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Stress, strain and technical magnetic properties in `exchange-spring' Nd2Fe14B [iopmath latex="$+\alpha$"] + [/iopmath] -Fe nanocomposite magnets

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A modified Williamson-Hall analysis was applied to synchrotron transmission x-ray diffraction data obtained for a series of three melt-spun magnetic nanocomposite `exchange-spring' materials comprised of Nd2Fe14B and varying amounts of [iopmath latex="$\alpha$"] [/iopmath] -Fe ranging from 0-32 vol%. Analysis of the data demonstrates that there is no measureable contribution from microstrains or other lattice distortions in the Nd2Fe14B phase. This result indicates that the Nd2Fe14B Bragg peak broadening arises from particle size only (on the order of 440 Å) and that the shape of the melt-quenched Nd2Fe14B grains is approximately spherical. In contrast, the [iopmath latex="$\alpha$"] [/iopmath] -Fe phase exhibits Bragg peak broadening that may be attributed to a combination of both anisotropic strain and particle-size broadening effects. The average particle size of the [iopmath latex="$\alpha$"] [/iopmath] -Fe phase is 360 Å, significantly larger than that determined from the Scherrer formula, and the average stress, deduced to be tensile, of the [iopmath latex="$\alpha$"] [/iopmath] -Fe particles is approximately 1 GPa, which translates to an rms strain [iopmath latex="$\tilde{e}=\langle e^2\rangle^{1/2}$"] = e21/2 [/iopmath] of approximately 0.1%. These results provide a physical basis to explain both the marginal exchange-derived remanence enhancement and the anomalous elevated-temperature coercivities found in these alloys.

Document Type: Miscellaneous

Affiliations: 1: Materials and Chemical Sciences Division, Department of Energy Sciences and Technology, Brookhaven National Laboratory, Upton, NY 11973-5000, USA 2: Magnequench International, Inc. (MQI), 6435 Scatterfield Road, Anderson, IN 46013, USA

Publication date: May 1, 2001

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