Abstract
Nanoanalytical TEM characterization in combination with finite element micromagnetic modelling clarifies the impact of the grain misalignment and grain boundary nanocomposition on the coercive field and gives guidelines how to improve coercivity in Nd-Fe-B based magnets. The nanoprobe electron energy loss spectroscopy measurements obtained an asymmetric composition profile of the Fe-content across the grain boundary phase in isotropically oriented melt-spun magnets and showed an enrichment of iron up to 60 at% in the Nd-containing grain boundaries close to Nd2Fe14B grain surfaces parallel to the c-axis and a reduced iron content up to 35% close to grain surfaces perpendicular to the c-axis. The numerical micromagnetic simulations on isotropically oriented magnets using realistic model structures from the TEM results reveal a complex magnetization reversal starting at the grain boundary phase and show that the coercive field increases compared to directly coupled grains with no grain boundary phase independently of the grain boundary thickness. This behaviour is contrary to the one in aligned anisotropic magnets, where the coercive field decreases compared to directly coupled grains with an increasing grain boundary thickness, if Js value is > 0.2 T, and the magnetization reversal and expansion of reversed magnetic domains primarily start as Bloch domain wall at grain boundaries at the prismatic planes parallel to the c-axis and secondly as Néel domain wall at the basal planes perpendicular to the c-axis. In summary our study shows an increase of coercive field in isotropically oriented Nd-Fe-B magnets for GB layer thickness > 5 nm and an average Js value of the GB layer < 0.8 T compared to the magnet with perfectly aligned grains.
Highlights
The increasing demand of high-performance rare earth permanent magnets with a high coercive field and an energy density product value suitable for large scale applications in wind turbines and electrically powered automotive devices led to the development of heavy rare earth lean/rare earthfree Nd-Fe-B based magnets and to the optimization of the complex multiphase microstructure of the magnets [1]
The polycrystalline microstructure of a rapidly quenched MQU-F ribbon with isotropic orientated c-axis of hard magnetic Nd-Fe-B grains with a size ranging from 20 nm to over 100 nm is shown in the transmission electron microscope (TEM) bright field (BF) and high angle annular dark field (HAADF) images of Figure 4
The contrast of the TEM-BF image is originated by the combination of orientation/diffraction contrast and absorption contrast, which depends on the thickness and average density of the TEM specimen leading to the bright contrast of the grain boundary (GB)-phase
Summary
The increasing demand of high-performance rare earth permanent magnets with a high coercive field and an energy density product value suitable for large scale applications in wind turbines and electrically powered automotive devices led to the development of heavy rare earth lean/rare earthfree Nd-Fe-B based magnets and to the optimization of the complex multiphase microstructure of the magnets [1]. The hard magnetic properties are primarily controlled by the size, shape, and misalignment of the hard magnetic grains and their distributions and secondarily by the occurrence of other nonmagnetic and soft magnetic phases [2,3,4]. The coercive field strongly depends on the intergranular grain boundary (GB) phases separating the hard magnetic grains [5, 6]. The role of dopant elements, the thickness, and magnetic properties of the GB-phases have extensively been studied during the last 30 years [7, 8]. Local changes of the exchange coupling between grains and the decrease of the anisotropy field and demagnetizing field at/near intergranular phases considerably reduce the overall coercive field. First principles ab initio calculations claimed that even an antiparallel exchange coupling between a crystalline α-Fe phase and the prismatic {100} planes of Nd2Fe14B would be energetically favorable, while a positive exchange-coupling constant was predicted in the Nd2Fe14B (001)/α-Fe interface [9]
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