coercivity k, agree well with their directly determined analogs. The calculated a and α parameters provide support for the concept of increased intergranular exchange coupling upon die upsetting, and decreased intergranular exchange coupling with the addition of gallium. © 1996 American Institute of Physics.
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