Stress, strain and technical magnetic properties in 'exchange-spring' Nd2Fe14B+alpha-Fe nanocomposite magnets

A modified Williamson-Hall analysis was applied to synchrotron transmission x-ray diffraction data obtained for a series of three melt-spun magnetic n anocomposite 'exchange-spring' materials comprised of Nd2Fe14B and varying amounts of alpha -Fe ranging from 0-32 vol%. Analysis of the data demonstra tes that there is no measureable contribution from microstrains or other la ttice distortions in the Nd2Fe14B phase. This result: indicates that the Nd 2Fe14B Bragg peak broadening arises from particle size only ton the order o f 440 Angstrom and that the shape of the melt-quenched Nd2Fe14B grains is a pproximately spherical. In contrast, the alpha -Fe phase exhibits Bragg pea k broadening that may be attributed to a combination of both anisotropic st rain and particle-size broadening effects.. The average particle size of th e alpha -Fe phase is 360 Angstrom, significantly larger than that determine d from the Scherrer formula, and the average stress, deduced to be tensile, of the alpha -Fe particles is approximately 1 GPa, which translates to an rms strain (e) over tilde = [e(2)](1/2) of approximately 0.1%. These result s provide a physical basis to explain both the marginal exchange-derived re manence enhancement and the anomalous elevated-temperature coercivities fou nd in these alloys.