Exchange-coupling interaction, effective anisotropy and coercivity in nanocomposite permanent materials

Abstract

The inter-grain exchange-coupling interactions, effective anisotropy, and coercivity in nanocomposite Nd2Fe14B/α−Fe magnets were investigated. The effective anisotropy of nanocomposite magnets has been calculated starting from the statistics of boundaries between magnetically hard-hard, hard-soft, and soft-soft grains. The result shows that the effective anisotropy decreases with reduction in grain size and/or increase in soft phase components. When grain sizes reduce to 4–5 nm, Keff decreases to 1/3−1/4 of the ordinary value of K. The coercivity in nanocomposite magnets demonstrates a similar behavior. The decrement of coercivity is mainly due to the reduction of effective anisotropy. Considering the opposite varying trend the remanence demonstrates with respect to the effective anisotropy and the coercivity, we conclude that the mean grain size should be in the range of 10–15 nm and the volume fraction of soft phase should be less than 50% in order to achieve high energy product magnets.

Explosive shock processing of Pr2Fe14B/α–Fe exchange-coupled nanocomposite bulk magnets

“…Previous calculation has shown that the anisotropy constants K h and K s decrease with the reduction of grain sizes of both hard and soft magnetic phases. 32 The grain with the crystal structure 33 and the site preference of the transition metal ions in the crystal lattice in some cases. 34 The structural defects, such as dislocation or planar defect (twinning) regions introduced by the shock wave, will lead to the local changes in anisotropy constants.…”

Development of novel exchange spring magnet by employing nanocomposites of CoFe2O4 and CoFe2

“…450, 500, 550, 600, 650, and 700°C. it can be seen that the all of composite samples, show a smooth single hysteresis loop, certifying that the two different magnetic phases in the nanocomposites, means hard CoFe 2 O 4 and soft CoFe 2 are exchanged coupled to each other [24]. Indeed soft magnetic moments rotate along with the hard phase due to its external magnetic field and therefore the magnetization and demagnetization of the magnet have a single magnetic phase features.…”

Micromagnetic Simulations of Anisotropies in Coupled and Uncoupled Ferromagnetic Nanowire Systems

“…This value is significantly smaller than the Fe exchange length which exceeds 20 nm [27]. The other physical parameters were exchange constant A = 2 · 10 −11  J/m, magnetic polarization at saturation J s = 2.1 T, and the Gilbert damping constant α = 0.01 [28].…”