Irreversible magnetization and self-interaction investigated by thermal activation in Pr2Fe14B magnets

Abstract

The irreversible magnetization is investigated by thermal activation, which results from the nucleation of reversed domain from defect region at grain surface to perfect region within grain owing to the self-interaction. In the irreversible magnetization, the self-interaction unit involves two parts, i.e., the domain wall in perfect region and a small part in defect region. A larger volume of defect region involved in the self-interaction unit leads to the significant reduction of energy barrier, resulting in a lower coercivity. These investigations demonstrate that magnetization behavior could be manipulated by the self-interaction at grain outer layer.

Uniform magnetization reversal in dual main-phase (Ce,Nd)2Fe14B sintered magnets with inhomogeneous microstructure

“…Even with the hybrid structure, the squareness of the demagnetization curve does not deteriorate, which arouses the interest for unveiling the magnetization reversal process. Coercivity is determined in the irreversible magnetization [22]. Thermal activation, resulting from the energy barrier being overcome due to thermal fluctuation, is the irrevers ible magnetization reversal [23].…”

“…Thermal activation is magnetization reversal over the activation volume with the driving of the thermal [24][25][26], the activation size d active is in the several nanometer range. Due to the low anisotropy at the grain boundary, in sintered magnets and exchangecoupled magnets the irrevers ible magnetization occurs via the nucleation and free motion of the reversed domain wall at the grain outerlayer [22]. The size of the domain wall is several nanometers according to the formula of / δ π = A K m (A, 5.6-7.8 × 10 −12 J m −1 , is the exchange coefficient, and K, 1.5-5.6 MJ m −3 , is magnetoaniso tropy for Ce 2 Fe 14 B and Nd 2 Fe 14 B at a temperature of 300 K).…”

“…The size of the domain wall is several nanometers according to the formula of / δ π = A K m (A, 5.6-7.8 × 10 −12 J m −1 , is the exchange coefficient, and K, 1.5-5.6 MJ m −3 , is magnetoaniso tropy for Ce 2 Fe 14 B and Nd 2 Fe 14 B at a temperature of 300 K). The activation size d active is comparable to the domain wall size, so the magnetization reversal should undergo the nuclea tion of reversed domain wall and the subsequent domain wall free motion [22]. In conventional Nd 2 Fe 14 B sinter ed magnets, the nucleation of the reversed domain occurs at the grain boundary [27].…”

“…For a small addition to the amount of Ce 9.5 Nd 4 Fe 80 B 6.5 powders, the coercivity does not decrease largely. For this reason, the nucleation of the reversed domain cannot occur independently in the Cerich main phase, which implies the existence of exchange coupling against magnetization [28], resulting in the increase in the size of the thermal activation [22]. The exchange coupling effect leads to the decrease in the size of the domain wall in irreversible magnetization [29].…”

Abnormal variation of magnetic properties with Ce content in (PrNdCe) 2 Fe 14 B sintered magnets prepared by dual alloy method

“…[25] Magnetization reversal and coercivity are strongly dependent on the selfinteraction between perfect and defect regions at the grain outer layer. [34] As shown in Fig. 4(c), Ce enrichment whereas a lack of PrNd in the intergranular phase implies the inhomogeneous distribution of rare earth elements near the grain boundary, which possibly leads to an abrupt change in anisotropy and may vary the characteristics of self-interaction at the grain outer layer.…”